OFFSHORE STANDARD

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

DNV-OS-C201

Structural Design of Offshore Units (WSD Method)

APRIL 2012DET NORSKE VERITAS AS

FOREWORDDET NORSKE VERITAS (DNV) is an autonomous and independent foundation with the objectives of safeguarding life,property and the environment, at sea and onshore. DNV undertakes classification, certification, and other verification andconsultancy services relating to quality of ships, offshore units and installations, and onshore industries worldwide, andcarries out research in relation to these functions.

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 Systems Det Norske Veritas AS April 2012

Any comments may be sent by e-mail to rules@dnv.com

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-C201, April 2012Changes Page 3CHANGES

GeneralThis document supersedes DNV-OS-C201, April 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 ChangesGeneralUpdated references to latest DNV-RP-C104 instead of DNV-CN 31.5.Minor updates and some clarification in text.

Section 1 C: Definitions: C231: added Heave Damping Structure: Structure to increase added mass in heave and reduce the vertical

motions of the Deep Draught Semi units (DDS). Table D1: added DDS - Deep draught semi-submersible unit.

Section 3 D400: Lifeboat platforms in previous edition of document deleted.

Section 11, C700: Guidance note with reference to DNV-RP-C205 included. D104: Guidance note included.

Section 12 B302: Material Grade NV690 E is sufficient for thick rack plates. C403: Guidance note for dynamic amplification factors DAF 1.10. D202: Wind (profile and unrestricted service). F1100: Vertical position of heavy equipment during transit (Guidance note). F1200: New section on Simplified P effect (including Guidance note). G304: Spudcan design loads updated.

Section 14 A103: included examples specific to Deep Draught Semi (DDS) units for Structural categorization, A205: added Special connections, for example, grouted connections shall be fully qualified. The process

as presented in DNV-RP-A203 Qualification of New Technology can be used. C102: included guidance specific to Deep Draught Semi (DDS) units and Spars for Structural categorization.

C500: Guidance to Minimum Requirements: updated Figure 1 and included Figures 2 and 3 to captureexamples specific to DDS units.

D202: added mating, ballasting sequences to include DDS units. D203: changed to be specific to Spars. D204: added requirements specific to DDS units. D506: added Air gap and green water (Wave-on-Deck) are to be considered. Air gap is measured to the

bottom of the deck structure. I103: deleted list of DDF units eigenmodes. J102: added Relevant model testing should be considered for novel installation procedures. J103: added Stability check and ballast capacity design shall include assessment of relevant load

conditions during transport and installation. J301: changed text to be specific to Spars. K204: changed text to be specific to Spars, K205: added For DDS units, the finite element analysis shall cover all critical transient phases during

installation. L101: in previous edition of document is deleted. L204: included DDS specific connections. L207: added For mooring fairlead/riser porch analysis, the effect of disturbed wave field due to the

presence of columns etc. should also be taken into account. L209: added The fatigue analysis to riser/keel guide frames shall account for the interaction between risers

and guide frames where relevant.DET NORSKE VERITAS AS

L210: added text and Table L1 - to be in line with API requirements. M605: added The structure should be designed to withstand one compartment flooding under reduced

environment (e.g. 10-yr return probability).

Offshore Standard DNV-OS-C201, April 2012Changes Page 4CHANGES

M700: Riser Damage added. Subsection N: Dynamic Riser Design and Global Performance added.DET NORSKE VERITAS AS

Offshore Standard DNV-OS-C201, April 2012 Contents Page 5CONTENTS

Sec. 1 Introduction ...................................................................................................................................... 13

A. General .......................................................................................................................................................................... 13A 100 Introduction......................................................................................................................................................... 13A 200 Objectives ........................................................................................................................................................... 13A 300 Scope and application ......................................................................................................................................... 13A 400 Other than DNV codes........................................................................................................................................ 13A 500 Classification ...................................................................................................................................................... 14

B. References ..................................................................................................................................................................... 14B 100 General................................................................................................................................................................ 14

C. Definitions..................................................................................................................................................................... 15C 100 Verbal forms ....................................................................................................................................................... 15C 200 Terms .................................................................................................................................................................. 15

D. Abbreviations and Symbols .......................................................................................................................................... 18D 100 Abbreviations...................................................................................................................................................... 18D 200 Symbols .............................................................................................................................................................. 19

Sec. 2 Design Principles .............................................................................................................................. 22

A. Introduction................................................................................................................................................................... 22A 100 General................................................................................................................................................................ 22A 200 Aim of the design................................................................................................................................................ 22

B. General Design Considerations..................................................................................................................................... 22B 100 General................................................................................................................................................................ 22B 200 Overall design ..................................................................................................................................................... 22B 300 Details design...................................................................................................................................................... 22

C. Design Conditions......................................................................................................................................................... 23C 100 Basic conditions .................................................................................................................................................. 23

D. Loading Conditions....................................................................................................................................................... 23D 100 General................................................................................................................................................................ 23D 200 Load .................................................................................................................................................................... 24

E. Design by the WSD Method ......................................................................................................................................... 24E 100 Permissible stress and usage factors ................................................................................................................... 24E 200 Basic usage factors.............................................................................................................................................. 24

F. Design Assisted by Testing........................................................................................................................................... 24F 100 General................................................................................................................................................................ 24F 200 Full-scale testing and observation of performance of existing structures........................................................... 24

Sec. 3 Loads and Load Effects ................................................................................................................... 25

A. Introduction................................................................................................................................................................... 25A 100 General................................................................................................................................................................ 25

B. Basis for Selection of Loads ......................................................................................................................................... 25B 100 General................................................................................................................................................................ 25

C. Permanent Functional Loads......................................................................................................................................... 26C 100 General................................................................................................................................................................ 26

D. Variable Functional Loads ............................................................................................................................................ 26D 100 General................................................................................................................................................................ 26D 200 Variable functional loads on deck areas ............................................................................................................. 27D 300 Tank pressures .................................................................................................................................................... 27

E. Environmental Loads .................................................................................................................................................... 28E 100 General................................................................................................................................................................ 28E 200 Environmental conditions for mobile units ........................................................................................................ 28E 300 Environmental conditions for site specific units................................................................................................. 28E 400 Determination of hydrodynamic loads................................................................................................................ 29E 500 Wave loads.......................................................................................................................................................... 29E 600 Wave induced inertia forces................................................................................................................................ 29E 700 Current ................................................................................................................................................................ 30E 800 Wind loads .......................................................................................................................................................... 30DET NORSKE VERITAS AS

E 900 Vortex induced oscillations ................................................................................................................................ 30

Offshore Standard DNV-OS-C201, April 2012 Contents Page 6E 1000 Water level and tidal effects ............................................................................................................................... 30E 1100 Marine growth..................................................................................................................................................... 30E 1200 Snow and ice accumulation ................................................................................................................................ 30E 1300 Direct ice load ..................................................................................................................................................... 31E 1400 Earthquake .......................................................................................................................................................... 31

F. Combination of Environmental Loads .......................................................................................................................... 31F 100 General................................................................................................................................................................ 31

G. Accidental Loads........................................................................................................................................................... 32G 100 General................................................................................................................................................................ 32

H. Deformation Loads ....................................................................................................................................................... 32H 100 General................................................................................................................................................................ 32H 200 Temperature loads............................................................................................................................................... 32H 300 Settlements and subsidence of sea bed ............................................................................................................... 32

I. Fatigue loads ................................................................................................................................................................. 32I 100 General................................................................................................................................................................ 32

J. Load Effect Analysis..................................................................................................................................................... 33J 100 General................................................................................................................................................................ 33J 200 Global motion analysis ....................................................................................................................................... 33J 300 Load effects in structures and soil or foundation................................................................................................ 33

Sec. 4 Structural Categorisation, Material Selection and Inspection Principles .................................. 34

A. General .......................................................................................................................................................................... 34A 100 ............................................................................................................................................................................ 34

B. Temperatures for Selection of Material ........................................................................................................................ 34B 100 General................................................................................................................................................................ 34B 200 Floating units ...................................................................................................................................................... 34B 300 Bottom fixed units .............................................................................................................................................. 34

C. Structural Category ....................................................................................................................................................... 34C 100 General................................................................................................................................................................ 34C 200 Selection of structural category .......................................................................................................................... 35C 300 Inspection of welds ............................................................................................................................................. 35

D. Structural Steel ............................................................................................................................................................. 36D 100 General................................................................................................................................................................ 36D 200 Material designations .......................................................................................................................................... 36D 300 Selection of structural steel................................................................................................................................. 37D 400 Fracture mechanics (FM) testing ........................................................................................................................ 38D 500 Post weld heat treatment (PWHT) ...................................................................................................................... 38

Sec. 5 Structural Strength .......................................................................................................................... 39

A. General .......................................................................................................................................................................... 39A 100 General................................................................................................................................................................ 39A 200 Structural analysis............................................................................................................................................... 39A 300 Ductility .............................................................................................................................................................. 39A 400 Yield check ......................................................................................................................................................... 40A 500 Buckling check ................................................................................................................................................... 40

B. Flat Plated Structures and Stiffened Panels .................................................................................................................. 40B 100 Yield check ......................................................................................................................................................... 40B 200 Buckling check ................................................................................................................................................... 41B 300 Capacity checks according to other codes .......................................................................................................... 41

C. Shell Structures ............................................................................................................................................................. 41C 100 General................................................................................................................................................................ 41

D. Tubular Members, Tubular Joints and Conical Transitions.......................................................................................... 41D 100 General................................................................................................................................................................ 41

E. Non-Tubular Beams, Columns and Frames.................................................................................................................. 42E 100 General................................................................................................................................................................ 42

Sec. 6 Section Scantlings ............................................................................................................................. 43

A. General .......................................................................................................................................................................... 43DET NORSKE VERITAS AS

A 100 Scope................................................................................................................................................................... 43

Offshore Standard DNV-OS-C201, April 2012 Contents Page 7B. Strength of Plating and Stiffeners ................................................................................................................................. 43B 100 Scope................................................................................................................................................................... 43B 200 Minimum thickness............................................................................................................................................. 43B 300 Bending of plating .............................................................................................................................................. 43B 400 Stiffeners ............................................................................................................................................................. 44

C. Bending and Shear in Girders ....................................................................................................................................... 45C 100 General................................................................................................................................................................ 45C 200 Minimum thickness............................................................................................................................................. 45C 300 Bending and shear............................................................................................................................................... 45C 400 Effective flange................................................................................................................................................... 45C 500 Effective web ...................................................................................................................................................... 45C 600 Strength requirements for simple girders............................................................................................................ 46C 700 Complex girder systems...................................................................................................................................... 47

Sec. 7 Fatigue ............................................................................................................................................... 48

A. General .......................................................................................................................................................................... 48A 100 General................................................................................................................................................................ 48A 200 Design fatigue factors ......................................................................................................................................... 48A 300 Methods for fatigue analysis............................................................................................................................... 49A 400 Simplified fatigue analysis.................................................................................................................................. 49A 500 Stochastic fatigue analysis .................................................................................................................................. 50

Sec. 8 Accidental Conditions ...................................................................................................................... 51

A. General .......................................................................................................................................................................... 51A 100 General................................................................................................................................................................ 51

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

Sec. 9 Weld Connections............................................................................................................................. 53

A. General .......................................................................................................................................................................... 53A 100 Scope................................................................................................................................................................... 53

B. Types of Welded Steel Joints........................................................................................................................................ 53B 100 Butt joints............................................................................................................................................................ 53B 200 Tee or cross joints ............................................................................................................................................... 53B 300 Slot welds............................................................................................................................................................ 55B 400 Lap joint .............................................................................................................................................................. 55

C. Weld Size ...................................................................................................................................................................... 55C 100 General................................................................................................................................................................ 55C 200 Fillet welds.......................................................................................................................................................... 56C 300 Partly penetration welds and fillet welds in cross connections subject to high stresses..................................... 56C 400 Connections of stiffeners to girders and bulkheads, etc. .................................................................................... 57C 500 End connections of girders.................................................................................................................................. 58C 600 Direct calculation of weld connections............................................................................................................... 58

Sec. 10 Corrosion Control ............................................................................................................................ 60

A. General .......................................................................................................................................................................... 60A 100 Scope................................................................................................................................................................... 60

B. Techniques for Corrosion Control Related to Environmental Zones ........................................................................... 60B 100 Atmospheric zone ............................................................................................................................................... 60B 200 Splash zone ......................................................................................................................................................... 60B 300 Submerged zone.................................................................................................................................................. 61B 400 Internal zone........................................................................................................................................................ 61

C. Cathodic Protection....................................................................................................................................................... 62C 100 General................................................................................................................................................................ 62C 200 Galvanic anode systems...................................................................................................................................... 62C 300 Impressed current systems .................................................................................................................................. 63

D. Coating Systems............................................................................................................................................................ 64DET NORSKE VERITAS AS

D 100 Specification of coating ...................................................................................................................................... 64

Offshore Standard DNV-OS-C201, April 2012 Contents Page 8Sec. 11 Special Considerations for Column Stabilised Units .................................................................... 65

A. General .......................................................................................................................................................................... 65A 100 Assumptions and application .............................................................................................................................. 65

B. Structural Categorisation, Material Selection and Inspection Principles...................................................................... 65B 100 General................................................................................................................................................................ 65B 200 Structural categorisation ..................................................................................................................................... 65B 300 Material selection................................................................................................................................................ 66B 400 Inspection categories........................................................................................................................................... 66

C. Design and Loading Conditions.................................................................................................................................... 69C 100 General................................................................................................................................................................ 69C 200 Permanent loads .................................................................................................................................................. 69C 300 Variable functional loads .................................................................................................................................... 69C 400 Tank loads........................................................................................................................................................... 70C 500 Environmental loads, general.............................................................................................................................. 70C 600 Sea pressures....................................................................................................................................................... 70C 700 Wind loads .......................................................................................................................................................... 71C 800 Heavy components.............................................................................................................................................. 71C 900 Combination of loads.......................................................................................................................................... 71

D. Structural Strength ........................................................................................................................................................ 71D 100 General................................................................................................................................................................ 71D 200 Global capacity ................................................................................................................................................... 71D 300 Transit condition ................................................................................................................................................. 72D 400 Method of analysis.............................................................................................................................................. 72D 500 Air gap ................................................................................................................................................................ 73

E. Fatigue........................................................................................................................................................................... 73E 100 General................................................................................................................................................................ 73E 200 Fatigue analysis................................................................................................................................................... 73

F. Accidental Conditions................................................................................................................................................... 74F 100 General................................................................................................................................................................ 74F 200 Collision.............................................................................................................................................................. 74F 300 Dropped objects .................................................................................................................................................. 74F 400 Fire ...................................................................................................................................................................... 74F 500 Explosion ............................................................................................................................................................ 74F 600 Heeled condition ................................................................................................................................................. 74

G. Redundancy................................................................................................................................................................... 75G 100 General................................................................................................................................................................ 75G 200 Brace arrangements............................................................................................................................................. 75

H. Structure in Way of a Fixed Mooring System .............................................................................................................. 75H 100 Structural strength............................................................................................................................................... 75

I. Structural Details........................................................................................................................................................... 75I 100 General................................................................................................................................................................ 75

Sec. 12 Special Considerations for Self-Elevating Units............................................................................ 76

A. Introduction................................................................................................................................................................... 76A 100 Scope and application ......................................................................................................................................... 76

B. Structural Categorisation, Material Selection and Inspection Principles...................................................................... 76B 100 General................................................................................................................................................................ 76B 200 Structural categorisation ..................................................................................................................................... 76B 300 Material selection................................................................................................................................................ 76B 400 Inspection categories........................................................................................................................................... 77

C. Design and Loading Conditions.................................................................................................................................... 77C 100 General................................................................................................................................................................ 77C 200 Transit ................................................................................................................................................................. 77C 300 Installation and retrieval ..................................................................................................................................... 78C 400 Operation and survival........................................................................................................................................ 79

D. Environmental Conditions ............................................................................................................................................ 79D 100 General................................................................................................................................................................ 79D 200 Wind.................................................................................................................................................................... 79D 300 Waves.................................................................................................................................................................. 80D 400 Current ................................................................................................................................................................ 80DET NORSKE VERITAS AS

D 500 Snow and ice ....................................................................................................................................................... 80

Offshore Standard DNV-OS-C201, April 2012 Contents Page 9E. Method of Analysis ....................................................................................................................................................... 80E 100 General................................................................................................................................................................ 80E 200 Global structural models ..................................................................................................................................... 82E 300 Local structural models....................................................................................................................................... 82E 400 Fatigue analysis................................................................................................................................................... 82

F. Design Loads................................................................................................................................................................. 82F 100 General................................................................................................................................................................ 82F 200 Permanent loads .................................................................................................................................................. 83F 300 Variable functional loads .................................................................................................................................... 83F 400 Tank loads........................................................................................................................................................... 83F 500 Environmental loads, general.............................................................................................................................. 83F 600 Wind loads .......................................................................................................................................................... 83F 700 Waves.................................................................................................................................................................. 84F 800 Current ................................................................................................................................................................ 84F 900 Wave and current ................................................................................................................................................ 84F 1000 Sea pressures during transit ................................................................................................................................ 85F 1100 Heavy components during transit ....................................................................................................................... 86F 1200 Displacement dependent loads............................................................................................................................ 86F 1300 Combination of loads.......................................................................................................................................... 86

G. Structural Strength ........................................................................................................................................................ 87G 100 General................................................................................................................................................................ 87G 200 Global capacity ................................................................................................................................................... 87G 300 Footing strength .................................................................................................................................................. 87G 400 Leg strength ........................................................................................................................................................ 88G 500 Jackhouse support strength ................................................................................................................................. 88G 600 Hull strength ....................................................................................................................................................... 88

H. Fatigue Strength ............................................................................................................................................................ 88H 100 General................................................................................................................................................................ 88H 200 Fatigue analysis................................................................................................................................................... 88

I. Accidental Conditions................................................................................................................................................... 89I 100 General................................................................................................................................................................ 89I 200 Collisions ............................................................................................................................................................ 89I 300 Dropped objects .................................................................................................................................................. 89I 400 Fires .................................................................................................................................................................... 89I 500 Explosions........................................................................................................................................................... 89I 600 Unintended flooding ........................................................................................................................................... 89

J. Miscellaneous requirements.......................................................................................................................................... 89J 100 General................................................................................................................................................................ 89J 200 Pre-load capacity................................................................................................................................................. 89J 300 Overturning stability ........................................................................................................................................... 90J 400 Air gap ................................................................................................................................................................ 91

Sec. 13 Special Considerations for Tension Leg Platforms (TLP) ........................................................... 92

A. General .......................................................................................................................................................................... 92A 100 Scope and application ......................................................................................................................................... 92A 200 Description of tendon system ............................................................................................................................. 92

B. Structural Categorisation, Material Selection and Inspection Principles...................................................................... 94B 100 General................................................................................................................................................................ 94B 200 Structural categorisation ..................................................................................................................................... 94B 300 Material selection................................................................................................................................................ 95B 400 Design temperatures............................................................................................................................................ 95B 500 Inspection categories........................................................................................................................................... 95

C. Design Principles .......................................................................................................................................................... 96C 100 General................................................................................................................................................................ 96C 200 Design conditions ............................................................................................................................................... 96C 300 Fabrication .......................................................................................................................................................... 97C 400 Hull and Deck Mating......................................................................................................................................... 97C 500 Sea transportation ............................................................................................................................................... 97C 600 Installation .......................................................................................................................................................... 97C 700 Decommissioning ............................................................................................................................................... 97C 800 Design principles, tendons .................................................................................................................................. 97

D. Design Loads................................................................................................................................................................. 98D 100 General................................................................................................................................................................ 98DET NORSKE VERITAS AS

D 200 Load categories ................................................................................................................................................... 98

Offshore Standard DNV-OS-C201, April 2012 Contents Page 10E. Global Performance ...................................................................................................................................................... 99E 100 General................................................................................................................................................................ 99E 200 Frequency domain analysis............................................................................................................................... 100E 300 High frequency analyses ................................................................................................................................... 100E 400 Wave frequency analyses.................................................................................................................................. 100E 500 Low frequency analyses.................................................................................................................................... 100E 600 Time domain analyses ...................................................................................................................................... 101E 700 Model testing .................................................................................................................................................... 101E 800 Load effects in the tendons ............................................................................................................................... 102

F. Structural Strength ...................................................................................................................................................... 102F 100 General.............................................................................................................................................................. 102F 200 Hull ................................................................................................................................................................... 103F 300 Structural analysis............................................................................................................................................. 103F 400 Structural design ............................................................................................................................................... 103F 500 Deck .................................................................................................................................................................. 104F 600 Extreme tendon tensions ................................................................................................................................... 104F 700 Structural design of tendons ............................................................................................................................. 105F 800 Foundations....................................................................................................................................................... 105

G. Fatigue......................................................................................................................................................................... 105G 100 General.............................................................................................................................................................. 105G 200 Hull and deck .................................................................................................................................................... 105G 300 Tendons............................................................................................................................................................. 105G 400 Foundation ........................................................................................................................................................ 106

H. Accidental Condition .................................................................................................................................................. 106H 100 Hull ................................................................................................................................................................... 106H 200 Hull and deck .................................................................................................................................................... 107H 300 Tendons............................................................................................................................................................. 107H 400 Foundations....................................................................................................................................................... 107

Sec. 14 Special Considerations for Deep Draught Floaters (DDF)......................................................... 108

A. General ........................................................................................................................................................................ 108A 100 Introduction....................................................................................................................................................... 108A 200 Scope and application ....................................................................................................................................... 108

B. Non-Operational Phases.............................................................................................................................................. 108B 100 General.............................................................................................................................................................. 108B 200 Fabrication ........................................................................................................................................................ 109B 300 Mating ............................................................................................................................................................... 109B 400 Sea transportation ............................................................................................................................................. 109B 500 Installation ........................................................................................................................................................ 109B 600 Decommissioning ............................................................................................................................................. 109

C. Structural Categorisation, Selection of Material and Extent of Inspection................................................................. 109C 100 General.............................................................................................................................................................. 109C 200 Material selection.............................................................................................................................................. 110C 300 Design temperatures.......................................................................................................................................... 110C 400 Inspection categories......................................................................................................................................... 110C 500 Guidance to minimum requirements................................................................................................................. 111

D. Design Loads............................................................................................................................................................... 112D 100 Permanent loads ................................................................................................................................................ 112D 200 Variable functional loads .................................................................................................................................. 112D 300 Environmental loads ......................................................................................................................................... 113D 400 Determination of loads...................................................................................................................................... 113D 500 Hydrodynamic loads ......................................................................................................................................... 113

E. Deformation Loads .................................................................................................................................................... 114E 100 General.............................................................................................................................................................. 114

F. Accidental Loads ........................................................................................................................................................ 114F 100 General.............................................................................................................................................................. 114

G. Fatigue Loads .............................................................................................................................................................. 114G 100 General.............................................................................................................................................................. 114

H. Combination of Loads................................................................................................................................................. 114H 100 General.............................................................................................................................................................. 114

I. Load Effect Analysis in Operational Phase ................................................................................................................ 114I 100 General.............................................................................................................................................................. 114DET NORSKE VERITAS AS

I 200 Global bending effects ...................................................................................................................................... 115

Offshore Standard DNV-OS-C201, April 2012 Contents Page 11J. Load Effect Analysis in Non-Operational Phases....................................................................................................... 115J 100 General.............................................................................................................................................................. 115J 200 Transportation ................................................................................................................................................... 115J 300 Launching ......................................................................................................................................................... 115J 400 Upending........................................................................................................................................................... 116J 500 Deck mating ...................................................................................................................................................... 116J 600 Riser installations.............................................................................................................................................. 116

K. Structural Strength ...................................................................................................................................................... 116K 100 Operation phase for hull.................................................................................................................................... 116K 200 Non-operational phases for hull........................................................................................................................ 116K 300 Operation phase for deck or topside ................................................................................................................. 117K 400 Non-operational phases for deck or topside...................................................................................................... 117

L. Fatigue......................................................................................................................................................................... 117L 100 General.............................................................................................................................................................. 117L 200 Operation phase for hull.................................................................................................................................... 117L 300 Non-operational phases for hull........................................................................................................................ 118L 400 Splash zone ....................................................................................................................................................... 118L 500 Operation phase for deck or topside ................................................................................................................. 119L 600 Non-operational phases for deck or topside...................................................................................................... 119

M. Accidental Condition .................................................................................................................................................. 119M 100 General.............................................................................................................................................................. 119M 200 Fire .................................................................................................................................................................... 119M 300 Explosion .......................................................................................................................................................... 119M 400 Collision............................................................................................................................................................ 119M 500 Dropped objects ................................................................................................................................................ 120M 600 Unintended flooding ......................................................................................................................................... 120M 700 Riser Damage.................................................................................................................................................... 120M 800 Abnormal wave events...................................................................................................................................... 120

N. Dynamic Riser Design and Global Performance ........................................................................................................ 120N 100 Dynamic Riser Design ...................................................................................................................................... 120N 200 Global Performance .......................................................................................................................................... 121

App. A Cross Sectional Types .................................................................................................................... 122

A. Cross Sectional Types ................................................................................................................................................. 122A 100 General.............................................................................................................................................................. 122A 200 Cross section requirements for plastic analysis ................................................................................................ 122A 300 Cross section requirements when elastic global analysis is used ..................................................................... 123

App. B Methods and Models for Design of Column-Stabilised Units .................................................... 125

A. Methods and Models ................................................................................................................................................... 125A 100 General.............................................................................................................................................................. 125A 200 World wide operation ....................................................................................................................................... 125A 300 Benign waters or restricted areas ...................................................................................................................... 125

App. C Permanently Installed Units .......................................................................................................... 127

A. Introduction................................................................................................................................................................. 127A 100 Application........................................................................................................................................................ 127

B. Inspection and Maintenance........................................................................................................................................ 127B 100 Facilities for inspection on location.................................................................................................................. 127

C. Fatigue......................................................................................................................................................................... 127C 100 Design fatigue factors ....................................................................................................................................... 127C 200 Splash zone for floating units ........................................................................................................................... 128

App. D Certification of Tendon System .................................................................................................... 129

A. General ........................................................................................................................................................................ 129A 100 Introduction....................................................................................................................................................... 129

B. Equipment categorization ........................................................................................................................................... 129B 100 General.............................................................................................................................................................. 129

C. Fabrication Record...................................................................................................................................................... 130C 100 General.............................................................................................................................................................. 130

D. Documentation Deliverables for Certification of Equipment ..................................................................................... 130DET NORSKE VERITAS AS

D 100 General.............................................................................................................................................................. 130

Offshore Standard DNV-OS-C201, April 2012 Contents Page 12E. Tendon Systems and Components .............................................................................................................................. 131E 100 General.............................................................................................................................................................. 131E 200 Tendon pipe ...................................................................................................................................................... 132E 300 Bottom tendon interface (BTI).......................................................................................................................... 132E 400 Flex bearings .................................................................................................................................................... 132E 500 Foundations ...................................................................................................................................................... 133E 600 Top tendon interface (TTI) ............................................................................................................................... 133E 700 Intermediate tendon connectors (ITC) .............................................................................................................. 133E 800 Tendon tension monitoring system (TTMS) .................................................................................................... 133E 900 Tendon porch .................................................................................................................................................... 134E 1000 Tendon corrosion protection system................................................................................................................. 134E 1100 Load management program (LMP) .................................................................................................................. 134

F. Categorisation of Tendon Components....................................................................................................................... 134F 100 General.............................................................................................................................................................. 134

G. Tendon Fabrication ..................................................................................................................................................... 135G 100 General.............................................................................................................................................................. 135DET NORSKE VERITAS AS

Offshore Standard DNV-OS-C201, April 2012 Sec.1 Page 13SECTION 1INTRODUCTION

A. General

A 100 Introduction101 This offshore standard provides principles, technical requirements and guidance for the structural designof offshore structures, based on the Working Stress Design (WSD) method.102 This standard has been written for general world-wide application. Statutory regulations may includerequirements in excess of the provisions by this standard depending on size, type, location and intended serviceof the offshore unit or installation.103 The standard is organised with general sections containing common requirements and sectionscontaining specific requirement for different type of offshore units. In case of deviating requirements betweengeneral sections and the object specific sections, requirements of the object specific sections shall apply.

A 200 Objectives201 The objectives of this standard are to:

provide an internationally acceptable level of safety by defining minimum requirements for structures andstructural components (in combination with referred standards, recommended practices, guidelines, etc.)

serve as a contractual reference document between suppliers and purchasers serve as a guideline for designers, suppliers, purchasers and regulators specify procedures and requirements for offshore structures subject to DNV certification and classification.

A 300 Scope and application301 This standard is applicable to the following types of offshore structures:

column-stabilised units self-elevating units tension leg platforms deep draught floaters.

302 For utilisation of other materials, the general design principles given in this standard may be usedtogether with relevant standards, codes or specifications covering the requirements to materials design andfabrication.303 The standard is applicable to structural design of complete units including substructures, topsidestructures and vessel hulls. 304 This standard gives requirements for the following:

design principles structural categorisation material selection and inspection principles loads and load effect analyses design of steel structures and connections special considerations for different types of units.

Requirements for foundation design are given in DNV-OS-C101.

A 400 Other than DNV codes401 Other recognised codes or standards may be applied provided it is shown that the codes and standards,and their application, meet or exceed the level of safety of the actual DNV standard.402 In case of conflict between requirements of this standard and a reference document other than DNVdocuments, the requirements of this standard shall prevail.403 Where reference is made to codes other than DNV documents, the latest revision of the documents shallbe applied, unless otherwise specified.404 When code checks are performed according to other than DNV codes, the usage factors as given in theDET NORSKE VERITAS AS

respective code shall be used.

Offshore Standard DNV-OS-C201, April 2012 Sec.1 Page 14A 500 Classification501 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.

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

B. ReferencesB 100 General101 The DNV documents in Table B1 are referred to in the present standards and contain acceptable methodsfor fulfilling the requirements in this standard. 102 The latest valid revision of the DNV reference documents in Table B2 applies. See also current DNVList of Publications.103 The documents listed in Table B2 are referred in the present standard. The documents include acceptablemethods for fulfilling the requirements in the present standard and may be used as a source of supplementaryinformation. Only the referenced parts of the documents apply for fulfilment of the present standard.

Table A1 DNV Offshore Service SpecificationsReference TitleDNV-OSS-101 Rules for Classification of Offshore Drilling and Support Units DNV-OSS-102 Rules for Classification of Floating Production, Storage and Loading UnitsDNV-OSS-103 Rules for Classification of LNG/LPG Floating Production and Storage Units or InstallationsDNV-OSS-121 Classification Based on Performance Criteria Determined by Risk Assessment Methodology

Rules for Planning and Execution of Marine Operations

Table B1 DNV Reference DocumentsReference TitleDNV-OS-A101 Safety Principles and ArrangementDNV-OS-B101 Metallic MaterialsDNV-OS-C101 Design of Offshore Steel Structures, General (LRFD method)DNV-OS-C301 Stability and Watertight IntegrityDNV-OS-C401 Fabrication and Testing of Offshore StructuresDNV-OS-D101 Marine Machinery Systems and EquipmentDNV-OS-E301 Position MooringDNV-OS-F201 Dynamic RisersDNV-RP-A201 Plan Approval Documentation Types - DefinitionsDNV-RP-B401 Cathodic Protection DesignDNV-RP-C103 Column Stabilised UnitsDNV-RP-C104 Self-Elevating UnitsDNV-RP-C201 Buckling Strength of Plated StructuresDNV-RP-C202 Buckling Strength of ShellsDNV-RP-C204 Design against Accidental LoadsDNV-RP-C203 Fatigue Strength Analysis of Offshore Steel StructuresDNV-RP-C205 Environmental Conditions and Environmental LoadsClassification Note 30.1 Buckling Strength Analysis of Bars and Frames, and Spherical ShellsClassification Note 30.4 Foundations

Table B2 Other references Reference TitleAISC-ASD Manual of Steel Construction ASDAPI RP 2A WSD with supplement 1

Planning, Designing and Constructing Fixed Offshore Platforms Working Stress Design

API RP 2T Planning, Designing and Constructing Tension Leg PlatformsBS 7910 Guide on methods for assessing the acceptability of flaws in fusion welded structuresNACE TPC Publication No. 3. The role of bacteria in corrosion of oil field equipmentDET NORSKE VERITAS AS

SNAME 5-5A Site Specific Assessment of Mobile Jack-Up Units

Offshore Standard DNV-OS-C201, April 2012 Sec.1 Page 15C. DefinitionsC 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 Accidental condition: When the unit is subjected to accidental loads such as collision, dropped objects,fire explosion, etc.202 Accidental loads: Loads which may occur as a result of accident or exceptional events, e.g. collisions,explosions, dropped objects.203 Atmospheric zone: The external surfaces of the unit above the splash zone.204 Cathodic protection: A technique to prevent corrosion of a steel surface by making the surface to be thecathode of an electrochemical cell.205 Characteristic load: The reference value of a load to be used in the determination of load effects. Thecharacteristic load is normally based upon a defined fractile in the upper end of the distribution function forload.206 Characteristic strength: The reference value of structural strength to be used in the determination of thedesign strength. The characteristic strength is normally based upon a 5% fractile in the lower end of thedistribution function for resistance.207 Characteristic value: The representative value associated with a prescribed probability of not beingunfavourably exceeded during the applicable reference period.208 Classic spar: Shell type hull structure.209 Classification Note: The Classification Notes cover proven technology and solutions which is found torepresent good practice by DNV, and which represent one alternative for satisfying the requirements given inthe DNV Rules or other codes and standards cited by DNV. The Classification Notes will in the same mannerbe applicable for fulfilling the requirements in the DNV Offshore Standards.210 Coating: Metallic, inorganic or organic material applied to steel surfaces for prevention of corrosion.211 Column-stabilised unit: A floating unit that can be relocated. A column-stabilised unit normally consistsof a deck structure with a number of widely spaced, large diameter, supporting columns that are attached tosubmerged pontoons.212 Corrosion allowance: Extra wall thickness added during design to compensate for any anticipatedreduction in thickness during the operation.213 Damaged condition: The unit condition after accidental damage. 214 Deep draught floater (DDF): A floating unit categorised with a relative large draught. The large draughtis mainly introduced to obtain reduced wave excitation in heave and sufficiently high eigenperiod in heave suchthat resonant responses in heave can be omitted or minimised.215 Design brief: An agreed document presenting owner's technical basis, requirements and references forthe unit design and fabrication.216 Design temperature: The design temperature for a unit is the reference temperature for assessing areaswhere the unit can be transported, installed and operated. The design temperature shall be lower or equal to thelowest mean daily temperature in air for the relevant areas. For seasonal restricted operations the lowest meandaily temperature in air for the season may be applied.217 Driving voltage: The difference between closed circuit anode potential and the protection potential.218 Dry transit: A transit where the unit is transported on a heavy lift unit from one geographical location toanother.219 Dynamic upending: A process where seawater is filled or flooded into the bottom section of ahorizontally floating DDF hull and creating a trim condition and subsequent water filling of hull or moonpooland dynamic upending to bring the hull in vertical position.220 Environmental loads: Loads directly and