RECOMMENDED PRACTICE

DNVGL-RP-C203 Edition April 2016

Fatigue design of offshore steel structures

DNV GL AS

The electronic pdf version of this document found through http://www.dnvgl.com is the officially binding version. The documents are available free of charge in PDF format.

FOREWORD

DNV GL recommended practices contain sound engineering practice and guidance.

DNV GL AS April 2016

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

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

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GeneralThis document supersedes DNVGL-RP-0005, June 2014.

Text affected by the main changes in this edition is highlighted in red colour. However, if the changes

On 12 September 2013, DNV and GL merged to form DNV GL Group. On 25 November 2013 Det NorskeVeritas AS became the 100% shareholder of Germanischer Lloyd SE, the parent company of the GL Group,and on 27 November 2013 Det Norske Veritas AS, company registration number 945 748 931, changed itsname to DNV GL AS. For further information, see www.dnvgl.com. Any reference in this document to DetNorske Veritas AS, Det Norske Veritas, DNV, GL, Germanischer Lloyd SE, GL Group or any otherlegal entity name or trading name presently owned by the DNV GL Group shall therefore also be considereda reference to DNV GL AS.

On 12 September 2013, DNV and GL merged to form DNV GL Group. On 25 November 2013 Det NorskeVeritas AS became the 100% shareholder of Germanischer Lloyd SE, the parent company of the GL Group,and on 27 November 2013 Det Norske Veritas AS, company registration number 945 748 931, changed itsname to DNV GL AS. For further information, see www.dnvgl.com. Any reference in this document to DetNorske Veritas AS, Det Norske Veritas, DNV, GL, Germanischer Lloyd SE, GL Group or any otherlegal entity name or trading name presently owned by the DNV GL Group shall therefore also be considereda reference to DNV GL AS.

involve a whole chapter, section or sub-section, normally only the title will be in red colour.

Main changes April 2016 Sec.1 Introduction [1.2.1] The validity of the recommended practice for use in seawater with and without cathodic

protection is increased from 550 MPa to 690 MPa.

Sec.2 Fatigue analysis based on S-N data [2.4.6] New S-N curve for simple tubular joint for harmonisation with ISO 19902. [2.4.10] The HS curve for high strength steel is moved to [D.1] to be in the same appendix as the S-N

curves for subsea application in [D.2]. [2.5.1] New information on residual stresses after cold forming is inserted. This may for example be

important for design of penetrations in monopiles for J-tube transfer. [2.5.2] Text is added for additional explanation of content. [2.10.1] Text is revised to align with DNV-OS-F101 and to include improved S-N data derived in a joint

industry project. [2.10.2] The content is revised to be aligned with DNV-OS-F101.

Sec.3 Stress concentration factors [3.1.1] More background information on 0 is included. [3.1.2] Added SCF equation for the back side of thickness transition as asked for by the industry. [3.1.5] This section is revised with improved figures. It is noted that also a compressive force range in

the axial direction can give tensile stress normal to the main stress direction and lead to fatigue of fillet weld.

[3.3.4] The change to a higher S-N curve for simple tubular joints implies that some more information is needed also on stiffened tubular joints, and some more information is added.

[3.3.7.2] The value of 0 is reduced to make this safer based on new information about actual fabricated tolerances.

[3.3.7.3] Limit on thickness ratio for use of equation (3.3.5) is deleted. [3.3.9] Information is added on effect of placement of thickness transition.

Sec.4 Calculation of hot spot stress by finite element analysis [4.3.4] Section added for purpose of fatigue life calculation using time domain and Rainflow analysis. [4.3.6] More explanation is included as the use of this section is considered difficult to document in

projects. [4.3.9] Factor in equation (4.3.15) is increased due to possible start and stop positions around

circumference when welding.

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Sec.7 Improvement of fatigue life by fabrication

[7.3] Section added because a different requirement to roughness is made regarding coating than for fatigue strength in standards.The reference list has been updated.

App.D S-N curves for base material of high strength steel [D.2] A new section with S-N curves for high strength carbon and low alloy machined forgings for subsea

applications is included.

App.E Application of the effective notch stress method for fatigue assessment of structural details

App.E The content on notch stress analysis is moved from the commentary section and is included as a new appendix to get it in more line with accepted procedures when using finite element analysis.

App.F Commentary [F.5] ISO 5817 revised in 2014 and existing text had to be updated. [F.7] Table F-3 and Figure F-6 are corrected. The former was not precise with respect to confidence

level.

In addition to the above stated main changes, editorial corrections may have been made.

Editorial corrections

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CONTENTS

CHANGES CURRENT .................................................................................................. 3

Sec.1 Introduction.................................................................................................. 91.1 General...................................................................................................91.2 Validity of standard ................................................................................9

1.2.1 Material ........................................................................................91.2.2 Temperature .................................................................................91.2.3 Low cycle and high cycle fatigue ......................................................9

1.3 Methods for fatigue analysis ..................................................................91.4 Definitions............................................................................................101.5 Symbols and abbreviations..................................................................12

Sec.2 Fatigue analysis based on S-N data............................................................. 142.1 Introduction .........................................................................................142.2 Fatigue damage accumulation..............................................................152.3 Fatigue analysis methodology and calculation of stresses....................16

2.3.1 General ......................................................................................162.3.2 Plated structures using nominal stress S-N curves ............................162.3.3 Plated structures using hot spot stress S-N curves............................172.3.4 Tubular joints .............................................................................172.3.5 Fillet welds at load carrying joints ..................................................192.3.6 Fillet welds at doubling plates ........................................................192.3.7 Fillet welded bearing supports .......................................................20

2.4 S-N curves............................................................................................202.4.1 General ......................................................................................202.4.2 Failure criterion inherent the S-N curves .........................................202.4.3 S-N curves and joint classification for plated structures .....................212.4.4 S-N curves in air..........................................................................222.4.5 S-N curves in seawater with cathodic protection...............................242.4.6 S-N curves for tubular joints..........................................................252.4.7 S-N curves for cast nodes .............................................................252.4.8 S-N curves for forged nodes ..........................................................262.4.9 S-N curves for free corrosion in plated structures .............................262.4.10 S-N curves for base material of high strength steel...........................262.4.11 S-N curves for stainless steel.........................................................262.4.12 S-N curves for small diameter umbilicals .........................................272.4.13 S-N data for piles.........................................................................282.4.14 Qualification of new S-N curves based on fatigue test data ................28

2.5 Mean stress influence on fatigue strength............................................282.5.1 Base material without significant residual stresses............................282.5.2 Welded connections without significant residual stresses ...................29

2.6 Effect of fabrication tolerances.............................................................302.7 Requirements to NDE and acceptance criteri