os-c102_2012-10

OFFSHORE STANDARD

The electronic pDNV-OS-C102Structural Design of Offshore Ships

OCTOBER 2012DET NORSKE VERITAS AS

df version of this document found through http://www.dnv.com is the officially binding version

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DNV service documents consist of among others the following types of documents: Service Specifications. Procedural requirements. Standards. Technical requirements. Recommended Practices. Guidance.

The Standards and Recommended Practices are offered within the following areas:A) Qualification, Quality and Safety MethodologyB) Materials TechnologyC) StructuresD) SystemsE) Special FacilitiesF) Pipelines and RisersG) Asset OperationH) Marine OperationsJ) Cleaner EnergyO) Subsea SystemsU) Unconventional Oil & Gas Det Norske Veritas AS October 2012

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This service document has been prepared based on available knowledge, technology and/or information at the time of issuance of this document, and is believed to reflect the best ofcontemporary technology. The use of this document by others than DNV is at the user's sole risk. DNV does not accept any liability or responsibility for loss or damages resulting fromany use of this document.

Offshore Standard DNV-OS-C102, October 2012Changes Page 3

CHANGESGeneralThis document supersedes DNV-OS-C102, October 2011.

Text affected by the main changes in this edition is highlighted in red colour. However, if the changes involvea whole chapter, section or sub-section, normally only the title will be in red colour.

Main Changes

General LRFD method is moved from App.B to a new section, Sec.13. New App.B for life time extension of FPSOs. Editorial modifications and some figures included.

Sec.2 Material Selection and Inspection Principles B105: Guidance note regarding doubling plates added. B203: Service temperature. Changed distance from 0.5 to 0.6 m for interference from external structure. Table B1: Foundation attached to decks included. B405: regarding steel grade above minimum required added. C305: A guidance note for definition of critical details added.

Sec.3 Design Principles Table B2 Design basis for wave load analysis for ultimate strength (old B1) modified. Table B3 Design basis for wave load analysis for fatigue strength (old B2) modified. B600: New benign water requirement included and moved from Sec.11 and Sec.12. Old B500: Extreme condition removed and requirements changed. See B403. C400: Usage factor for fine mesh FE analysis moved from Sec.5.

Sec.4 Design Loads C300: Wind loads modified. C800: Combination of loads for topside and topside interface modified and moved from Sec.6. C900: Minimum tank densities for the strength and fatigue analysis included.

Sec.5 Hull Strength C400: Design loading conditions for the hull girder longitudinal strength for the survival condition. Table

D1 moved from Sec.11 and Sec.12. E: Local detail stress analysis. Tables modified and moved from Sec.11 and Sec.12. F: fatigue strength moved to Sec.8.

Sec.6 Strength of Topside Structures F: Global dynamic loads for topside structure is modified and combination tables moved to Sec.4 C800. H: Table H1 modified. Coefficient for global strength for topside load bearing set to 1.0. Old G Hull Deformation moved to F200 and item F205 regarding sliding supports has been added.

Sec.7 Topside Interface to hull structure B400: (old C.) Combination of loads modified.

Sec.8 Fatigue Capacity Assessment F: Fatigue sensitive details in the hull and topside supporting structures moved from Sec.11 and Sec.12. G: FMS notation requirements included.

Sec.12 Additional Provisions for Floating Production, Storage and Offloading Units C200: Green sea load. Hs min changed from 8.m to 10 m. D200: Quay requirement deviation given. Table H2 modified. Design fatigue factors (DFFs).

Sec.13 LRFD Method New section - LRFD format moved from old App.B. C: Global strength analysis of the topside structure included.DET NORSKE VERITAS AS

Offshore Standard DNV-OS-C102, October 2012Changes Page 4 App.A Conversion of Tanker to Floating Offshore Installation A200 Benign waters. Reduction of the rule wave loads in benign waters by reducing the rule wave

coefficient (Cw).

App.B Longitudinal Strength according to the LRFD Method New. Life time extension of FPSOs.

In addition to the above stated main changes, editorial corrections may have been made.Editorial CorrectionsDET NORSKE VERITAS AS

Offshore Standard DNV-OS-C102, October 2012 Contents Page 5CONTENTS

Sec. 1 Introduction ...................................................................................................................................... 10

A. General ......................................................................................................................................................................... 10A 100 General................................................................................................................................................................ 10A 200 Objectives ........................................................................................................................................................... 10A 300 Classification ...................................................................................................................................................... 10

B. Assumptions and Applications................................................................................................................................... 10B 100 General................................................................................................................................................................ 10

C. Definition...................................................................................................................................................................... 11C 100 Verbal forms ....................................................................................................................................................... 11C 200 Terms .................................................................................................................................................................. 11C 300 Symbols .............................................................................................................................................................. 12C 400 Abbreviations...................................................................................................................................................... 12C 500 References........................................................................................................................................................... 12

Sec. 2 Material Selection and Inspection Principles ................................................................................ 13

A. Introduction................................................................................................................................................................. 13A 100 General................................................................................................................................................................ 13

B. Selection of Material ................................................................................................................................................... 13B 100 General................................................................................................................................................................ 13B 200 Design and service temperatures ....................................................................................................................... 13B 300 Hull structure ...................................................................................................................................................... 13B 400 Topside structure and elements not covered by Rules for Classification of Ships Pt.3 Ch.1............................. 14

C. Inspection Principles................................................................................................................................................... 15C 100 General................................................................................................................................................................ 15C 200 Hull structure ..................................................................................................................................................... 15C 300 Topside structure and elements not covered by hull structure requirements...................................................... 15

Sec. 3 Design Principles .............................................................................................................................. 17

A. Introduction................................................................................................................................................................. 17A 100 Overall design principles .................................................................................................................................... 17

B. Design Conditions ....................................................................................................................................................... 17B 100 Modes of operation ............................................................................................................................................ 17B 200 Transit and non-operational conditions .............................................................................................................. 17B 300 Operating conditions........................................................................................................................................... 17B 400 Survival condition............................................................................................................................................... 17B 500 Benign waters or harsh environmental areas ...................................................................................................... 18B 600 Wave load analysis ............................................................................................................................................. 18

C. Working Stress Design, WSD .................................................................................................................................... 20C 100 General ............................................................................................................................................................... 20C 200 Load conditions................................................................................................................................................... 20C 300 Permissible usage factor .................................................................................................................................... 20C 400 Usage factor for peak stress ............................................................................................................................... 20

Sec. 4 Design Loads..................................................................................................................................... 22


B. Static Loads ................................................................................................................................................................. 22B 100 General................................................................................................................................................................ 22B 200 Still water hull girder loads................................................................................................................................. 22

C. Environmental Loads ................................................................................................................................................. 23C 100 General................................................................................................................................................................ 23C 200 Wave induced loads ............................................................................................................................................ 23C 300 Wind loads .......................................................................................................................................................... 23C 400 Green sea............................................................................................................................................................. 23C 500 Sloshing loads in tanks ....................................................................................................................................... 24C 600 Bottom slamming................................................................................................................................................ 24C 700 Bow impact ......................................................................................................................................................... 24C 800 Combination of dynamic loads ........................................................................................................................... 24C 900 Design density of tanks....................................................................................................................................... 26DET NORSKE VERITAS AS

Offshore Standard DNV-OS-C102, October 2012 Contents Page 6Sec. 5 Hull Strength .................................................................................................................................... 28

A. Introduction................................................................................................................................................................. 28A 100 General................................................................................................................................................................ 28A 200 Corrosion addition .............................................................................................................................................. 29

B. Compliance with Main Class Requirement, 1A1 ..................................................................................................... 29B 100 Local scantling requirement................................................................................................................................ 29

C. Hull Girder Longitudinal Strength ........................................................................................................................... 29C 100 Application.......................................................................................................................................................... 29C 200 General................................................................................................................................................................ 30C 300 Analysis model ................................................................................................................................................... 30C 400 Design loading conditions .................................................................................................................................. 30C 500 Hull girder yield check........................................................................................................................................ 31C 600 Hull girder buckling capacity check ................................................................................................................... 31

D. Transverse Strength.................................................................................................................................................... 32D 100 Application.......................................................................................................................................................... 32D 200 General................................................................................................................................................................ 32D 300 Analysis model ................................................................................................................................................... 32D 400 Design loading condition .................................................................................................................................... 32D 500 Yield strength criteria ......................................................................................................................................... 32D 600 Buckling capacity check ..................................................................................................................................... 33

E. Local Stress Analysis .................................................................................................................................................. 33E 100 Application.......................................................................................................................................................... 33E 200 General................................................................................................................................................................ 34E 300 Analysis model ................................................................................................................................................... 34E 400 Design loading condition .................................................................................................................................... 34E 500 Acceptance criteria.............................................................................................................................................. 34

Sec. 6 Strength of Topside Structures ....................................................................................................... 35

A. Introduction................................................................................................................................................................. 35A 100 General................................................................................................................................................................ 35A 200 Definition of load point....................................................................................................................................... 36

B. Local Static Loads....................................................................................................................................................... 36B 100 Local loads on topside structures........................................................................................................................ 36

C. Local Requirements to Plates and Stiffeners............................................................................................................ 37C 100 Plates ................................................................................................................................................................... 37C 200 Stiffeners ............................................................................................................................................................. 37C 300 Green sea loads ................................................................................................................................................... 38

D. Local Requirements to Simple Girders..................................................................................................................... 38D 100 General................................................................................................................................................................ 38D 200 Minimum thickness............................................................................................................................................. 38D 300 Effective flange of girders .................................................................................................................................. 38D 400 Effective web of girders...................................................................................................................................... 38D 500 Strength requirement of simple girders............................................................................................................... 38

E. Global Static Loads..................................................................................................................................................... 39E 100 Global static loads in topside structure ............................................................................................................... 39

F. Global Dynamic Loads ............................................................................................................................................... 39F 100 Global dynamic loads in topside structure.......................................................................................................... 39F 200 Load combinations.............................................................................................................................................. 39

G. Complex Girder Systems............................................................................................................................................ 40G 100 General................................................................................................................................................................ 40G 200 Design loads........................................................................................................................................................ 41G 300 Impact from surrounding structure ..................................................................................................................... 41

H. Acceptance Criteria .................................................................................................................................................... 41H 100 Usage factors....................................................................................................................................................... 41H 200 Local liquid tanks................................................................................................................................................ 41

I. Buckling Stability ........................................................................................................................................................ 41I 100 Bars, beams, columns and frames....................................................................................................................... 41I 200 Flat plated structures and stiffened panels .......................................................................................................... 41I 300 Tubulars .............................................................................................................................................................. 41I 400 Capacity checks according to other codes .......................................................................................................... 42DET NORSKE VERITAS AS

Offshore Standard DNV-OS-C102, October 2012 Contents Page 7J. Fatigue strength........................................................................................................................................................... 42J 100 General................................................................................................................................................................ 42

Sec. 7 Topside Interface to hull structure ................................................................................................. 43


B. Strength Assessment ................................................................................................................................................... 43B 100 Application.......................................................................................................................................................... 43B 200 Requirements to the FE model............................................................................................................................ 43B 300 Design load ......................................................................................................................................................... 43B 400 Load combination .............................................................................................................................................. 43B 500 Acceptance criteria.............................................................................................................................................. 44

C. Fatigue Assessment ..................................................................................................................................................... 44C 100 General................................................................................................................................................................ 44

Sec. 8 Fatigue Capacity Assessment ......................................................................................................... 45


B. Principles and Methodology....................................................................................................................................... 45B 100 Assessment principles......................................................................................................................................... 45B 200 Methods for fatigue capacity .............................................................................................................................. 45

C. Structural Details and Stress Concentration Factor (SCF) .................................................................................... 45C 100 General................................................................................................................................................................ 45

D. Design Loads and Calculation of Stress Ranges ...................................................................................................... 46D 100 Wave environment .............................................................................................................................................. 46D 200 Wind and current................................................................................................................................................. 46D 300 Local and global loads ........................................................................................................................................ 46

E. Design Fatigue Factor (DFF) ..................................................................................................................................... 46E 100 General................................................................................................................................................................ 46

F. Details to be checked................................................................................................................................................... 46F 100 General................................................................................................................................................................ 46

G. Class Notation FMS .................................................................................................................................................... 48G 100 General................................................................................................................................................................ 48G 200 Design fatigue factors (DFF) .............................................................................................................................. 48G 300 Fatigue analysis methodology ............................................................................................................................ 49G 400 Areas to be checked ............................................................................................................................................ 50

Sec. 9 Accidental Conditions ...................................................................................................................... 51


B. Design Criteria ............................................................................................................................................................ 51B 100 General................................................................................................................................................................ 51B 200 Dropped objects .................................................................................................................................................. 51B 300 Fires .................................................................................................................................................................... 51B 400 Explosions........................................................................................................................................................... 52B 500 Unintended flooding ........................................................................................................................................... 52B 600 Collision.............................................................................................................................................................. 52B 700 Loss of heading control....................................................................................................................................... 52

Sec. 10 Welding and Weld Connections...................................................................................................... 53

A. Introduction................................................................................................................................................................. 53A 100 General requirements .......................................................................................................................................... 53

B. Size of Welds................................................................................................................................................................ 53B 100 Double continuous fillet welds ........................................................................................................................... 53B 200 Fillet welds and deep penetration welds subject to high tensile stresses............................................................ 54B 300 Full penetration welds......................................................................................................................................... 54B 400 End connection of stiffeners ............................................................................................................................... 54B 500 Direct calculations .............................................................................................................................................. 54DET NORSKE VERITAS AS

Offshore Standard DNV-OS-C102, October 2012 Contents Page 8Sec. 11 Additional Provisions for Well Service and Drilling Units .......................................................... 56

A. Introduction................................................................................................................................................................. 56A 100 Scope and application ......................................................................................................................................... 56

B. Design Principles ......................................................................................................................................................... 56B 100 General................................................................................................................................................................ 56

C. Design Loads................................................................................................................................................................ 56C 100 Green sea............................................................................................................................................................. 56

D. Local scantling requirement ...................................................................................................................................... 56D 100 General................................................................................................................................................................ 56

E. Hull Girder Longitudinal Strength ........................................................................................................................... 56E 100 General................................................................................................................................................................ 56E 200 Design loading conditions .................................................................................................................................. 56

F. Transverse Strength.................................................................................................................................................... 57F 100 General................................................................................................................................................................ 57F 200 Design loading conditions .................................................................................................................................. 57

G. Local Stress Analysis .................................................................................................................................................. 57G 100 General................................................................................................................................................................ 57

H. Fatigue Strength.......................................................................................................................................................... 57H 100 Design loading conditions .................................................................................................................................. 57H 200 Mean stress effect .............................................................................................................................................. 57H 300 Design fatigue factor........................................................................................................................................... 57H 400 Details to be checked .......................................................................................................................................... 57

I. Inspection Principles................................................................................................................................................... 57I 100 Hull and topside structure ................................................................................................................................... 57

J. Corrosion Control ....................................................................................................................................................... 58J 100 Hull and topside structure ................................................................................................................................... 58

Sec. 12 Additional Provisions for Floating Production, Storage and Offloading Units ......................... 59

A. Introduction................................................................................................................................................................. 59A 100 Scope and application ......................................................................................................................................... 59A 200 Definition ............................................................................................................................................................ 59

B. Design Principles ......................................................................................................................................................... 59B 100 General................................................................................................................................................................ 59

C. Design Loads................................................................................................................................................................ 59C 100 Mooring loads ..................................................................................................................................................... 59C 200 Green sea............................................................................................................................................................. 59

D. Local scantling requirement ...................................................................................................................................... 60D 100 General................................................................................................................................................................ 60D 200 Quay requirement................................................................................................................................................ 60

E. Hull Girder Longitudinal Strength ........................................................................................................................... 60E 100 General................................................................................................................................................................ 60E 200 Design loading conditions .................................................................................................................................. 60

F. Transverse Strength.................................................................................................................................................... 60F 100 General................................................................................................................................................................ 60F 200 Design loading conditions .................................................................................................................................. 61

G. Local Stress Analysis .................................................................................................................................................. 61G 100 General................................................................................................................................................................ 61G 200 Turret interface and moonpool structure............................................................................................................. 61

H. Fatigue Strength.......................................................................................................................................................... 61H 100 Design loading conditions .................................................................................................................................. 61H 200 Mean stress effect .............................................................................................................................................. 62H 300 Design fatigue factors ......................................................................................................................................... 62H 400 Details to be checked .......................................................................................................................................... 63

I. Special Consideration ................................................................................................................................................. 63I 100 Bilge keels........................................................................................................................................................... 63I 200 Support of mooring equipment, towing brackets etc .......................................................................................... 63I 300 Loading Instrument............................................................................................................................................. 63DET NORSKE VERITAS AS

Offshore Standard DNV-OS-C102, October 2012 Contents Page 9J. Inspection Principles................................................................................................................................................... 63J 100 General................................................................................................................................................................ 63

K. Corrosion Control ....................................................................................................................................................... 63K 100 General................................................................................................................................................................ 63

Sec. 13 LRFD Method................................................................................................................................... 64


B. Hull Girder Longitudinal Strength ........................................................................................................................... 64B 100 Hull girder bending and shear checks................................................................................................................. 64B 200 Hull girder yield check........................................................................................................................................ 65B 300 Hull girder buckling capacity.............................................................................................................................. 65

C. Topside structure and topside interface to hull structure....................................................................................... 65C 100 General................................................................................................................................................................ 65

App. A Conversion of Tanker to Floating Offshore Installation .............................................................. 66

A. Introduction ................................................................................................................................................................ 66A 100 General................................................................................................................................................................ 66

B. Strength........................................................................................................................................................................ 66B 100 General................................................................................................................................................................ 66B 200 Benign waters operation ..................................................................................................................................... 66B 300 Local scantling requirement................................................................................................................................ 67B 400 Transverse strength ............................................................................................................................................. 67B 500 Longitudinal strength .......................................................................................................................................... 67B 600 Topside and topside interface to hull structure ................................................................................................... 67

C. Fatigue.......................................................................................................................................................................... 67C 100 General................................................................................................................................................................ 67C 200 Previous trade ..................................................................................................................................................... 67C 300 Operation ........................................................................................................................................................... 68C 400 Areas to be checked ........................................................................................................................................... 68C 500 Mean stress effect .............................................................................................................................................. 68C 600 Documentation.................................................................................................................................................... 68

App. B Lifetime Extension of Floating Offshore Unit/Installation........................................................... 69

A. Introduction................................................................................................................................................................. 69A 100 General................................................................................................................................................................ 69A 200 Survey extent ...................................................................................................................................................... 69

B. Structural strength...................................................................................................................................................... 69B 100 General................................................................................................................................................................ 69

C. Fatigue.......................................................................................................................................................................... 69C 100 General................................................................................................................................................................ 69C 200 Previous trades .................................................................................................................................................... 69C 300 Operation ............................................................................................................................................................ 69C 400 Areas to be checked ............................................................................................................................................ 70C 500 Mean stress effect ............................................................................................................................................... 70C 600 Documentation.................................................................................................................................................... 70DET NORSKE VERITAS AS

Offshore Standard DNV-OS-C102, October 2012 Sec.1 Page 10SECTION 1INTRODUCTION

A. General

A 100 General101 This standard comprises sections with provisions applicable to all types of offshore floating ship shapedunits, and sections with provisions for specific types of units such as well intervention/drilling units and FPSOs.102 This standard is based on the principles of the Working Stress Design (WSD). In WSD the targetcomponent level is achieved by keeping the calculated stress for different load combinations equal to or lowerthan the maximum stress. The maximum permissible stress is defined by multiplication of the capacity, of thestructural member with permissible usage factors.

A 200 Objectives201 The objectives of this standard are to:

provide an internationally acceptable standard for design of offshore ship-shaped units serve as a technical reference document in contractual matters between purchaser and manufacturer serve as a guideline for designers, purchaser, contractors and regulators specify procedures and requirements for units subject to DNV classification services base the design of the hull and topside on the same principles and methodology for all transit and

operational scenarios provide, as far as possible, consistent loads for both topside and hull design.

A 300 Classification301 Classification principles, procedures and applicable class notations related to classification services ofoffshore units are specified in the DNV Offshore Service Specifications given in Table A1.

302 Documentation for classification shall be in accordance with the NPS DocReq (DNV NauticusProduction System for documentation requirements) and DNV-RP-A201.

B. Assumptions and Applications

B 100 General101 It is assumed that the units will comply with the requirement for retention of the Class as defined in theDNV-OSS-101 or DNV-OSS-102.102 This standard is applicable to hull and topside of ship-shaped offshore units, such as well interventionunits, drilling units, floating production and storage units, constructed in steel for both non-restricted andrestricted operations.103 The environmental loads used for the longitudinal strength calculations for the survival condition isbased on the 10-2 annual probability of exceedance (100 year return period). 104 Shelf State requirements are not covered by this standard.

Guidance note:Governmental regulations may include requirements in excess of the provisions of this standard depending on thetype, location and intended service of the offshore unit or installation. The 100 year return period is used to ensureharmonisation with typical Shelf State requirements and the code for the construction and equipment of mobileoffshore drilling units (MODU code).

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Table A1 DNV Offshore Service Specifications Reference TitleDNV-OSS-101 Rules for Classification of Offshore Drilling and Support Units

DNV-OSS-102 Rules for Classification of Floating Production, Storage and Loading UnitsDET NORSKE VERITAS AS

Offshore Standard DNV-OS-C102, October 2012 Sec.1 Page 11C. Definition

C 100 Verbal forms101 Shall: Indicates a mandatory requirement to be followed for fulfilment or compliance with the presentstandard. Deviations are not permitted unless formally and rigorously justified, and accepted by all relevantcontracting parties.102 Should: Indicates a recommendation that a certain course of action is preferred or particularly suitable.Alternative courses of action are allowable under the standard where agreed between contracting parties butshall be justified and documented.103 May: Indicates a permission, or an option, which is permitted as part of conformance with the standard.

C 200 Terms201 Standard terms are given in DNV-OS-C101.202 Drilling unit: A unit used for drilling in connection with exploration and/or exploitation of oil and gas.The unit is generally operating on the same location for a limited period of time and is normally equipped withdynamic positioning system with several thrusters. The unit follows the normal class survey program.203 Well intervention unit: A unit equipped for performing wire-line intervention (without riser) of subseawells and or coiled tubing of subsea. The unit is generally operating on the same location for a limited periodof time and is normally equipped with dynamic positioning system with several thrusters. The unit follows thenormal class survey program.204 Floating production and offloading unit: A unit used for the production of oil with arrangement foroffloading to a shuttle tanker. The units normally consist of a hull, with turret or spread mooring arrangement,and production facilities above the main deck. The unit can be relocated, but is generally located on the samelocation for a prolonged period of time.205 Floating storage and offloading unit: A unit used for storage of oil with arrangement for offloading to ashuttle tanker. The units normally consist of a hull, with turret or spread mooring system. The unit is equippedfor crude oil storage. The unit can be relocated, but is generally located on the same location for a prolongedperiod of time.206 Floating production, storage and offloading unit: A unit used for the production and storage of oil witharrangement for offloading to a shuttle tanker. The unit is equipped for crude oil storage. The unit is normallymoored to the seabed with production facilities on the main deck. The unit can be relocated, but is normallylocated on the same location for a prolonged period of time.207 Floating production, drilling, storage and offloading unit: A unit used for drilling, storage andproduction of oil with arrangement for offloading to a shuttle tanker. The unit is equipped for crude oil storage. 208 LNG/LPG Floating Production and Storage units: A unit with facilities for oil and gas production andstorage. The unit is typically permanently moored. Due to the complexity of the unit more comprehensivesafety assessment are typically carried out. The unit is normally equipped with solutions for quickdisconnection of mooring lines between the shuttle tanker and the oil and gas producing and storage unit.209 Turret: A device providing a connection point between the unit and the combined riser- and mooring-systems, allowing the unit to freely rotate (weather vane) without twisting the risers and mooring lines.210 Temporary mooring: Anchoring in sheltered waters or harbours exposed to moderate environmentalloads.211 Structural design brief: A document providing criteria and procedures to be adopted in the initial stagesof the design process. The structural design brief should include analytical methods, procedures andmethodology used for the structural design taking all relevant limiting design criteria into account. Ownersadditional specification, if any, should be clearly described in the structural design brief.212 Service life: The expected life time of the unit213 Fatigue life: Service life Design Fatigue Factor (DFF)214 Probability of exceedance:

10-4: Appr. daily return period10-8: Appr. 20 years return period10-8.7: Appr. 100 years return period

215 Annual probability:10-1: 10 years return period10-2: 100 years return periodDET NORSKE VERITAS AS

Offshore Standard DNV-OS-C102, October 2012 Sec.1 Page 12C 300 Symbols301 The following Latin characters are used in this standard:

302 The following Greek characters are used in this standard:

C 400 Abbreviations401 The abbreviations given in Table C3 are used in this standard. Definitions are otherwise given in DNV-OS-C101 Design of Offshore Steel Structures, General (LRFD method).

C 500 References501 The offshore standards and rules given in Table C4 are referred to in this standard.

Table C1 Latin characters used V Speed in knots CW Wave coefficient as given in DNV Rules for Classification of Ships Pt.3 Ch.1 Sec.4 av Vertical accelerationat Transverse accelerational Longitudinal accelerationMwv Vertical wave bending momentMwh Horizontal wave bending momentQwv Vertical wave shear force

Table C2 Greek characters used 0 Basic usage factor Coefficient depending on type of structurep Permissible usage factor

Table C3 Abbreviations Abbreviation In fullDFF Design fatigue factor NDT Non-destructive testing SCF Stress concentration factors WSD Working stress design

Table C4 DNV Offshore Standards, Rules, Classification Notes and Recommended PracticeReference TitleDNV-OS-C101 Design of Offshore Steel Structures, General (LRFD method) DNV-OS-C201 Structural Design of Offshore Units (WSD method) DNV-OS-C401 Fabrication and Testing of Offshore Structures DNV-OS-B101 Metallic Materials DNV-OS-C301 Stability and Watertight IntegrityDNV-RP-C201 Buckling Strength of Plated Structures DNV-RP-C202 Buckling Strength of Shells DNV-RP-C205 Environmental Conditions and Environmental Loads Classification Note 30.7 Fatigue Assessment of Ship Structures DNV-RP-C203 Fatigue Strength Analysis of Offshore Steel Structures DNV-RP-C206 Fatigue Methodology of Offshore ShipsDNV-OS-E301 Position MooringDET NORSKE VERITAS AS

Offshore Standard DNV-OS-C102, October 2012 Sec.2 Page 13SECTION 2MATERIAL SELECTION AND INSPECTION PRINCIPLES

A. IntroductionA 100 General101 This section describes the selection of steel materials and inspection principles to be applied in designand construction of offshore ship-shaped units.

B. Selection of MaterialB 100 General101 A material specification shall be established for all structural materials. The materials shall be suitablefor their intended purpose and have adequate properties in all relevant design conditions.102 In structural cross-joints where high tensile stresses are acting perpendicular to the plane of the plate, theplate material shall be tested according to Rules for Classification of Ships Pt.2 Ch.2 Sec.1 to prove the abilityto resist lamellar tearing (Z-quality). Continuous deck plate under crane pedestal shall have Z-quality steel withminimum extension of 500 mm. If the pedestal is continuous through the deck plate within 0.6 L amidship, thepedestal plate shall have Z-quality steel with minimum extension of 500 mm above and below the deck.103 For stiffeners, the grade of material may be determined based on the thickness of the web.104 Structural elements used only in temporary conditions, e.g. lifting lugs and pad eyes, are not part of theclass scope and thus not covered in this standard.105 Longitudinal members not contributing to the hull girder longitudinal strength and welded to the strengthdeck or bottom plating and bilge strake, such as longitudinal hatch coamings, gutter bars, strengthening of deckopenings, bilge keel, are to be made of steel with the same yield and grade as the strength deck or bottomstructure steel.106 Doubling plates welded to the main strength deck or bottom structure within 0.6 L shall be made of steelwith the same yield and grade as the main plate.

Guidance note:Doubling plates should generally to be avoided if the doubler is exposed for high tensile force perpendicular to theconnecting main plate. Small doublers for pipe supports, small foundations, etc. may be accepted with lower yield andgrade than the base material but shall be special considered for each individual case.


B 200 Design and service temperatures 201 The design temperature for a unit is the reference temperature in air for assessing areas where the unitcan be transported, installed and operated. The design temperature is to be lower or equal to the lowest meandaily average temperature in air for the relevant areas. For seasonal restricted operations the lowest mean dailyaverage temperature in air for the season may be applied for external structures above the lowest ballastwaterline shall be set equal to the design temperature for the area(s) in which the unit is specified to operate.202 The service temperature is a reference temperature on various structural parts of the unit used as acriterion for the selection of steel grades.203 The service temperature for external structures above the lowest ballast waterline shall be set equal tothe design temperature for the area(s) in which the unit is specified to operate. External structure is defined,with respect to design temperature, as the plating with stiffening to an inwards distance of 0.6 metre from theshell plating.204 The service temperature for external structures below the lowest ballast waterline needs normally not tobe set lower than 0C.205 The service temperature for internal structures in way of permanently heated rooms needs normally notto be set lower than 0C. Crude oil tanks may be considered to be permanently heated.206 The service temperature for internal structures in oil storage tanks need normally not to be set lower than0C except for the upper strake in longitudinal bulkheads and top wing tanks.

B 300 Hull structure301 The material grade shall be selected according to the DNV Rules for Classification of Ships Pt.3 Ch.1Sec.2 if the design temperature is equal or above -10C based on lowest mean daily average temperature.302 Lower Design Temperatures than -10C may be specified. The requirements for material class and gradeshall then be based on the DNV Rules for Classification of Ships Pt.5 Ch.1 Sec.7.DET NORSKE VERITAS AS

Offshore Standard DNV-OS-C102, October 2012 Sec.2 Page 14303 In the Rules for Classification of Ships Pt.3 Ch.1 materials are categorized into Material Classes. Thepurpose of the structural categorization is to ensure adequate material and suitable inspection to avoid brittlefracture, and to ensure sufficient fracture resistance of a material (stress intensity factor) to avoid crack sizeswhich may develop into brittle fracture at certain stress situations.

B 400 Topside structure and elements not covered by Rules for Classification of Ships Pt.3 Ch.1401 Structural members are classified into Material Classes according to the following criteria:

significance of member in terms of consequence of failure stress condition at the considered detail that together with possible weld defects or fatigue cracks may

provoke brittle fracture.

Guidance note:The consequence of failure may be quantified in terms of residual strength of the structure when considering failureof the actual component.


402 Materials for topside modules, topside supporting structures, foundations and main supporting structuresof heavy equipment attached to deck and hull shall be selected according to the principles in B300. 403 Materials for the specific structural members are not to be of lower grade than those corresponding to thematerial classes specified in Table B1.

404 Alternatively materials for the specific structural members specified in Table B1 may be selected basedon equivalent structural category in accordance with the principles given in DNV-OS-C101. 405 Selection of steel grade above the minimum required will not lead to stricter requirements duringfabrication.

Table B1 Material ClassesMaterial class

Structural member Equivalent structural category in the DNV-OS-C101

I

Outfitting steel Mezzanine decks, platforms Pipe support structure Laydown platforms Doubler plates, closer plates and support infill steels in topside structures 1) Foundation attached to decks (excl. main deck) where weight of

equipment < 50 ton Secondary

II

Stair towers Topside modules (plate, web frame, girder) 2) Main structures in riser hold 4) Longitudinal and transverse bulkheads in way of moonpool and setback area Offshore Crane boom rest and main supporting structures Foundation attached to main deck where weight of equipment < 50 ton Foundation attached to other deck where weight of equipment > 50 ton

III

Main girders and columns in truss work type modules Topside support stools and main supporting structures Pipe/riser rack stanchions Main structures in drill-floor and substructure 4) Main supporting structures for helideck 3) 4) Crane pedestal (shipboard and offshore) Flare tower Foundations and main supporting structures of heavy machinery and

equipment: e.g. thruster, gantry and rail, chain stoppers, offloading riser fairleads, anchor line fairleads, winches, davits, hawser brackets for shuttle tanker, towing brackets etc.

Foundation attached to main deck where weight of equipment > 50 ton

Primary

IV

Deck and bottom corner plates in way of moonpool Foundations and main supporting structures for derrick (substructure) Foundations and main supporting structures for flare tower and offshore

crane pedestal Main supporting structures for turret 4)

Special

1) To have the same minimum yield strength as the material to which they are attached2) Stiffener and other local members (brackets, collar plates, web stiffeners) are classified as Class I.3) For material selection of helideck and substructure, see DNV-OS-E4014) Main structures and main supporting structures are primary load bearing members such as plates, girders, web frames/bulkheads

and pillarsDET NORSKE VERITAS AS

Offshore Standard DNV-OS-C102, October 2012 Sec.2 Page 15C. Inspection PrinciplesC 100 General101 The purpose of inspection is to detect defects that may grow into fatigue cracks during service life. 102 When determining the locations of required non-destructive testing (NDT), consideration should begiven to relevant fabrication parameters including:

location of block (section) joints manual versus automatic welding start and stop of weld.

C 200 Hull structure 201 For the inspection principles for hull structure, see unit specific provisions Sec.11 and Sec.12.

C 300 Topside structure and elements not covered by hull structure requirements301 Fabrication and testing of topside structure shall comply with the requirements in DNV-OS-C401. Therequirements are based on the consideration of fatigue damage and assessment of general fabrication quality. 302 The inspection categories are related to the material class and structural categories as shown in Table C1.

303 The weld connection between two components shall be assigned inspection category according to thehighest of the joined components. For stiffened plates, the weld connection between the plate and stiffener,stringer, and girder web to the plate may be inspected according to inspection category III. 304 If the fabrication quality is assessed by testing, the extent of inspection required for elements withinmaterial class III may be reduced, but not less than for inspection category III. 305 Critical details within material class II or III shall be inspected according to requirements in inspectioncategory I.

Guidance note:Critical details are localized attached structures such as supports of which failure is critical for the overall safety orpersonnel.


306 Welds in fatigue critical areas not accessible for inspection and repair during operation shall be inspectedaccording to requirements in inspection category I.307 The extent of NDT for welds in block joints and erection joints transverse to main stress direction shallnot be less than for inspection category II. 308 Topside stools, or topside - hull connections, material class III, shall be inspected according to therequirements in inspection category I for the areas shown in Figure 1. 309 Inspection categories for offshore crane pedestals connection to deck are given in Figure 2.

Table C1 Inspection categoriesInspection Category Material Class Equivalent structural category in the DNV OS- standards

I IV SpecialII III PrimaryIII I, II SecondaryDET NORSKE VERITAS AS

Offshore Standard DNV-OS-C102, October 2012 Sec.2 Page 16Where:Length 'a' to be 0.35l, minimum 120 mm'a' does not need to be longer than 500 mm

Figure 1 Inspection categories for topside stool with soft nose brackets

Figure 2 Inspection categories for offshore crane pedestalDET NORSKE VERITAS AS

Offshore Standard DNV-OS-C102, October 2012 Sec.3 Page 17SECTION 3DESIGN PRINCIPLES

A. IntroductionA 100 Overall design principles101 This section defines the principles for design of the hull, topside structures and topside supportstructures.102 The overall principles are based on the following:

safety of the structure can be demonstrated by addressing the potential structural failure mode(s) when theunit is subjected to loads scenarios encountered during transit, operation and in harbour.

structural requirements are based on a consistent set of loads that represent typical worst possible loadingscenarios

unit has inherent redundancy. The units structure works in a hierarchical manner and as such, failure ofstructural elements lower down in the hierarchy should not result in immediate consequential failure ofelements higher up in the hierarchy

structural continuity is ensured. The hull, topside structures and topside interface to the hull structureshould have uniform ductility.

B. Design ConditionsB 100 Modes of operation 101 All relevant modes of operation shall be considered. Typically, the assessment of the unit shall be basedon the following operational modes:

all transit conditions all operating conditions, intact and damaged, at the design location(s) all inspection and repair conditions.

102 Changes in the design conditions of offshore ship-shaped units are usually accompanied by significantchanges in draught, ballast, riser connections, mooring line tension, etc. Limited variation of some of theseparameters may be contained within a specific design condition.103 The suitability of offshore ship-shaped units is dependent on the environmental conditions in the areasof the intended operation. A well intervention/drilling unit may be intended for World Wide operation oroperation in a specific region or site(s). A production unit may be planned to operate at a specific site. Such asite may be harsh environment or benign waters.

B 200 Transit and non-operational conditions201 Unrestricted transit is defined as moving the unit from one geographical location to another, and shall bebased on the DNV Rules for Classification of Ships Pt.3 Ch.1.202 Non-operational conditions like e.g. survey condition are considered to be covered by DNV Rules forClassification of Ships Pt.7.203 The design accelerations for the topside structures and topside interface to hull may be taken from eithera wave load analysis or DNV Rules for Classification of Ships Pt.3 Ch.1.

B 300 Operating conditions301 Operating conditions are defined as conditions wherein a unit is on location for purposes of production,drilling or other similar operations, and combined environmental and operational loadings are within theappropriate design limits established for such operations.302 The maximum allowable sea state (max Hs) shall be established based on the unit's criteria for abortingthe operation.The sea state shall be stated in the structural design brief document.303 The operating condition shall account for the combination of wave effects and wind effects.304 All the operating limitation used for the design and safe operation shall be stated in the operating manual.

B 400 Survival condition401 A condition during which a unit may be subjected to the most severe environmental loadings for whichthe unit is designed. Drilling or similar operations may have been discontinued due to the severity of theenvironmental loadings. The unit may be either afloat or supported on the sea bed, as applicable.DET NORSKE VERITAS AS

Offshore Standard DNV-OS-C102, October 2012 Sec.3 Page 18402 The survival condition shall account for the combination of wave effects and wind effects. 403 If the unit intends to leave the site and seek for sheltered waters in extreme weather, the survivalcondition may be considered as an accidental condition.

B 500 Benign waters or harsh environmental areas501 If the unit is intended to operate in benign waters, the strength requirements given in B400 for thesurvival condition may be omitted. The longitudinal strength requirement for the Transit condition accordingto DNV Ship Rules Pt.3 Ch.1 Sec.5 shall be complied with. Operation conditions may still be relevant designconditions for benign waters areas and shall be defined if relevant. 502 The Benign waters criteria are defined in Table B1.

a) If Hs100 year < 8.0 m (or 10.0 m depending on the ship length) is specified and documented for the actualsite specific location, no wave load analysis is required to demonstrate that the actual area is benign waters.

b) If Hs100 year > 8.0 m (or 10.0 m depending on the ship length) or if no sufficient information of actual Hs100 yearfor actual location exists, the MwSite-100year need to be determined by using a wave load analysis for the survivalcondition as described in Table B2 in order to demonstrate that MwRule-20 year > MwSite-100year, i.e. Benignwaters

If neither the a) or the b) requirements given above are satisfied, the unit is defined to operate in harshenvironmental area and not in benign waters.

Guidance note:The significant wave height (HsSite-100year) for the survival condition can be estimated using 2-parameter Weibullparameters (s s) for different scatter diagram as present in DNV-RP-C205 Appendix C together with the formula:

where N is the number of maxima for the sea state in a time period t. N = t/ and is the is the duration of eachshort-term variation (normally taken as 3 hours). E.g. for a time period of 100 year the value N is then N=10036524/3= 292000.


B 600 Wave load analysis601 The wave load analysis may be carried out for operating conditions with specific wave environments atthe considered site, and may also be carried out for transit conditions as alternative to the requirements givenin the DNV Rules for Classification of Ships Pt.3 Ch.1 Sec.5.602 The design loading conditions for ultimate strength shall be based on the units loading manual and shallin addition include part load conditions as relevant for the specific type of unit. The most severe loadingconditions defined in the loading manual shall be used as basis for the wave load analysis. 603 If short term response analysis is carried out for ultimate strength, the combination of significant waveheight (Hs) and spectral peak period (Tp) or zero-crossing period (Tz) for all sea states along a contour lineshall be considered. The most probable largest (MPL) value may be used for calculating the wave responses.

Table B1 Benign waters criteriaRule ship length

L= 100 m L > 200 m

Hs100 year 8.0m or whenMwRule-20 year > MwSite-100year

Hs100 year 10.0m or whenMwRule-20 year > MwSite-100year

Between L=100m and L=200m, linear interpolation may be used.

Hs100 year: Significant wave height at site specific with a 100 year return period

MwRule-20 year: Rule wave bending moment based on a 20 year return period in the North Atlantic

MwSite-100year: 100 years linear wave bending moment at the specified location

DET NORSKE VERITAS AS

Offshore Standard DNV-OS-C102, October 2012 Sec.3 Page 19Guidance note:The MPL value corresponds to the 37% percentile, i.e. 63% probability of exceedance.


604 The wave heading profile given in Table B2 and B3 should be used for the ultimate strength and fatiguestrength respectively, unless otherwise documented.605 The sectional loads are normally calculated at the neutral axis of the section considered. 606 The wave shear forces shall be determined at a sufficient number of sections along the hull to fullydescribe the limit curve for the maximum value.

607 Non-linear correction factor shall be included for the survival condition when using the wave bendingmoments and shear forces from a linear wave load analysis. Typical non-linear correction factors for ships withtraditional hull shape are given in Table B4 and shall be used, unless otherwise documented.

Guidance note:For areas exposed to normal winter storms the increase of Hs between the 100 year storm and the 10 000 year stormis normally less than 25% which is the difference between the acceptance criteria between condition c) and conditionb) defined in Table C1. Condition b) is then normally governing and condition c) may be omitted.


Table B2 Design basis of wave load analysis for ultimate strengthTransit Operation4) Survival

Wave environment North Atlantic based on a 20 year return period

Maximum significant wave height (Hs) and corresponding period (Tp or Tz) defined by the project

Site specific based on a 100 year return period 5)

Wave spectrum PM spectrum Specified spectrum 1) Specified spectrum 1)Wave heading profile All headings included

(0 to 360)Head sea: 60% 2) 15: 30% 30: 10%

Head sea: 60% 2) 15: 30% 30: 10%

Wave spreading Cos 2 Cos 2 None3)1) JONSWAP spectrum is normally used, ref. DNV-RP-C2052) For weather vanning units. Other heading profile for operation and survival condition may be used, if documented. For units with

spread mooring arrangement, all heading with same probability may be considered3) Accelerations for topside interface analysis shall be based on Cos 2 wave spreading4) Operation is mainly related to drilling operation e.g. max sea state for drilling operation, stand-by condition, etc.5) For units intended for unrestricted service (World Wide operation), North Atlantic scatter diagram shall be used.

Table B3 Design basis of wave load analysis for fatigue strengthTransit Operation

Wave environment World Wide Site specific3)

Probability of exceedance 10-4 10-4

Wave spectrum PM spectrum PM spectrumWave heading profile All headings included

(0 to 360)Head sea: 60%2) 15: 30% 30: 10%

Wave spreading 1) Cos 2 Cos 2

1) Cos 2 shall be used, unless otherwise documented.2) For weather- vaning units. Other heading profile for operation condition may be used, if documented. For units with spread

mooring arrangement, all heading with same probability may be considered3) For units intended for unrestricted service (World Wide operation), the World Wide scatter diagram should be used provided dry

docking of the unit every 5th year. Normally only relevant for drill and well intervention units.

Table B4 Non-linear correction factor for ultimate strengthItem Sagging HoggingWave bending moment 1.1 0.9Wave shear force 1.1 0.9DET NORSKE VERITAS AS

Offshore Standard DNV-OS-C102, October 2012 Sec.3 Page 20C. Working Stress Design, WSDC 100 General 101 The WSD principles and the acceptance criteria for the different combinations are applicable to hull andtopside structure for direct calculations when the structural requirements are not covered by DNV Rules forClassification of Ships Pt.3 Ch.1, or if direct calculations are used to replace the structural requirements in theRules for Classification of Ships Pt.3 Ch.1.

C 200 Load conditions201 Each structural member shall be designed for the most unfavourable of the loading conditions given inTable C1.

C 300 Permissible usage factor 301 The permissible usage factors depend on load combination, failure mode and importance of strengthmember.302 The usage factor is defined as the ratio between the calculated nominal stress and the correspondingminimum specified yield stress of the material used. The calculated usage factor based on the von Misesequivalent membrane stress at centre of a plane element (shell or membrane) shall not exceed the permissibleusage factors defined in 303.303 The permissible usage factor, p, is calculated by:

p = 0

The basic usage factor 0 accounts for:

possible unfavourable deviations of specified or expected loads uncertainties in the model and analysis used for determination of load effects possible unfavourable deviations in the resistance of materials possible reduced resistance of the materials in the structure, as a whole, as compared to the values deduced

from test specimens deviation from calculated strength resistance due to fabrication.

Guidance note:The von Mises equivalent stress is defined as follows:

Where x and y are element membrane stresses in x- and y-direction respectively, is element shear stress in the x-y plane, i.e. local bending stresses in plate thickness not included.


C 400 Usage factor for peak stress 401 The peak usage factor is defined as the ratio between the calculated peak stress and the correspondingminimum specified yield stress of the material used.402 Local peak stress by fine mesh Finite Element (FE) analysis in areas with pronounced geometricalchanges, such as moonpool corners, frame corners etc., may exceed the permissible usage factor in C201provided plastic mechanisms are not developed in the adjacent structural parts.403 Local peak stress criteria are based on the element mesh size of either 50 50 mm or 100 100 mm or200 200 mm depending on the actual thickness and geometrical complexity of local details to be checked.

Table C1 Load conditionsConditions Description Basic usage factor, 0

a) static loads 0.60b) maximum combined static and dynamic loads 0.80c) accidental loads and associated static loads 1.00

Notes:Condition a) and b) represent operational and survival conditions.Condition c) represents accidental conditions with low probability of occurrence such as explosions, fire, dropped objects etc.

where:0 = basic usage factor, see Table C1 = 1.0 for hull structure. For topside structure, see Sec.6

222 3 ++= xxyxeqDET NORSKE VERITAS AS

Offshore Standard DNV-OS-C102, October 2012 Sec.3 Page 21The calculated usage factor based on the von Mises equivalent membrane stress at centre of a plane element(shell or membrane) shall not exceed the permissible peak usage factors given in Table C2.

peak = peak Material yield stress

404 For fatigue critical connections local peaks are not accepted, ref. Sec.8.

Table C2 Permissible peak usage factor (Peak) for fine mesh FE analysisStructural

component 1)Designmethod Load combination

Mesh size50 50 mm 100 100 mm 200 200 mm

Hull in general Ship Rules Static + Dyn.(10-4 level) 2) 1.53 1.33 1.13

Hull and topside interface structures

WSD method Static 1.36 1.18 1.0Static + Dyn.

(10-8 or 10-8.7 level) 2) 1.7 1.48 1.25

1) For details to be checked, see Sec.5 E2) See Sec.1 C200 for definition.

Note:

Load level is to be consistent according to the applied design method and load combination. The peak stress criteria may also be used if using the LRFD method described in Sec.13.DET NORSKE VERITAS AS

Offshore Standard DNV-OS-C102, October 2012 Sec.4 Page 22SECTION 4DESIGN LOADS

A. IntroductionA 100 General101 The requirements in this section define and specify load components to be considered in the overallstrength analysis as well as design pressures applicable for local scantling design. 102 Design load criteria given by operational requirements shall be fully considered. Examples of suchrequirements may be:

drilling, production, workover and combinations thereof consumable re-supply procedures and frequency maintenance procedures and frequency possible load changes in most severe environmental conditions.

B. Static LoadsB 100 General101 The still water loads consist of the permanent and variable functional loads.102 Permanent functional loads relevant for offshore units are:

mass of the steel of the unit including permanently installed modules and equipment, such asaccommodation, helicopter deck, cranes, drilling equipment, flare and production equipment.

mass of mooring lines and risers.

103 Variable functional loads are loads that may vary in magnitude, position and direction during the periodunder consideration.104 Typical variable functional loads are:

hydrostatic pressures resulting from buoyancy crude oil ballast water fuel oil consumables personnel general cargo riser tension. mooring forces mud, brine and drill water.

105 The variable functional loads utilised in structural design shall normally be taken as either the lower orupper design value, whichever gives the more unfavourable effect.106 Variations in operational mass distributions (including variations in tank filling conditions) shall beadequately accounted for in the structural design.

B 200 Still water hull girder loads201 All relevant still water load conditions shall be defined and permissible limit curves for hull girderbending moments and shear forces shall be established for transit and operating condition separately. 202 The permissible limits for hull girder still water bending moments and hull girder still water shear forcesshall be given at least at each transverse bulkhead position and be included in the loading manual. Separatelimits will normally be given for sagging and hogging bending moments, and positive and negative shearforces.203 Actual still water shear forces shall be corrected for structural arrangement acc

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