DP31E

ExxonMobil ProprietaryMARINE TERMINAL Section Page

OFFSHORE BERTH DESIGN XXXI-E 1 of 66

DESIGN PRACTICES December, 2001

ExxonMobil Research and Engineering Company – Fairfax, VA

CONTENTSSection Page

SCOPE ............................................................................................................................................................6

REFERENCES.................................................................................................................................................6

GENERAL .......................................................................................................................................................7TYPES.....................................................................................................................................................7

Multiple Buoy Moorings (MBM).............................................................................................................7Single Point Moorings (SPM)................................................................................................................8

SELECTION OF SYSTEMS ..................................................................................................................10ADDITIONAL REFERENCES................................................................................................................10

DESIGN CONSIDERATIONS ........................................................................................................................11LISTING OF HANDLED PRODUCTS ...................................................................................................11THROUGHPUTS...................................................................................................................................12TANKER LISTING.................................................................................................................................12ENVIRONMENTAL DATA .....................................................................................................................12LOCATION ............................................................................................................................................12

Pipeline Length...................................................................................................................................12Maneuvering.......................................................................................................................................12Water Depths......................................................................................................................................14Other Location Considerations ...........................................................................................................15

GEOTECHNICAL / GEOPHYSICAL......................................................................................................15INITIAL FACILITY SIZING.....................................................................................................................15SUPPORT FACILITIES .........................................................................................................................15FACTORS INFLUENCING MOORING LOADS.....................................................................................15

Waves.................................................................................................................................................16Wind and Current ...............................................................................................................................16Additional Considerations for Single Point Moorings ..........................................................................17Conclusions ........................................................................................................................................17

DETERMINATION OF ENVIRONMENTAL DESIGN CRITERIA...........................................................17

MULTI BUOY MOORING SYSTEMS ............................................................................................................18ORIENTATION......................................................................................................................................18LAYOUT ................................................................................................................................................18

CBB Ship Anchor Layout Considerations ...........................................................................................19CBB Buoy Layout Considerations ......................................................................................................20ABB Buoy Layout Considerations.......................................................................................................22

MOORING BUOYS ...............................................................................................................................22General Buoy Requirements ..............................................................................................................23Bouyancy Requirements.....................................................................................................................23Description of Buoys and Advantages / Disadvantages .....................................................................23

MOORING LEGS ..................................................................................................................................24Chain ..................................................................................................................................................26Anchors ..............................................................................................................................................26

Changes shown by ➧

ExxonMobil ProprietarySection Page MARINE TERMINAL

XXXI-E 2 of 66 OFFSHORE BERTH DESIGNDecember, 2001 DESIGN PRACTICES


CONTENTS (Cont)Section Page

PREVENTER WIRES ...........................................................................................................................29ANCILLARY EQUIPMENT....................................................................................................................29

Quick Release Buoy Hooks................................................................................................................29Marker Buoys - General .....................................................................................................................30Navigational and Buoy Lights.............................................................................................................30

ADVANTAGES AND DISADVANTAGES..............................................................................................31

CATENARY ANCHOR LEG MOORING SYSTEMS .....................................................................................31BACKGROUND ....................................................................................................................................31BUOY COMPONENTS .........................................................................................................................34

Hull .....................................................................................................................................................34Rotating Deck.....................................................................................................................................34Fluid Swivel ........................................................................................................................................35Piping .................................................................................................................................................35Navaids and Electrical........................................................................................................................36Winch, Handling Equipment, Maintenance Aids.................................................................................36Component Design Requirements .....................................................................................................36

ANCHOR LEGS ....................................................................................................................................37MOORING LINES AND TANKER FITTINGS ........................................................................................37

Derating Factors.................................................................................................................................39Tankers 100,000 - 350,000 DWT.......................................................................................................39Tankers larger than 350,000 DWT .....................................................................................................40Safety Factors ....................................................................................................................................40

FLOATING HOSES...............................................................................................................................40UNDERBUOY HOSES..........................................................................................................................40OPERATING CONDITIONS..................................................................................................................40

Severe Environment Catenary Mooring Designs................................................................................41STABILITY AND BOUYANCY...............................................................................................................41WATER DEPTH LIMITATIONS OF CATENARY MOORINGS .............................................................41INSTALLATION.....................................................................................................................................41

Tensioning Chain Legs.......................................................................................................................41Buoy Transportation ...........................................................................................................................41Commissioning...................................................................................................................................42

ADVANTAGES AND DISADVANTAGES..............................................................................................42

SINGLE ANCHOR LEG MOORING (SALM) ................................................................................................42BACKGROUND ....................................................................................................................................42SALM DESIGNS ...................................................................................................................................45COMPONENTS ....................................................................................................................................45

Mooring Buoy .....................................................................................................................................45Riser...................................................................................................................................................45Foundation Base ................................................................................................................................45Swivel Assembly ................................................................................................................................45






MOORING LINES AND TANKER FITTINGS ........................................................................................45HOSE SYSTEM.....................................................................................................................................45DESIGN CONSIDERATIONS................................................................................................................46ADVANTAGES AND DISADVANTAGES ..............................................................................................48

DESIGN LOADS, ANALYSIS AND MODEL TESTS.....................................................................................48MULTI-BUOY MOORING SYSTEM ......................................................................................................48

Environmental Loads on Vessel .........................................................................................................48Mooring Loads....................................................................................................................................49Analysis ..............................................................................................................................................49Mooring Buoy Design Load ................................................................................................................49

SINGLE POINT MOORING SYSTEM ...................................................................................................49Wind and Current Loads.....................................................................................................................50Wave Loads and Motions ...................................................................................................................50Analysis ..............................................................................................................................................50Fatigue................................................................................................................................................50Model Testing .....................................................................................................................................50Offshore Lift Loads .............................................................................................................................51Loadout / Transportation Loads..........................................................................................................51

MODEL TESTS .....................................................................................................................................51

MOORING LEG DESIGN ..............................................................................................................................51MOORING LEGS ..................................................................................................................................51

Loads..................................................................................................................................................51Safety Factor ......................................................................................................................................51Corrosion Allowance...........................................................................................................................51Length.................................................................................................................................................51Wire Rope...........................................................................................................................................52

ANCHOR SYSTEMS.............................................................................................................................52Holding Power of Steel Anchors .........................................................................................................52Holding Power of Chain on Seafloor...................................................................................................52Factors of Safety ................................................................................................................................52Number of Mooring Anchors...............................................................................................................52Holding Power of Ship's Anchors (MBM)............................................................................................52Piling...................................................................................................................................................52

SUBMARINE PIPELINES..............................................................................................................................53ROUTE EVALUATION ..........................................................................................................................53SIZE OF PIPELINE ...............................................................................................................................53PIPELINE CONFIGURATIONS FOR MULTI-PRODUCT HANDLING ..................................................53

Common Pipeline/Hoses ....................................................................................................................53Pigging through Looped Pipelines ......................................................................................................54Segregated Pipeline System ..............................................................................................................54Pigging from Tanker Deck ..................................................................................................................54





CORROSION PROTECTION................................................................................................................54Coatings .............................................................................................................................................54Cathodic Protection ............................................................................................................................54

WEIGHT COATING...............................................................................................................................55PIPELINE BURIAL REQUIREMENTS ..................................................................................................55PIPELINE END MANIFOLD (PLEM).....................................................................................................55

Multi Buoy Mooring Systems..............................................................................................................56Single Point Mooring Systems ...........................................................................................................56

SHORE-SIDE PIPELINE VALVES........................................................................................................57DESIGN CONSIDERATIONS ...............................................................................................................57INSTALLATION.....................................................................................................................................57

Bottom Pull Method ............................................................................................................................57Floating Method..................................................................................................................................57Lay Barge Method ..............................................................................................................................58

HOSES ..........................................................................................................................................................58NUMBER OF HOSE STRINGS.............................................................................................................58

MBM Berths........................................................................................................................................58SPM Berths ........................................................................................................................................58

DIAMETER OF HOSE...........................................................................................................................58Flow Rates .........................................................................................................................................58Lifting Considerations.........................................................................................................................58Presentation Flange ...........................................................................................................................59

LENGTH OF HOSE STRING ................................................................................................................60MBM Berths........................................................................................................................................60SPM Berths ........................................................................................................................................61

DISPOSITION OF STORED HOSES AT MBMs ...................................................................................61TYPES OF HOSES...............................................................................................................................62

Surface Hoses....................................................................................................................................62Submarine Hoses...............................................................................................................................62Underbuoy Hoses...............................................................................................................................62Single versus Double Carcass ...........................................................................................................62

GENERAL DESIGN CONSIDERATIONS .............................................................................................62HOSE COUPLERS ...............................................................................................................................63

Vessel Manifold ..................................................................................................................................63Breakaway Couplers ..........................................................................................................................63

COATING SYSTEMS ....................................................................................................................................63HARDWARE .........................................................................................................................................63SUBMARINE PIPING............................................................................................................................63MBM AND SPM BUOY SYSTEMS .......................................................................................................64ADDITIONAL REQUIREMENTS FOR SPMs........................................................................................64






SPM FABRICATION AND INSTALLATION CONSIDERATIONS ................................................................65FABRICATION ......................................................................................................................................65Transportation and INSTALLATION......................................................................................................65QUALITY CONTROL.............................................................................................................................65

SPM CERTIFICATION AND CLASSING ......................................................................................................65CERTIFICATION...................................................................................................................................65

Required Information ..........................................................................................................................66Fabrication Surveys............................................................................................................................66

CLASSING ............................................................................................................................................66

TABLESTable 1 Relative Advantages / Disadvantages of Types of MBM Buoys ......................................24Table 2 Estimated Maximum Fluke-Tip Penetration of Some Drag-Anchor Types on Sands

and Soft Clayey Silts (Mud) .............................................................................................29Table 3 Typical Maximum Flow Rates and Hose Weight .............................................................59Table 4 CALM, SALM, And Conventional Mooring Buoy Coating Requirements .........................64

FIGURESFigure 1 Types of Multiple Buoy Berths ...........................................................................................8Figure 2 Types of Single Point Mooring Berths................................................................................9Figure 3 aneuvering at Conventional MBM’s .................................................................................13Figure 4 Layout of SPM’s...............................................................................................................14Figure 5 Layout of MBM's ..............................................................................................................19Figure 6 Layout of MBM.................................................................................................................21Figure 7 Swamped Mooring Legs ..................................................................................................22Figure 8 Mooring Leg Components ...............................................................................................25Figure 9 Anchor Holding Power .....................................................................................................28Figure 10 Quick Release Buoy Hook...............................................................................................30Figure 11 Schematic of Catenary Anchor Leg Mooring ...................................................................32Figure 12 Typical Design of Catenary Anchor Leg Mooring ............................................................33Figure 13 Ship with More Than 150,000 Tonnes Deadweight Fitted with Two Bow Stoppers.........38Figure 14 Typical Arrangement to Facilitate Connection / Disconnection of Pick-Up Ropes ...........39Figure 15 Typical Design of Single Anchor Leg Mooring .................................................................43Figure 16 Deeper Water Single Anchor Leg Mooring ......................................................................44Figure 17 Schematic of Single Anchor Leg Mooring........................................................................47Figure 18 Submarine Loading Hose ................................................................................................60Figure 19 Semi-Floating Hose Scheme ...........................................................................................61

Revision Memo

12/01 Original Issue of Design Practice XXXI-E.




SCOPEThis Design Practice, along with the references, provides criteria and guidelines for planning and design of offshore berths.Included in the scope are multiple buoy moorings (MBM) and single point mooring systems (SPM). The single point mooringsystems covered include catenary anchor-leg mooring systems (CALM) and single anchor-leg mooring systems (SALM). Alsoincluded are the associated offshore pipeline and hose systems.This design practice provides an overview of the various offshore berths and their components, including the pipeline and hosesystems. Issues that must be considered in the design and their impact on the design are discussed. Information is providedto assist with the selection of the appropriate berth type and to locate and evaluate the design of the system. Designrequirements for offshore pipelines and requirements for specification and supply of offshore hose systems are also included.Fabrication and installation considerations for SPMs are addressed and installation methods for offshore pipelines arediscussed. Certification and classing considerations for SPMs are also provided.The information included herein is intended for the planning and design of new facilities. The information can also be used forreplacement of individual components on existing systems or for evaluation of existing systems.Deepwater mooring systems for production facilities are not covered in this design practice.Because the basic units for this technology are metric, the usual convention of using customary first has been reversed for theunits.

REFERENCESDESIGN PRACTICESDP XXIII-A Basic Loading SystemDP XXXI-I Minimum Safety Requirements for Marine Terminals

GLOBAL PRACTICEGP 19-1-1 Paint and Protective Coatings

OTHER REFERENCESAmerican Bureau of Shipping

ABS 2, Rules for Classifying and Building Steel VesselsABS 8, Rules for Building and Classing Single Point MooringsABS 39, Certification of Offshore Mooring Chain

American Petroleum InstituteAPI RP 2A, Recommended Practice for Planning, Designing, Constructing Fixed Offshore PlatformsAPI 2SK, Recommended Practice for Design and Analysis of Stationkeeping Systems for Floating StructuresAPI RP 14F Recommended Practice for Design and Installation of Electrical Systems for Fixed and Floating OffshorePetroleum Facilities for Unclassified and Class 1, Division 1 and 2 Locations

American Society for Testing and MaterialsASTM A36 / A36M, Standard Specification for Carbon Structural SteelASTM A153-01, Standard Specification for Zinc Coating (Hot Dip) on Iron and Steel Hardware

American Society of Mechanical EngineersASME B31.3, Process PipingASME SEC VIII D2, SISI Units (Boiler and Pressure Vessel Codes)ASME SEC IX, BPVC SECTION IX, Qualification Standard for Welding and Brazing Procedures, Welders, Brazers, andWelding and Brazing Operators Addenda - 1995; Addenda - 1996; Interfiled (Boiler and Pressure Vessel Codes)

Crane Manufacturer's Association of America, CMAA 70, Specification for Electrical Overhead Travelling CranesDet norske Veritas (DnV)

Rules for the Design and Construction and Inspection of Offshore Tanker Loading SystemsRules 2 pt 3 ch 2 F100

Lloyds Register of Shipping, 2000, Rules and Regulations for the Classification of ShipsNACE RP0394, Application, Performance, and Quality Control of Plant-Applied, Fusion-Bonded Epoxy External Pipe Coating;Errata





REFERENCES (Cont)Naval Facilities Engineering Command (NAVFAC) Design Manual 26.5, Fleet Moorings - Basic Criteria and PlanningGuidelinesOil Companies International Marine Forum

OCIMF-2, Buoy Mooring Forum Hose GuideOCIMF-3, SPM Hose Ancillary Equipment GuideOCIMF-4, Buoy Mooring Forum SPM Hose System Design CommentaryOCIMF-6, Standards for Mooring Equipment Employed for Ships at Single Point MooringsOCIMF-7, Hawser Test ReportOCIMF-10, Guide to Purchasing HawsersOCIMF-11, Procedures for Quality Control and Inspection During the Production of HawsersOCIMF-14, Predictions of Wind and Current Loads on VLCCsOCIMF-19, Single Point Mooring Operations and Maintenance GuideOCIMF-20, Guide to Purchasing, Manufacturing and Testing of Loading and Discharge Hoses for Offshore Moorings

Society for Protective CoatingsSSPC SP 5, Joint Surface Preparation Standard White Metal Blast Cleaning, NACE No. 1: 1999 (Steel Structures PaintingManual, Ch 2 - Surface Preparation Specs.)SSPC SP 10, Joint Surface Preparation Standard Near-White Metal Blast Cleaning, NACE No. 2: 1999 (Steel StructuresPainting Manual, Ch 2 - Surface Preparation Specs.)

GENERALTYPES

Multiple Buoy Moorings (MBM)

There are two main categories of Multiple Buoy Moorings (MBM) (Refer to Figure 1):• Conventional Buoy Berth (CBB) - Offshore berths in which the ship’s bow is held in position by the ship’s own anchors

while 3 to 7 mooring buoys are used to secure the stern. CBBs are the most common type of MBM.• All Buoy Berth (ABB) - Offshore berths in which both the ship’s bow and stern are held in position by a total of four to eight

mooring buoys. ABB’s are generally located where bottom conditions prevent the use of ship’s anchors or whereadditional mooring restraint is needed for the expected environmental conditions.

At buoy berths, submarine pipelines run from shore tankage to the area of the buoys. Loading or unloading operations areaccomplished through a system of flexible hoses that are connected between the submarine pipeline end manifold and theship’s manifold.




GENERAL (Cont)

FIGURE 1TYPES OF MULTIPLE BUOY BERTHS

CONVENTIONAL BUOY BERTH (CBB) ALL BUOY BERTH (ABB)

Buoy Chain

Anchor

Hose

Pipeline

Mooring lines

Ship's Anchorand Chain

Hoses

Pipelines

Anchors

Mooring lines

Mooring lines

End Manifold Pipelineto Shore

DP31Ef01(May have 3,5,or 7 Bouys, Shown Above)

Single Point Moorings (SPM)

At SPMs, ships are moored with only a bowline to a single buoy or tower. Consequently, the ship is free to weathervanearound the point to which the bowline is attached. The cargo is transferred by rubber hoses between the ship’s manifold and acargo swivel located on the SPM. The tanker can freely swing around the mooring in response to the environment whilesimultaneously transferring cargo.The two most common types of SPM’s are (refer to Figure 2):• CALMs - Catenary Anchor Leg Moorings are comprised of a single buoy restrained by four to eight conventional

chain-anchor legs. The cargo is conveyed between the submarine pipeline and the buoy by submarine hoses. Themooring line is connected to the cargo swivel on the buoy by a lever arm. This device is used to rotate the swivel with themooring line. Floating hoses connect the cargo swivel on the buoy to the vessel manifold. CALMs are the most commontype of single point mooring.

• SALMs - Single Anchor Leg Moorings are comprised of a buoy moored by means of one vertical chain or pipe attached toa single base anchored to the seabed. The cargo is transferred from a submerged cargo swivel to the ship’s manifold byrubber hoses. The swivel is turned by the cargo hoses. The ship’s bow mooring line is attached to the buoy.





GENERAL (Cont)

FIGURE 2TYPES OF SINGLE POINT MOORING BERTHS

TYPICAL CALM SYSTEM

Turntable

SubmarineHose Strings

Seabed

FloatingHose Strings

ManifoldMooring Assemblies

Anchor Chain

MSL

Pipeline EndManifolds

SubmarinePipeline Anchors or

Anchor Piles

TYPICAL SALM SYSTEM

MSL

Manifold

Seabed

Hose Strings

Piles

Mooring Buoy

Mooring

Anchor Leg Swivel

Anchor Leg

Fluid Swivel Assembly

UniversalJoints

Jumper hose

Pipeline End Manifold

DP31Ef02




GENERAL (Cont)

SELECTION OF SYSTEMSBuoy moorings in general, and SPMs in particular, provide the flexibility to operate a terminal in areas where the environmentalconditions would make it impossible or impracticable to operate a conventional pier. MBM berths are typically operated inareas where wave heights are up to 1.2 m (4 ft). SPMs, requiring the use of tugs and support craft, are typically operated inwaves up to 2.5 m (8.2 ft). Other advantages of offshore berths over conventional piers include:• Fabrication and installation costs are generally advantageous.• Planning and installation can be completed in a short period of time.• Design flexibility facilitates application to a variety of circumstances.• Berthing and unberthing frequently does not require tugs.

Disadvantages to offshore berths are primarily operational. The most significant considerations include:• MBMs are typically designed to handle a certain class of ship and may not be suitable to handle other classes with

different dimensions.• Deteriorating weather conditions may require shutdown of cargo conditions and in some cases may require the ship to

vacate the berth.• The terminal typically is required to provide a shore based mooring gang to assist with mooring / unmooring and hose

hook-up / disconnect.• The terminal typically is required to provide one or more seagoing mooring launches.• At SPMs the terminal will be required to provide the hawser, and at some exposed MBM locations the terminal may also be

required to provide mooring wires permanently attached to the buoys.• Offshore pipelines are more expensive to install than onshore piping.• Offshore pipelines can be long and frequently loading / discharge rates are lower than at conventional piers.• It may be difficult to clear the pipelines between transfers if more than one product is handled.• Hose handling can be complicated and time consuming.• Supply craft are required to provide bunkers and fresh water, unless separate pipelines are provided.• Access must be provided to/from the moored vessel. This can involve transfer of stores and large quantities of provisions.• Inspection and maintenance work typically requires specialized craft and divers.

ADDITIONAL REFERENCESAPI 2SK Recommended Practice for Design and Analysis of Stationkeeping Systems for Floating Structures provides rationalmethods for analyzing, designing and evaluating the SPM mooring systems described in this Design Practice. Additionalguidelines for model testing, design and analysis of the SPM hawser can be found in Exxon Report No. EE-17E.T77 Guidelinesfor Deepwater Port Single Point Mooring Design.





GENERAL (Cont)The latest editions of the following OCIMF publications provide additional technical, operating and maintenance backgroundand guidance for offshore facilities. Although most of these documents are specific to SPM systems, some of the design andanalysis methodologies, model testing, and anchor leg hardware can be selectively applied to MBM terminals.• Single Point Mooring Maintenance and Operations Guide - Provides guidelines to operators of SPM terminals to assist

in the preparation of operating and maintenance manuals.• Guide to Purchasing, Manufacturing and Testing of Loading and Discharge Hoses for Offshore Moorings - This

publication gives the minimum specification of acceptable technical requirements to ensure the satisfactory performance ofrubber, reinforced, smooth bore, oil suction and discharge hoses for offshore moorings. The specification is divided intotwo parts: technical requirements for commercial hoses and technical requirements for prototype hose approval. Notes tothe purchaser are included listing items which should be specified or agreed with the manufacturer. It includes referenceto double-carcass hoses, lifting lug requirements, float design requirements, standardization of bead float collars andstandards for packing.

• Guidelines for the Handling, Storage, Inspection and Testing of Hoses in the Field - Gives guidance on the handling,storage, inspection and testing of hoses under service conditions.

• SPM Hose Ancillary Equipment Guide - Gives guidance on common descriptive terminology and technical requirementsfor the designer and operator of SPM systems.

• SPM Hose System Design Commentary - A commentary on the two most common types of SPM (CALM and SALM)reflecting modern practices, providing descriptive terminology together with an outline of present practice in the design ofhose systems.

• Recommendations for Oil Tanker Manifolds and Associated Equipment - Gives recommendations aimed atintroducing conformity in manifold arrangements for all ocean going tankers engaged in the transport of crude oil and bulkliquid petroleum products, including guidance on vapor recovery manifolds.

• Recommendations for Equipment Employed in the Mooring of Ships at Single Point Moorings - Givesrecommendations aimed at facilitating correct matching between the SPM and the ship. Recommends that mooringequipment available at terminals be brought into line with these standards and that ships be provided with bow stoppersand associated fittings designed to secure and accept standard chafe chains.

• Hawser Test Report - Gives a complete description of the first major large rope testing program. This was undertaken todetermine the actual new and used strengths and rate of strength reduction of large synthetic ropes similar to those usedas SPM hawsers. All the test data and analyses are included together with conclusions and recommendations. It will be ofinterest to designers and operators of SPMs and to all who manufacture or use large, synthetic ropes for lifting andmooring.

• Hawser Guidelines - These three booklets are produced with the intention that rope manufacturers establish completeand detailed documentation of their synthetic fiber products, particularly those primarily intended for use at single pointmoorings.Volume 1 - Guide to Purchasing HawsersVolume 2 - Procedures for Quality Control and Inspection During the Production of HawsersVolume 3 - Prototype Rope Testing

DESIGN CONSIDERATIONSIn planning the design of an offshore berth, the design should consider the site, environment, tanker, mooring operations andphysical parameters of the mooring and hose handling system. Planning studies are normally a joint effort among Engineering,Marine Transportation and Operations. A brief discussion follows concerning the major points in planning an offshore berth.

LISTING OF HANDLED PRODUCTSMost often, offshore berths handle a single product, which requires only one fluid path throughout the system. However,MBM's capable of handling as many as five products have been installed. A complete listing of all products to be handled,including ballast and bunkers, should be developed, along with discharge/receipt operations. This listing is needed in order todetermine the compatibility of products using the same fluid pipeline and the number of fluid transfer operations that need to becarried on simultaneously.




DESIGN CONSIDERATIONS (Cont)

THROUGHPUTSThroughputs by product and parcel sizes, should be established to evaluate the number and size of facilities required.

TANKER LISTINGA complete listing of tankers by class size, is required. Also required is a breakdown of the arrival frequency on an annualbasis.

ENVIRONMENTAL DATAThe effects severe weather can cause exceptional problems, especially with regard to port closure resulting in potentialthroughput cutbacks and demurrage. Data on wind, current, wave, swell, fog, tides, ice, etc., should be collected and evaluatedwith regard to buoy operations, including the effects on the various size classes of tankers and support craft necessary toconduct operations.

LOCATIONNearshore buoy terminals should be sited / located based on the following objectives and considerations:

Pipeline Length

Submarine pipelines often represent the major portion of the berth investment. Therefore, minimizing the length of the pipelineroute should be a primary consideration. Routing the submarine pipelines in a straight line perpendicular to shore is not alwayspossible. It is also important that pipelines be routed away from rock outcrops and achieve an elevation profile that minimizes /avoids high and low spots (maintenance and reliability consideration).

Maneuvering

Multi Buoy Mooring Systems - The necessity for providing adequate maneuvering room imposes the largest limitation onCBB layout. The following considerations should be evaluated when locating / siting an MBM berth:• Adequate room should be provided for maneuvering (safety and operability consideration). For MBM’s, it is critical that a

nautical advisor defines the vessel approach route(s), distance needed to stop and turn the vessel, level of tug assistance,and maneuvering procedure into the berth. The vessel must be able execute these maneuvers during the expectedoperational environmental conditions while remaining a safe distance from any obstructions (property lines, anchorage,submarine lines / cables, buoys, etc.) and in an area of sufficient water depth. The role of tugs in determining theappropriate safe distances should be considered. The berth should be situated such that vessels have a safe escaperoute in the event that the maneuver must be aborted (e.g. due to mooring difficulties). Refer to Figure 3.

• The vessel should be able to swing on its anchor without grounding during normal maneuvers and in the event of a vesselbreak out.

• The vessel should be able to swing clear without fouling the buoys. Impact with a buoy could cause the buoy or its anchorchain to be damaged or the ship's propeller to be fouled and damaged. In order to allow the vessel to swing on its anchorand back into the berth without tug assistance, normally requires that mooring buoys be located at the stern of the vesselonly, and far enough from the vessel to insure they will not be struck.





DESIGN CONSIDERATIONS (Cont)FIGURE 3

MANEUVERING AT CONVENTIONAL MBM’S

DP31Ef03

BerthL

BuoysShore Range

Drop Port Anchor

Port Anchor

StarboardAnchor

SparBuoy

Drop Stb'dAnchor

Berth Area

C

Buoys

60°-90°

Buoys

Entry and Exit

(Bi-Directional)

ALL BUOY MOORING

CONVENTIONAL BUOY MOORING




DESIGN CONSIDERATIONS (Cont)Single Point Mooring System - Most SPMs are positioned so that the largest tanker expected to use the buoy can approachor leave the buoy from any heading. The minimum water depth within the approach or maneuvering circle is determined asdescribed here. The maneuvering radius is normally established as five ship lengths of the largest (longest) tanker.For SPM’s, the maneuvering area is defined as a circle with a radius equal to 3 to 4 times the vessel LOA. There should besufficient water depth within the maneuvering area to achieve the required underkeel clearance. Refer to Figure 4.

FIGURE 4LAYOUT OF SPM’S

Safe ManuveringDepth

SPM

Swinging Circle(L + 60m)

Manuvering Circle(3-4 L)

L60m

DP31Ef04

Water Depths

Transit - Water depths are critical throughout the tanker transit from and to deep water. Once the maximum design draft hasbeen established for each leg of the transit, the required water depths can be calculated. This involves consideration of waves,bow sinkage, seabed material, bathymetry inaccuracies and underkeel clearance (UKC) requirements, which are oftenapproximately 15 percent of the deepest draft tanker.

Berth - The offshore berth should be located to ensure adequate water depth is available in the immediate berth area (safetyconsideration). Methods for estimating the required underkeel clearance at conventional piers are described in Section XXXI-I.A wave response allowance must be added to the requirements in Section XXXI-I for determining the water depth andminimum underkeel clearance for buoy berths. For vessels moored at buoy berths, the maximum wave response for earlyscreening purposes may be assumed equal to 1.9 times the operational wave height limit (expressed as significant height). Forplanning purposes, a minimum underkeel clearance can be set at 30% of the maximum vessel draft. Additional water depthconsiderations are listed below:• For MBM’s, the berth should be situated in sufficient water depth to achieve the required underkeel clearance between the

vessel hull and obstructions on the seabed, i.e. PLEM, pipelines, cargo hoses, and the seabed when moored during theexpected operating wave conditions. The berth area should encompass the range of possible vessel positions whilemoored. This range should provide an allowance for misalignment and the elasticity of the mooring system for themaximum operating environmental conditions. Refer to Figure 3.






• For MBM’s that are not aligned with currents, additional underkeel clearance may be necessary to reduce / limit current-induced loads on the mooring system.

• For SPM’s, the berth area should be also be situated in sufficient water depth to achieve the required underkeel clearancebetween the vessel hull and obstructions on the seabed, i.e., SALM base, PLEM, pipelines, seabed, etc. when mooredduring the expected operating wave conditions. The berth area is defined as the swinging area (see Figure 4) around thebuoy. The swing circle can be estimated by using a radius equal to 60 + LOA (meters).

Other Location Considerations

Areas with steep underwater slopes should be avoided (installation consideration). For MBMs, the berth should be located inan area where the water depth is relatively uniform (extending to include buoy anchor legs).

GEOTECHNICAL / GEOPHYSICALSoil borings at PLEM and anchor sites should be performed prior to contracting for SPM, and the soils report should beincluded as an attachment for the bid package. The soils report should provide sufficient information in order to design thefoundation for the PLEM and to evaluate anchor and/or piles for the mooring legs.An underwater hazard survey should cover the planned maneuvering area and the approach channel. The hazard surveyreport should be made part of the bid package. A hazard survey and documented report normally include the followingelements:1. Sidescan Sonar2. Shallow Seismic3. Magnetometer4. Bathymetry

INITIAL FACILITY SIZINGOnce throughputs, fleet composition and environmental conditions have been established, the initial sizing (such as pipelinerates and tankage volume) can take place. This should preferably be done by computer simulation techniques; however, forsimple cases, hand calculations may suffice. The entire transportation / storage system should be simulated, since allcomponents are interactive, and any one can affect the others. As the project progresses, and changes are made, the siteselected, and layouts developed, additional simulation runs may be required to verify and/or re-establish facility sizes. Refer toSection XXIII-A for additional loading system considerations.

SUPPORT FACILITIESThe support facilities that should be provided may vary widely, depending upon the berth site in relation to a built-up port and itsport's infrastructure, consideration should be given to such items as navigation aids, mooring launches, pilot boats,maintenance craft, etc. Additionally, the potential for environmental pollution should be addressed and contingency plansdeveloped, in order to determine the specialized equipment required to protect the environment.

FACTORS INFLUENCING MOORING LOADSCertain factors are vitally important in establishing design loads, while other factors have only minor effects. The designershould be aware of the significance of the various factors, and the possible effects that a change in a parameter may have onmooring loads and mooring performance. The basis for a mooring system design depends upon the peak mooring loads thatthe tanker exerts on the system, and the lateral excursion of the buoy for SPMs. Factors that have the dominant effect onmooring loads include wave height, wave period, wind, current and vessel-mooring system interaction.At SPM berths, the tanker and buoy are coupled by an elastic mooring hawser. The SPM itself acts as a lateral spring systemwith a non-linear response. Moreover, the motions of a vessel, moored to an SPM, in waves, wind and current can beexceedingly complex, especially if the three environmental components are not co-linear. Both the moored vessel and theSPM buoy are free to respond in three translational degrees of freedom: surge, sway, and heave; and in three rotationaldegrees of freedom: yaw, pitch and roll. Altogether, this results in a non-linear system with 12 degrees of freedom, some ofthem coupled and acted upon by irregular environmental forces. It is a complicated problem to deal with analytically, yet thereare ways of breaking the problem down to manageable proportions. For analysis purposes and for an understanding of therelationship of mooring loads to the environment, the effects of waves, wind, and current are often considered separately, asdiscussed in the following sections.





Waves

Like ocean waves in general, waves at typical tanker loading sites are usually irregular. That is, they cannot be adequatelyrepresented in terms of a regular sinusoidal wave function having a single amplitude and a single frequency. Indeed, it isdifficult, if not impossible, to completely describe ocean waves analytically. However, for most practical engineering purposes,statistical properties are used to define the important characteristics of irregular waves in the ocean. These include thesignificant wave height, peak period, significant period, dominant wave direction and the power energy spectrum.

Wave Height - It is conventional to characterize an irregular sea by its "significant wave height." Specifically, the "significantwave height" is defined as the average of the highest one-third of the peak-to-trough waves in a wave record. For SPM's, themooring hawser load has been found to increase roughly in proportion to the square of significant wave height in the absenceof wind, current and tanker propulsion assistance.

Wave Period - The term "significant wave period" applied to irregular waves has not achieved the degree of agreement assignificant wave height. "Significant wave period" refers to the average of the periods of the highest one-third of the waves in awave record. "Peak wave period" is the reciprocal of the frequency of peak energy in the wave energy spectrum. Generally,peak wave period data are more readily available.The wave period and the type of wave spectrum can also influence mooring loads. At SPM berths with a constant significantwave height, mooring loads generally increase with shorter wave periods (higher frequency), as shorter period waves tend toexcite the mooring buoy motion relative to the moored tanker.On the other hand, long period wave phenomena can have a major influence on the response of a moored tanker, and thus onmooring loads. Long period phenomena occur in the form of swell and wave groups. Swells result from large storm wavestraveling very long distances, even thousands of miles; however, over time and distance high wave frequencies damp out,leaving dominant low frequencies. Wave grouping results from the combining of waves of different frequencies. When they aresuperimposed, new waves with the sum/difference frequencies of the combining waves result. The envelope of the wavegroups are, in fact, the difference frequencies, which can be very low. If those low frequencies approach the natural frequencyof the mooring system, then resonance and high mooring loads can occur.

Response to Waves - At SPM berths a vessel moored in waves alone tends to align itself into the waves and respondsprimarily in surge. In regular waves, the vessel will surge back on the mooring until the force in the hawser equals the meanwave-induced force on the vessel, which will remain essentially in equilibrium at this position. Small variations will occuraround the mean hawser tension, as the buoy responds to the waves. Larger variations occur if the wave periods are inresonance with a natural frequency or a harmonic of the mooring system.In irregular waves, there is a mean wave force, the low frequency wave forces associated with wave groups, and the force ofeach individual wave. The moored vessel will respond to each of these components.The tanker / mooring system response will be especially pronounced, if the period of swell or wave grouping corresponds to thenatural period of the mooring system in surge. In that case, peak mooring forces will be significantly higher than wouldotherwise be experienced in waves of the same height.In planning an offshore berth, a study of oceanographic data should be performed to determine if these long period phenomenaexist at the site. If they are expected, they should be accounted for in the analysis, and model testing for SPM's.

Wind and Current

Unlike the loads imposed on the tanker by waves, the static loads imposed by wind and current can be established with relativeaccuracy. However, due to the nature of the response of the tanker to wind and current loads, the influences of wind andcurrent on SPM mooring loads may be difficult to determine. "Loads" in this case includes both forces and moments. Becausemoments are imposed when the tanker hull is at an angle to wind or current, the tanker may move to apparently unusualpositions under the combined effects of waves, wind and current.

Wind Loads on the Vessel - Wind loads are proportional to the square of the wind velocity and the projected area exposed.Wind force coefficients for Very Large Crude Carriers (VLCCs) have been developed from wind tunnel tests, and are publishedby OCIMF (refer to OCIMF-14). In general, a one-minute average wind velocity, taken at a 10 m (33 ft) elevation, is used tocalculate wind forces and moments on the tanker.

Current Loads on the Vessel - Current force and moment coefficients for typical tanker hulls have also been developed andpublished by the OCIMF (refer to OCIMF-14). As with the wind, current loads increase proportionally to the square of thevelocity of flow. Current loads will decrease with a decrease in draft. For a fixed draft, current loads will decrease with anincrease in the water-depth-to-draft ratio. This effect is important up to a depth-to-draft ratio of about 1.5, and less pronouncedbeyond that ratio.





DESIGN CONSIDERATIONS (Cont)For SPM's, it is possible to see large dynamic responses of a tanker moored in constant wind or current alone. Theseresponses in steady wind or current are due to the variations of moments and forces that occur, as the tanker hull changes itsheading to the direction of the steady wind or current field. Changing tanker heading causes alternating induced lift forces fromwind and current on the hull, which results in yaw and sway of the tanker.

Additional Considerations for Single Point Moorings

Influence of the Tanker - Variations in tanker size or shape do not affect mooring loads at SPMs as much as do variations inthe environment. Nevertheless, where different size tankers are to moor at an SPM, consideration should be given to theinfluence of tanker size on mooring loads. The larger tankers do not always impose the highest mooring loads.The state of tanker loading – that is, whether the tanker is loaded or light ballasted – can have a major influence on tankerresponse, and thus on mooring loads at SPMs. Consideration should be given to the loading condition in which tankers willmost probably be exposed to severe environments.In general, the mooring forces will increase proportionally to the square root of the tanker size. However, mooring loadsmeasured in model tests, for tankers of 500,000 DWT and larger, have generally been no higher than those measured ontankers in the size range of 200,000 to 400,000 DWT. This is because the smaller tankers move around more than the largerones, and high mooring loads result from the arresting of the tanker momentum. Therefore, not only the maximum size tanker,but also the smaller ones should be considered in designing an SPM.For a given tanker size, different mooring loads will be experienced in the loaded condition, and in the ballasted condition. Themore critical case of ballasted or loaded tanker cannot be assumed; therefore, both cases should be considered.The use of the tanker's main propulsion or thrusters and/or tying a tug to the stern of the tanker can have a significant effect onmooring loads. This assistance is often used in severe environment locations to reduce the effects of steady environmentalforces, or to maintain a steady mooring load and reduce surge loads.

Influence of the Mooring System - The design of the mooring system has a major influence on the mooring loads. The mostimportant mooring system parameters are those that affect the elasticity or stiffness of the mooring system. A very stiff mooringsystem will severely constrain the response of the tanker to waves, resulting in very high mooring loads. Conversely, if themooring system is very soft, the tanker may respond too freely. This builds up momentum as the tanker moves under theinfluence of waves, wind, and current, which exerts high loads on the mooring as it comes to the limits of mooring systemelasticity.

Conclusions

In determining design mooring loads of an offshore berth, many influences should be taken into account. The engineer shoulddetermine the dominant factors in a given situation and develop design loads accordingly, using the tools at his disposal.Because of the sometimes complicated interplay of forces and responses, it is important that realistic wave, wind and currentenvironments be used in analysis, and model testing for SPM's. Realistic wave spectra should be used. Long periodphenomena, such as swell and wave grouping, should also be reflected in the wave record, if they exist in nature at the site. Itis also important that the wind, and current be accurately represented as to direction, velocity, and their moment-producingeffects on the vessel.

DETERMINATION OF ENVIRONMENTAL DESIGN CRITERIAOffshore berths should be designed for both survival conditions (tanker disconnected) and maximum operating conditions(tanker moored at the berth).• Survival conditions should be determined based on the 100 year return period of wind, wave and current conditions

resulting in the highest loads on the buoy system with the vessel disconnected. Two sets of criteria are investigated:1. The 100 year waves with associated wind and current. Typically the governing load case for the MBM and SPM

systems considered in this Design Practice.2. The 100 year wind with associated waves and current.





• Maximum operating conditions are determined from an evaluation of the required frequency of loading / off-loading and thelong term weather statistics for the area. The required off-loading frequency is a function of the size of tankers, amount ofstorage and the product transfer rates. The maximum operating conditions may determine the type of offshore berth, assome are better adapted to rough sea conditions than others. The design environmental criteria should be developed fromthe environmental information described in this section and may also include a risk analysis where prior experience islimited. The risk analysis may include: historical experience, the planned life and intended use of the mooring,consideration of safety of operating personnel, the estimated cost of the mooring designed to environmental conditions forseveral average expected recurrence intervals, and the probability of mooring damage or loss when subjected toenvironmental conditions with various recurrence intervals, and the financial loss due to mooring failure. The typicallimiting wave height for MBM operations is about 1.2 m (4 ft) to allow the mooring gang to safely disconnect the lines fromthe buoys. The typical operating wave height for SPMs is 2.0-2.5 m (6.5 ft).

Acquisition of valid weather statistics may require several years of data measurement.

MULTI BUOY MOORING SYSTEMSThis section includes a description and general requirements for MBM buoys, anchor legs and ancillary equipment. Specificguidelines for MBM layout and orientation are also included. Additional design considerations for buoys, anchor legs, hosesand ancillary equipment are provided in subsequent sections. Orientation and layout considerations should always bediscussed with experienced marine personnel before final design to insure the operability of the berth.

ORIENTATIONIn general, MBMs should be aligned parallel to the direction of the predominant environment in order to minimize loads on themoored ship and reduce the probability of mooring incidents. Long period wave / swell action is normally the governingcondition.Consideration must also be given to the method of tanker approach and departure from the berth. Normally the tanker shouldbe capable of maneuvering into a CBB without the assistance of tugs. Furthermore, the tanker must be capable of leaving theberth quickly in the event of worsening weather. These criteria usually result in a berth oriented perpendicular to the coast lineand heading out to sea.In locations which experience high currents parallel to the coast, an ABB berth oriented parallel to the coast line may berequired. In other locations, where governing environmental conditions vary throughout the year, the berth may be given twoorientations.Specific guidelines that should be considered when locating and orienting the berth include:• MBMs should normally be sited with sufficient sheltering such that significant wave heights are less than 1.2 m (4 ft) more

than 90% of the time. Somewhat higher waves (up to 1.5 - 2 m (5 - 6.6 ft )) may be accommodated if the wave sector iswell defined and narrow, and the vessel aligned such than waves strike her bow.

• MBMs, where wave heights do not exceed 1.2 m (4 ft), should be aligned parallel with the direction of the strongestcurrent, if the velocity exceeds 1 knot.

• MBMs, where there are minimal currents or waves, should be oriented such that strong prevailing winds strike the stern ofthe vessel.

LAYOUTIf local conditions permit, a conventional 3 or 5 buoy berth layout (Figure 5) normally results in the most economical andworkable scheme. Therefore, the CBB layout is discussed first followed by the ABB layout.In laying out a CBB, three main factors must be considered:• Optimum disposition of vessel's anchors• Provision of adequate maneuvering room• Accommodation of full range of tanker sizes





MULTI BUOY MOORING SYSTEMS (Cont)

FIGURE 5LAYOUT OF MBM'S

3 BUOY BERTH SIMILAR EXCEPT DELETE BREAST BUOYS

30°-45°

Pipeline

Min. Ship

Max. Ship

Breast Buoy Quarter Buoy

30°-45°

90m-150m

35°-45°

35°-45°

Stern Buoy

Hose

Spar Buoys

1

2

3

4

5

15°15°

DP31Ef05

CBB Ship Anchor Layout Considerations

For economical and safe design of a CBB, the vessel’s anchors (2) should be utilized to their full capacity. Normally, the ship’sanchors are the only moorings holding the bow of the vessel against wave, wind, and current forces. At times, the ship’sanchors are the weak link in the mooring system, particularly in areas where the seabed consists of soft materials and theanchors are not capable of developing high holding power. Estimates of the holding power of ship anchors are given in sectionMOORING LEG DESIGN.To fully develop the anchor’s holding power, a sufficient length of chain must be used so that the pull on the anchor will beapproximately parallel to the sea bottom. If the chain is too short, the end of the chain will be inclined to the sea bottom and theanchor will tend to pull out and drag before developing its full capacity.For small tanker berths (< 30 KDWT), the layout should be based on a minimum chain length of 160 - 220 m (6 - 8 shots, where1 shot = 90 ft). For larger berths or berths located in exposed locations, or in deep water, the layout should consider the fulllength of chain carried by the ship. The length of chain carried by a ship is specified by ship classification societies (e.g.Lloyds, American Bureau of Shipping) according to the calculated equipment number for the vessel. The equipment numberis a function of vessel displacement, beam and depth, and wind area of superstructures. The equipment number and theanchor weight and length of chain for some typical size tankers are provided in the table below.

TANKER ANCHOR CHAIN LENGTH

LENGTH*SHIP DWT EQUIP. NO.

m ft25,000 2080 - 2330 300 1,00050,000 2870 - 3040 330 1,100100,000 4000 - 4200 360 1,200250,000 6100 - 6500 385 1,300




* Length of chain for each anchor

MULTI BUOY MOORING SYSTEMS (Cont)The angle between the bow anchor chains should be between 60 - 90° (see Figure 5). A high angle between anchors isnormally preferable, although not always possible, when strong beam or quartering forces must be resisted. If the berthexperiences high head seas and very little beam winds or currents, a 60 - 70° anchor spread may be more appropriate. Themaximum bow anchor spacing can be limited by the length of chain carried by the vessel (less the water depth) since the shipmust pay out the chain from one anchor while maneuvering to drop the second anchor.

CBB Buoy Layout Considerations

The necessity for providing adequate maneuvering room imposes the largest limitation on a CBB layout. To allow the vessel toswing on its anchor and back into the berth with or without tug assistance, normally requires that mooring buoys be located atthe stern of the vessel only, and far enough from the vessel to insure that they will not be struck. The vessel should be able toswing clear without fouling any buoys. Consideration must also be given to the possibility of a vessel breaking out of itsmoorings and swinging on its bow anchors.There is a difference of opinion among operators as to the need for or desirability of a 5 buoy CBB compared to a 3 buoy CBB.The use of breast buoys limits the available maneuvering room and makes berth entry and exit more difficult. Central Americansea berths handling vessels up to 36 KDWT have operated successfully without breast buoys. However, where larger vesselsare handled or more severe environmental conditions exist, 5 buoys are normally required. At very large ports, 7 buoys areused.Once the initial layout is determined, the mooring loads are analyzed. If analysis indicates that the vessel's anchors and/or thestern mooring lines are overloaded for a conventional layout, the following alternatives are available:• Add buoys• Add "swamped" moorings• Use an all buoy berth• Limit vessel operations

Number of Buoys - The majority of marketing terminals are conventional 3 or 5 buoy berths as shown on Figure 5. Aminimum of 3 stern buoys is required for tanker berths in open sea areas, although two stern quartering buoys may beadequate for barge berths. The buoys should be located on each quarter and directly astern of the vessel. Stern breast buoysshould not be used in lieu of stern quartering buoys in a 3 buoy berth, as this will normally result in an overload of the hawsersattached to the buoy located directly astern of the vessel.Where larger tankers are handled or more severe environmental conditions exist, the addition of breast buoys in a five-buoyarrangement may be necessary. If the critical environmental condition has a defined direction sector, an additional buoy mayalso be beneficial.For berths handling larger crude oil tankers, additional breast buoys and a forward bow buoy may be necessary. The buoylocated off the bow of the vessel can be used in conjunction with the ship's anchor as shown Figure 6. This buoy leg can bedesigned for higher mooring loads through the use of a suitable permanent anchorage system and a number of mooring linesfrom the vessel. Therefore, if the buoy is located on the side of the vessel which is exposed to the highest forces, it mayincrease the capacity of the berth.






FIGURE 6LAYOUT OF MBM

Bow Buoy

60°-90°

Forward Breast Buoy DP31Ef06

(Vessel Approach and Exit from this Side)

The addition of breast buoys limits the available maneuvering room and makes berth entry and exit more difficult. Forwardbreast buoys can greatly restrict the maneuvering room at the berth and are subject to damage in the event that the tankeroverrides the buoys. If forward breast buoys are used for a CBB, they are to be restricted to one side only, and should bereviewed with a nautical specialist.

Accommodation of Full Range of Vessels - Minimum berth dimensions are governed by the maneuvering room required forthe maximum size design vessel; whereas maximum berth dimensions are limited by mooring considerations for the smallestdesign vessels. The manifold of all vessels using the berth must line up at the same location in the vicinity of the submarineloading hoses. Therefore, the stern of the smaller vessels will be located farther from the mooring buoys than the stern of thelarger vessels (Figure 5). The breast buoys should also be aligned within 15° of the position of the aft winches on the ship’smain deck.

Length of Mooring Lines - The length of mooring line from the vessel to the buoys should usually be 90 - 150 m (300 - 500 ft)depending on the size of the vessel. Longer lengths may be required on stern breast lines to provide sufficient maneuveringroom when very large tankers are handled.If the distance is too large, smaller vessels may not have mooring lines long enough to reach the buoy. Tankers in MBBservice, which are greater than 35 kDWT, normally carry 220-250 m (720-820 ft) ropes. Smaller tankers and chartered vesselsusually carry shorter synthetic ropes, often purchasing 365 m (1200 ft) ropes and then cutting them in half or thirds. Vesselswith insufficient mooring lines will either not be able to moor or will have to join lines together to gain sufficient length.The vertical angle between the hawsers and the buoys should be limited to less than 10 degrees. If the hawser is too short, thevertical load on the buoy will be high, tending to lift the buoy out of the water, and decrease the elasticity of the mooring leg.

Maneuvering Room - For berths with 5 or less buoys, the distance between the tanker rail and mooring buoy should not beless than 90 m (300 ft) in order to allow for adequate maneuvering room. For seven buoy berths, the breast buoys should belocated 140 - 210 m (450 - 690 ft) from the side of the vessel to provide sufficient maneuvering room. For berths handlingcoasters and barges only, the buoys may be located closer to the vessel.

Swamped Moorings - Figure 7 shows a typical swamped mooring. It normally consists of a messenger line and a wiremooring line anchored to the sea bed with a chain, anchor and possibly a concrete block. When not in use, the wire is left onthe sea bed. The messenger line is pulled out of the way of berthing vessels and connected to a mooring buoy or small markerbuoy.Upon arrival of a vessel, the launch hauls the wire to the tanker and it is heaved aboard and tied off to bitts.




MULTI BUOY MOORING SYSTEMS (Cont)The main advantage of swamped mooring is that they can be used as forward breast lines or bow lines without restrictingvessel maneuvering. Swamped moorings are used successfully at a number of large tanker terminals to supplement the buoylegs or the vessel's anchors in areas that have poor holding ground.Preferably the sea bed in the area of the swamped leg should be smooth and free of obstructions. If the sea bed is rocky, thestored wires will become entangled in the rocks and will be very difficult to tension properly. Soft mud bottoms that allow thewire to sink will also cause operating difficulties. Most terminals that have swamped legs only use them during periods of roughweather, rather than as a standard procedure.

FIGURE 7SWAMPED MOORING LEGS

60°-90°

DP31Ef07

Ship'sAnchor Swamped

MooringLeg

Buoy

Ship'sHawser

SwampedLeg

Several disadvantages of swamped mooring legs should be noted when considering their use. The stiff wires must bemanhandled around the tanker's bitts and are difficult to pretension and tend. Small tanker crews are inexperienced withhandling wires and may even refuse to do so. Also, some terminals may not have a launch that is capable of hauling the wires.Another difficulty is that the mooring does not have much elasticity or resilience as compared to a buoy leg. Therefore, theswamped leg will assume higher loads than adjacent buoy legs. One way of alleviating this problem may be to insert a lengthof nylon rope in the swamped mooring to increase its elasticity.

ABB Buoy Layout Considerations

An all-buoy berth (see Figure 3) may be required where a rock or deep mud bottom precludes the use of the vessel's anchorsor where bi-directional approach is desired due to changing wind or current conditions. At a typical ABB, the vessel maneuversbetween the stern quarter buoys, normally with tug assistance and without the use of the ship’s anchors. Therefore, the vesselcan approach the berth and moor in either of two opposite headings. An ABB enables breast buoys to be used on both sides ofthe vessel, and therefore stronger beam forces can be resisted. However, the vessel normally does not use its anchors, andthe bow buoys resist head forces.

MOORING BUOYSThere are many types of commercially available mooring buoys. However, buoys normally used at the MBMs can be dividedinto the following categories:• Cylindrical buoys (horizontal)• Drum buoys (vertical)• Peg top buoys• Lamgar or Stayryt buoysThe above buoys are listed in the order of increasing cost. Experience with all types of buoys is varied and no clearrecommendations can be made as to the optimum type for a particular location.






General Buoy Requirements

The following factors should be considered in evaluating mooring buoys for a particular location:• Initial cost• Maximum net buoyancy of buoy• Ease of operation• Safety of operation• Maintenance requirementsThe ease and safety of operation is of vital concern in the selection of mooring buoys. One of the major considerations iswhether launch personnel will board the buoys to place and/or release mooring lines. If buoys are to be boarded, they shouldprovide a safe stable platform for the launch personnel. Stability is particularly important for handling heavy wire lines. Besidesproviding a stable platform, the buoys should be equipped with grab rails and footholds for personnel safety.The question as to what type of launch will be used at the port and whether men will or will not board the buoys should bediscussed with the affiliate marine personnel and local harbor authorities.If buoys are not to be boarded, the mooring hook on the buoy must be situated to be easily accessible to launch personnel.Therefore the mouth of the mooring hook should be located close to the edge of the buoy. Preferably the hook should also becapable of rotating in a horizontal plane, to enable the hawsers to be placed from any direction.Buoys must be capable of withstanding the impacts from the launch. This will require that the buoy or the launch be fendered.Buoys are normally located in exposed locations. The buoys must be capable of sustained and dependable service withminimum maintenance. When maintenance is required, it is preferable that it can be performed with locally availableequipment. Maintenance of large, heavy buoys requiring major lifting equipment may have to be contracted out at considerablecost.

Bouyancy Requirements

As the mooring line load on a buoy is increased, the amount of anchor chain which is lifted off the sea bed is increased. This,in turn, increases the vertical downward load which is applied on the buoy. The overall dimensions of the buoy govern itsmaximum net buoyancy (total available buoyancy minus weight of buoy) and hence determine at what mooring line load thebuoy will submerge.Buoys may be designed to submerge or not to submerge under maximum design loads. Maximum safety and mooring legflexibility will result if the buoy does not submerge. This requires the buoy to have a maximum net buoyancy equal to or greaterthan the submerged weight of the total length of anchor chain which is lifted off the sea bed under the design hawser pull on thebuoy. However, this criterion requires a very large buoy which may not be justified, especially when considering acceptability ofexisting facilities.Most of the buoys in service submerge under high hawser loads. There are several advantages gained by permitting buoys tosubmerge. First, a smaller buoy can be used, which is less costly, easier to maintain and possibly easier to operate. Second,a buoy, which submerges at a given load, gives the mooring master a good indication of the mooring load on the buoy.However, there are also several disadvantages involved with permitting the buoys to submerge. First, when a buoysubmerges, the anchor chain straightens out and much of the resilience of the mooring leg is lost. Therefore, the mooring legwill not be as capable of resisting surge forces due to vessel movements. Secondly, once the mooring hook on the buoysubmerges, it is impossible to run additional hawsers to the buoy, as is sometimes done during periods of changing orworsening weather. Thirdly, the buoy must be designed to withstand the hydrostatic pressures resulting from submersion. Andfinally, any lights provided on the buoy would have to be suitably designed.To minimize the above disadvantages, buoys should be designed not to submerge at a hawser load less than the normalworking load on the buoy (i.e., the load which is developed under normally experienced environmental conditions). Althoughthe design load on the mooring buoys is usually 60 - 100 tons, the normal working load is usually 30 - 50 tons. Whether it isjustified to use a large buoy of higher buoyancy, depends upon an evaluation of the above mentioned factors for the particularconditions existing at the terminal.

Description of Buoys and Advantages / Disadvantages

Cylindrical Buoys - Cylindrical buoys are large cylindrical cans which lay horizontally in the water. Most of the cylindricalbuoys in service have a diameter of about 1.8 - 2.1 m (6 - 7 ft) and a length of 3.7 - 4.3 m (12 - 14 ft). They are relatively light-weight (3 - 5 tons). A tension bar extends diametrically through the longitudinal centerline of the buoy. A mooring hook isattached to one end of the tension bar and the anchor chain is attached to the other end of the bar.




MULTI BUOY MOORING SYSTEMS (Cont)Drum Buoys - Drum buoys are cylindrical cans which sit vertically in the water. The buoys in service usually vary from about2.4 - 4.3 m (8 - 14 ft) in diameter and 1.2 - 2.1 m (4 - 7 ft) deep.

Peg Top Buoys - Peg top buoys have a conical shape to their bottom surface. They usually have a greater depth than drumbuoys and may be slightly more stable in short choppy seas.

Lamgar and Stayryt Buoys - Lamgar and Stayryt are trade names for buoys manufactured by Brown, Lenox and Co., andNorth British Electric Welding Company, respectively.

Advantages and Disadvantages - The major advantages and disadvantages of each type buoy are summarized in Table 1.

TABLE 1RELATIVE ADVANTAGES / DISADVANTAGES OF TYPES OF MBM BUOYS

BUOY TYPEFACTOR

CYLINDRICAL DRUM PEG TOP LAMGAR / STAYRYTInitial Cost + + 0 —

Maximum Net Buoyancy — 0 0 +Ease of Boarding — 0 (1) 0 (1,4) +Safety of Boarding — — (3) 0 +Ease of Mooring + (1) + (1) + (1) +Mooring Capacity — (2) 0 0 +Ease of Fabrication + + 0 —

Maintenance Frequency + 0 0 0

Ease of Maintenance + + 0 —

Remarks Generally used at smallterminals

Greater mooringcapacity than cylindricalbuoy

No significantadvantages over drumbuoy

Generally used for largevessels in rough seas

Notes:(1) For small buoys(2) Larger buoys can be fabricated but are more difficult to use thus these buoys are primarily used at small tanker terminals(3) Marginally better than cylindrical buoy(4) Marginally better than drum buoy

Legend:+ = Advantage for the factor / buoy type– = Disadvantage for the factor / buoy type0 = Neither a significant advantage or disadvantage for the factor / buoy type

MOORING LEGSA description and general requirements for the anchor chain and anchor systems are described below. More detailed designinformation is provided in Section MOORING LEG DESIGN. A cross section of typical mooring legs is shown in Figure 8.






FIGURE 8MOORING LEG COMPONENTS

Riser Section

Dip Section

AnchorChain

Swivel

Ground Leg

(No Sinker Block Shown)

Anchor

Mooring Buoy(No Load Position)

Mooring Line

PelicanHook

(Leg Shown Under Load)

Anchor Chain

Galvanized Wire

10T-15T Sinker Block

Conventional Buoy Leg

Swamped Mooring LegAnchorDP31Ef08





Chain

Chain can be manufactured as either open link or stud link. Due to its greater rigidity and strength, stud link chain is usedexclusively for buoy anchorage systems and is the only type discussed herein. There are two general types of stud link chainavailable:• Forged (flash welded) steel chain• Cast steel chainFor offshore anchorage systems, chain should have the following characteristics:• High strength• High abrasion resistivity• High corrosion resistivity• Low cost• Fatigue resistance (where necessary)

Grades - All major chain classification societies, such as the American Bureau of Shipping, Lloyd's Register and BureauVeritas, have established testing requirements for three grades of chain, Grade 1, Grade 2 and Grade 3. The higher the grade,the higher the alloy steel, and/or heat treatments. Grade 1 and Grade 2 are not recommended for major mooring operations.Besides manufacturing the standard grades of chain, many firms also produce an extra strong chain for offshore drilling rigs orlarge mooring operations. The catalogue breaking strength for some of these chains can be up to 40% higher than for theequivalent Grade 3 chain. Refer to API 2SK for additional information on higher grades of chain.

Connection Links - Detachable connection links are used between standard shots (one shot equals 25 m or 90 ft) of chain.Connecting links must be as strong or stronger than the anchor chain itself. Numerous types of connecting links are available.

Swivels - A swivel should be placed in the anchor chain near the connection to the mooring buoy. The swivel will prevent thechain from twisting if the buoy is turned or spins. A swivel may also be required in the chain near the anchor connection.

Sinker Blocks at MBM Berths - Sinker blocks are large concrete blocks or clumps which may be connected to the anchorchain at an intermediate point between the buoy and the anchor. The main purpose of the sinker is to reduce the length ofanchor chain required. If the sinker block is located near the dip section of the chain, it serves the dual purpose of holding thebuoy in place when a vessel is not in the berth. Sinker blocks are normally 5 - 10 tons in weight.

Anchors

This section discusses several types of anchors used to secure the mooring buoy anchor chain to the sea bed and the relativeholding power of each.The three major classifications of anchorage systems are:• Steel anchors• Concrete deadmen or concrete anchors• Stake pilesThe anchors discussed are not the only types available for this service but are intended to illustrate the considerations involvedin providing an anchorage system. Other types such as mushroom anchors may be equally acceptable.

Steel Anchors - Commercially available steel anchors are the most common form of buoy anchorage. Anchors for permanentmoorings should bite quickly into the sea bed and develop a high holding power. Furthermore, if a permanent anchor doesdrag, it should remain stable and not roll or pull out of the sea bed. The holding power of various types of anchors is discussedlater in this section.Admiralty and Navy stockless anchors have been commonly used for permanent moorings, despite the fact that they are poorlysuited for that purpose. Standard stockless anchors are designed for shipboard use and must handle and stow easily, besuitable for all types of sea beds and be of rugged construction. Furthermore, stockless anchors have been shown to berotationally unstable. Standard stockless anchors should not be used for permanent moorings. Stockless anchors aredesigned for shipboard use and since they must be broken out of the sea bed after each use, they are not designed to digdeeply or develop high holding power.





MULTI BUOY MOORING SYSTEMS (Cont)High holding power anchors have been developed which are more suitable for permanent anchorages. The most common ofthese anchors are the LWT, the Danforth and the Stato anchors. To avoid rotation of LWT and Danforth anchors in poorholding material it is customary to provide a swivel in the anchor chain adjacent to the anchor.

Concrete Wedge Anchor - Another type of anchor is the concrete wedge anchor, which can be fabricated locally andinexpensively. This anchor does not develop high holding power in sand and requires a high weight to be effective at all. Theanchor does not dig in deeply and shoves a mound of sand ahead of it when it drags. When the sand scours away, theprocess is repeated.In mud, the wedge anchors sink in and hold largely by a suction effect. On a hard bottom the wedge slides until it becomeslodged against an obstruction. The best use for this type anchor is as a deadman where it is inserted in an artificially dug holeand then covered with backfill, as discussed later.

Security Anchors - The security anchor is fabricated from steel and a filler material of pig iron and/or concrete. It gets itsholding power mainly from its heavy mass. The security anchor is also used as a deadman. No data are available on specificholding power values for these anchors.

Deadmen and Concrete Anchors - The term, deadmen, usually refers to a large concrete block or poured in place masswhich is used for anchorage. Deadman are often used for anchoring buoys to a very soft clay bottom. A hole is dug to firmmaterial. The deadman is then formed with tremie concrete or, a precast block is used, over which granular backfill is placed.The pullout value of the anchorage depends upon the shearing strength of the surrounding soil. Deadmen can also be usedwhen the sea bed is rock, but blasting is required in this case.When deep layers of very soft material are encountered, a large security anchor is sometimes used as a deadman. The anchorcan be provided with jet pipes for jetting into firmer material.

Stake Piles - Stake piles are steel piles that are driven into the sea bed for anchorage. The top of the piles are cut off near thesea bed and the anchor chain is normally connected near the top of the piles. Stake piles are used where high pulls must beresisted and where poor holding ground or rock bottom is encountered near the surface.Pile design should account for pile bending stresses as well as ultimate lateral pile capacity. Pile embedment should also besufficient to develop the axial capacity to resist vertical loads with an appropriate factor of safety. Pile design should be inaccordance with API RP2A.Stake piles are not commonly used at MBMs.

Holding Power of Steel Anchors - Holding power of an anchor is normally defined as the ratio of the actual pull it canwithstand without dragging to the weight of the anchor in air. For example, a 5 ton anchor which has a holding power of 10under given conditions can withstand a horizontal pull of 50 tons without dragging. The holding power of an anchor varies withthe following parameters:• Distance anchor has been dragged• Vertical inclination of anchor chain at anchor• Soil conditions at site• Type of anchor and fluke angleThe effects of each of these factors on anchor holding power is illustrated in Figure 9.The fluke angle of the anchor also affects holding power. Fluke angle is the angle between the center line of the shank and theflukes, when the flukes are in the extreme open position. The optimum fluke angles is 30° - 35° for sand and 50° for mud. If ananchor with a 50° fluke angle is used in sand, it will often plow a shallow furrow in the sand and fail to dig deeply. The highholding power anchors can be purchased with a fluke angle of 30° - 35° or 50°.





FIGURE 9ANCHOR HOLDING POWER

12

10

8

6

4

2

0

12

10

8

6

4

2

00 20 40 60 80 0 5 10 15 20

0

Sea Bed

Anch

or H

oldi

ng P

ower

Anch

or H

oldi

ng P

ower

Anchor Drag Distance (ft.) Chain Inclination atAnchor (Degrees)

Fluke Angle

34°

39°

49°

(18,000 Lb. StatoSand Bottom)

AC-14 Anchor

Admiralty Anchor

Note: Holding Powers are Representative for 5-10 Ton Anchors, but Values Vary with Anchor Weight and Soil Conditions

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REPRESENTATIVE HOLDING POWERSTYPE OF ANCHOR

FIRM SAND BOTTOM SOFT MUD BOTTOMNavy Stockless with Stabilizers 4 – 6 2Admiralty Stockless 3 – 4Danforth or LWT 10 – 12 3 – 6Stato 12 – 20 10AC-14 8 – 10

Holding Power of Chain on Sea Floor - The length of anchor chain laying on the sea bed adds marginally to the holdingpower of the system. Tests by the U.S. Navy indicated that 60 m (200 ft) of 70 mm (2-3/4 in.) chain had a holding power ofabout 12 tons in sand and 3 tons in mud.The holding capacity from friction of chain on the sea floor is typically ignored for MBMs, but can be calculated as described inAPI 2SK.

Depth of Anchor Penetration - The depth of penetration required for an anchor to develop its full holding power should beknown to evaluate the soil conditions at the proposed anchorage site. Similarly the depth of penetration of a ship's bowanchors should be known when determining burying requirements for submarine pipelines.Depth of penetration will vary with type and weight of anchor and type of sea bed. Representative values are listed Table 2taken from Navfac Design Manual 26.5.






TABLE 2ESTIMATED MAXIMUM FLUKE-TIP PENETRATION OF SOME DRAG-ANCHOR TYPES

IN SANDS AND SOFT CLAYEY SILTS (MUD)

NORMALIZED FLUKE-TIP PENETRATION(FLUKE LENGTHS)ANCHOR TYPE

SANDS / STIFF CLAYS MUD (1)

Stockless 1 3 (2)

MoorfastOffdrill II

1 4

StatoStevfixFlipper DeltaBossDanforthLWTGS (type 2)

1 4-1/2

Bruce Twin ShankStevmud

1 5-1/2

Hook 1 6

Notes:(1) For example, soft silts and clays(2) Fixed-fluke Stockless

PREVENTER WIRESAt MBM terminals, tankers must use shipboard mooring equipment to resist environmental loads from all directions. The tankermooring equipment alone may not be suitable, especially in more severe offshore locations. Accordingly, to prevent breakoutsfrom occuring, a steel mooring line known as a preventer wire should be run between the tanker and each mooring buoy. Thepreventer wire is permanently attached to the buoy. Preventer wires should not be used with synthetic lines on the vessel dueto the difference in elasticity.Preventer wires are tied around the mooring bitts with little or no pretension and are generally difficult to handle.

ANCILLARY EQUIPMENT

Quick Release Buoy Hooks

All mooring buoys should be equipped with quick release hooks (often called pelican hooks) to make the operation of releasingthe mooring line much easier and safer since launch personnel do not have to board the buoy. A lanyard is permanentlyattached to the release arm of the hook and is laid on the deck of the buoy. To release the mooring lines the lanyard is broughtaboard the launch with a boat hook. A schematic of a quick release hook is shown in Figure 10.The hook should be situated so that it is easily accessible to launch personnel. The hook should be capable of pivoting about ahorizontal axis to accommodate the changes in the inclination of the mooring line as the vessel discharges or as the buoyheels.





FIGURE 10QUICK RELEASE BUOY HOOK

LanyardPull

Position of Hookwhen open

Hook set in position on Buoy

Mooring Buoy

DP31Ef10

Marker Buoys - General

Marker buoys must be provided to designate the end of each submarine loading hose and the end of the submarine pipeline.The buoys should be color coded for identification.Marker buoys can have any shape which is practical to handle and maintain. Buoys and their anchorage should be selected tominimize damage to the buoy or the ship if a ship overrides their position, as often occurs. Buoys which can be fabricatedlocally are usually preferable to minimize costs.Long cylindrical spar buoys have good stability. The buoys are anchored with a taut chain and tidal or wave fluctuations aretaken up on the buoy itself, which remains essentially in stable position. The thin buoy also exerts minimum buoyant force onthe anchorage system.Small surface buoys, such as steel drums or wooden floats, may require that a large amount of slack be provided in the anchorchain, depending on tidal conditions. At low water these buoys could be considerably displaced from their intended location.Buoys should be sized to remain visible under normal operating conditions. The anchor chain or rope should be sized not tobreak if the buoy were entirely submerged. The lifting chain should be sized to lift the weight of hose full of product, or water.Some terminals prefer open link rather than stud link chain to allow the vessel to take intermediate bights when raising thehoses.

Pipeline Marker Buoys - Spar buoys, 4.5 - 12 m (15 - 40 ft) in length, are normally preferred for marking the end of thesubmarine pipelines. The buoys should be anchored to a concrete weight adjacent to the pipeline and not to the pipeline itself.Anchoring the buoy to the pipeline may result in damage to the pipeline during severe storms or from tankers dragging the buoyor forcing the buoy downward when overriding.

Hose Marker Buoys - Hose marker buoys should be attached to the ends of the submarine hoses by chain. The buoy ishoisted aboard the tanker when the hoses are raised with the ship's boom. Lifting rings should be placed every 4.5 - 6 m (15 -20 ft) along the anchor chain for lifting the hose. For ease of handling onboard the tanker, small buoys are normally used. Thebuoys need only sufficient buoyancy to remain visible under operating conditions. If the buoys are oversized, they will bedifficult to handle and will tend to lift the end of the hose off the sea bed under high wave action. Steel drums and spheres aswell as wooded floats are used successfully.

Navigational and Buoy Lights

The requirements for navigational lights, anchor ranges, or buoy lights should be discussed with affiliate's marine personneland local port authorities.






ADVANTAGES AND DISADVANTAGESMajor advantages and disadvantages of the MBM berths are highlighted in the table below.

ADVANTAGES DISADVANTAGES

• Depending on the number and length of submarinepipelines, MBM's generally require the lowest capitalinvestment of any alternative.

• Suitable for offshore installations, where servicing deepdraft ships are required.

• Can be designed and constructed rapidly.

• Can be changed, relocated, or salvaged.

• Vessel maneuvering usually does not require tugassistance.

• Buoys are not as vulnerable to severe damage.

• Can operate in more severe environments thanconventional pier.

• The number of pipelines and usually the number ofincompatible products is limited to three.

• Handling small ships and barges is often precluded byinadequate hoselifting gear onboard the vessel.

• Loading hose sizes limited by ships hose lifting gear.

• Locations are generally exposed to severe weatherconditions which can lead to high berth outages.

• Bunkers, fresh water, steam, or compressed air are costlyto provide, requiring either separate pipelines or offshoresupply.

• Handling of stores, repair parts, or containerized petroleumproducts requires a service barge.

• The berthing area is large and must be kept clear of othershipping. In certain locations, this area may have to beleased.

• Continuous launch service is required for safety, personnelaccess, and operating reasons.

• The berth is difficult to vacate in emergencies or heavyweather conditions.

CATENARY ANCHOR LEG MOORING SYSTEMSBACKGROUNDCatenary mooring type SPMs transfer tanker mooring forces to the sea bed by means of catenary moorings, composed ofeither all chain, or a combination of chain and wire. The principle of the catenary mooring is based on a combination of seabed friction, anchors, and the geometric properties of the catenary. It is a direct application of the principle of a ship's bowanchor.In the case of a ship anchored by its bow anchor, the ship pulls against a catenary chain that is attached to an anchorembedded in the sea bed. An equilibrium position is reached, when the environmental forces acting on the ship are balancedby the force in the catenary, with the horizontal force being ultimately transmitted to the seabed through the grounded chainand anchor. As additional offsetting forces (wind, waves, current) move the ship to a new position further from the anchor, thecatenary shape flattens, which produces a shallower chain angle, so that more chain is picked up from the sea bed. Thisincreases both the total chain load and the horizontal component at the ship, until forces become balanced at the new offsetposition. The mooring system "stiffness" (horizontal displacement per restoring force) is thus defined.A catenary type SPM to which a tanker moors has a number of mooring legs (typically of chain and 4-8 in number), distributedaround 360°, with a buoy supporting them in the middle. Although the tanker mooring load is shared by several of the mooringlegs, the principle is the same as with the ship's bow anchor. Horizontal offset increases the restoring force of the mooringlegs, and a new equilibrium position is established. The further the offset, the stiffer the system becomes. Horizontal forcesare transmitted to the seabed through chain-soil friction and through the anchor itself (see Figure 11).The overall stiffness of the catenary mooring is a function of the weight of the catenary legs and their initial pretension. Themooring system stiffness dictates how far the buoy will move horizontally (offset) in response to environmental forces acting onthe tanker. The mooring system stiffness is selected to accommodate the "reach" of the under buoy hose system (i.e., thedistance the buoy, to which the hose is attached, can be offset horizontally from its equilibrium position).




CATENARY ANCHOR LEG MOORING SYSTEMS (Cont)

FIGURE 11SCHEMATIC OF CATENARY ANCHOR LEG MOORING

Buoy

Anchor Chain

D

T

F1

F2

T = Horizontal Mooring Force = F1 - F2

F = Horizontal Anchor Chain ForceDP31Ef11

The most straightforward example of a catenary type of mooring is the "Catenary Anchor Leg Mooring" (CALM). A typicalCALM system, as shown in Figure 12, consists of the mooring buoy, the chain and anchor system, mooring assemblies, andfloating and submarine hose strings. This is the most widely used SPM design, owing to its inherent reliability, robustness andtrack record for reliable service.

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GNote: Anchor ChainsOmmited for Clarity

Bouyancy Tanks(or Bead Floats)

Pipeline

Plem

Moored Tanker

Tanker Manifold

Mooring Lines

AnchorGravity or Piled

Floating Hoses

Pipe Arm

Mooring Arm

Fluid Swivel Assembly (FDU)

Navigation Aids

Underbouy Hose"Chinese Lantern"

Configuration

Fenders

Central Chamber Piping

Hawse Pipewith Chain Stopper

Anchor Chains

Plem

Bead Floats

PipeLine

Fender Skirt

Launch Landing andCounterbalance Arm

UnderbouyHose"Lazy-S"Configuration

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BUOY COMPONENTSThe tanker is moored to a rotating deck or "arms" on the buoy, which permits the ship to weather vane about the buoy inresponse to changes in wind, waves and current. In a typical CALM design, shown in Figure 12, the buoy is moored in a fixedposition, and the turntable rotates about the buoy. At least one manufacturer provides a central mooring turret, which is held ina fixed position by anchor chains, while the buoy is free to rotate about the turret. The major components of a CALM buoy andtheir design requirements are listed below.

Hull

The hull is a welded steel structure, either circular or square, divided into at least six water tight compartments by steelbulkheads. Buoy size is based on the design buoyancy requirements and the space necessary for piping and mooringfacilities. Typical buoy sizes range from 12 - 14 m (40 - 45 ft) in diameter and from 3 - 5 m (10 - 16 ft) in depth.A submerged fender skirt is provided near the bottom of the buoy to keep tankers from contacting the buoy hull and/or productpiping and to protect the floating hoses from being crushed between the buoy and the tanker bow. The buoy side shell platingis protected by a system of skewed fenders, consisting of half rounds of pipe welded to the shell plate. The junction of the sideshell and top deck plate is also protected by a ring fender.Some buoy hull designs incorporate monocellular foamed flotation material in the buoy compartments to prevent the buoy fromsinking in the event of hull damage. However, foaming of compartments limits access for inspection, and is difficult to removefor hull repair/maintenance. It should be avoided whenever possible.Other features of the hull that are typically provided include:• Watertight storage compartments with access ladders• Lifting lugs designed to carry, as a minimum, the weight of the complete buoy with one compartment flooded• Chain stoppers and chain lockers• Recessed mooring bitts around the hull for use by small service craft

Rotating Deck

The rotating deck has three main components: the pipe arm, the mooring arm and the balance arm. The complete deck issupported on roller bearings, or on a combination of roller bearings and bogie wheels that run on the upper deck of the hull.The pipe arm supports the product piping, the expansion joints and the valves. The mooring arm carries the miscellaneousmooring fittings that transmit the mooring loads to the buoy hull. The balance arm supports equipment such as a winch andboat fenders. It is designed to counterbalance the other sections of the rotating deck, so that the arms do not produce a list onthe buoy. The rotating deck also carries a protective framework of small diameter pipe to prevent mooring lines from snaggingon the topside equipment. The rotating deck can be locked in a fixed position to permit undisturbed maintenance by smallservice craft moored to the buoy.The turret type CALM (where the entire hull rotates around a central turret) has these same facilities mounted directly on thebuoy, which in turn is free to rotate about the anchored central turret.The rotating portion of the buoy shall be designed to rotate in response to the mooring line pull of a weather vaning tanker, yetfree enough to rotate in response to forces on the hose, without damaging the hose. The rotating deck components shouldalso meet the following specific guidelines:

Pipe Support Section - This section shall contain the support structure for the product piping leading from the swivel to theedge of the buoy. The pipe supports shall be offset by approximately 60 degrees clockwise from the mooring bracket. This willkeep the floating hose string from coming into contact with the hull of the moored tanker and from fouling with the mooring lineswhen the CALM is not in use. The product piping support design shall ensure that forces acting on the piping do not result inunacceptable stresses being transferred into the fluid swivel.

Mooring Platform Section - This section shall have platforms for attachment of a mooring hawser chain bridle. There shall betwo mooring attachment points consisting of padeyes. Suitable provisions shall be made for reacting uplift loads and protectingthe structure from damage by the chain bridle. The chain bridle triplate shall be supported by the buoy mooring platform.Consideration shall be given to protecting the mooring platform with a wearing surface such as hard wood blocks.





CATENARY ANCHOR LEG MOORING SYSTEMS (Cont)On a typical CALM, a hawser guard of 300 mm (12 in) pipe, with an opening of approximately 2 m by 1 m (6.6 ft by 3.3 ft), isprovided at the outboard side of the mooring bracket platform. Steel chafing guards or doubler plates are provided on the innersurfaces of the hawser guard.

Counterweight Section - This section shall be designed to balance the off-center weight of the mooring bracket, hose armsections and other eccentric weight, so as to maintain the buoy floating in a horizontal attitude. This section shall alsoincorporate a boat landing for access to the buoy.

Anti-fouling Structure - This structure shall be provided to prevent lines from becoming fouled on the buoy equipment. Theanti-fouling structure can be constructed of tubular members of minimum cross-section OD 102 mm (4 in.), Schedule 160. Thestanchion landing points should terminate with a gusseted base plate, bolted to a corresponding pad plate that is welded to theunderlying rotating turntable structure. The structure should be of the least possible elevation and profile, so as to just containthe highest element of the equipment mounted on the rotating assembly (such as the navigational light on the central rotatingpipework).

Buoy Rotation System - The buoy rotating system may be the enclosed roller bearing type or the wheel-on-rail type. Regularlubrication is critical; therefore, consideration shall be given to reliable and durable automated lubricant dispensing systems.The wheel on rail design should meet the following additional requirements.• The wheel mechanism shall be designed for a minimum service life of 10 years and shall allow change out of wheels on

site. Wheels shall be mounted for balanced loading. A watertight grease seal and a wiper seal shall be provided for thewheel bearings. Provisions shall be made to prevent water entering or accumulating around the rotating mechanism.

• The rails shall be replaceable on site, without altering the structure of watertight compartments. Rails shall have aminimum service life of ten years. Rails and wheels shall meet recognized national and international standards for the useto which they are intended, including CMAA 70.

A locking assembly should be provided to enable the rotating assembly to be easily locked and unlocked in any randomposition.

Fluid Swivel

This unit, sometimes referred to as the Product Distribution Unit (PDU), maybe of single or multi-product design and is locatedon the rotational axis of the hull and the rotating deck. A typical fluid swivel consists of the following main parts:• The lower housing assembly, fixed to the hull structure and connected to the central chamber piping.• The rotating upper swivel assembly, connected to the overboard piping.• Seals and bearings between the housing and swivel assemblies.• Central Chamber and Piping. A circular chamber, concentric with the hull, is provided in the hull for housing the piping and

valves. Piping in this chamber connects the fluid swivel to the underbuoy hoses.• Overboard Piping. These pipes connect the fluid swivel to the floating hoses. All piping, valves and expansion joints are

supported on the pipe arm structure of the rotating deck.The fluid transfer swivel shall be designed in accordance with ASME SEC VIII D2 and ASME SEC IX to withstand the specifiedworking pressure. The swivel shall have primary and secondary seals, with leak detection ports between them. The seals shallbe continuous and suitable for the specified services and environment.

Piping

The piping system should be designed in accordance with ASME B31.3 and should be API Grade B. A piping supportstructure shall be designed to ensure the integrity of the piping system and a structural analysis shall be performed for the pipearm and other critical pipe supports. Piping loads that result from pressure effects, temperature effects and end loads inducedby the eccentric rotation of the pipe arm shall be considered.Pipe flanges at the bottom of the buoy, to which underbuoy hoses connect, shall be suitably angled to minimize hose bendingin service. The pipe overside termination shall be oriented so as to minimize hose bending in service and facilitate hosechange out. Pipe flanges shall be raised-face.All piping shall be arranged and supported such that the fluid swivel is not over-stressed from thermal or mechanical stresses.All piping with open ends shall be fitted with blind flanges.Flanged valves shall be provided upstream and downstream of the fluid swivel. They shall be located in the center well andjust outboard of the pipe arm expansion joint on deck. Ball valves have been used for most installations in the past, but othertypes may be considered.





Navaids and Electrical

Aids to navigation shall meet the greater of either local requirements or the following provisions:• One lantern signal installed on the CALM buoy, meeting IALA requirements, with 8 km (5 mile) range. The "Tideland"

Nav-Light ML300, including a TF-3B Flasher synchrostat 12V, code "U" with acrylic lens and focus mount or its equal, isrecommended.

• A foghorn with a 3.2 km (2 mile) range. The "Penwalt" Auto-Power model SA 8501A, frequency 840Hz, LAMDX, InputVolts = 14V, MDX - 20 VM or its equivalent, is recommended.

• Primary batteries shall be lead-acid, 1.25 V D.C., 3,000 Amp Hr, replaceable every six months. The "Penwalt" Auto-Powermodel SA 8501A, frequency 840Hz, LAMDX, Input Volts = 14V, MDX - 20 VM or its equivalent, is recommended. Thebattery box should be of corrosion resistant construction, sized to contain and weather protect the navigation aid batteries.A properly designed solar panel and battery charging system shall be provided to charge batteries during the shortestdaylight period.

• A radar reflector may be required by local authorities. If required, it shall not be mounted above the rope guard or in theway of the navigation light.

• Any electrical systems shall be designed in accordance with API RP 14F.

Winch, Handling Equipment, Maintenance Aids

A 10 ton (9.1 tonne), nominal capacity, air driven winch with the following features shall be installed on the buoy.• An adequate number of fairleads and a davit of sufficient size shall be provided to enable hose and hawser change out.• A block arrangement and rigging points shall be provided to enable mooring chains to be tensioned.• The air winch shall be suitable for long term operation in the splash zone and be provided with a canvas cover when not in

use. It shall also have a hand brake with a "dead-man" feature and a full drum of wire rope.The contractor shall prepare a plan showing how hose change out, hawser change out, battery change out and chaintensioning should be performed.

Component Design Requirements

Steel - Structural components of the buoy should be constructed of plate material manufactured to the latest issue of ASTMA36 / A36M Grade B or equivalent. Steel for special components, such as heavy weldments and padeyes, should be inaccordance with API RP 2A.The structural framing must be designed to withstand operational and survival load conditions, with safety factors not less thanthat allowed by ABS #39.Scantlings should be sized based on 1 m (3.3 ft) of water above the main deck. The load path from the mooring bracket to theanchor chain stopper must be analyzed for relevant combinations of static and dynamic loads on the buoy. Welded joints alongthe mooring load path shall be full penetration welds.Sufficient access and aids shall be provided to facilitate the change out of underbuoy hoses, surface hoses and mooringhawsers, including wire hawser pipe, rigging points, padeyes and work platforms.

Corrosion Protection - The buoy should be equipped with sacrificial anodes to provide a minimum 10-year protection for thebuoy. Doubler plates shall be provided at all anode attachments to the hull.Painting requirements are specified in section COATING SYSTEMS. The following markings should also be provided:• Draft Marks: Three lines of draft marks shall be provided on the buoy sides, indicating the total draft in meters. These

draft marks are to consist of welded-on Arabic numerals. Each draft mark shall have a height of 100 mm (4 in.) and shallbe painted white. Markings shall be done every 200 mm (8 in.). The letter "M" shall follow each set of numbers.

• Chain Hawser Numbers: Numbers 1 to 6 (or the number of chain legs) shall be provided to identify each hawser or chainstopper location. The application of the numbers shall be performed as prescribed for the draft marks.

• Compartment Numbers: Compartment numbers, welded-on, shall be provided next to the manhole.• Identification: A steel identification plate that complies with local regulations shall be provided to identify such items as

owner and location.






ANCHOR LEGSThe number of anchor legs, (normally six), is based on design parameters. Mooring configurations with less than six anchorlegs are not recommended. Failure of a mooring leg would cause excessive buoy excursions, which in turn would subject theunderbuoy hoses and the PLEM piping to excessive stresses and possible failure. High-strength steel, stud link chains arenormally used for these anchor legs. Each anchor leg is fixed to the seabed by means of either anchors or piles.The length of the anchor chain legs depends upon water depth, loads developed by environmental forces under various buoyconditions and the size of chain selected. Typical anchor chain length for 30 m (100 ft) water depth would be 300 m (1000 ft)for each leg.The anchorage system can consist of either piles or drag anchors. Two anchors, the Bruce Twin Shank Flat Fluke and theVryhtot Stevpris, have been tested full scale and proven to have high holding power. These should be considered first forCALM's, if drag embedment anchors are required.Refer to section MULTIPLE BUOY MOORING SYSTEMS for additional information regarding chain grades, anchor types andanchor performance. Methods to estimate the capacity of anchor legs are also described in section MOORING LEG DESIGN.

MOORING LINES AND TANKER FITTINGSThe mooring connections between the tanker and SPM generally consist of two ropes (hawsers) permanently attached to thebuoy. These consist of the following elements:• A chain section at the buoy end. The tanker mooring lines are permanently connected to the buoy by a chain bridle, which

is sized to withstand all mooring loads and contact (abrasion) with the buoy hull.• A synthetic rope hawser. This portion of the system is lightweight and provides the required elasticity to absorb energy

and snatch loads. When not in use, the ropes float on the sea surface by means of attached floats.• A chain section for the tanker end. This portion of the mooring line, called the chaffing chain, is connected to the tanker. It

extends outboard of the tanker's chocks to prevent abrasion of the synthetic ropes. Open links are provided to fit aspecially designed chain stopper or a special hydraulic stopper aboard the tanker strapped to tanker bitts (refer toFigure 13).

• A chain buoy. This buoy provides flotation for the chain section of the mooring lines, while the lines are not in use.• A messenger float rope. A suitable length of self-float rope is provided for hauling the mooring lines aboard the tanker.

The mooring lines are then fastened to the tanker.The hawsers shall be specified and manufactured in accordance with OCIMF-10 and OCIMF-11. The hawser pick-upassembly, shown in Figure 14, includes the chafe chains, pick-up rope, support buoys and connections. Guidelines for thepick-up assembly are specified in OCIMF-6. Guidelines for shipboard mooring equipment including the type and location ofsecuring devices and fair leads, and the safe working loads of securing devices, are also provided in OCIMF-6. Floatation forthe hawser must be provided.Hawser eyes shall be protected by galvanized thimbles. There are two types of commonly used thimbles: Bridon, which musthave the shackle pin pass through the eye; and Sampson, which requires the bow pass through the eye.If the design calls for two tanker attachment points, it shall be confirmed with the marine department that ships to moor at theSPM are so configured.In the equally loaded hawser system (recommended herein for all installations), the synthetic hawsers shall be a matched pair,as established at the manufacturing facility. Their length shall be measured while under reference load, with an allowablelength tolerance of 0.5 percent between hawsers. Other hawser arrangements may be considered, subject to the Company’sapproval.





FIGURE 13SHIP WITH MORE THAN 150,000 TONNES DEADWEIGHT FITTED WITH TWO BOW STOPPERS

DP31Ef13

Bow Stopper

Pick-Up Rope

Chafe Chain A or B

Mooring Hawser

76mm Dia Chain






FIGURE 14TYPICAL ARRANGEMENT TO FACILITATE CONNECTION / DISCONNECTION OF PICK-UP ROPES

DP31Ef14

Chafing ChainSupport Buoy

Wire Pendant(May be replaced byequivalent Floating Rope)

Floats

Mooring HawserWire PendantSupport Buoy

Chafe Chain

Following guidelines can be used to evaluate requirements for hawsers:

Derating Factors

If the breaking tests are performed on straight, non-spliced rope, the effective breaking strength of the hawser shall be taken as90 percent of the manufacturer's catalog breaking strength, in order to account for defects and splices. If the catalog breakingstrength is based on tests of already spliced constructions, no derating is necessary. If nylon rope is used, the catalog drybreaking strength shall be derated by 20 percent to account for reduced wet strength and short-term fatigue damage.

Tankers 100,000 - 350,000 DWT

• If peak hawser loads are less than 200 tonnes (220 tons), a single hawser attachment point on the tanker shall be used.This hawser arrangement shall consist of twin hawsers joined by a triplate with a single chafe chain to the tanker bow. Therope components shall be arranged such that no unequal loading of the rope occurs.

• If peak mooring loads are greater than 200 tonnes (220 tons), a two hawser arrangement shall be used on ships over150,000 DWT.





Tankers larger than 350,000 DWT

• If peak hawser load is less than 250 tonnes (275 tons), a single hawser attachment point on the tank shall be used,configured as above.

• If peak hawser loads are greater than 250 tonnes (275 tons), a two hawser arrangement in accordance with OCIMF-6 shallbe used.

Safety Factors

The hawser shall be designed for the determined design operating loads at the facility with an appropriate safety factor. Thehawser shall be the "weak link" in the overall mooring system; however, this requirement may be waived if a load monitoringsystem is provided.For equally loaded systems, a minimum safety factor of three should be used. For example, in an intact two-rope hawser, thedesign load will produce a tension level in each of the rope components of 33 percent of its effective breaking strength; if onerope fails, the tension level in the remaining rope will be 67 percent of its effective breaking strength. The minimum safetyfactor may be increased due to the following factors:• Confidence in predicted maximum mooring force• Material• Inspection and replacement frequency• Past experience, if any, at facility

FLOATING HOSESThis system usually consists of two independent strings. Their sizes and lengths are based on specified pumping rates and onthe size of the largest tanker to use the buoy. Separate flotation for each string is provided by either bead floats, or morecommonly by integral flotation material built into the hose. Hose sections with extra reinforcement are provided at theconnection to the buoy. At the tanker connection end, hoses are usually smaller in diameter. They terminate in a section ofhose having lightweight properties with extra flexibility, and are provided with flotation collars or extra built-in flotation to supportthe hose-end fittings.Generally, the outer hose string away from the tanker is one hose length longer than the inner hose string. The outer hosestring is normally provided with several evenly spaced, battery-powered winker lights operated by sun switches to provide nighttime illumination. The tanker end of each hose string is normally provided with a blind flange, quick-coupler device, abutterfly-type valve with spool piece, hang-off chain, and pickup buoy.For guidelines and requirements for offshore hoses refer to section HOSES.

UNDERBUOY HOSESThe underbuoy hoses can consist of either a single or multiple hose strings, connecting the PLEM to the buoy piping. Twohose configurations, the Lazy-S and the Chinese Lantern, are commonly used (refer to Figure 12). The hose string lengths arechosen to be compatible with the buoy excursions. Underbuoy hose configuration is maintained by underwater bead floats,adjustable flotation tanks, or both. It is important to check the maximum and minimum excursions, both with the tanker moored(maximum operating condition) and without the tanker moored (survival condition). Either of these extreme conditions couldlead to over-bending the hoses and overstressing the PLEM and buoy flange connections. Hose configurations are typicallychecked for the buoy motion envelope by computer hose-analysis programs. In areas where currents exceed 3 knots, flowinduced vibration effects should be investigated.For guidelines and requirements for offshore hoses refer to section HOSES.

OPERATING CONDITIONSThe structure and mooring legs of a CALM are designed to withstand some maximum hawser load. This "design hawser load"corresponds to the maximum weather conditions in which it is expected the tanker will remain moored to the CALM. Maximumhawser loads typically range to about 300 tons, which roughly corresponds to a maximum wave height of around 5 m (16 ft). Inpractice, local mooring policy may require the tanker to disconnect from the mooring at substantially lower hawser loads. Inmore severe conditions, the mooring threshold can be increased by the use of the ship's propulsion.





CATENARY ANCHOR LEG MOORING SYSTEMS (Cont)The hawser tension can be monitored through an instrumented pin in the hawser attachment point. Mooring load measurementand recording can be used to develop hawser replacement criteria at a given site. This could extend the replacement period ofhawsers, with attendant cost savings. In areas where high hawser loads are frequently encountered, load monitoring can beused for establishing disconnect criteria.

Severe Environment Catenary Mooring Designs

The traditional CALM design does not perform as well in a severe environment. The buoy becomes lively in moderate seastates, which results in difficult access and working conditions. The low freeboard allows the deck to be washed by breakingwaves in moderate or severe sea conditions, which exposes the topside equipment and the structure to sea water. In a severeenvironment, the shallow draft, large diameter buoy is kept in motion a greater percentage of the time, resulting in significantlyhigher wear on the hoses and the mooring chains.

STABILITY AND BOUYANCYThe mooring buoy must be sized so that it has adequate stability and buoyancy to support the anchor chain and otherequipment mounted on it. The typical minimum freeboard is 1.68 to 1.83 m (5-1/2 to 6 ft) of freeboard, and may be increasedfor severe environments.The diameter of the buoy must be large enough to provide adequate stability. The heel angle should not exceed 10 degreesunder static load conditions when the buoy is subjected to maximum mooring loads. Stability should also be adequate enoughto prevent the buoy from capsizing during installation or in the event any combination of anchor legs fail in undamagedcondition. Every effort shall be made to design the buoy to minimize motions during the expected operating conditions.The buoyancy capacity must be adequate to prevent the buoy, with chains attached, from sinking in the event any twocompartments are flooded.

WATER DEPTH LIMITATIONS OF CATENARY MOORINGSAs the basic, chain leg CALM design moves into deeper and deeper water, the buoy must grow in size to support the additionalweight of chain. As the buoy grows in size, it attracts more wave loading, which in turn, requires larger, heavier chains. Theadditional weight of chain leads to the need for higher pretension, in order to limit the buoy offset and prevent damage to theriser hoses. Higher pretension results in a larger buoy.For water depths beyond about 150 m (500 ft), designers solve this problem in one of two ways. Mooring legs may belightened by using a combination of chain and wire; the chain at the ends where wear is greatest and the wire in between.Alternatively, adding buoyancy to the mooring leg, usually in the form of one or more subsurface "spring" buoys attached to thechain leg, also serves to reduce the weight supported by the main buoy, and thus reduce the hull buoyancy requirements.

INSTALLATIONThe installation of a catenary mooring SPM usually occurs after the pipeline and pipeline end manifold (PLEM) have beeninstalled. Installation of the catenary mooring consists of installing the anchor points, laying and tensioning the anchor legs,connecting the buoy, attaching the underbuoy and surface hoses, and commissioning the testing. This work is performed byconventional offshore construction vessels.

Tensioning Chain Legs

The location of the buoy relative to the PLEM plays a key role in the performance of the mooring and the life of the underbuoyhoses. It is important, therefore, that the chain legs are laid straight from the anchor points to the PLEM location, and that allkinks and slack are removed by tensioning before the final chain angles are measured. Anchors shall be tensioned to themaximum expected loads and held for one hour. Otherwise, the kinks and slack will gradually be pulled straight during theservice of the mooring. The buoy will then end up off center from the PLEM, which can result in accelerated hose wear.

Buoy Transportation

The buoy is usually not constructed in the same area as the installation site, and shall therefore be transported after it is fullyfabricated. While it is possible to tow a mooring buoy, this practice is not recommended for open ocean transits, due to thepossibility of damage to the onboard equipment and the bearings, in the event of sustained rough weather. For open oceantransportation, it is preferable to deck load the buoy, and off-load it whereas relatively calm weather tow to site can beundertaken.





Commissioning

All the equipment, steel structure and piping should be inspected and tested in the fabrication yard before transportation.Consequently, commissioning tests are undertaken to ensure that no damage has occurred during the transportation andinstallation process. After the buoy is correctly installed in its mooring, all equipment on board shall be functionally tested andthe piping system pressure tested for leaks. Although torque measurements of the rotating portion of the buoy are difficult toperform offshore with any accuracy, it is very important to determine that the rotating portions of the buoy are in fact rotatingfreely. The best that can be expected is to ascertain that all bearings are turning freely in both directions through 360 degrees,with no unusual noises or rough spots. The piping is pressure-tested to detect any leaks in the flange connections madeoffshore, and shall be specified accordingly. The commissioning phase shall also include the first tanker loading.

ADVANTAGES AND DISADVANTAGESMajor advantages and disadvantages of CALM berths are highlighted in the table below.


• Mature design with an established track record. TheCALM is best suited to relatively mild environmentalconditions (approximately 2.5 meter waves or lower).


• The catenary mooring is a relatively simple design toconstruct, install, and maintain.



• Multiple mooring legs provide redundancy on failure of amooring leg.

• The number of lines and usually the number ofincompatible products is limited.

• Catenary chain legs require periodic inspection.

• There is a risk of tanker interference with legs.




• Locations are generally exposed to severe weatherconditions which can lead to high berth outages. While thecatenary mooring system can be adapted to severeenvironments and deep water, the cost of doing so can besubstantial.


SINGLE ANCHOR LEG MOORING (SALM)BACKGROUNDThe Single Anchor Leg Mooring (SALM) consists of a mooring buoy at the sea surface, which is attached to a base on the seafloor by a single anchor leg (refer to Figures 15 and 16). The buoy is drawn down against its buoyancy by tension in theanchor leg. Tankers moor to the buoy with mooring hawsers. A swivel in the anchor leg or in the buoy allows the tanker toweather-vane around the mooring point. A fluid swivel is mounted concentric about the anchor leg, either on top of the base oron top of the riser pivoted from the base. Cargo hoses connect to an arm on the fluid swivel and rise to the surface, where theyfloat and extend to the tanker's manifold.

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Moored Tanker

Mooring Lines

Floating Hoses

Anchor Chain

Tanker Manifold

Anchor Swivel

Base Hose

Plem Pipeline

Mooring Buoy

Navigation Light

Bouyancy Tanks

Underwater Hose

Universal Joints

Hose Arm

Fluid Swivel Assembly (PDU)

Base

Piles




SINGLE ANCHOR LEG MOORING (SALM) (Cont)

FIGURE 16DEEPER WATER SINGLE ANCHOR LEG MOORING

DP31Ef16

Mean Sea Level

Beacon Light

Mooring Hooks and Pins

Fendering

Mooring Buoy

Anchor Chain

Anchor Swivel

Fluid Swivel Assembly

Hose Arm

24" Hose

Buoyancy Chamber

Underbuoy Pipe

Swivel Joints

Universal Joints

24" Hose

Base

Rip Rap

Submarine Line

Pipe Piles

The SALM design, shown in Figure 15, was first developed for mooring very large tankers in more severe environments. TheSALM has been successfully applied at many locations throughout the world, both for conventional oil import and exportterminals, and for deep water production and loading facilities. It is a departure from the CALM design, both in the means ofreacting to the mooring loads, and also in the use of large mechanical components in the mooring load path. In general, theSALM provides a higher mooring threshold than the CALM for a given water depth and tanker size.As with the CALM, the tanker is allowed to weather vane around the mooring buoy. Instead of using a series of catenarymooring chains individually fixed to the seafloor with anchors or piles, however, the SALM is anchored by a single verticalanchor leg, which is fixed to the seafloor by a foundation structure. Mooring forces are transferred to the seabed through





tension and lateral loads on the foundation, instead of only lateral loads through the chain and anchor reactions of catenarymooring legs. The pipeline end manifold (PLEM) is usually designed as part of the foundation.


SALM DESIGNSA typical shallow water SALM is shown in Figure 15. In this design, the fluid swivel assembly is mounted directly on themooring base, and a continuous stud-link anchor chain leg connects the mooring buoy to the mooring base. In a SALMdesigned for water depths beyond 50 m (164 ft), a solid tubular member is used for the lower portion of the anchor leg, asdepicted in Figure 16. The fluid swivel assembly is located either at the top of the riser or in the bottom of the buoy.

COMPONENTSBoth types of the SALM system consist of the following major components:

Mooring Buoy

This is a cylindrical, all-welded steel structure with mooring bitts and anchor chain attachments. The structure is divided intowatertight compartments by radial bulkheads. A mechanical device on the center shaft of the buoy may be used to adjust thefreeboard of the buoy, so as to meet the design requirements of the anchor chain. The buoy is fitted with a full-length fenderingsystem and battery-powered navigation light.

Riser

The riser is either heavy anchor chain or a solid structural member. It is concentric with the buoy and the mooring base, and isfitted with top and bottom universal joints and a swivel. The universal joints and the swivel permit the buoy to oscillate freelyunder all sea and operating conditions.

Foundation Base

In contrast to a purely catenary mooring arrangement, which only transmits lateral mooring loads, the SALM designaccommodates both vertical and lateral mooring-load transfer to the seabed. The typical SALM foundation utilizes some formof gravity base in combination with either piles or a penetrating skirt arrangement. Both skirts and piles can react to lateral andvertical mooring loads, but to differing degrees, depending upon the design. The amount of excess weight or gravity in thebase is taken as some percentage of the static pretension of the buoy, in order to reduce the amount of vertical load reacted bythe skirt or pile friction. The foundation base is typically an all-welded steel structure with diagonal structural framing, whichtransfers mooring loads between the riser and the seabed.The choice of piles or skirt for the foundation depends upon soil conditions and economics. In thick sand or well consolidatedsoil, a penetrating skirt design with excess weight in the base structure is usually more cost effective than a pile design.Product piping is brought onto the base, where it is connected to the swivel or to hoses and riser piping. Shut-off valves toisolate the pipeline from the SALM piping are provided on the base.

Swivel Assembly

This assembly is concentric with the base and buoy. It consists of the Product Distribution Unit (PDU) and hose arm piping,which connects to the underwater hose.

MOORING LINES AND TANKER FITTINGSMooring lines and tanker fittings are similar at SALMs and CALMs. Refer to section CATENARY ANCHOR LEG MOORINGS.Buoy connections are made to bitts that are fixed to the buoy deck.

HOSE SYSTEMIn shallow water SALMs, the hose system consists of two sections: the surface floating section; and the underwater section,from the fluid swivel assembly connection to a point where the hose surfaces. The hoses in the underwater section areprovided with buoyancy tanks or bead floats to achieve the desired configuration. The hose in the floating section and thetanker handling and connection equipment are similar to those in the corresponding CALM system. Refer to sectionCATENARY ANCHOR LEG MOORINGS. In rigid riser SALMs, the hose string may consist of underwater hoses only,positioned with bead floats and/or buoyancy tanks.





DESIGN CONSIDERATIONSThe single anchor leg is held in tension by excess buoyancy of the buoy. The SALM reacts to tanker mooring loads bydeflecting in a small arc about its foundation. The manner in which the SALM deflects with an applied mooring load must beunderstood, in order to design its various components. The SALM acts as an inverted pendulum under the influence of anapplied load. As the buoy is pulled to the side, its arc of motion pulls the buoy further under water, increasing its buoyancy andrestoring forces (refer to Figure 17).As with other single point mooring designs, the elastic response characteristics of the system shall be within certain limits tominimize the mooring forces. The mooring system should be elastic enough to allow the moored vessel to move under theinfluence of waves and other forces, but shall be stiff enough to limit the extent of this motion. In the design of any SPMsystem, the optimum elasticity shall be established, which will minimize the motions and the resultant forces due to thesemotions. Elasticity is derived from two mechanisms: the restoring force generated by the incremental displacement of the buoyby a mooring force (inverted pendulum effect), and the elasticity of the mooring hawser.The design of the components of the SALM is based on a probable peak load and on fatigue. The peak load is predictedstatistically from analysis and model test data. Both maximum tanker moored conditions and survival (without tanker moored)conditions are evaluated to determine the peak loads to be used in the design.Fatigue life is an equally important design criterion. In deeper water depths, where long steel risers and buoys are used,fatigue can become a major concern, due to larger bending stress ranges in the long, slender structures.The SALM design depends upon large mechanical components, many of them located under water. These mechanicalcomponents represent potential single-point failures in the structural load path; therefore, thorough engineering is criticalthroughout design and fabrication.Structural components of the buoy should be constructed of plate material manufactured to the latest issue of ASTMA36 / A36M Grade B or equivalent. Steel for special components, such as heavy weldments and padeyes, should be inaccordance with API RP 2A.The buoy should be equipped with sacrificial anodes to provide a minimum 10 year protection for the buoy. Doubler platesshould be provided at all anode attachments to the hull. Painting requirements are specified in section COATING SYSTEMS.Refer to section CATENARY ANCHOR LEG MOORING SYSTEMS for requirements for navigational aids and electricalsystems.






FIGURE 17SCHEMATIC OF SINGLE ANCHOR LEG MOORING

DP31Ef17

D

L

T

BBuoy

Anchor Leg

Foundation

Center ofSubmergedBuoyancy

T =D B

L2 - D2

T = Horizontal ForceD = Horizontal DisplacementB = Net BuoyancyL = Legnth of Pendulum





ADVANTAGES AND DISADVANTAGESMajor advantages and disadvantages of SALM berths are highlighted in the table below.


• Mature design with an established track record.


• The catenary mooring is a relatively simple design toconstruct, install, and maintain.



• The anchor chain is positioned directly beneath the buoy,instead of extending out radially from the buoy, as with acatenary anchored SPM. Consequently, marine vessels(work boats, tugs, tankers) may drop their anchors, ifnecessary, without danger of fouling the mooring. Thesmall "foot-print" of the SALM can also be advantageousif seafloor congestion is a problem.

• The SALM mooring buoy can be small and ruggedly built.Furthermore, the fluid and anchor swivels and the hoseconnections are located below the keel of the tanker.Consequently, the possibility of serious damage to eitherthe buoy or the tanker, as a result of a collision, isminimized. Tankers can maneuver close to the SALMwith less concern of collision.

• The SALM is potentially less expensive than a CALM indeeper water depths, as the 6 to 8 catenary mooringchains are replaced by a single anchor leg.

• The number of lines and usually the number ofincompatible products is limited.

• Mechanical joints and hose connections locatedunderwater require diver inspection, and reduce theamount of field maintenance that can be performed.

• The single anchor leg provides no redundancy in the eventof chain or connecting hardware failure.




• Locations are generally exposed to severe weatherconditions which can lead to high berth outages. Whilethe SALM mooring system can be adapted to severeenvironments and deep water, the cost of doing so canbe substantial.


DESIGN LOADS, ANALYSIS AND MODEL TESTSMULTI-BUOY MOORING SYSTEMThis section discusses the calculation techniques and considerations for sizing / evaluating the components of the MBMsystem.

Environmental Loads on Vessel

The layout of a multiple buoy mooring berth and the size of the mooring legs are governed by the magnitude of the wind,current and wave loads exerted on the moored vessel. Wind, wave and current forces/moments on vessels at MBM's can bedetermined as discussed in section DESIGN CONSIDERATIONS. Wind and current forces should be calculated based on thecoefficients given in OCIMF-14. For buoy berths environments from all directions can be important since there is no structureto restrain the vessel.Although greater movements are permitted at MBM's (often up to 4.5 - 6.0 m or 15 - 20 ft) the system elasticity and limitedrestraint capacity usually restricts the maximum permitted wind speeds for vessels in the berth to 11 - 16 m/s (25 - 35 m/h).Current loads can further limit operations if velocities are significant and the berth is not in line with the prevailing currentdirection.Rough seas and swell are often the critical conditions affecting the tenability of a MBM. Wave action can develop considerabledynamic loads in vessel's mooring lines and anchor chains. These loads are very difficult to evaluate except through modeltesting, although some analytical approaches have been developed and may provide cost effective information.





SINGLE ANCHOR LEG MOORING (SALM) (Cont)Generally, mooring leg loads due to wave / swell vary as follows:• For the same wave period and wave height, the mooring leg loads are much greater for beam waves than for head waves.

Whereas a berth may remain tenable in 4.5 m (15 ft) head seas, it could be limited to 1.2 m (4 ft) beam seas.• The more elastic, or softer, the mooring system, the lower the mooring leg loads due to waves.• The mooring leg loads decrease as water depth increases.• The mooring leg loads are maximum when the tanker is in the loaded condition.

Mooring Loads

The mooring legs provide the restraint for the vessel against the imposed environmental loads. The mooring legs should bedesigned to handle the following load cases:• Working load equal or greater than the breaking strength of the strongest single mooring line connected to the buoy.• Working load equal or greater than the maximum combined load developed in all mooring lines run to a single buoy due to

the worst case of wind, waves and current acting on the vessel. Consideration should be given to the vessel load conditionand mooring equipment.

• Ultimate load equal or greater than the breaking strength of one line plus 50 percent of the breaking strength of anyadditional lines.

The design load for the mooring leg will generally be the breaking strength of a single mooring line. However, combinedmooring loads should always be considered when evaluating an existing facility.

Analysis

Computer analysis of a buoy berth is generally warranted due to the complexity/configuration of the mooring legs. The mooringline loads for MBM's can be calculated through static analysis using the PCSMART computer program with the appropriateallowance for dynamic wave loading and mooring system elasticity. Special application of the input of the vessel and berthdata is required with PCSMART. Other mooring software programs are available that include dynamic wave loads. Analysismethods for establishing design wave loads and vessel motions are similar to those described for SINGLE POINT MOORINGSYSTEMS in this section.The allowable movement of the vessel in the berth should be limited to a 4.5 m (15 ft) radius based on the limits of the cargotransfer systems (refer to section HOSES). The following load criteria should also be used to establish the operating conditions(e.g., worst case environmental conditions that cargo transfer can be performed). A factor of safety of 1.4 to 1.6 should beapplied to these criteria based on local conditions and operating experience at the terminal:• 65% of the new steel wire breaking strength• 55% of the synthetic new line breaking strength• Rated brake capacity of the tanker's mooring winch or allowable capacity of the tanker's mooring bitt

Mooring Buoy Design Load

As noted above, the buoy and its anchorage system will generally be designed for the breaking strength of the largest hawserused between the buoy and the vessel which is normally in the range of 60 - 100 tons, depending on ship size. Dynamic loadshigher than the breaking strength of one line could be placed on the buoy due to vessel motions during maneuvering or stormconditions. However, loads of this magnitude rarely, if ever, occur. Anchor chains are designed with sufficiently high safetyfactors to allow for slight overload without damage. Unusually high dynamic loads cause the anchor to drag, therebyredistributing loads on the mooring system and relieving highly loaded lines.

SINGLE POINT MOORING SYSTEMThe basis for the design of mooring components and the primary structure of a buoy typically result from the peak mooringloads that the tanker exerts on the system. These loads can occur under any combination of the maximum connectedoperating conditions. Survival conditions, with no tanker connected, must be checked; however, the peak connected loadstypically dominate.The design process must consider a wide variety of conditions in order to determine the true peak design loads. These includedifferent tanker sizes in loaded and ballast states, at both in-line and cross current conditions. Each site-specific design mustbe based on all known environmental conditions.Transportation to site, lifting and other loads associated with the buoy installation should also be considered.





Wind and Current Loads

Wind loads and current loads on the tanker can be developed based on the information discussed in section DESIGNCONSIDERATIONS. Wind and current force coefficients are given in OCIMF-14. Other load coefficients from recognizedindustry sources or from site-specific model tests may be used in lieu of OCIMF data, when approved by the Company.Wind loads on the buoy are insignificant compared to those on the tanker and may be ignored. Drag loads due to current onother system components and chains may be computed using Reynolds dependent drag coefficients.

Wave Loads and Motions

The steady wave drift forces and second-order motions shall be determined using model tests or radiation / diffraction analysismethods. Empirical scaling or other approximate estimating methods is only be acceptable for the initial sizing of the system,not for preliminary or detail level design.The first order motions of the vessel shall be determined from model tests, 2D or 3D radiation/diffraction methods or strip theorymethods. The first-order motions of the buoy shall be determined from model tests or 3D radiation / diffraction methods. Thestiffness and damping of the mooring system shall be taken into account for the first-order analysis of the buoy and the secondorder analysis of the vessel and/or two-body system.In some cases the above analysis requirements may be waived if the conditions and design criteria are sufficiently similar toprevious installations for which adequate engineering data is available.

Analysis

Peak loads in the hawser and primary structural components are governed by the combined motions of the SPM and thevessel. To adequately evaluate these loads, a coupled, time-domain analysis must be performed for the entire system:mooring, buoy, hawser and vessel. The runs shall be made in the appropriate design wave spectra (random waves) and eachanalysis shall run at least three hours real-time or until statistical relevance can be assured.Other design methodologies may be acceptable. Selection of the most appropriate methods will depend upon the contractor'sdatabase, the site-specific conditions and criteria and similarity in the design to previous installations. The exact methodologyto be used, including the specific analysis programs and model test plans, as well as the general methodology, must beapproved by the Company.Acceptable analysis procedures for the SPM system are described in API 2SK. Design of the mooring and hawser shallrequire:• Quasi-static and dynamic analyses for the connected and disconnected conditions• Coupled-time-domain analyses for the maximum connected conditionsThe quasi-static analyses shall be used to develop the initial sizing of the components. In these analyses, the steady windforce (or time varying force, if an appropriate spectrum is used), steady current force and steady wave drift force shall becombined to determine the steady offset of the SPM / vessel.The first and second order motion amplitudes shall then be calculated about that position, using frequency domain methods(the mooring / hawser stiffness at the steady offset position shall be used).The total offset and thus maximum quasi-static tensions shall be determined by combining the resulting first and second ordermotion amplitudes, using the method described in API 2SK.After the system is sized, using the above quasi-static analysis, the effect of mooring-line dynamics must be evaluated by eitherfrequency or time domain methods.Additional considerations for SALM's are given in API 2SK.

Fatigue

Fatigue design is required for the SPM mooring components. A predicted mooring component fatigue life of three times thedesign service life is recommended. Recommended procedures for fatigue analysis are given in API 2SK.

Model Testing

After all analyses have been completed and the design has been developed to a preliminary level, model tests should beconducted to verify and benchmark the computer analysis results. Model test results may also be used to establish peakmooring and survival loads. Under certain circumstances, the requirement for model testing may be relaxed. Namely, if thedesign conditions (tanker sizes, water depth, environment, system stiffness) are essentially identical to previous designs thecontractor has performed and can be used for confirmation of the analysis process. Guidelines for model testing can be foundin Exxon Report No. EE-17E.T77 Guidelines for Deepwater Port Single Point Mooring Design.






Offshore Lift Loads

A lifting load analysis shall be performed in accordance with API RP 2A. Lifting points on the buoy shall be designed to enablethe buoy to be lifted offshore using a load amplification factor appropriate to the local conditions. The dead weight of the buoyshall include the weight of any compartment flooded. A structural analysis of lifting padeyes and associated buoy structuresshall also be performed.

Loadout / Transportation Loads

All lift, movement or support conditions in connection with loadout and transportation shall be considered in the structuraldesign of the buoy.

MODEL TESTSDuring the final design stages, physical model experiments such as wave basin, towing basin and wind tunnel tests are oftenperformed to compensate for limitations of analysis and to verify analytical predictions. The primary of objectives of model testsare the following:• Determine maximum response for design purpose, for example, motions, hawser and mooring leg tensions, forces at the

mooring interfaces, and so forth.• Quantify important parameters and thereby calibrate computer programs.• Confirm, using the physical model, that no important parameter has been overlooked.Model testing has limitations and there are numerous sources that can cause errors in test results. Therefore, numericalpredictions and model experiment results are complimentary to each other. Procedures for designing and performing modeltests to determine SPM mooring loads are described in Exxon Report No. EE-17E.T77 Guidelines for Deepwater Port SinglePoint Mooring Design. The principles described in this reference can also be selectively applied to MBM model tests.

MOORING LEG DESIGNMOORING LEGSMooring legs shall normally consist of chain; however, if justified by mooring analysis, combination wire and chain may beused for CALM systems. Normally, wire shall not be used in the dip section or at the connection to the buoy, due to abrasionand fatigue. Anchor chain and connecting hardware should be specified and manufactured in accordance with ABS #39 orequivalent classing authority.

Loads

For sizing buoy anchor chain, the design load should be assumed to act horizontally. The chain leg should be designed usingstandard catenary equations (refer to NAVFAC Design Manual 26.5) considering the highest mooring leg load from theanalysis described in section DESIGN LOADS, ANALYSIS AND MODEL TESTS.

Safety Factor

Common industry practice is to size the chain for a design load equal to about 33% of the breaking strength (factor of safetyagainst breaking = 3.0). Although not normally stated, this design procedure includes an allowance for corrosion and abrasionon the chain. A minimum factor of safety of 2.0 is considered adequate for a chain in service.

Corrosion Allowance

Chain in the dip section, and sometimes the riser section (Figure 8), is subject to excessive abrasion and corrosion. The dipsection abrades due to continual pounding on the sea bed. The riser section sometimes receives high inter-link wear in areasof rough seas. It is advisable to increase chain size in the areas of excessive wear to maximize replacement periods. ForMBM's a corrosion allowance of 6 to 12 mm (1/4 to 1/2 in.) has been used. Industry practice for SPM's is described inAPI 2SK.

Length

The length of chain must be calculated to provide flexibility in the leg and to insure adequate anchor holding power. Anchorholding power is decreased as the inclination at the anchor increases. The length of chain should be sized to limit theinclination at the anchor to 0 - 3°. Use of an intermediate sinker block will reduce the length of chain required.




MOORING LEG DESIGN (Cont)

Wire Rope

Wire rope shall be designed and produced in accordance with DnV RULES 2 Pt. 3 Ch. 2 F100. Normally, wire rope will onlybe used in deep water. Since this limits the effectiveness of cathodic protection, fully jacketed rope shall be considered.

ANCHOR SYSTEMS

Holding Power of Steel Anchors

Anchors should be sized for the design load in the mooring leg.Prediction of an anchor's holding capacity is very complex. The holding power of the anchor should be based on an analysis ofdocumented holding power tests which takes into account the reliability of the tests and the variation between the test soil andthe soil at the site. Exact holding power is best determined after an anchor is deployed and load tested.Representative holding powers of some typical anchors are listed on Figure 9. Since holding powers vary greatly with soilproperties, judgement is required in establishing design values. If anchor performance for specific site and soil conditions isunavailable, the curves in API 2SK may be used to provide estimates of anchor holding capacity. The data needed to usethese curves are the general soil conditions (mud, sand), anchor type, and required anchor holding capacity. Anchormanufacturers are a source of recommendations for their anchors.

Holding Power of Chain on Seafloor

The capacity form friction of chain on the seafloor can be estimated in accordance with API 2SK.

Factors of Safety

The lateral loads on the anchor points shall be taken as the highest mooring leg load from the analysis described in sectionDESIGN LOADS, ANALYSIS AND MODEL TESTS. As a general rule, the maximum horizontal pull at MBMs should notexceed 70% of the anchors holding power. As a minimum, the factors of safety provided in API 2SK should be considered forCALM's.Factors of safety for anchor loads are substantially lower than those for the mooring legs. The rationale is to have the anchormoved instead of the mooring line broken in the event of mooring overload. Anchor movements of the most loaded leg wouldnormally cause favorable redistribution of the mooring loads among the legs resulting in lower tensions and anchor loads forthese legs.

Number of Mooring Anchors

In some cases, it may not be feasible to use a single anchor due to installation limitations. Since anchors must be pulled intoposition, the heavier the anchor, the greater the pull which must be exerted to set it properly. Therefore, limited constructionequipment may require the use of two smaller anchors. Three anchors are not recommended.If two anchors are required, they can be arranged either in a tandem pattern or in a spread pattern. The holding power of thesystem will be less than the combined holding power of the individual anchors.

Holding Power of Ship's Anchors (MBM)

The majority of vessels in service are equipped with Navy stockless or Admiralty stockless anchors, or similar. These are lowholding power anchors. Assuming a firm sand bottom and an adequate scope of chain, they have a holding power of about4 - 7. Some of the newer tankers are equipped with higher holding power anchors, such as the AC-14 anchor. However,berths must be designed for the trade which includes the older vessels.

Piling

Piles shall be designed and installed in accordance with API RP2A. A pile driveability analysis shall be performed for the pilesto ensure they can be driven to design penetration.If the chain padeye on the pile is below the mud line, then the chain configuration below the mud line and the resultant mooringloads on the pile shall be conservatively calculated, considering the chain-soil interaction that occurs.





SUBMARINE PIPELINESSubmarine pipelines often represent the major portion of an offshore berth investment. Key considerations involved in theplanning / evaluation of submarine pipeline systems are discussed in this section and include:• Route evaluation• Size of pipelines• Pipeline configurations for multi-product handling• Corrosion protection• Weight coating• Burial requirements• Pipeline end manifold• Pipeline valves

ROUTE EVALUATIONThe pipeline route should minimize both material and installation costs. Preferably the seabed contours along the route shouldslope gently away from the shoreline, eliminating the need for trenching or filling to get a smooth profile. Rocky or very softbottoms should be avoided where possible. Decreased installation costs may justify the selection of a site that requires longerpipelines than alternative sites.

SIZE OF PIPELINEAt receiving terminals the pipelines should be sized to permit maximum discharge rates with the pressure available at thetanker's rail. Due consideration must be given to pressure losses in the loading hoses and to possible future expansion of theberth. Marketing terminals usually require 250 or 300 mm (10 or 12 in.) pipelines. Crude pipelines are typically 900 to1200 mm (36 to 48 in.) diameter.At loading terminals the pipeline should be sized for optimum loading rates which can be accepted by the largest tankers to behandled.The required wall thickness of the pipe depends upon pipe diameter, pipe material and method of installation. Presentation ofcalculation procedures is beyond the scope of this Design Practice.

PIPELINE CONFIGURATIONS FOR MULTI-PRODUCT HANDLINGThe following alternatives are available for handling multiple products at offshore berths:• Common pipelines, displacing one product with another• Pigging through a looped pipeline• Individual pipelines/hoses for each product• Pigging to/from the deck of the tankerOf the above schemes, the first two are the most commonly used when more than three products are handled. The mainconsiderations involved in choosing the most suitable scheme for a particular location are capital and operating costs andallowable contamination limits.

Common Pipeline/Hoses

As an example, Central and South American MBM's frequently use common pipeline/hose systems. Separate pipelines aregenerally provided for gasolines and distillates. A separate line is also used for dark products such as cutback asphalt and fueloil. Products are not separated in the pipelines; one product will be displaced by the succeeding product. Pumping sequencesare carefully worked out to minimize product quality give-away.At some receiving terminals seawater is pumped through the pipeline between light products. The water settles to the bottomof the tanks and is then drained to skimming pits. Seawater must not be left in the pipelines or tanks due to corrosionproblems. If the product in the submarine pipelines must be displaced at the end of vessel discharge, fresh water is pumpedfrom shore to the tanker and left in the pipeline. The use of seawater washing is not recommended.Switchover of products at the shore manifold is normally made by using a sight gauge and taking line samples. Walkie-talkiecommunications between the ship and shore manifolds signal when the vessel is ready to switch products.




SUBMARINE PIPELINES (Cont)

Pigging through Looped Pipelines

Normally, looped pipelines are restricted to tanker loading terminals. Parallel pipelines are manifolded together on shore andjoined by a loop on the offshore end. A hose(s) connects the loop to the tanker manifold. When the loading of one product iscomplete, a pig is inserted at the shore manifold and circulated around the loop by the following product. When the pig reachesthe shore manifold, the entire loop is lined out with the next product. Samples are then taken; and, if they meet specifications,pumping to the tanker is resumed.At receiving terminals, inert gas or water is needed to push the pig around the loop between products. This process requiresmore time and is operationally less attractive than at loading terminals.

Segregated Pipeline System

Submarine pipelines are costly. Therefore, the number of pipelines must be minimized. Rarely are more than three pipelinesused at an offshore berth. If more than three products are handled, common lines or line displacing are used.

Pigging from Tanker Deck

Pigging from the deck of a tanker is technically feasible, although rarely done. Spherical pigs are passed through the smoothbore submarine hose. To change products at a receiving terminal, a pig is inserted at the tanker manifold and moved throughthe hose and submarine pipeline by the succeeding product. The pig is removed at the shore manifold.Generally, pigging from the tanker deck is difficult and involves additional time, manpower and investment. A specialpig-handling device is required, which must be brought out to each tanker or left on the end of the submarine hose.Pigging could also be accomplished to the deck of a tanker at a loading terminal although this is not presently done. The pigwould be inserted on shore and moved through the pipeline and hose by product movement. Flow velocity would be reducedbefore the pig reaches the tanker deck and stopped when the pig reached the trap. Valves would then be adjusted on thetanker. This operation would be slower than at a receiving terminal due to the difficulty involved in getting a warning of pigarrival at the tanker.

CORROSION PROTECTIONSubmarine pipelines must be coated and cathodically protected to minimize corrosion. For purposes of design of coating orcathodic protection systems any concrete weight coating should be ignored due to porosity and crack potential. In addition aminimum of 1.6 mm (0.05 in.) corrosion allowance should be provided.

Coatings

Two types of coating should be considered for submarine pipelines:• Protective• InternalProtective coatings are used to minimize the required output, and hence cost, of the cathodic protection system. Refer tosection COATING SYSTEMS for submarine pipeline system coating requirements.An internal pipeline coating may be required for lines carrying a corrosive product or for fresh water lines. Typically internalcoatings would be a baked epoxy phenolic. Cement lining would be used for salt or corrosive water lines. Additionallycorrosion inhibitors should be considered for this service. To overcome potential problems at pipeline welded joints thru-koteconnectors should be used.

Cathodic Protection

Cathodic protection is normally provided by an impressed current arrangement although a sacrificial anode system isoccasionally used. Where cathodic protection is in use, an insulating flange must be inserted at the shore end of the pipelineabove the highest water level to prevent current drainage to the onshore section of the pipeline system. An electricallydiscontinuous hose must be inserted at the seaward end of the pipeline to isolate the pipeline form the tanker. Refer to sectionHOSES for additional information.In the tidal zone piping should be installed in a trench to permit effective use of the cathodic protection by surrounding it with anelectrolyte.





SUBMARINE PIPELINES (Cont)

WEIGHT COATINGWeight coating, consisting of a wire mesh reinforced concrete jacket (typically 25 - 125 mm or 1 - 5 in. thick), is often requiredto give the pipeline sufficient negative buoyancy, particularly for pipelines over 300 mm (12 in.) in diameter. The concretejacket also protects the pipeline coatings during installation and service. The weight of the pipe must overcome buoyancy, plusthe drag and lift forces, which result from currents or storm conditions. The optimum specific gravity (s.g.) of the coatedpipeline depends upon the direction and magnitude of currents at the ocean floor, the nature of the ocean floor, whether the linewill be buried or not and the severity of storms in the area. Specific gravities of 1.3 to 1.6 during service condition are mostcommon, while a s.g. of at least 1.1 is required for laying strength. Concrete coating should be ignored when evaluating thepipeline strength, however it may be considered in evaluating buckle propagation. A detailed study of weight coatingrequirements should be made for each installation.Rather than using concrete coatings, the pipeline could be weighted with precast concrete collars that straddle the pipeline atset intervals. Another alternative is the use of mechanical anchors that also form a collar around the pipeline and are drilledinto the seabed. These anchors are often used where rock bottoms make trenching costs prohibitive.

PIPELINE BURIAL REQUIREMENTSPipelines may have to be buried for one or more of the following reasons:• To protect them from damage caused by dragging ship's anchors• To protect them from wave and current forces or attendant scouring action• For protection in the surf zone• To comply with local regulationPipeline crossings are marked as a prohibitive anchorage. However, if there is a possibility that a vessel could accidentallydrag its anchor over the area, it is usually advisable to bury the pipelines. For effective protection, the pipelines must be buriedbeneath the depth of expected anchor penetration. Anchor penetration will depend upon type and weight of anchor and type ofsoil. With the development of high holding power ship's anchors such as the AC-14, which are provided on some vessels, acover of about 10 ft (3 m) is probably required to ensure adequate protection.Pipeline burial is also required where strong currents or wave action cause scouring of the seabed. There are a number oflocations where seabed contours vary considerably from year to year. To preclude long unsupported lengths of pipelineresulting from shifts in the seabed or scouring, the pipelines should be trenched and backfilled. The depth of trenching willdepend upon local conditions, with 1.5 - 1.8 m (5 - 6 ft) of cover normally being suitable. Burying may also be preferable inlocations, which experience typhoons or hurricanes.Burying is often used to protect the pipeline in the surf zone. However, trenching may be impossible or may be very costlyalong rugged coasts, or where surf conditions are severe. In these situations a number of schemes have been used forpipeline protection. The most attractive schemes appear to be encasing the pipeline in concrete or installing a sleeveconsisting of a larger sized pipeline over the product pipeline and then sealing the sleeve ends.When trenching and backfilling are used, the backfill should be sufficiently dense and cohesive to prevent it from scouring awayor becoming fluidized. A poor soil may become fluid when agitated and have a buoyant effect on the pipeline.In soft sea-bottom conditions it is recognized practice to bury the pipe by jetting it into the seabed, in which case the jettingequipment is often incorporated in a sledge which is towed along the pipeline. In shallow water however, sections of pipelineare occasionally buried with the aid of an airlift.When harder seabed conditions are encountered, it is necessary to excavate the required trench by dredging or blasting. Oncompletion of the excavation work the pipeline is deposited in the trench which is subsequently back filled. Whenever possible,a diver should inspect the pipeline before it is covered.

PIPELINE END MANIFOLD (PLEM)The PLEM design must be compatible with the pipeline size and the hose configuration. The PLEM shall be connected to thepipeline using a flange connection. The use of a lap joint flange (swivel ring) to ease installation should be considered. Lapjoint flanges can also be used on the PLEM side of the bottom hose connection, so as to ease the connection of hoses andfacilitate bolt hole alignment.




SUBMARINE PIPELINES (Cont)Pipeline damage is very difficult and costly to repair. It is preferable that the hose or its connection part before the pipeline isdamaged. Consideration should be given to the incorporation of features to protect the pipeline from excessive hose tensionthat can result from any of the following:• Mooring failure• Storm waves• Excessive pipeline surge pressure• Vessel collision with SPM buoyIt may be desirable to incorporate a weak-link or breakaway coupling, so as to limit the maximum load that can be transmittedby the hoses to the PLEM. Where breakaway couplings are installed, the foundation system for the PLEM should be designedto resist the failure or breakaway load limit of the hose system to improve protection of the pipeline.A means for flushing the underbuoy hoses should be provided and a blind flange should be provided on the end of the PLEM atSPMs. Sufficient rigging points and padeyes should also be provided on the PLEM to facilitate the installation of the underbuoyhoses.Requirements and operational procedures regarding the PLEM valves vary substantially between operators and geographicalregions. In some areas, the PLEM valves are used only for maintenance and emergencies, while in others the PLEM valvesare closed after every tanker loading.

Multi Buoy Mooring Systems

Anchorage of the end of the pipeline can be accomplished with precast or poured-in-place concrete anchors. Where theseabed has a deep layer of mud, a piled foundation may be required to hold the pipeline above the mud line.The layout and design of the PLEM and associated valves at MBM terminals should address the following considerations:Excessive Length of Hose - Due to the need for periodic removal of cargo transfer hose strings for inspection and hydrostatictesting, PLEMS should be located to limit the length of the hose string (i.e. a horizontal distance from the vessel manifold to thePLEM of not more than 60 m (200 ft).

Minimum Spacing From Vessel Manifold - Hoses should be deployed and stored completely outside the outline of the hull ofthe tanker. Orientation and profile of the hoses needs to be carefully checked to ensure that there is sufficient room to avoidinterference with the vessel hull, the hose string(s) and other submerged equipment for the full range of tankers and manifoldpositions. The curvature of the hose strings should consider the bend radius limitations and towing power of the mooring craft.Terminals have found it advantageous to locate the end of the submarine pipeline 15 - 30 m (50 - 100 ft) aft of the centerline ofthe moored vessel's manifold and 7.5 - 15 m (25 - 50 ft) away from the side of the vessel. A short spool piece is welded to theend of the pipeline to permit the hose to be laid out along the seabed and parallel to the side of the vessel without any hosebending at the pipeline connections. Although this requires long lengths of hose, it minimizes hose kinking and underwatercontact with the bilge keel. It also alleviates the problem of severe hose bending when the hose is connected to the vessel,thereby prolonging hose life.In areas where high undertow currents exists that may cause the hoses to move around on the sea bed, short hose lengthsmay be necessary. The end of the submarine pipeline can be located directly at the vessel centerline, and about 4.5 - 7.5 m(15 - 25 ft) away from the side of the vessel. Although this procedure results in more severe hose bending and entanglementthan if the pipeline was offset, it has worked satisfactorily.

PLEM Valves - Although it is important that the subsea valve arrangement be kept as simple as possible to reduce underwatermaintenance and the possibility of actuator equipment failure, each hose should be fitted with a gate valve to facilitate hoseisolation and change-out.

Marker Buoy - A marker or “spar” buoy is frequently used to mark the position of the PLEM or other reference point foralignment and positioning of the ship’s manifold. Refer to section Component Design.

Single Point Mooring Systems

In addition to the above general requirements for pipeline end manifolds, the PLEM at an SPM berth should meet the followingrequirements:

PLEM Valves - Valves to isolate each of the underbuoy hoses shall be provided. They shall be full-bore ball valves withhydraulic actuators and manual overrides. The manifold and valves shall be capable of being removed from the base.

PLEM Valve Actuation System - A valve actuation system is to be provided for operation of the subsea valves located on thePLEM. New systems are to be powered by bottled nitrogen located on the buoy. Nitrogen capacity is to be sufficient to fullycycle (open and close) the valves a minimum of fifty (50) times.





SUBMARINE PIPELINES (Cont)The system shall consist of bottled nitrogen, hydraulic reservoir, piping, sufficient valving, etc., for regular operation andmaintenance of the system and a subsea umbilical from the buoy to the subsea valve(s). The subsea umbilical is to bestrapped to the subsea loading hose between the PLEM and the buoy.In highly sensitive environmental areas, the PLEM valves are operated via a radio-telemetry connection between the loadingtanker, shore and the loading buoy. In this scenario, a portable radio control is held on the tanker deck, which is capable ofinitiating a sequence to shut down the loading pumps and then close the PLEM valves in case of emergency. Note that thedesign must not allow rapid closure of the valves while the pipeline is flowing, as the sudden increase in pressure caused bythe momentum of the fluid flowing can cause severe damage to the pipeline.The requirement for radio-telemetry, closure of valves between loadings and the required bottled nitrogen capacity shall beconsidered in light of local regulations, local operating practices, frequency of loading and environmental sensitivity of theloading area.

SHORE-SIDE PIPELINE VALVESReceiving terminals should install check valves as well as block valves on the shore end of the pipelines. These valves limitany spills, which may occur due to a failure in the pipeline or hose during tanker discharge. At loading terminals a single blockvalve at the shore end of the pipeline is required.

DESIGN CONSIDERATIONSSpecific consolidated design requirements for submarine pipelines are beyond the scope of this Design Practice; however, thefollowing should be considered in specifying, designing or evaluating submarine pipelines:• Inaccessibility makes submarine pipelines difficult to install, inspect, maintain and repair.• Design depends upon installation method, environment, water depth and soil conditions.• Pigging may be required though pipeline, manifold and possibly hoses.• Soil conditions for burial and support of pipe.• Typical minimum yield strength of pipe should be 358.5 MPa (52 ksi).• Corrosion allowance in addition to coating and cathodic protection.• Maximum stress on pipeline during installation should not exceed 80% of yield strength.

INSTALLATIONThe main objectives when laying a submarine pipeline are to ensure that the line is deposited on the sea bottom without over-stressing, permanent bending or kinking. At the same time it is necessary to avoid damage to the coating or gunite jacket. Themost widely used techniques for laying submarine lines are:

Bottom Pull Method

This is a simple method, and is particularly attractive in areas where sea conditions are rough and liable to hinder thealternative procedures mentioned below. It is probably the most common method used, and is frequently employed whenlaying the smaller and shorter product lines that normally serve marketing installations.The pipeline is assembled on the shore, launched into the water, and pulled along the sea bottom to its final position.Where launching space is limited, the pipeline is usually divided into sections, and thereafter the puffing is carried out in stages.This allows sufficient time for the subsequent sections to be tied in. To simplify launching, the approach to the water's edgeshould be graded, and the pipeline mounted on skids, rollers or preferably rail mounted trolleys. As an added precaution atlocations where the launching area and/or seabed is composed of hard material, wood slats are often strapped to the outerpipe coating to provide a measure of protection from abrasion. Pipeline pulling / towing is normally carried out by a suitablypowered tug, or alternatively by a winch mounted on a barge that is securely anchored offshore.

Floating Method

The assembly and launching procedures are similar to those described for the bottom pull method except that additionalbuoyancy is attached to the pipeline to keep it afloat during pulling operations. If the line is to be assembled in sections, theconnections, i.e. tie-ins, can either be carried out on shore before launching as in the bottom pull method, or after launchingwith the aid of a tie-in barge, which supports the end of one section until the next section arrives and the connection is made.Once the complete line has been launched and its seaward end is in the desired position, the previously attached floats arereleased systematically which allows the pipeline to sink to the seabed.




SUBMARINE PIPELINES (Cont)This method has the disadvantage in that the pipeline is somewhat vulnerable to damage from crosscurrents and/or waves,and is therefore more suitable for calm and sheltered locations or areas in which reliable weather and se condition forecastscan be obtained.

Lay Barge Method

This technique is normally used where the submarine line is of a considerable length and is laid in the open sea.Lengths of pipe, often pre-coated, are assembled on the lay barge and welded together using assembly line techniques. Thelay barge slowly proceeds along the pipeline route, as each pipe joint progresses through the various pipe construction stages,i.e., welding, radiography, coating, etc., before passing over the stem and onto the ramp or stinger, which supports and guidesthe completed pipeline to the sea bottom. As these pipe handling procedures require a stable work deck, laying operations areliable to be affected by inclement weather or adverse sea conditions. In most cases, work on the lay barge is suspended whenwave heights exceed 2.0 m (6 ft).

HOSESHose systems can cause considerable operating and maintenance difficulties at the berth. Care should be exercised in theirselection and maintenance. All hoses shall be designed, manufactured, tested and handled in accordance with OCIMF-20 andthe manufacturer recommendations. Surface and subsurface hose arrangements for SPMs shall be configured in accordancewith OCIMF-4.Submarine hoses connect the submarine pipelines to the tanker manifolds at MBMs, the PLEM to the buoy piping at CALMsand the underwater section form the fluid swivel assembly to the surface at SALMs. Floating hoses are used to connect thetanker to the buoy piping at SPMs.

NUMBER OF HOSE STRINGS

MBM Berths

The minimum number of hoses is normally governed by the number and type of products handled. Flow rates at mostmarketing terminals do not warrant more than one hose per product.The maximum number of hoses at an MBM berth should be limited to three, with two being preferable. A larger number ofhoses may cause serious operational problems due to tangling.

SPM Berths

Generally, two strings are used at SPMs, otherwise one. Use of a single larger diameter string at an SPM shall be consideredto reduce the number of spares. However, this shall be weighed against the danger of a facility shutting down in the event of afailure in the one string. This particularly applies to the underbuoy hose string, which takes time to replace and sometimesrequires the mobilization of heavy floating equipment from a distant source.

DIAMETER OF HOSE

Flow Rates

Minimum hose diameter should be established based on required flow rates and allowable pressure losses. Flow rates inhose, providing pressure drops are no problem, are usually limited by the flow velocity recommended by the manufacturer.This velocity is established to prevent damage to the internal hose lining. Good quality hose is designed for a flow velocity of50 ft/s (15 m/s). Higher flow velocities may be used; but hose life will decrease and hose warranty will normally be voided. Onthe other hand lower flow velocities will tend to increase hose life.Maximum flow rates for various size hoses at a flow velocity of 15 m/s (50 fps) are listed in Table 3.

Lifting Considerations

The submarine hose at MBMs normally rests on the sea bed when not in use. A lifting chain is attached to the free end of thehose and a marker buoy is attached to the free end of the lifting chain. The tanker raises the hose by hoisting on the liftingchain with the midship derrick. Floating hoses at SPMs are raised in a similar fashion. Lifting procedures and equipment aredescribed in OCIMF-19.In some instances, the maximum hose diameter may be limited by the hoisting capacity of the tankers midships' derrick. Mosttankers between 15,000 dwt and a 50,000 dwt have a 5 tonne derrick. Smaller vessels may have a 3 tonne derrick, and largertankers a 10 tonne derrick.





HOSES (Cont)Figure 18 demonstrates how to calculate hose lift weight once the hose diameter and length are established. The hose weightis the sum of the total deadweight of the hose lifted out of the water. For MBMs, the submerged weight of hose lifted off thebottom but remaining under water must also be considered. The hose will normally be full of product. A dynamic load factor of1.5 times the total hose lift deadweight is normally required if sea conditions during lifting regularly exceed a height of 1.0 m(3 ft).It may be necessary to limit hose diameter or the minimum size vessels to be handled to insure that the lift weight will notexceed the ship's capacity. Where hose lift weight is a concern and pressure drop problems call for the use of larger hose, thelast few sections of hose may be a smaller size. At MBM berths the lower portion of the hose which is never raised above thesea surface may be a larger size and the portion that is raised may be larger. The largest rail hose size is typically not greaterthan 406 mm (16 in.) diameter.Another alternative to minimize the hose weight is to provide for draining or displacing the hoses.

Presentation Flange

Over-the-rail hoses should match the presentation flange for the majority of ships anticipated, using pipe reducers (enlargers)for other ships as needed.

TABLE 3TYPICAL MAXIMUM FLOW RATES AND HOSE WEIGHT

WEIGHTS lb/ft (5) FLOW RATE @ 50 fpsHOSE

DIAMETER(in.) EMPTY HOSE (1)

PRODUCTS.G. = 0.7 (2)

PRODUCTS.G. = 1.0 (2)

BUOYANCY INSEA WATER BBL/hr Tonne/hr (3)

6 21 9 13 22 6300 7908 31 15 22 35 11,250 1290

10 48 24 34 50 17,500 219012 59 34 49 69 25,250 3150

16 (4) 84 61 81 107 40,900 5100

Notes:(1) Hose weight includes weight of fittings and flanges every 10m (33 ft). Weight are representative but values do vary among

manufacturers.(2) Weight of product in hose.(3) Tonnes/Hr. based on S.G. = 0.80(4) Actual bore = 387mm 15.25 in.(5) Lifting weights should be multiplied by a factor of 1.5 if sea conditions are expected to exceed 1.0 m (3 ft) height.

ACCEPTABLE UNIT EQUIVALENTS1.5 kg/m = 1 lb/ft0.4 m/s = 1 fps25.4 mm = 1 in.1 m = 3.28 ft




HOSES (Cont)

FIGURE 18SUBMARINE LOADING HOSE

DP31Ef18

Tanker

15' -

20'

Max

. Fre

eboa

rd

Wei

ght i

n Ai

r of H

ose

Subm

erge

d W

eigh

t of H

ose

Wat

er D

epth

HHW

Example:

Data: Vessel freeboard: 35 ftWater depth @ HHW: 40 ftHose diameter: 10 in.Product S.G.: 0.7Hose bent over rail

*Total Lift = 40 ft (48 lb/ft + 24 lb/ft - 50 lb/ft) +(35 ft + 20 ft) (48 lb/ft + 24 lb/ft)

= 4840 lb

*Refer to Table 3 for Hose Weights

LENGTH OF HOSE STRING

MBM Berths

The following factors must be considered in determining the length of submarine hoses:• Water depth• Maximum tidal variation• Freeboard of largest vessel in its lightest operating condition• Offset of vessel centerline from end of pipelines• Vessel movements in the berthThe maximum freeboard, which the vessel will have in the berth, should be obtained from the marine department.Requirements concerning the location of the PLEM with respect to the vessel manifold are given in section SUBMARINEPIPELINES. Another consideration is the accuracy with which a vessel can be located in the berth. This depends upon shipsize, whether both bow anchors are utilized, and environmental conditions; and should be established with the marinedepartment. Lacking data, a reasonable basis would be that the vessels manifold could be initially located anywhere within a7.5 m (25 ft) diameter circle around the end of the pipeline (or the spar buoy if pipeline is offset).Besides providing for error in the initial location of the vessel, allowance must also be made for vessel movements in the berth.Vessel movements must be estimated based on vessel size, and wind, wave and current conditions. Normally, a surge of atleast 4.5 m (15 ft) in either direction should be allowed for together with a drift (transverse movement) of 4.5 m (15 ft). Verticalmovement at the side of the vessel will normally not exceed 1 - 1.5 m (3 - 5 ft).Once the above parameters have been established, a cross-sectional layout should be made showing the hose in the mostextended position and in the most tucked-in position. Sufficient hose length should be provided to prevent over extension ofthe hose. Also, the minimum allowance bend radius of the hose should not be exceeded. Overly long hoses should beavoided since they can move around and abrade on the sea bed or tanker side during discharge and magnify tanglingproblems when stored.





HOSES (Cont)

SPM Berths

Surface hose length shall be compatible with hawser length and maximum tanker size. The hose shall be long enough to allowfor reasonable slack when hooked-up to a tanker. The buoy manufacturer should provide recommendations regarding thelength of hose.

DISPOSITION OF STORED HOSES AT MBMsWhen not in use, submarine hoses should be laid out on the sea bed to minimize movement of the hose and to precludetangling. The practice at many small MBM's is to lower the hoses with the ship's boom without attempting to string them out.This often results in hose piling up and/or tangling on the sea bottom. Cases have been reported where the end of thesubmarine pipeline or the hose lifting chains were broken when the tanker attempted to raise hoses which were tangled on thesea bed.Preferably stored hoses should be strung out parallel to the side of the vessel. For small diameter hoses this can be done by alaunch attaching a line to the hose as it is lowered individually. This operation is more difficult with 12 in. and 16 in. hoses.When the sea bottom consists of thick mud or has rock outcroppings hoses should, if possible, be kept off the sea bed whennot in use. Hoses sink in mud and the resulting suction effect makes them difficult or impossible to retrieve. Rock will severelyabrade the hose. A number of schemes have been tried to keep hoses off the sea bottom. One scheme uses floats on thehose to keep it off the bottom (refer to Figure 19). Split floats are securely fastened to the hose at set intervals to give the hosestring the proper shape. Stored hoses are held back from the berthing face by a small buoy which is in turn anchored to thesea bed. The floats must withstand the external hydrostatic pressure.

FIGURE 19SEMI-FLOATING HOSE SCHEME

DP31Ef19

Swivel Joint

Stake Pileor Deadweight

Deadweight

Spar Buoy

10"BunkeringLine

Tanker Connection

LiftingChainBuoy

Float (Typical)

Tanker Connection



HoseMooringBuoy

LiftingChainBuoy

12" Hose




HOSES (Cont)

TYPES OF HOSES

Surface Hoses

All surface hose, except the first hose off the buoy, shall be self-floating. The first hose off the buoy shall use a combination ofstiffness and floating to minimize wear at the flange connection, in accordance with OCIMF-4. The rail hose must have enoughfloatation to support the hose end equipment, such as valves and flanges.

Submarine Hoses

Submarine loading hoses are normally specified as heavy duty hose having a working pressure of 1.6 MPa (225 psi) and aminimum burst pressure of 7.8 MPa (1125 psi). Smooth bore hose is normally used. Rough bore hose is used only whenproduct characteristics, such as very high temperatures, preclude the use of smooth bore hose.Kinking of the submarine hose at the point where it touches the sea floor is a problem at multiple buoy berths. Wire helixreinforced hose does not flatten during normal handling, but will permanently deform when kinked or crushed. Soft wall orwireless hose is now available from many manufacturers. The soft wall hose can withstand kinking and crushing and restore tonormal shape.Most hose manufacturers offer several alternative hose constructions and designs, suitable for different situations. Whenrequesting hose bids, the general system design may be specified, requesting each manufacturer to propose the hose or hosesbest suited for the design.OCIMF-20 should be specified for hose larger than about 150 mm (6 in.) diameter. All hoses should be tested by themanufacturer before shipment. Inspection of the finished hose product by an experienced hose inspector should be carriedout, especially for large orders.

Underbuoy Hoses

Hose lengths at either end of the underbuoy hoses shall be reinforced. The underbuoy hoses shall be fitted with detachablefloats of sufficient number to provide optimum configuration for long hose life. Proper hose configuration, in relation to buoyand pipeline end manifold for the CALM, shall ensure that hose stresses are minimized and that lengths are sufficient toprevent damage under maximum sea conditions and buoy / tanker excursions. The hose configuration shall not permitadjacent hose strings to contact each other, the anchor lines or the seabed. The hose connected to the PLEM shall beelectrically discontinuous.

Single versus Double Carcass

Double carcass hoses can reduce the risk of a spill. This is achieved at extra cost, which must be considered. Double carcasshose is not regarded as a means to increase hose life but as an insurance against a spill. It does not reduce the operator'sresponsibility to remain vigilant in inspecting and testing hoses for defects. If double carcass hose is selected, it is essentialthat a reliable telltale system be supplied which alerts the operators to fluid entering or existing in the interstitial space betweenthe two carcasses, indicating failure of the inner (primary) carcass.

GENERAL DESIGN CONSIDERATIONSRequirements for the hose system must be addressed during design to simplify operation and maintenance. Improper designof the hose system will result in high maintenance costs and possibly in hose leakage with resultant product loss and pollution.The use of soft wall hose is preferred for all positions in an underwater hose string except the rail hose. Unless properlysupported, the soft wall hose will flatten on the tanker hose rail and restrict flow. Thus wire helix reinforced hose should beused at this rail position. If wire helix reinforced hose is used throughout the system and kinking at the pipeline manifoldposition is a concern, then specially reinforced hose may be used at this position.Floats may be used on the mid portion of the hose string to minimize hose lifting weight and to reduce the occurrence of hosebending and kinking at the point where the hose touches the sea floor. Hard-shell split collar floats which can be removed andreplaced by divers underwater should be used. Integral foam flotation should not be used on submarine hose because itcompresses with increased water depth/pressure and looses buoyancy with time.Electrically conductive hose can be used to provide cathodic protection for flanges and nipples. All hose should be electricallycontinuous, except the rail hose and tail hose, which should be electrically discontinuous. The electrical discontinuity at theend of the hose prevents the conduction of stray currents, while the continuity in the body of the hose accommodates cathodicprotection of the flanges.





HOSES (Cont)The rail hose at SPMs should be equipped with a butterfly valve and marker buoy as described in OCIMF-3. Floating hosestrings shall be provided with battery powered winker lights to furnish some nighttime illumination.Additional guidance on handling, storage, inspection and testing of hose in the field is contained in OCIMF-2.

HOSE COUPLERS

Vessel Manifold

It is important at an offshore berths that the hoses be released quickly in an emergency. At MBMs hoses can be bolted orclamped to the tanker manifold. AT SPMs the rail hose should be equipped with a camlock connector.Couplers manufactured by Marine Moisture Control, FMC Europe, MIB or Gall Thompson may be used to facilitate emergencydisconnection. Additionally some of these devices may be capable of facilitating normal connection / disconnection of thehoses. Couplers are normally left on the end of the hose and therefore must be very robust, mechanically simple and corrosionresistant. Some terminals have found it necessary to take the couplers off the ends of the hose due to corrosion and fouling orexcessive lift weight. These terminals either take the couplers out to the vessel each time it arrives or bolt the hoses to thetanker manifold.Since some terminals also support hoses with snubbing chains during cargo transfer, devices must be provided to enable quickdisconnection of the chain as well in an emergency. Such devices are commercially available from several chainmanufacturers.

Breakaway Couplers

Depending on the severity of local environmental loading conditions, consideration shall be given to incorporating breakawayconnectors and emergency shutdown systems (refer to section SUBMARINE PIPELINES). At SPM berths, breakawaycouplings are recommended in both the floating hose and under buoy hose systems.

COATING SYSTEMSSpecific coating system requirements for the components described in this design practice are given below.

HARDWAREAll steel hardware, including chains, connecting nuts, bolts, and washers should be galvanized per ASTM A153-80 ZincCoating (Hot Dip) on Iron and Steel Hardware.

SUBMARINE PIPING

Submarine piping should be coated in accordance with the requirements specified in GP 19-1-1 for underground andunderwater piping. Alternatively, fusion bonded epoxy (FBE) may be applied per NACE RP0394. For additional information oninternal coatings, concrete jackets and cathodic protection refer to section SUBMARINE PIPELINES.




HOSES (Cont)

MBM AND SPM BUOY SYSTEMSRecommended coatings for CALM, SALM and conventional buoys and ancillary equipment are given in Table 4 below:

TABLE 4CALM, SALM, AND CONVENTIONAL MOORING BUOY COATING REQUIREMENTS

Superstructure (Area above splash zone)

Surface Preparation Near-white metal blast cleaning SSPC SP10Prime Coat Inorganic zinc silicate (2 - 3 mils)Intermediate Coat 2 coats high-build epoxy (5 - 6 mils) eachFinal Coat Acrylic polyurethane enamel (2 - 4 mils)Remarks Includes piping and miscellaneous steel on deck of SPM

Interior Surfaces of CALM and SALMSurface Preparation Near-white metal blast cleaning SSPC SP10Prime Coat Inorganic zinc silicate (2-3 mils)Additional Coats None requiredRemarks Includes only those surfaces that are normally sealed but are occasionally

opened to service equipment contained therein

Submerged SteelSurface Preparation White-metal blast cleaning to SSPC SP5(2)

Prime Coat High-build epoxy (8 - 10 mils)Intermediate Coat High-build epoxy (8 - 10 mils)Final Coat 2 coats vinyl anti-fouling (1.5 mils each) (3)

Remarks Includes all sea bed level steel. Piles should be coated from 5 ft below seabedto top of pile.

Splash Zone

Surface Preparation White-metal blast cleaning to SSPC SP5(2)

Prime Coat High-build epoxy (8 - 10 mils)Intermediate Coat High-build epoxy (8 - 10mils)Final Coat High-build epoxy (8 - 10mils)

Notes:(1) Coating requirements are specified in nominal dry film thicknesses.(2) Near-white metal blast cleaning SSPC SP10 is acceptable for new steel without rust. SSPC SP10 allows no mill scale to

remain on the prepared surface. Therefore, for new steel without rust, specifying SSPC SP10 should provide for a preparedsurface nearly equivalent to SSPC SP5 at a lower cost.

(3) Two coats of epoxy may be suitable where marine fouling is not expected.

ADDITIONAL REQUIREMENTS FOR SPMSFollowing are requirements for marking SPMs:• Draft marks should be provided on the buoy sides, indicating the total draft in meters. These draft marks should consist of

welded-on Arabic numerals. Draft marks should have a height of 100 mm (4 in.) and should be painted white. Markingsshould be done every 200 mm (8 in.). The letter "M" should follow each set of numbers.

• Numbers 1 to 6 (or the number of chain legs) should be provided to identify each hawser or chain stopper location. Theapplication of the numbers should be performed as prescribed for the draft marks.

• Compartment numbers, welded-on, should be provided next to the manhole.• A steel identification plate that complies with local regulations shall be provided to identify such items as owner and

location.





SPM FABRICATION AND INSTALLATION CONSIDERATIONSFABRICATIONThe typical time frame for construction of a 12 m (40 ft) diameter CALM buoy is around 7 months. SALM systems may take upto around 14 months due to the universal joint, chain swivel, non-standard fluid product swivels and non-standard bearings.Factors that determine the construction time for an SPM terminal include:• Shipyard experience level• Availability of non-standard materials• Availability of main bearings• Availability of swivels, and• Amount of engineering time required

TRANSPORTATION AND INSTALLATIONThe buoy and associated equipment are typically transported dry from the fabrication site to the installation site by cargo bargeor as deck cargo. Wet towing may be permitted where the fabrication yard is near the installation and environmental conditionsare benign.A detailed transportation and installation procedure must be developed, with supporting calculations, to verify that sufficientengineering has been done to ensure a successful SPM installation. This should include complete data concerning themethods, equipment to be used and scheduling of the transportation and installation of the SPM. Proposed methods shouldprovide for the safety of all personnel and the prevention of damage or loss of the SPM.Wind and wave height limits shall be specified for each phase of the transportation and installation procedure. Proceduresshould be delineated for actions to be taken in the event of rough weather or seas, at any point in the operation.Installation is a critical phase of the work. During installation the SPM is at a greater risk of damage. Moreover, mistakes madein placing or connecting up the system may not show up for some period of time. The contractor will usually be on a fixed-costcontract, while hiring marine vessels on a day-rate basis, and will be anxious to complete the work as soon as possible. It istherefore important to have full-time Company representation on site during installation, so as to ensure that the installation iscorrectly accomplished and that any damage is duly noted.The placement of the SPM system at the site and the final positioning and orientation of both the buoy and anchor pointsrelative to the PLEM, must be within 1.5 m (5 ft), in accordance with OCIMF-4 or as established in the design.For CALM buoys, accurate placing the chain leg anchors, and laying out the chain legs without any slack, is critical. Slack leftin the chain legs will gradually work out over time, causing the buoy to move out of its original position, and thus puttingadditional strain on the underbuoy hoses. After the buoy is connected to the mooring legs, the line tensions shall be adjustedso that the buoy is in its correct position relative to the PLEM. Pretension angles are measured by a chain angle protractor andshall be in accordance with the design pretension values, within the tolerances given in OCIMF-9. A careful record of thisprocess and the final position and chain tension readings is essential for future reference by the operator.

QUALITY CONTROLSPMs are offered by specialist contractors, and are usually procured on a lump-sum basis. Sufficient provision should bemade in the contract to permit the client a voice in its execution. Namely, the ability to approve design loads and overallstructural design and arrangements, to approve transportation and installation procedures, to ensure that specified design andconstruction codes and standards have been met, and to verify that all out-sourced items meet specification.

SPM CERTIFICATION AND CLASSINGCertification or classification of the SPM may be stipulated by local regulations or may be required by the Company, dependingupon the specific insurance or quality assurance requirements of the project.

CERTIFICATION"Certification" of a design is the process of verifying that it conforms to stated codes, rules or guidelines. Certification isperformed by a third-party, usually for the purpose of satisfying insurance underwriters or governmental agencies that a designhas been properly executed, constructed and installed. Certification can be performed to any recognized design code or rules(e.g., ABS, DnV). When certification is required, a list of qualified Certification Authorities is provided by the agency orunderwriter. Certification Authorities are private engineering firms (e.g., DnV, Lloyds, ABS).




SPM CERTIFICATION AND CLASSING (Cont)The certification process amounts to a third party check of the work, which can include construction and installation, as well asdesign. The certification process can be made part of an overall project quality control plan. However, two important facts shallbe kept in mind. First, a certification authority will seldom accept responsibility for the consequences of his actions in terms offailure of the design, or the project cost and schedule delays; Second, the objectives, scope, and time frame of the certifyingauthority's reviews seldom match those of the client.The certification process is usually not viewed as a substitute for in-house engineering review of the design. However, wherepast experience justifies, the review of the certifying authority may be accepted on some aspects of the design.The requirement for certification, if desired, shall be stipulated in the work scope of the procurement contract.

Required Information

For single point moorings, the following information is typically required by the certifying authority:• Site Conditions, including environmental, bathymetric, and soils data.• Design Data, including design load derivation, structural, mechanical, and piping design calculation, model test results, and

stability calculations.• Design drawings, including scantlings, arrangements and details of principle parts of the structure, the joints, and welding.

Fabrication Surveys

After design review and approval, fabrication surveys are performed by the certifying authority. Established certifying agencies,such as ABS and DnV, have a network of field surveyors throughout the world. The certifying agency surveys the materialsand workmanship during construction. At the completion of the work, it issues a survey report and (all being well) a certificationthat the structure was constructed per the approved design drawings.

CLASSINGAs single-point moorings are floating structures, they may be classed by the American Bureau of Shipping (ABS) andLloyds. The classing process and requirements are explained in ABS Rules for Building and Classing Single Point Moorings.Classing involves much the same submission, reviews, and approvals as the certification process, but it is morecomprehensive. Certification can apply to specific portions of the system (e.g., buoy only) and project phases (e.g., design andconstruction only), while classing applies to the design, construction, and installation of the entire single point mooring. UnderABS, the entire installed system then becomes classed: "A1-Single Point Mooring," and will be carried in the ABS published"Record." Maintenance of the Class designation requires periodic inspection by the operator and ABS. Once classed, periodicinspections are required to keep the system "in class.".

DP31E

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