HSE Report on Mooring Integrity

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HSEHealth & Safety

Executive

Floating production system

JIP FPS mooring integrity

Prepared by Noble Denton Europe Limited

for the Health and Safety Executive 2006

RESEARCH REPORT 444


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Crown copyright 2006

First published 2006

All rights reserved. No part of this publication may bereproduced, stored in a retrieval system, or transmitted inany form or by any means (electronic, mechanical,photocopying, recording or otherwise) without the priorwritten permission of the copyright owner.

Applications for reproduction should be made in writing to:Licensing Division, Her Majesty's Stationery Office,St Clements House, 2-16 Colegate, Norwich NR3 1BQor by e-mail to [email protected]


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HSEHealth & Safety

Executive

Floating production system

JIP FPS mooring integrity

Noble Denton Europe Limited

No 1 The Exchange

62 Market Street

Aberdeen

AB11 5PJ

The main objective of this report is to improve the integrity of the mooring systems on Floating ProductionSystems (FPSs). It is intended to be read and understood by non mooring specialists such as FPSOperational staff - so that the people who live and work on FPSs will be better able to become more

involved in the vital task of looking after their own mooring systems. Meanwhile the included feedback onthe actual performance of mooring systems in the field should assist designers and manufacturers toimprove future mooring designs. Hence, the report attempts to identify gaps in the existing knowledge of

mooring behaviour and components to provide a road map for future work. Appendix C includes a paperpresented at the 2005 Offshore Technology Conference (OTC) which represents a stand alone summaryof the key points of the JIP.

This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents,including any opinions and/or conclusions expressed, are those of the authors alone and do notnecessarily reflect HSE policy.

HSE BOOKS


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12.2 Implications of Mechanical Repairs 168

13 GENERAL TRENDS AND STATISTICS 169

13.1 Questionnaire Process 16913.2 Summary Statistics for Unit Type and Geographical Area 171

13.3 HSE UK Sector and Norwegian Statistics 174

14 CONNECTORS AND TERMINATIONS 177

14.1 Background 177

14.2 What Type of Connectors Can be Considered for Long Term Mooring (LTM) 17714.3 Terminations General 182

14.4 Connector/Termination Design Flow Chart 186

14.5 Detailed Design Guidance 18914.6 Proof Load Testing and Its Impact on Fatigue 193

15 OUT OF PLANE BENDING CHAIN AND ROPES (FIBRE + WIRE) 197

15.1 Tension Bending at a Wildcat and its Effect on Fatigue 197

15.2 Tension Bending at Chainhawse 20515.3 Tension Bending In Wire Rope 215

15.4 General Implications of Tension Bending Fatigue for the FPS Industry 219

15.5 Recommendations 222

16 FRACTURE MECHANICS AND CRITICAL CRACK SIZE 223

16.1 Required Data 223

16.2 Fracture Mechanics and Chains State of the Art Summary 224

16.3 Fracture Mechanics Critical Crack Size Implications 225

17 LINE STATUS MONITORING AND FAILURE DETECTION 226

17.1 Instrumentation Status - Survey Results 226

17.2 Existing Failure Detection Systems 227

17.3 Future Failure Detection Systems 232

18 INSPECTION, REPAIR & MAINTENANCE (IRM) 238

18.1 In Air-Inspection 238

18.2 Where to Inspect on a Mooring Line 239

18.3 In-Water Inspection 242

18.4 Marine Growth Removal 24618.5 Manufacturing Tolerances and the Inspection Implications 247

18.6 Wildcat/Gypsywheel Inspection 247

18.7 Inspection Frequency Code Requirements 255

18.8 Outline Method To Break Test Worn Mooring Components 257

19 SPARING OPTIONS 261

19.1 Contingency Planning - Spares and Procedures 261

20 THE IMPORTANCE OF A COMPREHENSIVE MOORING DESIGN

SPECIFICATION 264

20.1 Installation Parameters 265

21 KEY CONCLUSIONS & FUTURE WORK RECOMMENDATIONS 267

21.1 Overview 26721.2 Key Conclusions 268

21.3 Recommendations for Further Study 270

22 REFERENCES AND BIBLIOGRAPHY 272

23 APPENDIX A - SUMMARY OF PAST RELEVANT JIPS 278

24 APPENDIX B MOORING INTEGRITY QUESTIONNAIRE (EXCEL) 279

25 APPENDIX C 2005 OTC JIP PAPER 280

26 APPENDIX D HSE SAFETY NOTICE 3.2005 FLOATING PRODUCTIONAND OFFLOADING (FPSO) MOORING INSPECTION 281


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LIST OF TABLES

Table 1-1 - North Sea Mooring Line Failure Data, 1980 to 2001 [Ref. 1] ...................................12Table 1-2 - Indicative Single Mooring Line Failure Costs ...........................................................13Table 3-1 Summary of Chain Design Parameters (modified from Vicinay Chain Catalogue)..41Table 3-2 Comparison of Manufacturing Parameters ................................................................41Table 3-3 Chain Geometry Implications for Inspection and Maintenance................................42Table 3-4 - Summary of Ship or Marine Grade Chains [Ref. 13].................................................45Table 3-5 Example of Indicative Surface Hardness Values for Various Chain Grades (courtesy

of Vicinay).............................................................................................................................46Table 3-6 Illustration of Indicative Wire Rope Material Properties [Ref. 2].............................49Table 3-7 - Comparison of the Advantages of Spiral and Six Strand Wire (courtesy of Bridon) 50Table 3-8 - Comparison of the Cons of Spiral and Six Strand Rope ............................................50

Table 3-9 - Wire Rope Recommendations for Varying Field Lives (courtesy of Bridon) ...........50Table 3-10 - Stipulated MBL and Proof Load Values for Various Sizes and Grades of Chain(courtesy of Vicinay).............................................................................................................62

Table 5-1 - Line Failure Cost Estimate, 50,00bpd North Sea FPSO ............................................83Table 5-2 - Line failure Cost Estimate, 250,000bpd West African FPSO....................................84Table 7-1 - Example of Specified Corrosion and Wear Allowances from One Classification

Society.................................................................................................................................102Table 12-1 - Summary of the Pros and Cons of Sliding and Roller Bearings [Ref. 48].............167Table 13-1 - Example of the First Page of the Questionnaire see appendix B for a Full Listing

.............................................................................................................................................169Table 13-2 - UK Sector of the North Sea Data [Ref. 49]..........................................................174Table 13-3 - UK Sector of the North Sea Data [Ref. 49]...........................................................174

Table 13-4 Number of Anchor Incidents in the Period of 1990-2003 in the Norwegian Sector[Ref. 50] ..............................................................................................................................174Table 15-1 Comparison between Chain Tension-Bending Fatigue Parameters Note that values

in italics are derived from BOMEL measured stress factor. ...............................................203Table 15-2 : Wire Rope Fatigue Reduction Due to Tension Bending [Ref. 31].........................216Table 15-3 - S-N Parameters for Mooring Chain Fatigue..........................................................220

LIST OF FIGURES

Figure 1-1 - Red Arrows Indicate Key Areas subject to Degradation on a ..................................14Figure 2-1 - JIP Organisation ........................................................................................................19

Figure 2-2 - CTR Breakdown........................................................................................................19Figure 2-3 Participants at the Steering Committee meeting in Paris .........................................21Figure 3-1 Typical Turret Moored FPSO...................................................................................22Figure 3-2 Shallow and Steep Mooring Line Angle Illustration................................................23Figure 3-3 Line Heading Illustration..........................................................................................23Figure 3-4 Definition of Windward and Leeward Lines + Environmental Offset.....................24Figure 3-5 Offset Position and Tension Effect...........................................................................25Figure 3-6 Illustration of Load Excursion Curve [Ref. 2]..........................................................25Figure 3-7 - Typical Spread Moored Unit, Girassol FPSO offshore West Africa (courtesy of Stolt

Offshore) ...............................................................................................................................26Figure 3-8 Illustration of Catenary System................................................................................28Figure 3-9 - Typical Spread Moored Catenary System (Courtesy of Vryhof) .............................28

Figure 3-10 Illustration of Taut-Leg system ..............................................................................29Figure 3-11 - Typical Spread Moored Taut-Leg System (Vryhof)...............................................29


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Figure 3-12 Illustration of Surge, Sway, Heave, Roll, Pitch and Yaw ......................................30Figure 3-13 Example of Optimising the Stiffness of the Load offset Curve ............................32

Figure 3-14 - Illustration of Long Crested (Unidirectional) Seas .................................................35Figure 3-15 - Illustration of Short Crested (Confused) Seas.......................................................35Figure 3-16 - Example of a Wave Breaking on a Column of a Semi-Submersible ......................36Figure 3-17 - Illustration of Deepwater Breaking Wave Types (Plunging Break on the Left and

Spilling Breaking on the Right) ............................................................................................37Figure 3-18 - Illustration of the Damage Caused to Schiehallions Bow by an Unusually Steep

Wave (courtesy of BP) ..........................................................................................................37Figure 3-19 - Model Illustration of the Effect of a Breaking Wave on a FPSO (Courtesy of APL

website) .................................................................................................................................38Figure 3-20 - Isambard Kingdom Brunel in front of Studded Chain for the Great Eastern steam

ship, circa 1858 .....................................................................................................................39Figure 3-21 Comparison of the Geometry of Modern Studded and Studless Chain..................40

Figure 3-22 - SPATE Contour Map of a 76mm Loose Stud Chain Link [Ref. 8].....................43Figure 3-23 - Example of the Arrangement of an Asymmetric Stud ............................................44Figure 3-24 Indication of the Manufacturing Tolerances of Studless Links (courtesy of

Vicinay).................................................................................................................................47Figure 3-25 Studlink Manufacturing Tolerances (courtesy of Vicinay)....................................48Figure 3-26 - Illustration of the Make Up of Different Wire Rope Types (courtesy of Bridon) ..49Figure 3-27 Chronology of Deep Star Funded Synthetic Mooring Studies ............................51Figure 3-28 - Accurate Drag Anchor Placement by Crane in Good Weather Conditions (courtesy

of Stolt Offshore) ..................................................................................................................52Figure 3-29 Installation and Normal (Vertical) Load Position (courtesy of Vryhof ) .................53Figure 3-30 Anchor Pile + Chain Tail Deployed by a Twin Crane Construction Vessel (courtesy

of Stolt Offshore) ..................................................................................................................53

Figure 3-31 Suction Anchor Deployment (courtesy of Stolt Offshore).......................................54Figure 3-32 - Example of a Tensile Test on a Steel Sample cut out from a Chain Link .............58Figure 3-33 - Example of a Chain Sectioned for Material Testing..............................................59Figure 3-34 - Example of Terminology during a Tensile Test (courtesy of Ashby & Jones, [Ref.

17]) ........................................................................................................................................59Figure 3-35 - Stress Strain Curves for R3, R4 and R5 Chain Steel (Data courtesy of Vicinay)..61Figure 3-36 Approximation of the Stress Distribution in a Typical Chain Link .......................63Figure 3-37 - Illustration of a Finite Element Representation of a Chain Link ............................64Figure 3-38 Finite Element Representation of a Shackle Body .................................................64Figure 4-1 Illustration of some of the Main Factors which Influence Mooring Integrity..........65Figure 4-2 - North Sea Pioneer on the Argyll Field .................................................................66Figure 4-3 Fulmar SALM after Breakaway (courtesy of BBC film clip)..................................68

Figure 4-4 Schematic of the layout of the Fulmar SALM .........................................................69Figure 4-5 - Extract from On this Day BBC Website ...............................................................70Figure 4-6 Illustration of a Typical Lightship Weathervaning Mooring ...................................71Figure 4-7 - Helicopter Rescue from the Free Drifting North Carr Lightship after Mooring

Failure [Ref. 20] ....................................................................................................................72Figure 4-8 - Illustration of the North Carr Link Failure Relative to a 1999 North Sea FPSO Link

Failure (fatigue cracking followed by ductile rip out) ..........................................................73Figure 4-9 - Dutch Lightship Number 11 whose Mooring Failed in a Force 10 Gale in October

1991 which also broke a number of semi-sub moorings see Section 4.2...........................73Figure 4-10 - Fifth Generation Deepwater Nautilus Broke free of all her Moorings during

Hurricane Ivan.......................................................................................................................75Figure 4-11 - Petrojarl 1 which experienced two broken lines at the same time when hit by a

steep wave .............................................................................................................................76Figure 5-1 - Summary of a Single Line Failure Scenario .............................................................78


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Figure 5-2 Illustration of Riser Stretch After Loss of Position Following Mooring LineFailure....................................................................................................................................79

Figure 5-3 - Potential Multiple Line Failure Scenario ..................................................................80Figure 5-4 - Example of how Mooring Integrity Philosophy can affect Production ....................81Figure 6-1 - Spooling Fibre rope onto a Powered Reel from Standard Containers ......................85Figure 6-2 - Illustration of the Weight and Handling Issues Associated with Mooring

Components (Courtesy of Stolt Offshore) ............................................................................86Figure 6-3 - Red Arrows Show Examples of Mooring Dog-Legs.............................................87Figure 6-4 - Illustration of Twist on a FPSO Mooring Line during Recovery ............................90Figure 6-5 Illustration of a Hockle in Spiral Strand Wire during Recovery of a FPSO

Mooring System ....................................................................................................................90Figure 6-6 - Example of Damage to the Bend Stiffener on an Open Socket ................................91Figure 6-7 Illustration of Spiral Strand Wire Kinking during Installation.................................91Figure 6-8 - Mid Line Buoy Swivel Connection Link (courtesy of MoorLink AB). ...................92

Figure 6-9 Pre-Stretching Polyester lines During Installation to Minimise the Requirement forFuture Line Length Adjustments [Ref. 27] ...........................................................................96

Figure 6-10 - Illustration of the Potential Difficulty in offshore alignment of pins on largeDiameter Rope [Ref. 26] .......................................................................................................97

Figure 6-11 - Sledge used to Protect H Connector during Deployment over the Stern Roller(Courtesy I. Williams)...........................................................................................................97

Figure 7-1 The Balmoral Benchmark FPV which has been continuously on station since 1986(Courtesy of CNR) ................................................................................................................99

Figure 7-2 Plan View of Mooring Incidents at Balmoral........................................................100Figure 7-3 Illustration of the Extent of General Corrosion on a Recovered Floating Production

Unit Mooring Line after 16 years service ...........................................................................103Figure 7-4 Illustration of the Extent of Corrosion Pitting..........................................................104

Figure 7-5 Example of the Damage Caused to the Crown of the Links ..................................105Figure 7-6 Arrow shows the Apparent Grinding Action on the Inner Face of One of the Links

.............................................................................................................................................105Figure 7-7 Example of the Damage Caused to a Hanging Shackle Pin on a FPSO Mooring Line

.............................................................................................................................................106Figure 7-8 Finite Element Stress Contour Plot (compare red areas with Figure 7-6) [Ref. 8] ..106Figure 7-9 - Example of Thrash Zone Wear ..............................................................................107Figure 7-10 - Illustration of the Extent of Pitting Corrosion......................................................108Figure 7-11 - Example of Wear and Pitting Corrosion on the Shackle Pin ...............................109Figure 7-12 -Test Rig Set Up for Break Testing of Mooring Components (Studless Chain in the

instance) ..............................................................................................................................111Figure 7-13 Illustration of Biologically Induced Pitting Corrosion in a Ballast Tank.............112

Figure 7-14 - Crack Growth per Cycle versus Stress Intensity Range [Ref. 2] ..........................113Figure 7-15 Illustration of Excessive Chain Wear on a CALM Buoy [Ref. 34]......................115Figure 7-16 Typical Temperature and Salinity Profile in the Tropical Oceans .......................116Figure 7-17 Indicative Oxygen Concentration versus Water Depth (courtesy of BP).............116Figure 7-18 Gulf of Mexico Snap Shot of Bottom Oxygen Concentration .............................117Figure 7-19 - Measured Wear Rates of U3 and U4 Chain at 8,170lbs (300 tonnes equivalent)

[Ref. 34] ..............................................................................................................................119Figure 8-1 Illustration of Line Tension Variations during a Payout/Pull-In Test....................123Figure 9-1 - Turret Design in which Chain Lengths can be Adjusted (courtesy of Chevron-

Texaco)................................................................................................................................127Figure 9-2 Generic Turret Design in which the Chains are Stoppered off at the Turret Base

(courtesy of Bluewater).......................................................................................................127

Figure 9-3 - Spread Moored FPSO Single Axis Chain Stopper (courtesy of SBM)...................128


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Figure 9-4 - External Cantilever Turret which experienced Chain wear at the Trumpet Weldswhich was halted by use of UMPHE (courtesy of Shell)....................................................130

Figure 9-5 - Example of the Level of Inspection Detail which can be achieved using a TypicalWorkclass ROV (courtesy of I.Williams)...........................................................................131

Figure 9-6- Test Tank Mock-Up of Micro-ROV inspection of Chain Emerging from TurretTrumpet (courtesy of I. Williams) ...................................................................................132

Figure 9-7 - Micro-ROV Photograph of Chain Wear Notches where Chain Emerges at theTrumpet Bell Mouth (courtesy of I. Williams) ...................................................................132

Figure 9-8 - Indication of the Extent of the Wear......................................................................133Figure 9-9 - Artificially Introduced Notch on to Spare Chain Links, note also Red Circular

Infrared Target (courtesy of I. Williams) ............................................................................134Figure 9-10 - Example of Stretched Chain during Break Testing, the Blue Mark Shows the

Location of a Typical Notch (courtesy of I. Williams) .......................................................134Figure 9-11 - Example of a Special Cobalt Chromium Anti-Wear Coating (courtesy of I.

Williams).............................................................................................................................135Figure 9-12 - Photograph of a Recovered Link Showing a Wear Notch (courtesy of I. Williams)

.............................................................................................................................................136Figure 9-13 - An Example of the Chain Damage noted after the Notched Chains had been

recovered back to Shore (courtesy of I. Williams)..............................................................136Figure 9-14 - Turret Arrangement where the Chain Stopper (in red) is Behind the Rotation Point

(2 black concentric circles) .................................................................................................137Figure 9-15 Illustration of Potential Wear at Metal to Metal Contact (courtesy of I. Williams)

.............................................................................................................................................138Figure 9-16 - Fairlead Chain Stopper where the Chain Stopper is in Front of the Rotation Point

(used on some Spread Moored FPSOs) (courtesy of Maritime Pusnes).............................138Figure 9-17 - As Installed Photo Graph of the Design Shown in Figure 9-16 (courtesy of

Maritime Pusnes).................................................................................................................139Figure 9-18 Typical CALM Buoy Chain Stopper (courtesy of The Professional Divers

Handbook [Ref. 38])..........................................................................................................140Figure 9-19 - Amoco CALM Buoy- Note Inclusion of Rubber Casting (courtesy of [Ref. 38])140Figure 9-20 - Comparison of Alternative Fairlead Arrangements (courtesy of Bardex)...........142Figure 9-21 Example of a Wire Rope Bending Shoe (courtesy of API RP25K) .....................143Figure 9-22 - Example of a Chain Bending Shoe Design [Ref. 39]............................................143Figure 9-23 - Bending Shoe Design which includes an Angle Sensor [Ref. 40]........................144Figure 10-1 Examples of the Subjectivity Associated with Assessing IWRC Rope Conditions

[Ref. 43] ..............................................................................................................................145Figure 10-2 - Illustration of the Mooring Layout and Connections............................................146Figure 10-3 - Photograph of Disconnected Socket on the Sea-Bed (courtesy of BP/Stolt

Offshore) .............................................................................................................................147Figure 10-4 - Note End Plate also seems to be Falling Off on the Right Hand Side (courtesy of

BP/Stolt Offshore)...............................................................................................................147Figure 10-5 - End Connection Detail .........................................................................................148Figure 10-6 - Illustration of Socket Minus End Plate .................................................................148Figure 10-7 - Repair Utilised Bigger Bolts and Allowed the Socket Pin to Rotate....................149Figure 10-8 - Example of Retrofitted Anodes to Control Corrosion Rate..................................150Figure 10-9 - Example of Disconnected Anodes after approximately 12 months of Service.....150Figure 10-10 - Example of Detached Clump Weight on the Sea-Bed ........................................151Figure 10-11 - Example of Recovered Clump Weights ..............................................................151Figure 10-12 Illustration of Where the Damage Occurred on the Mooring Catenary .............152Figure 10-13 - Example of a Parallel Chain Excursion Limiter (courtesy of I. Williams).........152

Figure 10-14 - Weighted Chain Option Utilising Parallel Chain Sections (courtesy ofN.Groves) ............................................................................................................................153


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Figure 10-15 - Red Arrow Illustrates the Local Wear can take place when utilising ParallelChain (courtesy of N. Groves) ............................................................................................153

Figure 10-16 - Example of Mid-Line Buoy Failures on a European FPSO................................154Figure 10-17 - Gripper chock showing chain damage ................................................................156Figure 10-18 - Upper Gypsy Wheel Arrangement before Failure ..............................................156Figure 10-19 - Gypsy wheel structure after failure, i.e. Gypsy Wheel No Longer Present ........156Figure 10-20 - Illustration of a New Design of Kenter Shackle intended to have improved

Fatigue Performance ...........................................................................................................157Figure 10-21 - Example of Windlass Crack (Red Arrow) due to Stress Raiser caused by Sharp

Corner (courtesy of BP) ......................................................................................................158Figure 11-1 - General Arrangement of the Brent Spar Mooring System (courtesy of Shell) .....160Figure 11-2 - Brent Spar Fairlead Chain Stopper in the Hull (courtesy of Shell).......................161Figure 11-3 - Close Up of the Stopper (courtesy of Shell) .........................................................161Figure 11-4 - Indentation from where the chain bore down on the Stopper (courtesy of Shell) 162

Figure 11-5 Red Arrow Illustrates wear on the chain, where it sat on the stopper (courtesy ofShell) ...................................................................................................................................162

Figure 11-6 - Brent Spar Wire Sample Y1 prior to cleaning [Ref. 41].......................................163Figure 11-7 FLP Mooring General Arrangement (courtesy of Shell)......................................163Figure 11-8 - Example of Short Trumpets on a Long Term Moored Floating Loading Platform

(courtesy of Shell) ...............................................................................................................164Figure 13-1 - Comparison of Mooring Line Inspection Periods for Different FPS Categories. 173Figure 13-2 Historical Failure Rates for Different Types of Units .........................................176Figure 14-1 - Special Joining Shackle (courtesy of Vicinay Catalogue) ....................................179Figure 14-2 - H Shackle Pin Configuration (courtesy of I. Williams) ....................................180Figure 14-3 Illustration of Subsea Connectors which have been used on Pre-Installed Mooring

Lines ....................................................................................................................................181

Figure 14-4 - Example of a Special Joining Plate - Note Electrical Isolating Bush ...................181Figure 14-5 Example of the Make Up of a Typical Closed Spelter Socket (courtesy of Bridon)

.............................................................................................................................................182Figure 14-6 - Example of an Open Socket..................................................................................183Figure 14-7 - Example of a Closed Socket .................................................................................183Figure 14-8 - Connector or Termination Design Flow Diagram - Initial Phase ........................187Figure 14-9 - Connector (Termination) Detailed Design Flow Chart.......................................188Figure 14-10 Illustration of a Purpose Designed connector allowing limited compliance in Two

Planes ..................................................................................................................................190Figure 14-11 - Example of a Dynamic Analysis to Estimate the Angle for the V Slot Size on

the H Shackle...................................................................................................................191Figure 14-12 - Example of Material with a Non Clearly Defined Yield Point..........................194

Figure 15-1 Broken Link from Fairlead.....................................................................................197Figure 15-2 Mechanical Damage on Fairlead Link................................................................... 197Figure 15-3 - Support of a Link in a Wheel Fairlead.................................................................198Figure 15-4 - Photograph of Test Link Showing Bearing Plates [Ref. 10]................................. 199Figure 15-5 - General View of Tension Bending Test Rig (protective screens removed for

clarity) [Ref. 10]................................................................................................................. 199Figure 15-6 - Broken Hardened Plates at the end of the First Test [Ref. 10] .............................200Figure 15-7 - Twisted Link Due to Mis-aligned Butt Weld [Ref. 10] ........................................201Figure 15-8 - Simple Out of Flatness Twist Measurement Jig [Ref. 10]....................................201Figure 15-9 - Illustration of Failed Link Due to Tension Bending [Ref. 10]..............................204Figure 15-10 - Girassol Offloading Buoy [Ref. 55]...................................................................205Figure 15-11 - Girassol Offloading Buoy Failure in Chain Link 5 [Ref. 55] .........................206

Figure 15-12 - Girassol Offloading Buoy Failure in Polyester Rope [Ref. 55]......................206Figure 15-13 - Chainhawser Arrangement and Location of Critical Link [Ref. 55] .................207


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Figure 15-14 - Out of Plane Bending Mechanism (See Section 25 [Ref. 56].........................208Figure 15-15 - Schematic of SBM Test Rig [Ref. 55] ...............................................................209

Figure 15-16 - Photograph of SBMs Test Rig [Ref. 55]............................................................210Figure 15-17 - Typical FPSO Chain Stopper Arrangement........................................................211Figure 15-18 Illustration of Wire Rope Failure Modes (courtesy of Bridon)..........................217Figure 15-19 - The 1.0MN Wire Rope Bending-Tension Fatigue Test Machine.......................218Figure 15-20 - Tension Bending at Wheel Fairlead (Bearing Load Eccentricity) and Tension

Bending from Interlink Friction (Torque at Contact)..........................................................219Figure 15-21 - Comparison between Various Mooring Chain S-N Curves...............................221Figure 17-1 - Sonar Fish for Deployment through Turret (courtesy Chevron Texaco).............227Figure 17-2 Sonar Fish Deployment Method (courtesy Chevron Texaco)..............................227Figure 17-3 - Sonar Display Screen Showing 12 Mooring Lines and 2 Risers Close to the Centre

(courtesty Chevron Texaco) ................................................................................................228Figure 17-4 - Simple Pre-Installed Inclinometer with + or 1 Degree Accuracy ......................229

Figure 17-5 - Illustration of a Football Sized ROV (Courtesy of I. Williams) .......................229Figure 17-6 - Instrumented Load Pin Shackle Link (courtesy of BMT/SMS).........................230Figure 17-7 - Indication of the Data Available from Instrumented Mooring Lines (courtesy of

BMT/SMS)..........................................................................................................................231Figure 17-8 - Illustration of a New Sonar System due to be Installed in the North Seas (courtesy

of I. Williams) .....................................................................................................................233Figure 17-9 - Close Up of the Proposed Sonar Head (courtesy of Ian Williams)......................233Figure 17-10 - Response Learning Without Line Tension Input ................................................234Figure 17-11 - Illustration of Riser Monitoring Instrumentation (courtesy of 2H) ....................236Figure 18-1 - Red Arrows and Black Line Indicate Key Areas subject to Degradation on a

Mooring System (leeward likely to have worst wear) ........................................................239Figure 18-2 - Example of a Weight Discontinuity which may Result In Enhanced Wear .........240

Figure 18-3 - Typical Turret Cross Section Illustrating that the key Mooring Components areSubmerged...........................................................................................................................241

Figure 18-4 - Chain Stopper View Prior to Chain Installation with Pull in Rigging Present(compare to Figure 18-3).....................................................................................................242

Figure 18-5 - Illustration of ROV Deployed Optical Calliper Measurement System (courtesyof Welaptega Marine Ltd) ...................................................................................................244

Figure 18-6 Illustration of Heavy Marine Growth on Long Term Deployed Chain...............246Figure 18-7 - In-Situ Inspection of a Wildcat Pocket by Abseillers ..........................................248Figure 18-8 - Close Up Of Fairlead Pocket Note Slight Lip on the Right...............................248Figure 18-9 - Example of Chain Wear From Sitting in a Wildcat Pocket ..................................249Figure 18-10 - Red Zones Highlight the Importance of Checking all Relevant Structural

Connections (Courtesy of CNR) .........................................................................................249

Figure 18-11 - Example of a Parted Lubrication Line Feeding a Submerged Wildcat or GypsyWheel (Courtesy of CNR)...................................................................................................250

Figure 18-12 - Example of a Non Flat Link................................................................................251Figure 18-13 - Buchan FPS Wire Rope NDT Inspection Head.................................................252Figure 18-14 - Proposed Wire Rope Inspection Toll Delpoyed from a ROV............................253Figure 18-15 - Example of a Difficult Area to Inspect ..............................................................256Figure 18-16 - Partially Buried Shackle Illustrates the Difficulties in checking locking pins

(courtesy of ENI).................................................................................................................256Figure 18-17 - Example of the Wheel Tappers Approach Used for Detecting Cracks on Railway

Carriages and Locomotives.................................................................................................258Figure 18-18 - Example of Anchor Handling and Heading Control Tugs during a Mooring Line

Repair Operation (courtesy of I. Williams)........................................................................259

Figure 18-19 - Use of Divers from a RIB to open up the Chain Stopper during a FPSO MooringLine Repair (coutesy of I. Williams)...................................................................................260


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Figure 19-1 - Example of a Plate Shackle which may be useful for a Temporary Repair (courtesyof Balmoral Marine)............................................................................................................262

Figure 19-2 - Temporary Mooring Line Winch Deck on a Gulf of Mexico Spar.......................263


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1 EXECUTIVE SUMMARY

1.1 Overview of the Report

The main objective of this report is to improve the integrity of the mooring systems onFloating Production Systems (FPSs). It is intended to be read and understood by nonmooring specialists such as FPS Operational staff - so that the people who live andwork on FPSs will be better able to become more involved in the vital task of lookingafter their own mooring systems. Meanwhile the included feedback on the actual

performance of mooring systems in the field should assist designers and manufacturersto improve future mooring designs. Hence, the report attempts to identify gaps in the

existing knowledge of mooring behaviour and components to provide a road map forfuture work. Appendix C includes a paper presented at the 2005 Offshore TechnologyConference (OTC) which represents a stand alone summary of the key points of theJIP.

1.2 Introduction

Unlike trading ships, FPSs stay at fixed positions year after year without regular drydocking for inspection and repair. Since they cannot move off station they mustwithstand whatever weather comes their way. Hence, depending on location, at times

their mooring systems need to withstand high storm loadings. Typically, during designfor harsh environments, mooring systems do not have much reserve capacity abovewhat is required to withstand survival conditions. Therefore, deterioration of the linesover time can increase the likelihood of single or multiple line failures. Multiple linefailure could conceivably result in a FPS breaking away from the moorings and freelydrifting in the middle of an oil field as has been seen in the past see Section 4.Failure of two adjacent mooring lines mooring lines at the same time due to waveshock loading has been seen and this could have serious implications if the risers are

pressurised at the time.

This JIP is concerned with assessing how mooring systems have performed in the fieldto identify the level of degradation which has taken place. Hence, the JIP has looked atFPSOs, Semi-submersible production units and Spars through out the world. From the

survey it has become apparent that certain, potentially significant, problems haveoccurred and thus the JIP wishes to publicise these so that they can be taken account ofduring inspection of existing units and during the design of future units.


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1.3 Indicative Mooring Statistics

At the beginning of the project it was hoped that it would be possible to gain data onthe mooring performance on most of the FPSs (turret and spread moored FPSOs,

production semis and Spars) in the world. In practice the best data which has beenobtained is for North Sea Units, partly due to local contacts and also the rigorousreporting regime in this area. In the absence of more comprehensive information, itthus seems prudent to consider official statistics for this region to be the best availableindicator of the likelihood of mooring failure. Exactly how these statistics can berelated to milder environments is difficult to quantify based on the presently availabledata set.

Table 1-1 summarises failure statistics for North Sea operations for different floatingunits covering the period 1980 to 2001. It is clear from these statistics that the

probability of line failure per operating year is relatively high.

Type of Unit Number of Operating

Years per Failure

Drilling Semi-submersible 4.7

Production Semi-submersible 9.0

FPSO 8.8

Table 1-1 - North Sea Mooring Line Failure Data, 1980 to 2001 [Ref. 1]

Given the safety critical nature of mooring lines and the likelihood of failure one mightimagine that they would be heavily instrumented with automatic alarms which wouldgo off in case of line failure. The following indicative statistics, based on data from themajority of North Sea based FPSOs, give an indication that instrumentation is not as

prevalent as might be expected for such a heavily regulated region:

50% of units cannot monitor line tensions in real time,

33% of units cannot measure offsets from the no-load equilibrium position,

78% of units do not have line failure alarms,

67% of units do not have mooring line spares available,

50% of units cannot adjust line lengths.


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1.4 The Cost of Mooring Line Failure

If a multiple mooring line failure should occur in storm conditions the potential costand the implications for the whole FPS industry could be extremely high, depending oncircumstances. Even a single mooring line failure would be costly as is illustrated inTable 1-2 for two different case studies, further details can be found in Section 5.4.

Description Approx. Cost of

Single Line Failure

50,000bpd N. Sea FPSO 2M

250,000bpd W. African FPSO 10.5M

Table 1-2 - Indicative Single Mooring Line Failure Costs

1.5 Key Findings from the Survey

During the course of the project a few common themes emerged which are outlinedbelow:

Wear where the Lines Connect to the Surface Platform

Achieving material compatibility at the key turret interface is vital see Section 0.Whether the trumpet pivot point should be in board or outboard of the chain stopper

needs further consideration for new designs. In addition, whether rotation should be permitted in two planes, rather the one which is typically the case at present alsorequires addressing based on further in field experience. This may have particularimplications for spread moored FPSOs. Access for inspection of these areas also needsto be improved and this should be specified in the mooring design criteria see Section20.

Wear/Corrosion Allowance for Long-Term Moored Units

On two North Sea FPSs chain wear and corrosion has been found to be significantlyhigher than what is specified by most mooring design codes. More in field inspectiondata is needed to find out if this is a general finding, which could have long-termimplications for other FPSs in the North Sea and elsewhere.

Excursion Limiting Weighted Chain Designs

A number of excursion limiting weighted chain systems have experienced problems see Section 10.3. Great care is needed in the design of such systems; particularly ifthey are due to operate in adverse environmental conditions. Parallel chains seem tohave worked well, as opposed to clump weights (lump masses) or hung off chain tails.Clean catenaries, i.e. without buoys or clump weights seem to work best, althoughwater depth, riser offset limits and environmental conditions may mean that this is notalways possible.


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Local Design of Connectors

Connectors are vital components on any mooring system and they need to be verycarefully designed if they are to prove reliable over a long field life. Certain mooring

problems have been due to the local design of the connectors. Section 14.4 includes asummary of key items which should be considered during detailed connector design.There is an emerging need for the development of a fatigue resistant connector suitablefor use with mooring chains during repair/overhaul operations.

Friction Induced Bending

Friction induced bending fatigue appears to be a significant issue which has beensomewhat neglected and warrants further investigation. This was less of an issue forcatenary systems in moderate water depths. Deep water taut moored units seem to be

potentially particularly susceptible see Section 15.2.

1.6 Key Areas to Check on a Mooring System

Based on the survey results,

Figure 1-1 illustrates the key areas which should be inspected on a mooring line. TheFPS has been displaced by environmental forces, thus illustrating both windward andleeward mooring lines.

Figure 1-1 - Red Arrows Indicate Key Areas subject to Degradation on aMooring System (leeward likely to have worst wear)

From a number of units it has become clear that the less loaded leeward lines can besubject to greater degradation than the windward lines. This seems to be due to greaterrelative rotation on leeward lines since the line is typically under lower tension.


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1.7 What are the Best Ways to Detect Line Failures?

It is vital to detect line failures promptly or else there is a danger of a unit entering astorm while still producing and thus being at an increased danger of loosing anothermooring line.

Detecting a line failure in the mud is difficult since the catenary shape, depending onsea-bed conditions, may not change that much. Section 17 summarises the keydetection techniques available at present. It is clear that in-field trials are required toidentify what systems prove to be reliable over the long-term. Hence, this is an ongoing issue which requires monitoring, assessment and publicising of the key findings.

1.8 Inspection Technologies

Inspecting moorings lines in situ is desirable due to the danger of damage during linerecovery to the surface and also during re-installation. There is also a significant costinvolved in mobilising intervention vessels to recover lines to the surface and then re-install them.

In-water line inspection is difficult, particularly with respect to identifying possiblecracks. Despite this it has become clear that many possible problems can be identifiedearly on, using tweaks to existing technology. This has been successful as long assuitably experienced people are involved in planning the inspection process and

examining the results.Section 18 summarises the present available inspection technologies and includes a

prioritised list of possible future improvements.

1.9 Key Conclusions and Recommendations

The survey of past and presently operating FPS units has shown that serious incidentshave occurred in the past including loss of station. The survey has also shown thateven for more up to date designs, deterioration of certain areas of the moorings systemmay be more rapid than expected. As well as the detail issues there is a more generalissue that requires addressing, namely the manner in which mooring integrity ismanaged and audited on an on-going basis.

Since moorings are category 1 safety critical systems, if the deterioration is not detectedearly and monitored/rectified the consequences could be severe. Hence, it is vital thatwhoever offshore is responsible for the day to day operation and inspection of FPSmoorings should have a strong marine background, such as a Deck Officer or MarineEngineer. Such personnel have a suitable mindset in that they really understand theimportance of moorings and their likelihood to deteriorate significantly over time. It isimportant that these personnel should be provided with sufficient resources so that theycan be pro-active with regard to inspection and any possible repairs which may berequired.


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Semi-submersible units have accumulated hundreds of years of mooring experience forvaried world wide locations. A key point to learn from such units is that chains, wireropes, gypsy wheels, stoppers and connectors have finite lives and do wear out.Although drilling rigs deploy and recover lines fairly regularly, which can causedamage, the wear seen on production semis is still significant see Section 7.1.However, most large scale FPS with 20+ year design lives seem to have been built onthe expectation that the mooring lines will last for the life of the field and that safetywill not be compromised towards the tail end of the field life, when production rateshave dropped. If production rates have dropped there is less money available formooring line repairs. Hence, assessments should be undertaken during the field life toassess whether line change outs may be required in the future and if so contingency

money should be allowed for to cover this later expenditure.In general, moorings should be thought of as relatively vulnerable primary structuralmembers subject to constant dynamic motion. Expecting such systems to last for 20+years without overhaul may prove to be optimistic. The commercial risks associatedwith a line failure during the field life justify the selection of top quality equipmentfrom the outset. This equipment then needs to be regularly inspected and repaired asrequired to ensure that it is still fit for purpose.

Availability of mooring line spares including connectors is extremely variable. Giventhe several month lead-time associated with procuring new components, it isrecommended that each operator should identify short term remedial measures to repaira line if it fails. This would involve identifying commonly available components which

can be obtained at short notice from marine equipment rental companies. Outlineprocedures including the type of intervention vessel required should also be developed.

Mooring systems are not as simple as they first appear and they need carefulmanagement through out their design lives. Thus a life cycle approach to mooringdesign and operation is recommended. In this way designers can feedback theirinspection requirements to Operators and then learn from whatever is found duringinspection. Manufacturers should also be included in this feedback loop, since theymay be best placed to implement improvements to their products. Hence over timemooring design and manufacturing should improve. At present designers andmanufacturers are not always involved with the in field behaviour mooring systems.Therefore, they may not be aware of operational or inspection type issues. In generalthere seems to be a need for periodic Mooring Audits to re-assess original design

parameters and review inspection records to assess whether the system is still fit forpurpose.

It is clear from this state of the art review that to continue to improve mooring integritya number of topics still require further investigation. A bullet point list is included inSection 21.3.


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2 INTRODUCTION AND SCOPE

2.1 The Need for a JIP

The number of Floating Production Systems (FPSs) operating in the world increasedsubstantially during the 1990s and there is now an ever-increasing body of FPSoperational experience. In 2001 Noble Denton was commissioned by the UK OffshoreOperators Association (UKOOA) to review available operational data from the Britishsector. The key results to emerge from this study were as follows:

There has been one FPSO line failure for every 5.4 operating years (thisfigure has been updated during this study);

Several cases occurred in which there was systematic damage to morethan one line;

Particular problems have been experienced at connectors and interfaces;

In no cases was the damage recognised immediately;

Long-term failure rates remain uncertain.

The study concluded that the potential for multiple line failure is greater than iscommonly perceived, and this should be a major cause for concern. The main reasonsfor this situation are:

Available inspection and maintenance provisions can allow long periodsin which single or multiple defects can remain undetected;

Most UK sector FPSOs can not detect if they have lost a mooring line;

The risk of mooring line failure is often underestimated and the majorityof operators do not carry spares or have systems in place for dealing witha line failure;

Design codes and standards give little guidance on terminations,connections, fair leads and stoppers which is where the majority offailures has been seen;

Similarly there is limited guidance on inspection, repair and maintenance.


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2.2 Scope Development

The JIP scope was developed to extend the previous UKOOA study to includeinternational experience, and reassess the conclusions of the UK sector study in moreglobal terms. In addition, a follow up has been carried on the recommendations of theUKOOA study to investigate the levels of exposure to duty holders, and developingmeasures to reduce the associated risks.

Specifically the work has covered the following:

Disseminate data gathered from international experience,

Develop guidance for designing mooring connectors and interfaceelements,

Provide guidance on mooring line inspection,

Summarise the pros and cons of line failure detection methods,

Take a look to future deepwater and taut leg applications

Investigate and report illustrative case studies

The JIP scope has been adjusted during the project to take into account results found todate and also the difficulties experienced in obtaining international data.

2.3 JIP Objectives

The basic objectives of the JIP are to:

Improve safety

Help to safeguard reputation of FPSO/FPS industry

Feedback operational and inspection to mooring designers

Publicise the importance and potential vulnerability of mooring systems

This report is intended to be read and understood by non mooring specialists such asFPS Operational staff. In this way the people who live and work on FPSs will be betterable to become involved in the vital task of looking after their own mooring systems.


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2.4 Project Organization

The project organisation is illustrated in Figure 2-1.

Figure 2-1 - JIP Organisation

The scope of work was broken down into Cost, Time, and Resource Modules [CTRs],

which were organized as follows:

Figure 2-2 - CTR Breakdown

Martin Brown

Project Manager

Consultants: I.D. Williams, R Stonor, R

Nataraja, D. Orr, R.V. AhilanND Group Resources & Subcontractors

STEERINGCOMMITTEE

Nigel Robinson

NDE Project Director

DESIGN AND CONSTRUCTION ISSUES

CTR 2 :Transportation, Handling & Installation

ChallengesCTR 3 :Design of Connectors & Interfaces

DESIGN AND CONSTRUCTION ISSUES

CTR 2 : Transportation, Handling & Installation

ChallengesCTR 3 : Design of Connectors & Interfaces

INTERNATIONAL SURVEY OF

MOORING PROBLEMS

CTR 1 : Survey of International FPSO/ FPS ExpCTR 4 : Consequences of Line Failure

INTERNATIONAL SURVEY OF

MOORING PROBLEMS

CTR 1 : Survey of International FPSO/ FPS ExperienceCTR 4 : Consequences of Line Failure

INTEGRITY MANAGEMENT

CTR 5 : Status Monitoring and Failure DetectionCTR 6 : Inspection, Repair & Maintenance, inc In

Water Survey

CTR 7 : Sparing Options

DISSEMINATION

OF RESULTS

(CTR 10)

Lessons Learned

Detailed Report

Integrity Check List

DISSEMINATION

OF RESULTS

(CTR 10)

Lessons LearnedBulletins/SteeringCommittee briefings

OTC paper

Detailed Report


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2.5 Project Participants/Sponsors

The following list details the organisations which have sponsored the JIP plus the personnel nominated to the Steering Committee. It is worth noting that the SteeringCommittee meetings provided an excellent mechanism to obtain and distribute data.Thanks are given to all members of the committee and the Chairman for their

participation.

1. ABS, Rod Yam and Ernesto Valenzuela

2. Ansell Jones

3. Balmoral Group, Doug Marr

4. Bluewater, Simon Stauttener5. BP, Richard Snell, Peter Gorf and Steve Barron

6. Bureau Veritas, Frank Legerstee and Michel Franois

7. Chevron Texaco, Matthew Brierley, Paul Devlin, and Jim Hughes(corresponding member)

8. ENI (Agip), Les Harley and Bill Nicol

9. Hamanaka Chains, Yoshiyuki Kawabe

10. HSE, Martin Muncer and Max English

11. IMS/Craig Group, Alan Duncan and Mark Prentice

12. Lloyds Register, Douglas Kemp, Richard Bamford and Alwyn McLeary

13. MARIN, Henk van den Boom and Johan Wichers

14. Maersk Marine Contractors, Graham Kennedy and Vere MacKenzie

15. National Oilwell/Hydralift-BLM, Philippe Gadreau

16. Norsk Hydro, Tom Marthinsen

17. Offspring International, Nigel Grainger and Russell Glen

18. Petro Canada, Sherry Power and Scott OBrien

19. SBM, Philippe Jean (Chairman)

20. Statoil, Kjell Larsen21. Vicinay Cadenas, Dave Nicol and Eduardo Lopez

22. Welaptega Marine, Tony Hall

Many people from various organisations helped out through out the JIP by providinginformation. It is impossible to list them all, but their combined support has beencrucial in enabling a comprehensive picture to be pulled together. Particular thanks are,however, given to Amerada Hess/Wood Group and Mr Ian Williams for making highlyrelevant data readily available to the JIP. Thanks also to Diane for all her assistancewith the layout and editing of this document.


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2.6 Steering Committee Meetings

The Steering Committee met four times during the course of the JIP in Monaco,Aberdeen, Paris and Houston, all being well attended. The meetings in Monaco andParis were part of the FPSO Forum/JIP Week. The Aberdeen meeting was a standalonemeeting.

The final meeting in Houston was at the end of the 2005 Offshore TechnologyConference (OTC).

Figure 2-3 Participants at the Steering Committee meeting in Paris


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3 MOORINGS OVERVIEW

3.1 Mooring Basics

3.1.1 Restoring Forces

To appreciate how to preserve the integrity of a mooring system it is helpful to have a

basic understanding of the different types of mooring systems and how they work. This

subject is covered in this chapter, which also includes a simple introduction to howsuch systems can be analysed.

The primary purpose of a mooring system is to maintain a floating structure on stationwithin a specified tolerance, typically based on an offset limit determined from the

configuration of the risers. The mooring system provides a restoring force that acts

against the environmental forces which want to push the unit off station. In thefollowing diagrams the main components of mooring system restoring force are

explained.

The connection between the mooring system and the body of the vessel is where the

restoring force of the mooring system acts, see Figure 3-1. At this connection point

there are two force components present; horizontal and vertical. The horizontal

component of the mooring lines tension acts as a restoring force. The vertical

component acts as a vertical weight on the vessel. In deep water the vertical force can be quite considerable. For some designs of FPS, with limited payload capacity, the

vertical mooring force can have significant design implications.

Figure 3-1 Typical Turret Moored FPSO


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It is informative to understand the significance of the mooring line angle as it departs

the point of connection to the vessel. A low angle to the vertical will generate a lowrestoring force, with significant vertical load on the vessel. If the angle here is large,then the restoring force will be increased while the vertical load on the vessel will bereduced. This relationship can be seen in Figure 3-2. The vessel needs to be able tosupport the applied vertical loading.

Figure 3-2 Shallow and Steep Mooring Line Angle Illustration

The relationship outlined in Figure 3-2 is adequate for considering a 2 dimensionalscenario. The mooring of a vessel, however, is a 3 dimensional problem and to this endit is necessary to consider the angle of the mooring line in the plane of the sea-surface.With reference to Figure 3-3 it can be seen that the tensions in a mooring line are splitinto two components; the restoring force that opposes the environmental loading, andthe lateral force, which may balanced by another mooring line.

Figure 3-3 Line Heading Illustration


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3.1.2 Environmental Loading

When there is no external loading on the system the vessel will not move from its staticequilibrium position. When environmental loading does occur an imbalance in thesystem will occur. To restore equilibrium the mooring system restoring force must

become equal to that of the environmental load. This is achieved through the vesseloffsetting from its original position. As this occurs the windward lines will pick uptension and the leeward lines will shed tension. This is shown in Figure 3-4.

Figure 3-4 Definition of Windward and Leeward Lines + Environmental Offset

The vessel will offset until the windward lines have generated a restoring force thatbalances the environmental loading. This means that the distance between the anchorand fairlead will increase, and thus the tension at the fairlead will also increase. This isshown in Figure 3-5.


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The relationship between environmental load and vessel offset is often represented in a

Load Excursion curve, as shown in Figure 3-6. This figure illustrates the loadexcursion characteristics of a 1,200m long, 76mm nominal diameter chain in 100mwater depth with a working or pretension tension of 100te. The plot emphasizes theneed to model the axial elasticity, even for chains, in order to get realistic results. Axialelasticity depends on geometry and material. Since there are new materials andgeometries available in the market, it is important that designers should confirm withmanufacturers that the values they are using agree with full scale testing values.

Figure 3-5 Offset Position and Tension Effect

Figure 3-6 Illustration of Load Excursion Curve [Ref. 2]


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3.1.3 Mooring Configuration

The most common mooring configurations are Spread Moored and Single PointMooring systems, which are taken to include turret systems. The key attributes of eachare discussed in this section.

Spread Mooring

This conventional mooring approach is widely adopted for semi-submersibledrilling/flotel/production units. For floating production applications, spread mooringsare used primarily with semi-submersibles and non-weathervaning FPSOs (i.e. noturret) see Figure 3-7. Since the wave loading on a semi-submersible is relatively

insensitive to direction, a spread mooring system can be designed to hold a semi onlocation regardless of the direction of the environment, although there is probably anoptimum heading. However, a spread system can also be applied to ship-shapedvessels, which are more sensitive to environmental directions, as long as theenvironmental conditions are relatively benign and the weather direction is fairlyuniform without strong cross currents. In a location such as the North Sea, the forceswhich can be generated on the beam of a spread moored FPSO, plus the motions insuch conditions, effectively prohibit such a mooring arrangement.

The mooring lines can be chain, wire rope, fibre rope or a combination of the three.Either conventional drag anchors or anchor piles can be used to terminate the mooringlines.

Figure 3-7 - Typical Spread Moored Unit, Girassol FPSO offshore West Africa(courtesy of Stolt Offshore)


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Spread moorings are typically cheaper than turret moorings since they are mechanicallyfar less complicated. However, they are limited to where they can be used and they can

make offloading operations by a shuttle tanker somewhat more involved.

Single Point Moorings (SPMs)

Single point moorings (SPMs), such as internal or external turrets, are used primarilyfor ship shaped units see Figure 3-1. They allow the vessel to weathervane, which isnecessary to minimise environmental loads on the vessel by heading into the prevailingweather. There is a wide variety in the design of SPMs, but they all perform essentiallythe same function.

3.1.4 Catenary and Taut Leg Moorings

Two main types of mooring system can be used for either the Spread or Single Pointsystem; Taut-Leg and Catenary. Both methods allow the system to withstand theapplied forces, but through different mechanisms.

A catenary system generates restoring force through the lifting and lowering of theline onto the seabed, plus a limited amount of line stretch. This is shown in Figure 3-8with a typical arrangement shown in Figure 3-9.

A taut-leg system makes use of the material properties of the mooring line, namely itselasticity, as shown in Figure 3-10. A typical taut-leg arrangement is shown in Figure3-11. Taut-leg moorings are relatively new and are typically used in deep water to limit

FPS offsets.


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Figure 3-8 Illustration of Catenary System

Figure 3-9 - Typical Spread Moored Catenary System (Courtesy of Vryhof)


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Figure 3-10 Illustration of Taut-Leg system

Figure 3-11 - Typical Spread Moored Taut-Leg System (Vryhof)


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3.1.5 Vessel Dynamics

Waves will cause a vessel to move in all six degrees of freedom; surge, sway, heave,roll, pitch and yaw. These degrees of freedom are illustrated in Figure 3-6.

The motion of the vessel to individual waves is called its wave frequency or first-orderresponse. As a mooring line moves through the water it will be subject to dynamic linedrag and inertia loading and sometimes a whipping effect. It is possible to take thisinto account by undertaking a dynamic mooring analysis, but this does increasecomputing time significantly.

Figure 3-12 Illustration of Surge, Sway, Heave, Roll, Pitch and Yaw


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The compliance of a mooring system is such that conventionally the presence of themooring system is not considered to affect the wave frequency response. The overall

mooring system stiffness and associated natural frequency will influence its secondorder or low frequency slow drift response.

In deep water for certain floating objects, such as deep draft Spars, the wave frequencymotion is attenuated to a certain extent by the mooring system due to the higher systemstiffness. Hence, a coupled analysis is sometime undertaken. The general conclusionfrom this type of analysis appears to be that the mooring quasi-static tension has animpact on a floater's wave frequency response, which in turn will affect the mooringdynamic tension. On the other hand, the effect of dynamic tension is less important to afloater's wave frequency response. For deep water the effect of risers on the vesselresponse becomes increasingly important and this should be taken into account.

The coupled wave frequency motion of a floater can be calculated in the time domainusing the wave force, wave frequency added mass and damping, and mooring force ateach time step. Usually a convolution method needs to be adopted in the radiationforce calculation. Although the coupled wave frequency motion calculation in the timedomain is slower than the Response Amplitude Operator (RAO) based wave frequencymotion calculation, it is still acceptable. Typically a 3 hour simulation will take a fewminutes. However if there is very high mooring stiffness or if a mooring dynamicanalysis is performed, then the computing time will be high.

3.1.6 Mooring Design

The tensions experienced by a mooring system at any time are driven by the following:

Static component from Wind, Mean Wave Drift and Current,

Wave frequency component, caused by 1st order wave frequency motions anddrag/inertia effects on the line,

Low frequency component, due to 2nd order low frequency waves and winddynamics.

The essence of mooring design is to optimise the behaviour of the mooring system suchthat the excursions of the surface vessel do not exceed the allowable flexible riseroffsets, while at the same time ensuring that the line tensions are within their allowablevalues. Thus the mooring system load offset curve should not be too hard or too soft see Figure 3-13. Hence, considerable iteration work may be required to optimise asystem for a particular location.


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It is worth noting that spring buoys (mid water buoys) and clump weights can also be

used to obtain an optimised mooring system stiffness by extending the resistive forcesover greater distances, hence allowing clearance over subsea features. However, theiruse should be treated with caution, particularly in areas subject to harsh environmentalconditions, where they have been known to come adrift see Section 9.3. Buoys andclump weights are also likely to introduce bending effects which may have anundesirable impact on the fatigue life see Section 15.

Figure 3-13 Example of Optimising the Stiffness of the Load offset Curve

3.1.7 Mooring Analysis Calibration with Full Scale Behaviour

The determination of maximum tensions for a multiple line system requires applicationof specialist computer programmes, which in many cases have been under continuousdevelopment for a number of years. Despite this, there are still uncertainties inestimating mooring loads using analysis software and model tests. Hence, it would be

desirable to compare the behaviour of a full scale FPS in known weather conditionsversus predictions. Surprisingly little work has been done on this topic, although this is

partly due to the difficulties associated with obtaining reliable weather andinstrumentation readings.


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3.1.8 Active Winching and Thruster Assistance

Since there are now hundreds of years of accumulated mooring experience from semi-submersible rigs, it is informative to understand the basis of their mooring operations.This is reviewed in this section, which considers active winching and thrusterassistance.

Active Winching

Active winching can be undertaken on semi-submersible production, drilling andaccommodation units. There are two basic options, namely:

1. Leeward line slackening,

2. All round length adjustment, including windward lines, so that the tensions are aswell balanced as possible at the limit of vessel surge.

If the leeward lines are slackened down too much this can result in greateryawing/surging and reduced direction control which can lead to higher line tensions. Inother words, if there is too much slack in the system, there is an increased danger ofhigh line snatch loadings.

Windward line tension optimisation can also be problematic. To quote from RobertIngliss informative 1992 paper [Ref. 3]:

in practise rig operators are reluctant to adjust windward line tensions insevere weather conditions and usually restrict adjustments, if any, toslackening leeward lines. This is partly to do with limitations in winch stallcapacity and the risk of a winch or brake failure, but most importantly themajority of rigs are not provided with suitable tension monitoring devices andcomputerised winch control systems which would make extensive line tensionoptimisation a realistic possibility. The general situation is that analystsfrequently utilise line optimisation to reduce tensions to meet acceptancecriteria but these line tension optimisation procedures are almost neverimplemented in practice on a rig.

Based on this type of feedback the latest mooring design codes (e.g. ISO [Ref. 4] + OSE301 [Ref. 5]) do not permit either windward or leeward active wincing to minimisemooring line tensions apart from going from one operational state to another.

Thruster Assistance

A number of semi-submersibles and a relatively small number of FPSOs are equippedwith thruster assistance. The thruster assistance can be categorised as either ThrusterAssistance (TA) or Automatic Thruster Assistance (ATA). TA is based on manual

joystick thruster control. ATA makes use of automatic remote control algorithmsystem to control the behaviour of the thrusters.


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It has been found that operation of the thrusters can be very effective in reducing peakline tensions; even though the thrust delivered can be modest. Typically in a mooring

analysis the thrusters are considered to reduce the mean load applied to the mooringsystem. However, thrusters also seem to damp down the magnitude of the slow driftsecond order offsets. They can also be helpful with respect to heading control. Thiscan be particularly useful on a production vessel, if a small change in heading can resultin reduced vessel motions, thus improving the efficiency of the oil/water separation

process.

In practical terms, when operating in manual thruster mode, high line snatch loads canbe avoided by applying thrust as the wave train approaches. This will tend to push thevessel in the direction of the advancing sea. As the wave passes it is necessary to easedown on the thrust to avoid over slackening the windward lines. If these become too

slack there is an increased danger of snatch loading when the next wave train passesthrough.

3.1.9 Metocean Parameters and their Impact on Mooring Integrity

For relatively benign environments, such as off West Africa, there is a much smallerdifference between operational and survival sea states compared to say the North Sea.This means that if the metocean parameters, or the response of the vessel due to these

parameters, is underestimated, there is significantly less of an in built safety margincompared to harsher climates, particularly with regard to fatigue.

The degree of spreading of the waves (see Figure 3-14 and Figure 3-15) can also affectmooring analysis results. The geographic area and fetch distance will influence thetype of waves likely to be encountered in practice. Conventionally, short crested seasare considered to result in reduced wave frequency response and hence reducedmooring line tensions - see section 3.3.2 of [Ref. 6]. However, recent model test resultsat DHI in Denmark has shown that for certain vessel sizes the mooring loads in shortcrested waves can be higher than in long crested waves [Ref. 7]. Thus the key point isto ensure that the response of the system is thoroughly evaluated for the worst expectedconditions (ie short or long-crested) both from a fatigue and a strength point of view.


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Figure 3-14 - Illustration of Long Crested (Unidirectional) Seas

Figure 3-15 - Illustration of Short Crested (Confused) Seas


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3.1.10 Rogue/Steep Breaking Waves and Shock Loading

Mariners have used phrases such as Freak Waves, Rogue Waves, Walls of Water oreven Holes in the Sea, to describe some of the conditions they have experienced atsea. Trading vessels are typically weather routed to avoid the worst of predictedweather conditions. However, permanently moored FPSs have to ride out whateverweather is thrown at them.

From a statistical sense the longer a FPS is on station the more likely it is to experience100 year + conditions. If an elderly FPS with a mooring system which has seen wear,corrosion and has accumulated some hair line cracks is subject to such conditions, thelikelihood of single or even multiple line failure is increased.

Very occasionally an unusually steep wave slam load could occur at the same time thata floating structure is around its maximum slow drift offset. The resulting shock orspike load on the mooring might be quite considerable. How much this shock loadingis transferred to the mooring lines will depend to a significant extent on the degree ofstructural damping in the hull structure, the vessel inertia, how long the load acts andwhere the moorings are relative to where the wave impacts. For a semi, where youmight get wave slam/slap right into one of the corners (see Figure 3-16), the amount ofstructural damping might well be less than compared say to a FPSO with an internalturret (see Figure 3-19). Hence the loading could be higher.

Figure 3-16 - Example of a Wave Breaking on a Column of a Semi-Submersible

In deep water steep elevated wave fronts with breaking or near breaking crests canoccur see Figure 3-17. In addition, a "Three Sisters" wave group can occur in whichthe second wave is generally the highest and is often preceded by a long trough.Hence, a moored object may ride the first wave, but then plunge submerged into the

base of the second steep fronted wave that then inflicts the greatest shock loading.


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Figure 3-17 - Illustration of Deepwater Breaking Wave Types (Plunging Break on theLeft and Spilling Breaking on the Right)

In November 1998 the Schiehallion FPSO was struck by a wave which was feltthroughout the vessel. The wave caused tears in the forward shell plating of theforecastle superstructure, buckling of supporting stiffeners and permanent deformationof the forecastle tween deck see Figure 3-18. Production was shut down and nonessential personnel were evacuated to a nearby drilling rig. In this instance no damagewas reported to the mooring system, but it illustrates the danger presented by infrequentsteep breaking waves.

Figure 3-18 - Illustration of the Damage Caused to Schiehallions Bow by an UnusuallySteep Wave (courtesy of BP)


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Present day standard mooring analysis tools do not evaluate this potential shock load

effect on the mooring systems. Hence it is difficult to quantify. But there is apossibility, based on the wave description, that it could have been a factor which led tothe virtually instantaneous multiple line failures experienced by Petrograd 1 in theearly 1990s (see Section 4.3). This might also be a factor in the relatively frequentmooring line failures experienced by semi-subs. It is recommend that this topic should

be investigated further and that appropriate cross checks should be made with the reallife recorded response of FPSs in severe/steep sea weather conditions. However, it alsoshould be noted that such weather conditions do not occur very often.

Figure 3-19 - Model Illustration of the Effect of a Breaking Wave on a FPSO (Courtesyof APL website)

The right hand side photograph of Figure 3-19 is perhaps an example of the type ofwave conditions which could impart a shock loading to the moorings, depending on theFPSO offset at the time. If a mooring line had already broken and its failure had not

been detected (due to a lack of failure of instrumentation) the chance of additional linefailures would be high in these conditions.


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3.2 Mooring Line Constituents

3.2.1 Introduction

Various different materials can be used to assemble a mooring line. This sectionprovides a brief description of the main components that typically constitute a mooringline. The pros and cons of the various types of line components are explained. Thishelps to aid understanding when considering how actual systems have performed insitu. Connectors and terminations are considered separately in Section 14.

3.2.2 History of Studded and Studless Chain

Early mooring lines tended to make use of simple links without studs. Development ofthis design led to usage of studded links, see for example Figure 3-20. Ease ofhandling and avoidance of kinking were the primary reasons for the introduction ofstuds. The resulting link geometry (see Figure 3-21) took advantage of the ability ofthe stud to resist some of the bending loads in the links. The studded link standardgeometry of length of 6 x Bar Diameter (D) and breadth of 3.6 x D was approved bythe British Admiralty in the 1860s.

Historically anchor chain used on ships was, in general, only required to meetintermittent short term loading and therefore, even over a long ship service life, fatiguewas unlikely to be a problem.

Figure 3-20 - Isambard Kingdom Brunel in front of Studded Chain for the GreatEastern steam ship, circa 1858


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Studless Link Studded Link

Figure 3-21 Comparison of the Geometry of Modern Studded and Studless Chain

[Note: DNV Cert Note 2.6, states 3.3D to 3.4D for the of studless link width]

Fairly recent long-term applications of chains in the moorings of floating productionsystems have brought about the development of studless chain. The studless chain link

has been redesigned with a smaller breadth to reduce the bending loads. These designsare increasingly used for long-term moorings because loose and missing stud problemsare eliminated. Unfortunately, however, the fatigue life of studless chain has beenshown to be half that of comparable studded chain, based on the results of fatiguetesting [Ref. 8]. In other words the fatigue endurance of studded chains is twice that ofstudless if the studs remain tight. Of the 70 fatigue failures reported in the Houston JIP,52% occurred at an inner Half-Crown position, 34% at an inner Crown position and14% at a mid leg position. The Crown refers the area of maximum bend and Half-Crown essentially refers to the area of the link where bending commences.

The studless link standard geometry of length of 6 x D and breadth of 3.35 x D came to

the market after 1989 as consequence of collaboration between DNV and Vicinay forthe Veslefrikk B project. For this chain the first tentative specification went out in1995 with the DNVs Certification Note 2.6. More recent developments includecustomised chain geometries also known as Variable Geometry and Weight (VGW) asdiscussed in OTC paper 8148, 1996 [Ref. 9]. VGW provides flexibility to modify linkgeometry and weight to suit a particular application see for example Section 18.8.2.

Table 3-2 and Table 3-3 summarise the relative merits of studless and studded chain interms of design, manufacturing, inspection and maintenance.


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RequirementRecommended

ChainReason

Lower static or dead weight in the catenary Studless Lower weight per metre

Access for shackle and accessoryconnection

Studless More interior link space

Versatility, similar to end links Studless Completely open links

Greater safety factor with same weight permetre in the catenary (strength to weightratio)

Studless Larger possible diameter with lessweight per metre

Greater stiffness in the mooring line Studlink Higher elasticity modulus

Higher Break Load Both Same break load, but differentproof loads

Transition through windlasses andfairleads

Both But studless more likely to knot ortwist

Long fatigue life Open to discussion See previous page

Table 3-1 Summary of Chain Design Parameters (modified from Vicinay ChainCatalogue)


ChainReason

Better inspection of weld and crown area Studless Greater access due to lack of stud

Elimination of stud locating problems Studless Lack of stud

Oversizing of the link in the weld zone Studless Elimination of the flattening andmaterial expansion in the weldzone

No links with stud looseness Studless Lack of stud

Table 3-2 Comparison of Manufacturing Parameters


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ChainReason

Eliminate premature fatigue due to loosestuds

Studless No stud, therefore no notch effect

Reduce inspection/repair costs Studless Easier access + no loose studs torepair

Eliminate galvanic reaction between thestud and the link

Studless No stud, therefore no possibility ofreaction

Increase reliability of the chain over time To be determined Although there are no loose studsissues with studless, the fatigue performance of studless is less

good than that of studded

Handling and connectability with Dshackles and hooks

Studless Better access for the through pin.Minimal requirements andrestrictions

Early indication of system degradation Studlink Condition of studs likely to berepresentative of system as a whole

Handling and Manoeuvrabili

HSE Report on Mooring Integrity

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