DNVGL-RP-F112 Duplex stainless steel - design...

The electronic pdf version of this document, available free of chargefrom http://www.dnvgl.com, is the officially binding version.

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RECOMMENDED PRACTICE

DNVGL-RP-F112 Edition June 2018

Duplex stainless steel - design againsthydrogen induced stress cracking

FOREWORD

DNV GL recommended practices contain sound engineering practice and guidance.

© DNV GL AS June 2018

Any comments may be sent by e-mail to [email protected]

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

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Recommended practice — DNVGL-RP-F112. Edition June 2018 Page 3Duplex stainless steel - design against hydrogen induced stress cracking

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CHANGES – CURRENT

This document supersedes the July 2017 edition of DNVGL-RP-F112.Changes in this document are highlighted in red colour. However, if the changes involve a whole chapter,section or subsection, normally only the title will be in red colour.

Changes June 2018

• GeneralThe aim of the 2018 update is to improve the 2017 revision with best practice based on today's knowledge,in-service experience and recent research. This is driven by the following factors:

— a need for operators and contractors to have a harmonized approach to the design of duplex stainlesssteel components exposed to cathodic protection (CP)

— a need for increased awareness of hydrogen embrittlement due to CP within the industry.

The RP includes two different assessment categories for design against hydrogen induced stress cracking(HISC) and provides more detailed guidance on how to perform diffusion assessments in order to determineif the HISC stress or strain criteria may be disregarded on surfaces without cathodic protection.

• Front pageTitle has been amended.

• Sec.2 Design philosophyDesign philosophy has been revised.

• Sec.3 LoadsLoads and conditions have been revised and divided into Sec.3 and Sec.4 .

• Sec.4 Design detailsDesign criteria have been revised and replaced by Sec.5.

• Sec.5 Hydrogen induced stress cracking assessmentMaterial requirements have been revised and renamed to Sec.6 .Design details have been added.

• Sec.6 Material and fabrication recommendationsNon-destructive testing has been removed.

• Sec.7 Procedure for assessment of austenite spacingThis section has been removed.

• Sec.7 ReferencesNew section has been added.

• App.A Practical measuresNew informative appendix has been added.

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• App.B Diffusion and technical reportingNew normative appendix has been added.

Editorial correctionsIn addition to the above stated changes, editorial corrections may have been made.

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AcknowledgementsThis recommended practice is developed based on results from a joint industry project (JIP). The followingcompanies, listed in alphabetical order, are acknowledged for their contributions to the JIP.

Aker Solutions

BP

GE Oil & Gas

Pacson Valves

Petrobras

Shell

Sintef

TechnipFMC

This recommended practice was developed in 2008 based on results from a joint industry project (JIP). Thefollowing companies, listed in alphabetical order, are acknowledged for their contributions to the originaldocument:

Aker Kværner Allegheny

BP Cameron

Chevron ConocoPhillips

FMC NKK Tenaris

Outokumpu Petrobras

Shell SINTEF

StatoilHydro Sumitomo

Technip Total

TWI VectoGray

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CONTENTS

Changes – current.................................................................................................. 3Acknowledgements.................................................................................5

Section 1 General....................................................................................................81.1 Introduction......................................................................................81.2 Objective...........................................................................................81.3 Scope................................................................................................ 81.4 Application........................................................................................81.5 References........................................................................................ 91.6 Definitions and abbreviations......................................................... 10

Section 2 Design philosophy................................................................................. 142.1 General considerations................................................................... 142.2 Design process................................................................................14

Section 3 Loads.....................................................................................................173.1 Loads to be considered...................................................................173.2 Loading scenarios........................................................................... 17

Section 4 Design details........................................................................................194.1 Wall thickness.................................................................................194.2 Stress magnification factor.............................................................194.3 Local surface penalty factor........................................................... 194.4 Local surface magnification factor..................................................194.5 Girth welds..................................................................................... 194.6 Fillet welds..................................................................................... 204.7 Bolted connection........................................................................... 204.8 Coating............................................................................................20

Section 5 Hydrogen induced stress cracking assessment......................................225.1 General........................................................................................... 225.2 Characteristic material properties.................................................. 225.3 Input for category 1 assessment....................................................225.4 Input for category 2 assessment....................................................275.5 Design criteria................................................................................ 29

Section 6 Material and fabrication recommendations............................................336.1 General........................................................................................... 33

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6.2 Material limitations.........................................................................33

Section 7 References.............................................................................................357.1 References...................................................................................... 35

Appendix A Practical measures (informative).......................................................36A.1 Mesh convergence.......................................................................... 36A.2 Local surface penalty and magnification factors based onlinearization..........................................................................................37A.3 Sequential loading.......................................................................... 38

A.4 Numerical estimation of Lres...........................................................38A.5 The material hardening curve based on testing..............................38A.6 Measuring the austenite spacing.................................................... 39

Appendix B Diffusion and technical reporting (normative)................................... 40B.1 Diffusion......................................................................................... 40B.2 Diffusion modelling.........................................................................41B.3 Reporting........................................................................................ 42

Changes – historic................................................................................................43


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SECTION 1 GENERAL

1.1 IntroductionBoth 22Cr and 25Cr duplex (ferritic-austenitic) stainless steels have been extensively used for subseaequipment. These types of steel have been used as rolled or extruded pipes, small bore tubing, hubs, fittingsand valve bodies manufactured by castings, forgings or hot isostatic pressing (HIP). To date the experience isgood but some significant failures have occurred.The main reason for these failures has been attributed to a combination of load or stress and ingress ofhydrogen formed at the steel surface due to the cathodic protection. In this document this is referred to ashydrogen induced stress cracking (HISC).Other materials commonly used in the offshore industry may also be prone to HISC when exposed tocathodic protection. However, only duplex stainless steels are addressed in this recommended practice.Testing of typical small-scale laboratory specimens has shown that the duplex stainless steels are susceptibleto HISC when exposed to elevated stresses in conjunction with cathodic protection potentials more negativethan about -800 mV relative to the Ag/AgCl reference electrode in seawater.This recommended practice presents requirements based on the resistance to HISC of duplex stainlesssteel grades. The choice of characteristic loads, load factors and target safety level is not described in thisdocument. This should either come from the project design standard or be based on company requirements.The design requirements herein are aimed at avoiding HISC. They are a supplement to, and not areplacement for, the selected design standard. In case of conflict between the selected design standard andthe requirements in this recommended practice (RP), the most stringent requirements shall apply.The requirements in this recommended practice are assumed to be conservative, implying that theprobability of HISC failure is acceptably low when stress and strain are below the allowable limits set forth.The probability of HISC failure for stress and strain levels above the limits is not known.

1.2 ObjectiveThe objective of this RP is to give guidance how to avoid HISC in the design of subsea equipment made fromduplex stainless steels.This document is intended to be the reference industry recommended practice for the design of duplexstainless steel components for subsea equipment exposed to cathodic protection.

1.3 ScopeThis RP covers the design of components made from duplex stainless steels that are installed subsea and areexposed to cathodic protection, by:

— providing recommendations on loads and conditions that should be considered in the design of subseasystems where duplex stainless steels will be used in conjunction with cathodic protection

— defining other parameters affecting the resistance to HISC, such as surface characteristics (i.e. coating),temperature and specific configurations requiring particular attention

— establishing a stress and a strain design criteria.Guidance note:Successful design for the avoidance of HISC is strongly dependent on accurate appraisal of the loads. In this appraisal,conservatisms that are appropriate to the level of uncertainty and associated risks should be applied.

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1.4 ApplicationThis RP is applicable to the design of all components made of duplex stainless steels that are installed subseaand are exposed to cathodic protection. These materials are generally referred to as 22Cr and 25Cr.


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This RP is applicable in conjunction with referenced DNV GL recommended practice and recognized designstandards (e.g. ASME B31.3, ASME B31.4, ASME B31.8, ASME BPVC-VIII, ASME BPVC II, ASTM E112-13,DNVGL-ST-F101, ISO 13623, ISO 13628, ISO 17781, EN 14161, ISO 10423/API 6A, API 17D, PD 8010,DNVGL-RP-B401, DNVGL-RP-D101, DNVGL-RP-F102, DNVGL-RP-F103, DNVGL-RP-F106, NORSOK M-001,NORSOK M-501, NORSOK M-601, NORSOK M-630, NORSOK M-650).

1.5 ReferencesThe latest revisions of the following documents apply.

Table 1-1 DNV GL documents

Document code Title

DNVGL-CG-0051 Non-destructive testing

DNVGL-RP-B401 Cathodic protection design

DNVGL-RP-C203 Fatigue design of offshore steel structures

DNVGL-RP-D101 Structural analysis of piping systems

DNVGL-RP-F102 Pipeline field joint coating and field repair of line pipe external coating

DNVGL-RP-F103 Cathodic protection of submarine pipelines

DNVGL-RP-F106 Factory applied external pipeline coatings for corrosion control

DNVGL-ST-F101 Submarine pipeline systems

Table 1-2 External documents

Document code Title

API 17D Design and Operation of Subsea Production Systems - Subsea Wellhead and TreeEquipment

ASME BPVC II Part D Boiler and Pressure Vessel Standard - Section II: Materials - Part D: Properties(Customary)

ASME BPVC VIII Div. 2 Boiler and Pressure Vessel Standard- Section VIII: Rules for Construction of PressureVessels - Division 2: Alternative Rules

ASME B31.3 Process Piping

ASME B31.4 Pipeline Transportation Systems for Liquids and Slurries

ASME B31.8 Gas Transmission and Distribution Piping Systems

ASTM E112-13 Standard test method for determining average grain size, 2013

EN 14161 Petroleum and natural gas industries - Pipeline Transportation Systems

ISO 10423/API 6A Petroleum and natural gas industries - Drilling and production equipment - Wellhead andChristmas tree equipment

ISO 13628-15/API 17P Petroleum and natural gas industries - Design and Operation of Subsea ProductionSystems - Part 15: Subsea structures and manifolds

ISO 13623 Petroleum and natural gas industries - Pipeline Transportation Systems


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Document code Title

ISO 17781 Petroleum and natural gas industries - Test methods for quality control of microstructureof ferritic/austenitic (duplex) stainless steels

NORSOK M-001 Material Selection

NORSOK M-501 Surface preparation and protective coating

NORSOK M-601 Welding and inspection of piping

NORSOK M-630 Material data sheets and element data for piping

NORSOK M-650 Qualification of manufacturers of special materials

PD 8010 Pipeline systems. Part 2: Subsea pipelines. Code of practice

Guidance note:In case of conflict between requirements of this recommended practice and a referenced design standard, the most stringentrequirement applies.The latest edition of the DNV GL documents may be found in the publication list at the DNV GL website www.dnvgl.com.


1.6 Definitions and abbreviations

1.6.1 Definition of verbal formsTable 1-3 lists the verbal forms used in the document.

Table 1-3 Definition of verbal forms

Term Definition

shall verbal form used to indicate requirements strictly to be followed in order to conform to thedocument

should verbal form used to indicate that among several possibilities one is recommended as particularlysuitable, without mentioning or excluding others, or that a certain course of action is preferredbut not necessarily required

may verbal form used to indicate a course of action permissible within the limits of the document

1.6.2 Definition of termsA definition of the terms used in this document can be found in Table 1-4.

Table 1-4 Definition of terms

Term Definition

cathodic protectionpotential

potential of the steel surface relative to the Ag/AgCl reference electrode in seawater

design temperature,maximum

the highest possible temperature to which the equipment or system may be exposedduring installation and operation

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Term Definition

design temperature,minimum

the lowest possible temperature to which the equipment or system may be exposed toduring installation and operation, irrespective of the pressure

diffusion mechanism for change in local concentration of hydrogen in the material over time

duplex stainless steel andsuper duplex stainlesssteel

stainless steels containing both a ferrite and an austenite phase

guidance note used for giving additional information, clarifications or advice that increases theunderstanding of the preceding textGuidance notes are not binding.

hydrogen induced stresscracking

cracking due to a combination of load and ingress of hydrogen formed at the steelsurface due to cathodic polarisation

load any action causing stress, strain, deformation, displacement, motion, etc. to theequipment or system

load effect effect of a single load or combination of loads on the equipment or system, such asstress, strain, deformation, displacement, motion

local surface penaltyfactor

local surface penalty factor accounting for increased strain due to local strainconcentrations

note especially important information

preload applied bolt pre-tension

pressure, design the maximum internal pressure defined by the design standard, referred to at a specifiedreference height, to which the system shall be designed

residual stress the stress origin from any mechanical, thermal and metallurgical processes remaining ina component in the absence of any external load

residual strain any permanent strain in a component in the absence of any external load

resistance the capacity of a structure, or part of a structure, to resist load effects

specified minimum tensilestrength

the minimum tensile strength prescribed by the specification or standard under which thematerial is purchased

specified minimum yieldstress

the minimum yield stress prescribed by the specification or standard under which thematerial is purchased

standard in the context of this document, the term standard shall be understood to coverdocument types such as codes, guidelines and recommended practices in addition tobona fide standards

stress magnificationfactor

estimated factor of increase in nominal stresses calculated using beam or piping analysisdue to transitions or misalignments

submerged zone the part of the system or installation below the splash zone, including buried parts


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1.6.3 Definition of symbols - Greek charactersa = factor to determine influence of residual stresses on acceptable stress levelaL = thermal expansion coefficientβ = factor to determine influence of weld and transition angle on acceptable stress levelεc = maximum allowable surface strainεmem = non-linear membrane strainεpeak = peak surface strainv = Poisson's ratioγ = material factor reflecting influence of microstructureσmax = maximum principal surface stress from FEAσlin = sum of membrane and bending stress from linearization in FEAσhoop = stress in hoop directionσaxial = axial stress from beam or piping analysisσbending, m = global bending stress based on mid-section modulusσbending,o = global bending stress based on outer section modulusσmem,long = membrane stress in the longitudinal directionσout,long = outer fibre stress in the longitudinal directionσmem,hoop = membrane stress in the hoop directionσlong,max = maximum longitudinal stress derived from piping analysisσmem,i = membrane stress in principal direction iσout,i = outer fibre stress in principal direction iσtherm = stress due to potential thermal gradients over the wall thickness

1.6.4 Definition of symbols - Latin charactersC = hydrogen concentrationC0 = initial surface hydrogen concentrationCh = bulk hydrogen concentrationCint = hydrogen concentration on surface not exposed to CPD = diffusion coefficientE = modulus of elasticityLres = length of zone assumed to be influenced by weld residual stressesLtran = length between root of geometrical transition and weld toeLSPFcat1 = local surface penalty factor category 1LSMFcat2 = local surface magnification factor category 2R = outer pipe radiusRm = tensile strengthRp0.2 = characteristic 0.2% yield stressSmag = stress magnification factorSmag,i = stress magnification factor in principal direction iSmag,hoop = stress magnification factor in hoop directionSmag,long = stress magnification factor in longitudinal directionSCF = stress concentration factor


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SCFanalytical = linear elastic analytic estimate of SCFt = wall thicknessT = temperature [oC]TK = temperature [oK]ΔT = temperature gradient over wall thickness

1.6.5 AbbreviationsTable 1-5 Abbreviations

Abbreviation Description

API American Petroleum Institute

ASME American Society of Mechanical Engineering

ASTM American Society for Testing and materials

CP cathodic protection

CTOD crack tip opening displacement

DSS duplex stainless steel

FE finitie element

FEA finite element analysis

HAZ heat affected zone

HIP hot isolatic pressing

HISC hydrogen induced stress cracking

LPI liquid penetrant inspection

LSPF local surface penalty factor

MPM multi-phase flow meter

NDT non-destructive testing

PT pressure transducer

RP recommended practice

SDSS super duplex stainless steel

SCF stress concentration factor

SIF stress intensity factor

SMTS specified minimum tensile strength

SMYS specified minimum yield stress

WPQ weld performance qualification


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SECTION 2 DESIGN PHILOSOPHY

2.1 General considerationsThe most common HISC field failure locations are near welds and stress raisers, and it is important thatdesign and construction take practical precautions to ensure robustness. Critical weld features expected toexperience the greatest loads or stresses (or for which the applied loads have the greatest uncertainty) needparticular attention. Practical measures should be taken to manage stresses at welded joints, ensure goodaccess for welding, and produce a build sequence that maintains good fit up and ensures ready access fornon-destructive testing (NDT). For NDT see DNVGL-CG-0051.In addition to loads and stresses the following aspects should also be considered during design of a system:

— Certain design details should be avoided, e.g. fillet welds, sharp edges, sharp stress raisers.— Despite high predicted stresses at an internal surface, HISC may still not be a concern if sufficient

hydrogen cannot diffuse through the thickness.— Small bore tubing systems, including welded tees (from extruded tubing products) and their associated

compression fittings are acceptable without stress or strain analysis provided that cold bend areas are notlocated within Lres. The small-bore tubing size is limited to tubing with an outer diameter of 2 inches.

— Heavily cold formed materials will be non-conservatively assessed based on criteria defined in this RP (see[5.3.6]).

— It is not required to apply specific checks against HISC for surfaces where all stress components arecompressive.

— If it can be shown that neither CP nor hydrogen can influence a given volume or surface in thecomponent, the limits in this RP do not need to be considered for that volume.

— Generic components (such as instruments and sensors) made from duplex stainless steel (DSS) shall alsomeet the requirements in this RP. However, such components are typically of standard design, and maybe verified on a type approval basis, not on a project specific basis. This requires that limitations on loadsand other conditions are clearly stated.Guidance note:The following aspects are not considered during a design process, but may be considered beneficial:

— Application of coating as mitigation to hydrogen charging at the surface (see [4.8]).

— There is evidence that cathodic protection of -800mV Ag/AgCl or higher (less negative) may mitigate the diffusion of hydrogenand subsequent threat of HISC. The installation of diodes is a possible option.


2.2 Design process

2.2.1 Analysis categoriesThis RP presents two different analysis categories for design against HISC. Within the limits of the differentcategories, demonstration of fulfilment of the criteria is equally valid. The semi-analytical analysis category1 is intended for cross-sections with rotational symmetry and moderate transitions and/or for componentswhere analytical evaluation is applicable. The complex structures/strain level analysis category 2 isrecommended for complex geometries and complex loading scenarios where pipe stress and/or analyticalevaluation are not applicable or when fulfilment of the criteria in category 1 has not been reached. Arecommendation for selecting analysis category for a subsea system is presented in [2.2.3].

Guidance note:Pipe stress analysis software may be used for the category 1 analysis.



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2.2.2 Design guidanceThe design against HISC should be carefully assessed by considering the following design guidance, which isbased upon historical HISC failure events.The majority of significant HISC failures are associated with welded connections, stress raisers, and highexternal loads. It is therefore important to assess such features, especially:

— High external loads are caused by pipeline or flowline movement caused by thermal or pressure expansionand loads heavily influenced by soil mechanics or subsidence. The adoption of flowline expansion loopshave been used as a potential mitigation measure to control high external loads.

— During installation, misalignment will increase the installation loads.— Historical failures have occurred because of poor design of welds and/or tapers or other stress raisers in

combination with poor assembly weld joint fit-up. A combination of good design and fabrication practice isrequired to avoid such failures.

— Historical failures have occurred at the outboard welded connection near the porch due to high appliedexternal loads and stress raisers. Less common failures have occurred inboard of the porch due toflexibility of the structural supports generating high stresses within the inboard piping. Appropriateanalysis tools and methods should be selected to capture the stiffness changes or stresses induced fromcomplex loading from thermal gradients, bolt pre-load and externally applied loads.

— Historical failures have occurred at weld repairs. Constraints can be very high for such welds and theresultant residual stresses and strains may be greater than the acceptance criteria of the RP. Thisemphasises the need for good fabrication specification and practice. Special attention should be paid tosingle pass welds, see [4.5] and [4.6].

2.2.3 Selection of analysis categoryBased on the design guidance in [2.2.2] the recommendations for selecting analysis category are shown inFigure 2-1.

Figure 2-1 Recommendations for selecting analysis category 1 and 2


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Guidance note:The stress magnification arising on the inner surface for standard pipe fittings such as tees, bends, wyes and similar componentsdo not have to be included in a HISC assessment, if it can be shown by hydrogen diffusion assessment according to [B.1] that thehydrogen does not reach the high stress location within the service life.The connector or porch design requires consideration against the constraint assumptions adopted against the structural steelinterface. It is common practice to interpret the porch structure to be fixed, resulting in conservative external loads outboard ofthe connector, but potentially non-conservative loads inboard within the manifold piping. Thus, a verification of the flexibility of theporch and structural steel design and any adverse influence on the manifold piping stress condition should be performed. If thesituation is unclear, then the stiffness and clearances of structural steel design should be incorporated in the piping stress analysisfor category 1.



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SECTION 3 LOADS

3.1 Loads to be consideredHISC is a non-ductile mode of failure caused by an interaction between stresses, a hydrogen charging system(cathodic protection most common) and a susceptible material. All relevant loads that can be transferred tothe component from the connecting system during installation and operation shall be considered. In addition,be aware that deformation loads, such as thermal stresses, seabed subsidence effects and residual stress,shall be included when designing for HISC resistance.For HISC to occur, a load shall be applied over a certain time interval. In laboratory tests, however, HISCfailures have been produced in a matter of hours. Hence, all loads except momentary loads (lasting less than1 minute), shall be considered.Loads to be considered in the design:

— external and internal design pressure acting while CP is applied— external mechanical loads acting while CP is applied— thermal loads due to restraints of the system acting while CP is applied— preloads applied subsea while CP is applied— preloads applied before CP is applied— loads due to pipeline expansion, settlement and soil resistance.

3.2 Loading scenarios

3.2.1 Pressure containmentThe design criteria for stress or strain given in this RP shall be fulfilled for all pressures scenarios to whichthe system will be exposed while in a submerged zone and the CP system is active. This also includes subseapressure testing.

Guidance note:The influence of pressure in subsea design needs to be considered correctly depending on the stress analysis method (analyticalbeam theory, finite element analysis (FEA) and piping software).


3.2.2 Incidental loadsA brief shock load (dropped objects, fishing gear/trawl impact, anchor interference, earthquake etc.) will notlead to failure by HISC.If the load is transient, but acts on the structure or component for longer than 1 minute, the strains andstresses shall be evaluated against the acceptance criteria in this RP.If the transient loading introduces permanent stresses, for example residual stresses from permanentdeformation, these shall be evaluated against the acceptance criteria in this RP.

Guidance note:This recommended practice does not set any requirements to protection against incidental loads, nor the acceptable levelof damage from such loads. Such requirements should be defined by the project, the project design standard or companyrequirements.Incidental loads, such as trawl impact, dropped object and earthquake, will have a short duration, and the loads from such eventscan be neglected from a HISC point of view. These loads are characterised by having a duration less than a minute.Incidental loads can introduce deformation in the structure, or change the soil conditions or support. A new assessment of externalloads may be necessary after an incidental load.



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3.2.3 Installation loadsAs long as the parts and components considered are submerged and cathodic protection is active, no loadsduring installation shall lead to higher stresses/strains than given in this recommended practice.For installation loads including tie-in loads that are applied for only a very short time (less than 1 minute)higher stresses/strains may be accepted, see [2.2.2] and [3.2.2].Reeling will introduce residual stresses in the material. This shall be evaluated, see also Sec.5.

3.2.4 Lifetime assessmentThe design shall cover the full design life of the subsea system. Significant changes of the loading conditionsover the lifetime of the system should be considered.

3.2.5 Residual stressesResidual stresses shall be taken into consideration. Residual stresses associated with welds and cold formingare addressed in Sec.5.

3.2.6 Temperature loadsThermal stresses act across the wall thickness and are developed due to restraints of the system. Thermalexpansion and thermal gradient acting across the wall thickness should be included in the stress/strainassessment. The thermal steady state condition should be established by either thermo- mechanical analysisor by simplified analytical evaluations where applicable. In the design phase the production flow conditionsare unknown thus the maximum design flow conditions should be used.Thermal loads (maximum design temperature) during a piping analysis are applied as uniform temperatureacross the thickness (e.g. the thermal gradient is not accounted for). Thus, in the global piping analysisassessed with a category 1 the mean wall temperature may be used to capture the global expansion loads.The local component assessment using category 1 should then include the local bending stress due tothermal gradients over the wall thickness as shown in [5.3.3].For the definition of the minimum and maximum design temperature, environmental as well as operationaltemperatures shall be considered.


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SECTION 4 DESIGN DETAILS

4.1 Wall thicknessHISC check should be performed on the minimum wall thickness taking possible losses due to mill tolerance,corrosion, erosion, wall thinning/thickening from bending into account. For bends extrados thinning/intradosthickening may impact the results and should be included in piping specification if considered in analysis.

4.2 Stress magnification factorTransitions or misalignments will give rise to secondary bending moments that again will lead to additionalsurface stresses which shall be included in the HISC assessment. The influence of such geometrical featuresare not included in simplified analytical beam or piping analyses, and a stress magnification factor, Smag,shall be applied if stresses from such analyses are used in HISC assessment. Guidance on possible ways todetermine Smag is given in Sec.5.

Guidance note:For misalignment, due to fabrication tolerances, the difference between the mean/nominal and maximum/minimum toleranceshould be used, if nothing else is specified. It is not necessary to use the difference between maximum and minimum tolerance.


4.3 Local surface penalty factorLocal features like fillets or weld toes will result in local increase in stresses and strains near the surface ofthe component. The allowable nominal stresses or strains will be limited in the presence of such features,and the reduction in critical nominal stress levels will depend on the magnitude of the local surface penaltyfactor, LSPFcat1. The value of LSPFcat1 shall be estimated in order to perform the design check. Guidance onpossible ways to estimate LSPFcat1 is given in Sec.5.

4.4 Local surface magnification factorThe allowable non-linear surface strain depends on the local geometry of the surface. The magnitude of theallowable local surface strain is determined through the local surface magnification factor, LSMFcat2, whichreflects the influence of the local geometry. Guidance on applicable procedures to estimate LSMFcat2 is givenin Sec.5.

4.5 Girth weldsTo reduce the risk of HISC it is considered good engineering practice in design to locate welds away fromgeometrical stress concentrations. Welds located at steep transitions or small fillet radii can be especiallydetrimental, as there will be two interacting stress concentrations: one due to the transition/fillet radii andone due to the weld toe. The uncertainty associated with the actual local geometry of the weld toe is part ofthis picture.Moreover although grinding of weld toes is considered beneficial from a geometrical stress concentrationpoint of view, care should be taken to ensure no detrimental effects of the grinding on the new surface.In addition, single-pass girth welds, especially in relation to start-stop areas, repair welds, and thin-walled(<10 mm) structures, may result in residual stresses significantly beyond the levels accounted for in thisRP. Thus, stress induced in the weld by design loading should be less than 50% of the allowable limits in theRP. Single pass and repair welds as well as start-stop areas may also be subjected to phase unbalance andhigher ferrite content. Specific guidance for how to assess girth welds are given in Sec.5.


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4.6 Fillet weldsFillet welds shall not be used as pressure controlling or pressure retaining welds in systems carryingservice fluids. Fillet welds shall not be used unless satisfactory weld geometry and microstructure can bedocumented. Fillet welds are in many cases more susceptible to high ferrite contents than butt welds due tolow heat input and faster cooling.Several of the known duplex stainless steel HISC failures are related to fillet welds. If fillet welds are usedthey require high attention, with regards to weld material quality, coating quality and stress levels. All filletwelds shall be subject to evaluation regarding the need for mitigation actions against HISC. This wouldinclude:

— Fillet welds require 100% visual and 100% liquid penetration inspection (LPI).— Weld performance qualification (WPQ) testing or production testing (including testing for geometry and

microstructure) on the fillet weld configurations used in production, but not qualified using butt weld. Thenumber of passes is an essential variable.

In addition, single pass fillet welds need particular care, due to the greater risk of high cooling rates and highferrite content.

Guidance note:Fillet welds used for attachment of doubler plates, pipe supports, etc. are not pressure containing. However, the following aspectsmay significantly challenge the conservativeness of this RP:

— welding method and execution

— fillet weld details may receive less attention during fabrication and can erroneously be considered as a non-critical weld

— flaws at the weld toe that may not be visible can initiate cracks, which may extend into the base material

— ensuring satisfactory weld quality of fillet welds

— proper finite element (FE) modelling of fillet welds are challenging.

Thus, the stress induced in the fillet weld by design loading should be assessed, including pressure and thermal mismatch betweenthe doubler plate and pipe.


4.7 Bolted connectionThe use of duplex stainless steel for subsea fasteners (e.g. bolts and studs) exposed to cathodic protection isnot recommended, hence designing and dimensioning of such fasteners is not included in this RP.Threaded holes are frequently used for subsea components, but to date there has been no known failures.Threaded holes should be designed in accordance with an applicable design standard.

4.8 CoatingPolymeric coatings have primarily been applied to reduce the current demand from the sacrificial anodes and/or for thermal insulation. They are then not expected to act as a 100% effective barrier. Even quite narrowcrevices associated with disbonded or damaged coating can lead to significant local hydrogen production andabsorption. If this coincides with a location with high stresses, HISC can occur. However, a high integrity thickinsulation coating (e.g. rubber) may mitigate the threat of HISC at the surface. The type of coating that isapplied on subsea components depends on the type and size of the component and the internal fluid andenvironmental conditions (primarily the operating temperature).Coating shall not be used as the only means of preventing HISC by CP unless agreed by the end user. Thecombined materials selection and design with respect to maximum allowable stress/strain shall be such thatHISC will not occur even if the coating is damaged or removed.


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Note:Some of the reported HISC failures were caused by upset loading conditions beyond the design value. Whenever practical,components in duplex stainless steel that may become exposed to high stresses during commissioning or in service (i.e. withCP applied) should therefore be coated with a coating system qualified for resistance to disbonding at the applicable operatingtemperature. Coating materials and application procedures shall be adequately qualified for resistance to damage and disbondingby mechanical and physical/chemical effects. The design life and possible coating degradation should be taken into consideration.For pipelines the weakest point in a coating system is in the field joints and where the factory coating has been deliberatelypenetrated (e.g. for fastening of anodes). In many cases this coincides with locations where high operational stresses occur.

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SECTION 5 HYDROGEN INDUCED STRESS CRACKING ASSESSMENT

5.1 GeneralThis section provides design and acceptance criteria for duplex stainless steels exposed to CP for category 1and category 2 assessment.The semi analytical linear-elastic assessment (category 1) has limitations of usein terms of complexity in geometry and loading scenario. These limitations are outlined in [2.2].Flow charts for the design processes are shown in Figure 5-3 and Figure 5-4. Requirements fordocumentation of the analysis in reporting are described in [B.3].

5.2 Characteristic material propertiesThe different material grades refer to mechanical properties (see [6.2.1]) at room temperature. Possibletemperature effects shall be considered for temperatures above room temperature.The design criteria in this recommended practice shall apply for temperature de-rated specified minimumyield stress (SMYS), specified minimum tensile strength (SMTS) and the corresponding stress-strain curve.The temperature derating shall be based on one of the following (in prioritised order):

— project design standard derating requirements valid for duplex material— testing of the material; testing according to design standard requirements— derating requirements from another applicable design standard e.g. ASME BPVC II Part D or DNVGL-ST-

F101.

Guidance note:Material derating should reflect local temperature in the area to be assessed (e.g. temperature dependent material propertiesthrough thickness may be used).


If the de-rating is based on testing of the material, see [A.5], such test data shall either be obtained from thematerial supplier, or based on project-specific testing.The microstructures assumed in design should be classified per component type as defined in Table 6-1 in[6.2.3]. Components not listed in Table 6-1 should be considered as having coarse austenite spacing unlessspecial measures are taken to demonstrate that assuming fine austenite spacing can be justified. See [A.6]for austenite spacing measurement.The material factor that shall be used in relation to the acceptance criteria on stress levels is defined in Table5-1.

Table 5-1 Material factor, γ, as a function of austenite spacing classification

γ

Fine austenite spacing 1.00

Coarse austenite spacing 0.85

5.3 Input for category 1 assessmentThe following stresses, stress magnification factors and penalty factors shall be established to evaluate thedesign criteria presented in [5.5.1].

σmem,i = the membrane stress in principal direction iσout,i = the outer fibre stress in the principal direction iσtherm = the stress due to potential thermal gradient over the wall thickness


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Smag,i = stress magnification factor in principal direction iLSPFcat1 = local surface penalty factora = residual stress penalty factorβ = penalty factor for welds in connection to transitions.

A flowchart for category 1 is provided in [5.5.3], Figure 5-3.Note:The Von Mises equivalent stress shall not be used in the HISC assessment.

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5.3.1 The membrane stressThe membrane stress, σmem,i, is the average stress acting normal to the path from the outside to the insideof the component in principal direction i.

Guidance note:For analytical beam approaches or piping analysis the following simplification may be applied:

(5.1)

where:

σmem,long = the membrane stress in the longitudinal direction

σaxial = the axial stress to due pressure, axial force and thermal expansion

σbending,m = the global bending stress due to applied bending moment using mid-section modulus.

(5.2)

where:

σmem,hoop = the membrane stress in the hoop direction

σhoop = the analytical hoop stress from the beam or piping analysis.


5.3.2 The outer fibre stressσout,i is the outer surface stress in the principal direction i.


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Guidance note:For analytical beam approaches or piping analysis the following simplification may be applied:

(5.3)

where:

σout,long = the outer fibre stress in the longitudinal direction

σaxial = the axial stress due to pressure, axial force and thermal expansion

σbending,o = the global bending stress due to applied bending moment using outer section modulus

σlong,max = the maximum longitudinal stress derived from piping analysis.

(5.4)

where:

σout,hoop = the membrane stress in the hoop direction

σhoop = the analytical hoop stress from the beam or piping analysis.


5.3.3 Stress due to thermal gradient over the wall thicknessσtherm is the surface stress arising due to thermal gradient through the wall thickness. This stress is notextracted in regular beam or piping analyses and shall thus be evaluated separately.

σtherm can be disregarded for components with thermal coating and when assessing the hotter side of thecomponent.

Guidance note:

σtherm may be estimated from the following equation:

(5.5)

where:

aL = the thermal expansion coefficient

E = the elastic modulus

ΔT = the temperature gradient in the steel over the effective wall thickness

v = Poisson's ratio.

The equation applies for a thin walled pipe. In case less conservative equations can be justified, e.g. for more thick-walledstructures, such equations may be applied. For pipes a component is considered thick-walled if D/t<15.


5.3.4 The stress magnification factorThe stress magnification factor, Smag, should provide the increase in surface stress compared to the outerfibre stress, σout, due to secondary bending moments created by geometrical features like misalignmentsor transitions. The stresses calculated using Smag do not include the effect of local features like notches orfillets.


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Guidance note:

Analytical formulas may be used to calculate Smag. For stresses acting in the longitudinal direction Smag,long may be evaluatedusing applicable expressions for stress concentration factor (SCF) found in DNVGL-RP-C203. In a category 1 screening analysis of

piping system and its components a conservative value of Smag,long=2 may be used in the HISC assessment.


Note:The standard stress intensity factor (SIF) commonly used for compliance with ASME B31.3/B31.8 shall not be used in the HISCassessment.

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5.3.5 The local surface penalty factorThe local surface penalty factor, LSFPcat1, is used to limit the allowable stresses due to primary loads andsecondary bending moments in presence of fillets or notches. The local stress level does not explicitly enterthe stress criteria, but is rather included through the LSPFcat1. The SCF expressions found in DNVGL-RP-C203 shall not be used to estimate LSPFcat1.

Guidance note:

LSPFcat1 may be evaluated using applicable analytical formulas and nominal stress levels. For fillets with radius, r, equal to

or larger than 10% of the wall thickness, LSPFcat1=3 may be used without further calculations. For local notches in machined

transitions with radius larger than 1mm LSPFcat1=3 may be used without further calculations. For welds LSPFcat1=3 may be

assumed for the weld toe. For welds that are flush-ground LSPFcat1=1 may be assumed. Notches, i.e. local features reducing the

wall thickness, should always be analysed using the category 2 approach. LSPFcat1 for category 1 may also be established byusing the calculation procedure presented in [A.2].


5.3.6 Residual stressThe influence of potential residual stresses for category 1 should be considered through introduction of apenalty factor, α, for:

— A location within the distance Lres from a weld centre line.— A component subjected to cold forming.

Lres represents the distance in which residual stresses from welding can be of influence and may beestimated from the following equation:

(5.6)

where:

R = the outer pipe radiust = the wall thickness.

Only residual stresses acting parallel to the applied stress level should be considered. E.g. residual stressesin the hoop direction of girth welds and in the longitudinal direction of seam welds may be taken as zero, anda = 1 may be assumed for such cases. For materials subjected only to normal straightening operations e.g.cold straightening of pipes, a = 1. The penalty factor, a, is defined in Table 5-2.


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Table 5-2 Penalty factor for residual stress, a

Scenario a

Inside Lres or for cold formed material 0.9

Other 1.0

Guidance note:

Lres may alternatively be established from a simplified numerical analysis according to [A.4]. Lres (Figure 5-1) should be definedas the distance from the weld centre line to the position where the residual stress has fallen to 10% of the maximum value in the

analysis. For detailed assessment of Lres weld analysis may be used.

Figure 5-1 Representation of Lres measurement


Note:Cases where residual stress assessment per the guidance above will be non-conservative are:

— welding, and especially repair welding, of thin components with unfavourable microstructure see [4.5]

— heavily cold formed components (> 20% plastic strain).

Special considerations should be made for such cases.After solution annealing of casting repair welds and seam welds in fabricated pipes, residual stresses and strain from welding areconsidered negligible and do not need to be considered for the HISC design criteria.

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5.3.7 Welds close to transitionsThe combination of weld toes and steep transition may potentially be detrimental. Therefore, an additionalpenalty factor, β, is introduced when such cases are considered, see Table 5-3.


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Table 5-3 Penalty factor β for welds near transitions

Geometry β

Weld toe + trasition of 30o or less 1.0

Weld toe + transition >30o and <45o 0.9

Weld toe + transition >45o 0.8

If the distance Ltran (Figure 5-2) between the weld toe and the root of the transition is larger than 5 mm β =1.

Figure 5-2 Representation of Ltran measurement

5.4 Input for category 2 assessmentTo perform a category 2 assessment, the following shall be established:

— non-linear material properties— εmem which is the applicable non-linear membrane strain— εpeak which is the maximum principal surface strain— LSMFcat2 is the local surface magnification factor— residual stress considerations.

The use of category 2 assessment requires a non-linear FEA analysis of a representative geometry. Guidancefor mesh convergence /1/ is shown in [A.1]. A flowchart for category 2 is provided in [5.5.4].

5.4.1 Selection of material parameters for non-linear analysisThe material hardening curve should to have the following characteristics:

— linear elastic to 0.1% total strain— 80% of SMYS corresponding to 0.3% total strain


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— SMYS corresponds to 0.5% total strain— an appropriate curve should describe the strain hardening after 0.5% strain.

It is not necessary to include cold-creep in the material representation/analysis.Guidance note:Stress/strain data measured by tensile tests on samples taken from the component may be used in the FEA if enough tests havebeen made to enable a statistical assessment (see [A.5]) of property variations.


Guidance note:Stress/strain characteristics based on ASME BPVC VIII Div. 2 may be used as an alternative to the recommendations in [5.4.1].


5.4.2 Calculation of the membrane strainThe membrane strain, εmem, should be extracted from the FEA by calculating the average strain normal tothe path through the thickness of the component.The path along which the strain should be evaluated shall be the minimum distance from the outside to theinside of the component. For complex geometries, it shall be substantiated that this will be the path with thehighest membrane load. For components with biaxial loading the most critical direction shall be identified.

5.4.3 Calculation of the applied strainThe applied strain should be extracted as the maximum principal strain in the surface of a FEA, εpeak,representing the actual geometry and accounting for non-linear material behaviour. The strain should includeelastic strains and plastic strains generated while CP is active.

Guidance note:There are no clear indications that moderate plastic deformation prior to application of CP may significantly reduce the material'sresistance to HISC.Plastic strains introduced prior to application of CP may be disregarded if e.g. a sequential analysis (see [A.3]) accounting for theloading history is performed.Thermal expansion/contraction of components leads to thermal strain in the material. However, this thermal strain is notnecessarily a risk factor for HISC. If the component is free to expand/contract, stresses will not be induced in the material.In case a category 2 analysis is necessary to perform for a weld one should model the weld toe with a radius of 2 mm in case oflocal grinding of the weld toe is performed. For untreated welds a local weld toe radius of 1 mm should be used. A situation withboth a small radius and an undercut may lead to higher local strain than a scenario with only a radius of similar size. Alternativeweld toe radii may be applied, but such radii should be representative of the production parts. This may require fabricationspecification and NDE that is more onerous than standard requirements. It is not necessary to model weld caps in category 2

analysis, unless it is nearby a transition (Ltran < 5).


5.4.4 Calculation of local surface magnification factorThe allowable maximum surface strain is defined to depend on LSMFcat2. For the category 2 assessmentLSMFcat2 may be determined from:

A FEA according to [A.2] with:

(5.7)


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where:

σmax = the maximum principal surface stressesσlin = the sum of the membrane and global bending stress averaged over the wall thickness.

or from applicable analytical formula based on nominal stresses using:

(5.8)

where:

SCFanalytical = the value of the stress concentration factor extracted from the analytical formula.

It is always acceptable to perform a category 2 assessment assuming LSMFcat2 = 1.Note:

The increase of allowable critical strain with LSMFcat2 defined in [5.5.2] is based on empirical observations from experimental

tests with primarily tension loading. The procedures to define LSMFcat2 for category 2 are formulated to avoid non-conservativeestimates due to overestimation of the applicable stress concentration factor. For the analytical equation, the factor 2 is introduced

in line with the potential default value of 2 for Smag. The SCF expressions found in DNVGL-RP-C203 shall not be used to calculate

LSMFcat2.

The LSMFcat2 shall be estimated according to [A.2] (e.g. it is not allowed to assume a SCFanalytical = 3 for a weld cap).

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5.4.5 Residual stress considerationsResidual tensile stresses are assumed to reduce the allowable total strain in the material and should beconsidered also for a category 2 analysis. Thus, the residual strain is explicitly accounted for by reducing theallowable strain with a factor of 0.05* LSMFcat2 in the non-linear criteria as given in Table 5-4.

5.5 Design criteria

5.5.1 Category 1 assessment design criteriaComponents designed in line with the category 1 assessment shall fulfil the following design criteria:

(5.9)

and

(5.10)

in all relevant directions i, where:

σmem,i = the membrane stress in direction i defined in [5.3.1]σout,i = the outer fibre stress in direction i defined in [5.3.2]σtherm = the additional stress due to thermal gradients over the wall thickness defined in [5.3.3]Smag,i = the stress magnification factor in direction i defined in [5.3.4]a = a factor reflecting the influence of residual stresses defined in [5.3.6]


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β = a factor reflecting the potential interaction between weld toes and transitions defined in [5.3.7]γ = a factor reflecting the microstructure categorization defined in [5.2]LSPFcat1 = penalty factor reflecting local stress concentrations due to fillet radi idefined in [5.3.5]SMYS = the applicable yield stress accounting for de-rating.

Guidance note:In case equally or more stringent membrane stress criteria are used as a part of other design standards applied, reference can bemade to this and explicit demonstration of fulfilment of the membrane stress criterions is not necessary. For cases applicable forcategory 1 assessment the relevant directions i to check will be the longitudinal and hoop directions.


5.5.2 Category 2 assessment design criteriaComponents designed in line with category 2 assessment need to fulfil the following design criteria:

(5.11)

and

(5.12)

where:

εmem = the average strain normal to wall thickness described in [5.4.2]εpeak = the applied maximum principal surface strain due to loads acting while CP is applied described in

[5.4.3]εc = the maximum allowable surface strain due to loads acting while CP is applied as defined in Table

5-4.

Table 5-4 Non-linear strain criterion

εc [%]

Outside Lres and no cold forming Inside Lres or cold formed material

Fine austenite spacing 0.50 · LSMFcat2 0.45 · LSMFcat2

Coarse austenice spacing 0.35 · LSMFcat2 0.30 · LSMFcat2

The LSMFcat2 should reflect the actual geometry, however, with the following restrictions:

1) LSMFcat2 used to determine the allowable strain shall be a maximum of 4 when assessing scenariosoutside Lres.

2) LSMFcat2 used to determine the allowable strain shall be a maximum of 3 when assessing scenariosinside Lres.

Guidance note:In case a 3D linear elastic FEA of a representative geometry is performed and the membrane stress criterion is met, and the

peak surface stress is not higher than γ · SMYS. The component is taken as acceptable without the need to perform a non-linearanalysis.



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5.5.3 Flow chart for category 1 assessmentThe suggested work flow for category 1 assessments is presented in Figure 5-3.

Figure 5-3 Category 1 flowchart


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5.5.4 Flowchart for category 2 assessmentThe suggested work flow for category 2 assessments is presented in Figure 5-4.

Figure 5-4 Category 2 flowchart


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SECTION 6 MATERIAL AND FABRICATION RECOMMENDATIONS

6.1 GeneralThis section gives recommendations for materials. This includes the following:

— Limitations related to materials that shall be met in order for the design criteria in this RP to be applicable.— Locations that may require additional precautions in fabrication practice.— Austenite spacing classification.

6.2 Material limitationsThis RP covers material generally referred to as 22Cr and 25Cr, DSS and SDSS. The limitations set onmaterials for which the design criteria in this RP apply are primarily based on limits in available test data.If materials outside the limits given in [6.2.1] are used, a detailed assessment should be carried out andqualification testing should be considered.Acceptable quality requirements for duplex stainless should be according to a recognised industry standarde.g. DNVGL-ST-F101, API 17P/ISO 13628-15, ISO 17781, NORSOK M-601 and M-630.

6.2.1 Mechanical propertiesThe field experience with duplex stainless steels is primarily from applications where the SMYS given in thematerial standards has been applied in design.For HISC assessment according to category 1, the following SMYS for room temperature shall be used:

— 450 MPa for 22Cr duplex stainless steel— 550 MPa for 25Cr super duplex stainless steel.

For HISC assessment according to category 2 one may take benefit from higher actual mechanical propertiesin design by testing as per [A.5].SMYS for material in the weld area should be taken equal to the SMYS of the base material.

6.2.2 Defect considerationThis RP is not intended for application to designs requiring crack tolerance, for example those based onapplication of fracture mechanics. However, design locations where the stress/strain utilisation approachesthe maximum allowable by this RP may benefit from additional precautions/mitigations in design andfabrication: The fracture toughness of hydrogen embrittled duplex/super duplex stainless steel is low(crack tip opening displacement (CTOD) < 0.05mm) and there may be uncommon instances where design/application/service conditions challenge the conservativeness of this RP. For these uncommon instances,additional mitigations may be needed.Applicable mitigations include but are not limited to: use of coating (reducing hydrogen uptake); appropriateNDE methods or procedures (reducing the probability of flaws); limitations on weld repairs (avoidance of highresidual stresses).

6.2.3 Austenite spacingA HISC crack generally propagates as a straight cleavage-like fracture through the ferrite phase. The crackmay be arrested or propagate around or through the austenite phase depending on crack size, hydrogenconcentration and stress levels. Consequently, all fabrication techniques that tend to decrease austenitespacing (free ferrite path) are favourable. Testing confirms that materials with fine phase spacing have agreater resistance to HISC than materials with coarse phase spacing.Table 6-1 provide a list of materials assumed to have microstructure with fine austenite spacing. Theaustenite spacing is considered fine if it is less than 30 µm.


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Table 6-1 Components and classification for fine austenite spacing

Component classified as having fine austenite spacing Comment

Manufactured with HIP materials

Weld metal However, the heat affected zone (HAZ) has the sameaustenite spacing as the base material.

Tube and pipe made by extrusion, seamless rolling ordrawing

All dimensions and wall thicknesses are included.This also includes fittings made from such pipes and tubes.

Rolled plate with wall thickness less than 25 mm This also includes pipes and fittings made from rolled platewith such wall thickness.

Materials that are not covered by the categories presented in Table 6-1 shall be considered to have coarseaustenite spacing, unless fine austenite spacing can be verified by testing (see [A.6]).

Guidance note:Tests and failures have shown that adverse grain flow, when the ferrite grains are oriented perpendicular to the principalstresses, can give increased susceptibility to HISC. For items with an anisotropic grain structure (forged or rolled material) themanufacturing route should be reviewed to ensure a favourable grain flow.



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SECTION 7 REFERENCES

7.1 References/1/ NAFEMS International Association for the Engineering Modelling Analysis and Simulation Community.

Knowledge base (internet). Available from: https://www.nafems.org/join/resources/knowledgebase/

https://www.nafems.org/join/resources/knowledgebase/


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APPENDIX A PRACTICAL MEASURES (INFORMATIVE)

A.1 Mesh convergenceThe guidance for mesh convergence presented in this appendix applies to static linear stress and non-linearanalysis and the use of finite-element software.

A.1.1 Convergence criteriaIn the region of interest a result parameter in form of e.g. stress, strain and/or linearized stress for throughthickness convergence is required. The result parameter should be plotted against level of mesh refinementto indicate when convergence is achieved. At least three points are needed to create a mesh convergencecurve. An example of a mesh convergence curve is illustrated in Figure A-1.

— Linear analysis: A mesh convergence is considered reached with a 3% convergence error on stress, whena halving of the local element size is applied.

— Non-linear analysis: A mesh convergence is considered reached with a convergence error of 5% on totalstrain, when a halving of the local element size is applied.

Figure A-1 Mesh convergence

A.1.2 Important factors in meshingThe most important factors in meshing are:

— For linear (straight sided) or quadratic elements, depending on the analysis type pros and cons exist forboth element orders. However, for analyses performed within extent of this document a proper meshconvergence check will ensure that the results will be independent of element order.


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— Linear elements require more elements to represent a junction, because of the geometric effect of usingstraight sided elements.

— Elements located at a distance away from a critical region may be significantly larger if the quality of themesh, stiffness, transitions and geometry representation are ensured.

A.1.3 Aspects of mesh convergence— Mesh convergence checks should only be extended to similar models with similar loadings (e.g. if stress

gradients differ, a new mesh convergence should be performed).— A significantly increased load magnitude will increase the stress/strain gradient, thus giving a lower

accuracy to the fixed allowable stress/strain.— Assuming that an element mesh is convergent for stress just because it has the same element size as a

converged mesh in a non-similar model, is not valid.— Assuming that an element mesh is convergent for stress just because it has the same element size as a

converged mesh at a different location in a similar model, is not valid.

A.2 Local surface penalty and magnification factors based onlinearizationUsing solid 3D FE-model for establishing LSPFcat1 and LSMFcat2 may be performed according to thefollowing procedure:

1) Establish FE-model including geometrical features where LSPFcat1 or LSMFcat2 shall be established.2) Establish linear/non-linear material properties.3) Include all relevant loading.4) Check mesh convergence.5) Perform linearization of stresses through the wall thickness at the root of the notch or transition

according to Figure A-2.

Figure A-2 Path for establishing LSPFcat1 and LSMFcat26) The stress linearization should be performed based on the maximum stress (normal to the path) through

the wall thickness.7) Calculate LSPFcat1 or LSMFcat2 according to Figure A-3.


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Figure A-3 Calculation LSPFcat1 and LSMFcat2Guidance note:

Either a linear or non-linear FEA may be used to generate LSMFcat2. If yielding has not occurred, the linear and non-linear analysiswill give equal results. In locations where yielding has occurred, however, the non-linear analysis will give more conservative

results, with lower LSMFcat2. Linear elastic FEA should be used to generate LSMFcat1.


A.3 Sequential loadingIt is acceptable to include the possible beneficial effect of proof loading performed before the CP systemis active by first performing an analysis of the proof load case and sequentially performing the analysis ofthe service loading. The strain to be considered is then the total tensile strain increment induced during theservice loading, i.e. the current elastic strain plus plastic strain increments induced during service. In thiscase a detailed non-linear FEA including non-linear materials behaviour should be performed.

A.4 Numerical estimation of LresLres may be estimated by performing a simplified linear FEA, by modelling the weld and locally reducing thetemperature in the weld until the Von Mises stress equals yield stress in the surface of the material. One endof the component should be fixed.Material properties relevant for room temperature should be used (i.e. no up-grading of the strength dueto the lower temperature). Lres is then equal to the distance from the weld where the von Mises equivalentstress has declined to 10% of the characteristic yield stress.

This simplified method is only applicable to determine Lres.

A.5 The material hardening curve based on testingThe characteristic engineering stress-strain curve used for non-linear elastic-plastic analyses in category 2shall be based on SMYS/SMTS.


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If statistically significant test results are available, the characteristic engineering stress-strain curve may bebased on:

— Characteristic yield stress, Rp0,2 char, defined as the mean value subtracted by three standard deviationswith 95% confidence from the yield stress (Rp0.2) test results.

— Characteristic tensile strength, Rm char, defined as the mean value subtracted by two standard deviationswith 95% confidence from the tensile strength (Rm) test results.

The characteristic strain at the tensile strength (εchar) is defined as the largest registered strain at the tensilestrength for the total population of test results.The characteristic yield stress shall not be defined higher than 650 MPa at room temperature.For temperatures, higher than 25°C both the yield stress and tensile strength shall be de-rated in accordancewith a recognized standard.

A.6 Measuring the austenite spacingAustenite spacing measurements should be carried out in accordance with ASTM E112-13. It will ensurethat general issues such as equipment calibration and reporting are carried out in accordance with generallyrecognized industry practice. Requirements relevant for measurement of austenite spacing are given inparagraph 17 of ASTM E112-13.A characteristic austenite spacing is defined as mean-two standard deviations with 95% confidence. If thischaracteristic austenite spacing is less than 30 micrometer, both at (close to) the external and internalsurface, the austenite spacing may be considered as fine in the context of this RP.


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APPENDIX B DIFFUSION AND TECHNICAL REPORTING(NORMATIVE)

B.1 DiffusionThe basic principle to be applied to disregard HISC acceptance criteria on the inside of the material isdemonstrating that hydrogen does not reach the inside of the material through diffusion. The criterion for thelatter is taken as an increase in hydrogen concentration of ≤ 1 ppm above the bulk hydrogen concentration(Cb) in the material, within the design lifetime for a representative steady state outer surface temperature.HISC acceptance criteria on the inside of the component may be disregarded if:

— The assumed temperature in Table B-1 is equal to or higher than the outer surface steady statetemperature of the component.

— The assumed time in Table B-1 is equal to or longer than the design life of the component.— The corresponding penetration depth for the given combination of assumed temperature and assumed

time period in Table B-1 is equal to or smaller than the wall thickness of the component.

The described criterion applies for both uncoated and coated components.

Table B-1 Hydrogen diffusion depth for C=Cb+1 [mm]

Temperature [oC]Time [years]

4 10 15 20 45 60 80 100 121 150

1 1,4 1,5 1,7 1,8 2,6 3,1 3,9 4,7 5,6 6,6

4 2,8 3,1 3,3 3,6 5,2 6,2 7,8 9,4 11,1 13,3

5 3,1 3,4 3,7 4,0 5,8 7,0 8,7 10,5 12,5 14,8

10 4,4 4,8 5,3 5,7 8,2 9,9 12,3 14,9 17,6 21,0

15 5,3 5,9 6,4 7,0 10,0 12,1 15,1 18,2 21,6 25,7

20 6,2 6,8 7,4 8,1 11,6 13,9 17,4 21,1 24,9 29,7

25 6,9 7,6 8,3 9,0 12,9 15,6 19,5 23,5 27,9 33,2

30 7,5 8,4 9,1 9,9 14,2 17,1 21,3 24,8 30,5 36,3

35 8,1 9,0 9,8 10,7 15,3 18,5 23,0 27,9 33,0 39,2

40 8,7 9,7 10,5 11,4 16,3 19,7 24,6 29,8 35,2 41,9

45 9,2 10,2 11,1 12,1 17,3 20,9 26,1 31,6 27,4 44,5

50 9,7 10,8 11,7 12,7 18,3 22,1 27,5 33,3 39,4 46,9

Note:Data in the table is conservative and takes no account of the mitigating effect of intact coating and insulation, or any calcareousdeposits that may form on exposed metal surfaces.

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Guidance note:Diffusion considerations addressed in this RP are limited to estimation of diffusion depths. The diffusion analyses used to derivethe values in Table B-1 are based on fitting data to experimentally measured hydrogen profiles from retrieved subsea components.As a part of these analyses an assumption about temperature independence of hydrogen solubility has been applied, as noquantitative information regarding this has been available from the literature.


B.2 Diffusion modellingThe following simple modelling procedure is suggested as an indicative procedure for FE-based mass diffusionanalysis.

1) Temperature estimation:a thermal analysis should be conducted in order to establish the temperature distribution in thecomponent if no real temperature data is available.

2) Mass diffusion analysis:a Fick's law analysis (available in most of the commercial FE software) may serve as basis for massdiffusion in the component under study. The parameters needed for these analyses should be establishedas follows:Diffusivity : The value is obtained from the following expression:

(B.1)

Equation (B.1) can be reproduced by Table B-2.

Table B-2 Diffusivity dependency on TKTK [oC]

TK [oC] D [mm2/sec] TK [oC] D [mm2/sec]

161.6 1.5E-07 60.2 2.0E-08

143.5 1.1E-07 49.4 1.6E-08

126.8 8.6E-08 39.4 1.2E-08

111.5 6.5E-08 29.9 8.9E-09

97.2 4.9E-08 21.0 6.7E-09

84.0 3.7E-08 9.8 4.6E-09

71.7 2.8E-08 2.0 3.5E-09

The diffusivity is obtained after the temperature distribution is established.Solubility : it is usually needed in FE analysis in order to carry out mass diffusion simulations. Thesuggested value is: 0.033 wppm·mm·N-1/2 (which is assumed to be temperature-independent);The following hydrogen boundary conditions should be used:

— Exposed surface concentration: defined based on the following equation.

(B.2)

For example, for a component at 4 oC, C0 = 219 wppm.— Internal surface: Cint = Cb = 2 wppm. Cb is the bulk concentration.


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3) Engineering assessment:all the areas of the component whose concentration is below Cb +1 wppm may be considered safeagainst hydrogen cracking.

B.3 ReportingThe HISC analysis report should as a minimum contain the following information:

a) Executive summary briefly describing the scope of the analysis and the main conclusion with reference tocompliance with applicable design standards and this document.

b) Description of the component, and its intended use with explanation of its functionality.c) Reference to governing design specifications and applicable pressure design standards.d) References to project design premises and a summary of applicable loads and other design premises.e) List of all relevant design drawings.f) Component geometry analysed with reference to drawings. Any simplifications done in the geometry

model should be discussed. This should include how fabrication tolerances and corrosion allowance areaccounted for.

g) Materials including designation and reference material standard or specification. Lists of relevant materialproperties within the design temperature range.

h) Categorization of loading and assumptions with respect to external loads including sensitivity tosubsidence (including uniform), soil resistance, flowline expansion and other credible influencing factors.

i) Clear conclusion of compliance with this document.For category 1 analysis:

j) Results of relevant stresses from piping/analytical evaluation presented in a tabular format includingidentification of assessed locations.

k) The basis and justification for choice of Smag, LSPFcat1 and σtherm.l) Justification of choice of penalty factors.

For category 2 analysis:m) The FE-model discretisation, with type of elements, discussion on element size with respect to accuracy

in calculated stresses and strains. A mesh convergence check according to [A.1] App.A may be used toconfirm the accuracy of the FE-model.

n) Where values for weld cap radii, other than those defined in this RP, are selected, these shall be reportedand justified.

o) Conclusion from FE model verification and load application verification including checking of reactionforces.

p) Description and colour plots of loads application, boundary conditions and contact elements used.q) Calculation of LSMFcat2.

Cha

nges

– h

isto

ric


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CHANGES – HISTORICThere are currently no historical changes for this document.

About DNV GLDNV GL is a global quality assurance and risk management company. Driven by our purpose ofsafeguarding life, property and the environment, we enable our customers to advance the safetyand sustainability of their business. We provide classification, technical assurance, software andindependent expert advisory services to the maritime, oil & gas, power and renewables industries.We also provide certification, supply chain and data management services to customers across awide range of industries. Operating in more than 100 countries, our experts are dedicated to helpingcustomers make the world safer, smarter and greener.

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