structural design of metallic components

7/21/2019 Structural Design of Metallic Components

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1 Copyright 2014 by ASME

Proceedings of the ASME 2014 Pressure Vessels & Piping ConferencePVP2014

July 20-24, 2014, Anaheim, California, USA

PVP2014-28712

STRUCTURAL DESIGN OF METALLIC COMPONENTS IN ISO 13628-7

Finn KirkemoStatoil ASA

Oslo, NorwayEmail: [email protected]

ABSTRACT

ISO 13628-7 (ISO-7) for completion and workover(C/WO) riser systems was published in 2005 and adopted backby API as API RP 17G 2ndedition 2006 (identical). ISO-7 givesrequirements and recommendations for the design, analysis,materials, fabrication, testing and operation of C/WO risersystems. This paper provides a brief introduction andbackground to some of the design and material requirements inISO-7. The main focus is on requirements for calculation ofstatic and cyclic (fatigue) capacities of metallic components inthe C/WO riser system subjected to pressure and external loads.

Some differences between ISO-7, API 6A (6A), API 6X (6X),API 17D (17D), ASME VIII Div. 3 (Div.3) and ASME VIIIDiv.2 (Div.2) are also included.

INTRODUCTION

The first edition of API 17G was released in 1995 as arecommended practice for completion/ workover risers. TheISO-7 committee saw the need to update API 17G toincorporate the latest industry practice and to be moreprescriptive and self-contained than the first edition of API17G. ISO-7 was a rewrite of the first edition of API 17G andincluded a major update regarding design, material and

fabrication requirements for pipes and connectors and riserconnector qualification requirements.

ISO-7 was written by a team including end users andmanufactures based primarily in Europe in the period 1999-2005. The first edition of ISO-7 was released in 2005 and wasadapted back by API as API RP 17G second edition (identical)in 2006. The Petroleum Safety Authority in Norway refers toISO 13628 series for subsea facilities; hence ISO-7 ismandatory in the North Sea for C/WO riser systems. ISO-7 hasbeen used since 2006 by Statoil for all new subsea field

development projects in the North Sea for completion/workoverrisers, wellhead systems and for connectors on subsea trees.

The ISO-7 standard is made up of eleven clauses, twonormative appendixes and seven informative appendixes. Theclauses and annexes addressed in this paper are Clause 6 fordesign requirements which refers to Annex C for determinationof cyclic capacities, Annex D for determination of staticcapacities, Annex I for qualification of connectors and Clause 7for materials and fabrication requirements. Materiarequirements in Clause 7 are directly linked to componentstructural capacities in Clause 6 to ensure ductile failure modesand that final mechanical properties of the components are

representative for the critical cross sections.API 17G is under revision from a recommended practice to

a specification; see Stawaisz et al (2014). Some proposedmodifications to ISO-7 which are in the proposed 3rd edition ofAPI 17G are also included in this paper and referred to as 17GThe modifications are based on experience with use of ISO-7.

The present paper will focus on structural designrequirements with associated material requirements. Emphasisis given to structural failure modes such as burst, plasticcollapse, brittle (unstable) fracture and fatigue failure opressure-containing components under combined tension andbending, i.e. in the primary load path of the riser. Some enduser experience is also included. The comments in this paper

are the personal opinions of the author and should not beconsidered as interpretations of ISO-7.

NOMENCLATURE

a surface flaw height (depth) intercept of design M-N curve with N axisB plate thicknessCALAS carbon and low alloy steelsCVN Charpy V-notch


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C/WO completion/workover2c surface flaw lengthd bore diameterDF fatigue design factorFM fracture mechanicsFd static design factorF

i,mean static load components

Fi,amp amplitude of dynamic load componentsFtot total design loadm inverse slope of the M-N curveM bending momentMc single load bending capacity (limit load)MT magnetic testingNDT nondestructive testingN number of cycles to failure at constant load

rangepd design pressurepb,c cylinder burst pressure capacity (closed end)pc single load pressure capacity (closed end)pi internal pressure

po external pressurept test pressureRWP rated working pressureRWL rated working loadS-Ncurve

graphical presentation of the dependence offatigue life (N) on fatigue strength (S)

t thicknessTe effective (applied) tensionTc single load effective tension capacityUT ultrasonic testingYu tensile strength temperature derating factorYy yield strength temperature derating factorM bending moment range (double amplitude)

equivalent von Mises plastic strain yield strength tensile strengthGENERAL APPROACH USED TO WRITE ISO-7

The guiding philosophy used in the development of ISO-7was to use present industry practice around year 2000 forproduction risers represented by DNV-OS-F201 (2001) and APIRP 2RD (1998) in combination with the high pressure codesDiv.3 (1997)and ASME B31.3 Chapter IX (1999). In additionto the referred codes, the following documents were consulted:

DNV-OS-F201: Kirkemo, et al (1999), Mrk, et al (2001),and Kirkemo (2001).

ASME B31.3 Chapter IX: Sims (1994).

Div.3: ASME VIII and III criteria (1969), Sims (1997) andMraz and Kendall (2000).

DNV-OS-F201 (2001) was used as the design factors fordifferent failure modes were calibrated to a consistent risk levelapplicable to offshore risers systems operating from floating

vessels. C/WO risers are high pressure systems; hence, both thehigh pressure section in ASME B31.3 (1999) and Div.3 (1997)were consulted in the preparation of ISO-7.

EDP

LRP

Figure 1 Open sea WOR system

NEW TECHNOLOGY

ISO-7 uses the latest (at year 2001) industry designmethods with associated design factors (safety factors) tooptimize design such as: minimize wall thickness (weightreduction) and optimize fatigue life. The ISO-7 design methodsgive increased insight into structural behavior compared witholder design methods (i.e. 6A). A pre-requisite for the use of

ISO-7 design methods and criteria is that materialmanufacturing procedures, mechanical testing, NDT andqualification guarantee the required quality of the components.

Some of the new design approaches included in ISO-7compared to API RP 17G 1stedition are:

Explicitly addressing relevant failure modes

Explicitly addressing of the effect of external loads

Introduction of accidental design conditions

Limit load based closed form pipe capacity equations

Elastic-plastic stress analysis of components

Fatigue design requirements

Use of offshore calibrated design factors

Global riser analysis requirements

Representative mechanical testing

Thermal derating of material strength above 50C (120F)

Material toughness requirements

Ultrasonic testing in lieu of radiographic testing

Connector qualification testing


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CLAUSE 1 - SCOPE AND LIMITATIONS

Clause 1 describes the scope of application of ISO-7. ISO-7 gives requirements and recommendations for the design,analysis, materials, fabrication, testing and operation of subseacompletion/workover (C/WO) riser systems run from a floatingvessel, see Figure 1. It is applicable to all new C/WO risersystems and may be applied to modifications, operation ofexisting systems and reuse at different locations and withdifferent floating vessels. ISO-7 is intended to serve as acommon reference for designers, manufacturers andoperators/users, thereby reducing the need for companyspecifications. ISO-7 is limited to pressure-containing andprimary load bearing components manufactured from carbonand low alloy carbon steels (CALAS). The term pressure-containing component means a component whose failure tofunction as intended results in a release of wellbore fluid to theenvironment, e.g. valve bodies, bonnets, pipes and boltingwhich join pressure-containing components.

ISO-7 is a system based standard. System engineering is

conducted to ensure that the C/WO system and its equipmentare designed, manufactured, fabricated, tested, operated andmaintained for their intended use, throughout its intended life.It is important to note that the manufacturers can design,manufacture and supply equipment independent of systemrequirements.

The design methodology in ISO-7 assumes ductile materialbehavior. ISO-7 is applicable for design pressure (ratedworking pressure) between 34,5 MPa (5 ksi) and 138,0 MPa(20 ksi) at minimum design (operating) temperature. Therecommended static design method does not have any pressurerating limitations. Design temperatures (temperature ratings)includes the range from temperature class K, -60C (-75F) to

temperature class Y, 345C (650F). Maximum temperature isbelow the creep limit and the stress relaxation limit for metallicmaterials. Minimum temperature is limited to the ductility andtoughness performance of the metal to ensure ductile failuremode. Temperature class Y is proposed to be removed from API17G. Cyclic design methods are limited to high cycle fatigue(stress life) and temperatures in the referred codes; i.e. DNV-RP-C203, BS 7608, BS 7910 and ASME FFS-1.

CLAUSE 4 - SYSTEM REQUIREMENTS

GeneralSystem requirements are given in clause 4 of ISO-7.

Clause 4 gives the principles for safety, design and operationand barrier requirements. The governing design principle ofISO-7 is that the overall system design shall be fail-safe.

A C/WO riser system is run and operated from a floatingvessel and it forms a high pressure conduit from a subsea treeor tubing hanger to the floating vessel. C/WO riser systems areused for well completion, well testing, well servicing withwireline and coiled tubing and well isolation. The riser isexposed to ocean environmental loads such as hydrodynamicloads from waves and current in addition to vessel motions. A

C/WO riser is classified as a temporary riser and normally has alimited operating envelope. In situations where operatingconditions are expected to exceed the allowable, the well is shutin and the riser is either disconnected and hung-off or retrievedIn addition it may serve as a running string for the subsea treeand for the tubing hanger.

A C/WO riser system includes riser joints, riser connectorsworkover control systems, surface flow trees, surface treetension frames, lower workover riser packages, valves, ramslubricator valves, retainer valves, subsea test trees, shear subsand tubing hanger orientation systems.

Wellhead connector

17D: DF=0,67

Subsea tree block

17D: DF=0,67

LRP connector

17D: DF=0,67

LRP block

17D: DF=0,67

Wellhead

17D: DF=0,67

EDP connector

17D: DF=0,67

EDP block

17D: DF=0,67

Stress joint

ISO-7: DF=1,00

Wellhead connector

17D: DF=0,67

Subsea tree block

17D: DF=0,67

THRT

17D: DF=0,67

Subsea test

tree

ISO-7:

DF=1,00

Wellhead

17D: DF=0,67

BOP

stack

BOP

connector

Tre mode (open sea) Tubing hanger mode (within BOP)

Figure 2 Equipment codes and design factors for accidenta

load condition

The C/WO riser interfaces the following 17D equipmentFigure 2:

Tree mode (open sea) operation: Christmas tree andwellhead system.

Tubing hanger (thru BOP/drilling riser) mode: Tubinghanger, tubing hanger running tool and wellhead system.

API 17D is a component specification where therequirement is to design and qualify equipment for rated


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working pressure (RWP), rated loads (normal operating) andhydrostatic test pressure. For C/WO riser applications, it isnormal industry practice to ensure that the load combinationsdetermined in ISO-7 (e.g. normal, extreme and accidentalloading conditions) do not exceed the rated capacity (i.e.normal capacity) of 17D equipment, see Figure 2. Tubinghanger mode in Figure 2 applies for horizontal tree. In caseswhere the rated capacity is exceeded, the equipment should bedesigned, manufactured and qualified in accordance with ISO-7. Compared to 17D, ISO-7 permits higher design stress levelsin materials for extreme and accidental load conditions, butrequires more extensive design analysis, as well as additionalnon-destructive testing, material toughness, mechanical testingand qualification of connectors. 17D mentions fatigueconsiderations but does not specify requirements and refers to

ISO-7. However, no reference to material, fabrication andquality requirements in ISO-7 are given in 17D, hence 6Amaterial requirements apply.

17G propose to include lower riser package (LRP) andemergency disconnect package (EDP) bodies with connectorsas part of 17G. In addition to design factors for normal (0.67)and extreme (0.8) an increased safety margin for accidental(0.9) is proposed for well barrier elements that remain in placefollowing an emergency disconnect, e.g. subsea test tree andLRP with connector.

Design processIn the design of C/WO riser components all relevant factors

shall be taken into account to ensure that the equipment will besafe in operation. Design of C/WO riser systems typicallyinvolves the following main steps:

Establishment of a design basis, e.g. minimum bore (drift)requirement, design pressure, design temperature, externalpressure and material selection of pipe and components;

Perform static design of pipe (initial sizing-wall thicknessby hand calculations):a. Internal pressure (burst) design with zero external

pressure, i.e. pipe sizing (wall thickness);b. Collapse due to external pressure only with the empty

pipe (zero internal pressure);c. Longitudinal load due to static effective tension and

internal pressure (plastic collapse) at top of riser;

Select/design C/WO riser system components/connectors:

a. Connector/connection selection based on static andcyclic capacities;

b. Weak link/safety joint sizing and static weak linkglobal analysis;

Perform global riser analysis to determine external loadeffects to:a. Determine operating limitations;b. Calculate fatigue lives and fatigue inspection intervals.

In order to obtain sufficient bending moment capacity ofstandard pressure rated components, components with higherpressure rating and/or larger size are normally selected.

Test pressure is usually the governing case for cylinderwall thickness design in accordance with 6A/17D. This is nothe case for ISO-7 where design pressure at operatingtemperature normally governs and not the test pressure. Insome cases, combined design pressure and external loadseffects may be governing.

CLAUSE 5 - FUNCTIONAL REQUIREMENTS

Equipment functional requirements for all equipmenwithin the scope of ISO-7 including workover control systemare specified in clause 5 of ISO-7. The functional requirementsare aligned with the principles for safety, design and operationand barrier requirements specified in clause 4 of ISO-7.

CLASUE 6 - DESIGN REQUIREMENTS

GeneralDesign requirements for all pressure-containing and/or

primary load bearing components, are given in clause 6 of ISO-7. The scope of clause 6 includes requirements for; structuradesign, component capacities, loads and load effects analysiscode checks and operating limitations. A failure mode baseddesign approach is used, where the C/WO riser system isdesigned against all relevant failure modes.

Clause 6 gives general design requirements and does notprovide cookbook formulae for design of components, since

it is anticipated that a design by analysis approach will be usedin most cases. Closed form solutions are provided for pipes andother cylindrical components.

Structural and functional design and safety (clause 6) asshown inFigure 3,is closely linked to materials and fabricationrequirements (clause 7), cyclic capacities (Annex C), staticcapacities (Annex D), qualification of connectors (Annex I)testing (clause 8) and maintenance and monitoring (clause 10)where there is an iteration between material selection, designrequirements and verification of component static and cycliccapacities.

The design shall incorporate appropriate design factors toobtain adequate safety margins against all relevant structuraand functional failure modes in a consistent manner. The C/WOriser components shall be designed for loadings appropriate toits intended use and other reasonable foreseeable operation

conditions. Failure mode, effects and criticality (FMECA) andhazard and operability (HAZOP) are useful tools in thiscontext.

Structural and functional integrity are obtained by usingdesign factors on static capacities and cyclic capacities (fatiguelife) for the relevant failure mode. The structural and functionalcapacities of components are established by calculations andvalidated by testing, especially to evaluate performanceparameters that cannot be studied adequately through


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calculations, e.g. leak tightness, galling resistance, contactfatigue performance, fretting performance.

Global riser analysisStatic and dynamic load effects

Cyclic load effects

Structural and functional safetyLoad or Load effect Fdx Satic capacity

Service life Fatigue life/DF

Operation and maintenanceMonitoring

Inspection/repair/replacement

Manufacture and fabricationManufacture procedure specification

Representative mechanical testing

Quality control and testing

Material engineeringMaterial selection

Material specifications

System designReduce failure probability

Reduce consequence of failure

Component design and

qualification testingStatic capacity

Cyclic capacity

TestingPressure and functional testing

System integration testing

Figure 3 Structural and functional safety flowchart

Material and quality control requirements are required toadequately support the design. It does not matters howelaborate or sophisticated a design calculation is, if the materialselection requirements are not met and the quality controlrequirements during manufacture and fabrication are not

adequate to support the material selection requirements, thecomponent will not perform as required. In short, the structuraldesign principle in ISO-7 is a complete package of design,material selection, quality control requirements and operationalrequirements and maintenance to ensure structural andfunctional integrity of the C/WO riser system during its lifetime(Figure 3).

Fundamental requirementsSection 6.2.2 gives the fundamental design requirements of

ISO-7. The goal of ISO-7 is to provide a safe C/WO risersystem and riser components for its intended use throughout itsintended life. This philosophy is put into practice by starting

with robust material selection, good mechanical designpractice, high level manufacture and fabrication standards,quality assurance/control standards, testing, system design,operating procedures, monitoring, maintenance and verificationof compliance with requirements by use of third parties, seeFigure 3. It is assumed that all work during design,manufacture, fabrication, operation and maintenance are carriedout by personnel who have appropriate qualifications andexperience and using qualified procedures. The design in ISO-7

includes the following fundamental design requirements focomponents and system:

Risk based design, i.e. the failure probability or safety fora given failure mode is linked to the consequence of thesame failure mode. A low failure probability or high safetyis required when the consequence of that failure is high andvice versa.

Ductile failure modewhich is obtained by:o Ductile and tough materials, i.e. yield before fracture;o Ductile design, e.g. avoiding sudden changes in

section properties (low constraint/triaxiality).

Safe-life fatigue design which is obtained by using highdesign fatigue factors for the service life or the scheduledinspection intervals.

Damage tolerant design which is obtained by requiringtoughness to ensures ductile failure mode for fabrication-or service-induced flaws for the service life or scheduledinspection intervals.

Structural and functional safety which is obtained byoperating within allowable structural and functional limits.

The overall system design shall be fail-safe, i.e. ensure thano single failure will cause an unacceptable risk.

Protection against accidental damage is done by thefollowing two principles:o Reduction of damage probability.o Reduction of damage consequences, e.g. by use of

load limiting devices (e.g. safety joint) to protect welbarriers.

Design considerationsSection 6.2.3 provides the general design considerations

As described in Section 6.2.3 the design of the C/WO risersystem, its components and details shall, as far as possibleinclude the following considerations:

Riser connectors should be stronger than the connectingpipe with respect to pressure and/or bending moment.

Simple load paths and smooth stress fields should be aimedfor in the design.

Components and details should be designed so that thecomponent will behave in a ductile manner by:o Avoiding sudden changes in section properties or deep

sharp notches which give high constraint (triaxiality).o Selection of ductile and tough material in actua

environment and temperature.

Robust material selection considering mechanical andphysical properties, brittle fracture, weldabilityhardenability, environmental stress fracture resistance andcorrosion resistance.

Sharp notches and stress concentrations should as far aspossible be avoided including minimizing contact stressconcentrations between contacting bodies.

Access for inspection at fatigue sensitive locations shouldbe provided.


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Cyclic loaded components should be preloaded up tonormal operating cyclic stress levels and hence reduce riskfor fatigue failure and fretting failure.

Inspection/replacement philosophy should be an integralpart of the design.

Static design format and design factorsISO-7 requires static design of components due toenvironmentally induced external loads. Although theenvironmental loads are time dependent static and dynamic, theresulting load effect is assumed to be static for the purpose ofcomponent design. The aim of the static design is to ensure thatthe components have adequate structural and functionalcapacity for the static loads.

The component static safety level is achieved by settinglimits on load or load effects to a fraction of the capacity of thecomponent as follows, seeFigure 4:

CapacityFEffector LoadLoad d (1)

Fdis design factor which is the inverse of the safety factor SF,seeTable 1.External load effects are typically a time dependentvariable Load or load effect can also mean applied stress orstrain. Capacity can mean yield strength, strain capacity or loadcapacity. The design factors depend on the failure mode andloading condition. This design format is named the allowableload design method or working stress design method.

The reference values for load effects and capacity used indesign are normally based on lower bound values for capacityand upper bound values for loads. The values of tension,pressure, bending moment to be used in design are the mostprobable maximum values corresponding to that load case. Thevalues used for capacities are minimum strength, minimumdimensions, lower/upper bound friction coefficients andminimum/maximum preload.

Figure 4 Structural safety and allowable load/stress format

Nominal target failure probabilities, Pf, design factors andsafety factors for static loading conditions are given inTable 1

for burst, hoop buckling (collapse) and plastic collapse, seesections 6.4 and 6.5 in ISO-7. The failure probability inTable1 is from structural reliability analysis and hence is a nominavalue and cannot be interpreted as an expected frequency offailure. Note that gross (human) errors are not included in thefailure probabilities inTable 1.For functional failure modes forconnectors, the design factor is 1,00. This corresponds to anominal target failure probability of 10-2-10-3. Lower designfactors shall be considered if material and fabrication are not inaccordance with Clause 7 in ISO-7.

The primary design format in DNV F201 is called Loadand Resistance Factor Design format using partial safetyfactors. Each partial safety factor is intended to reflect theuncertainty in the parameter it is multiplied by. Morebackground to target failure probabilities and applicable failuremodes are given in DNV F201. The design factors in ISO-7 arebased on a calibration to DNV F201 assuming a 50%/50% splibetween functional and environmental load effects. The factorsinTable 1 are similar to what is found in API RP 2RD (1999).

Table 1 Static structural design factorsStatic load condition Pf Fd SF RWL

factor4

Single load

Internal design (rated) pressure 10- 0.60 1.67

External design pressure 10-5 0.60 1.67

Pressure testing - FAT - 0.90 1.11

Combined external load

Normal (rated working load)2 10-6 0.67 1.50 1.00

Extreme2 10-5 0.80 1.25 1.20

Accidental, above well barrier 10- 1.00 1.00 1.50

Accidental, well barrier2,3 10-6 0.90 1.11 1.351For sizing, limiting static axial tension (sustained) combinedwith combined design pressure to Fd=0.55-0.60 will usuallyprovide a design that meets normal, extreme and accidentalconditions when dynamic loads are included.2Combined loading with pressure, temperature and externalloads are determined by global riser analysis.3Proposed in 17G to increase safety due to consequence offailure.4Rated working load factor (RWL) = Load/Rated load

Normal, extreme and accidental load conditions set limitson combined axial, pressure and bending loads. Since pressuredesign (internal and external) set limits on pressure, thecombined loading criteria set limits on the longitudinal load due

to axial and bending loads. In other words, the pressure limitapplies in addition to the combined load limits.

The normal load condition and the accidental condition forwell barrier elements have a very high consequence of failurehence a very low target failure probability is given. Extremeload conditions and accidental conditions for elements abovethe well barrier elements have a high consequence of failurehence a low target failure probability is given. The accidentalload conditions are associated with extreme-low-probability


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events (10-2 to 10-4). Load conditions with a probability lessthan 10-4are not considered. The failure probability for internaldesign pressure (wall thickness design) is 1 to 2 orders ofmagnitude lower than the general failure probability given inTable 1. This is in accordance with industry practice in DNVF201 and ASME B31.3 Chapter IX.

Experience has shown that for loads in excess of ratedworking loads and for hydrostatic FAT pressure testing loss ofpreload may occur due to limited local permanent deformationsin areas of stress concentrations/flange ring/bolting. Accidentalstatic load condition may result in permanent deformations thatnecessitate the removal of the component from service forinspection and repair of damage

Div.3 (1997) used the working stress design format;however Div.3 (2007 and later) uses the load and resistancefactor design format. It should be noted that API 6X (2014)includes the extreme condition in addition to the rated loadcondition. API 17D standard wellhead and Christmas tressconnectors is restricted to operate within the normal loadcondition (rated load factor equal to 1.00) for all external load

conditions.ISO-7 uses the same design factors for bolting as for

forgings and pipes. The allowable primary stress in Div.2 atRWP is one-third of the specified minimum yield strength attemperature. Div.3 limits the allowable primary stress at RWPto the yield strength at temperature divided by 1.8. 6A flangesdesign limits primary stress to the yield strength at roomtemperature divided by 2.0 based on ASTM A193 B7. Forbolted flanged connections designed in accordance with 6A andDiv.3, the allowable stresses leaves little margin left forexternal loads. This means that the RWL is relatively small atthe RWP and may not be suitable for components used inC/WO with high external loads. Industry practice is to select

15K flanges for 10K RWP. This practice results in limiting boltstresses at RWP to one-third of the yield strength will typicallyensure sufficient reserves for bending moments. It should benoted that the compact flanged connections in ISO 27509applied one-third of yield strength for RWP for bolting design.17G's proposal to bolt sizing is one third of 17G yield strengthfor RWP for connections in the riser load path to include somereserves for external loads.

Cyclic design format and design factorsIn principle, all components exposed to loads which cause

cyclic stresses, are liable to suffer fatigue cracking, andtherefore should be assessed for fatigue failure. ISO-7 requires

fatigue analysis of components due to environmentally inducedcyclic external loads, see sections 6.2.4 and 6.4.9. Design toresist fatigue is recognized as one of the main requirements forC/WO riser system components in harsh environments like inthe North Sea. The aim of the cyclic load design is to ensurethat the components have adequate structural and functionalcapacity for cyclic loads. To ensure this, one of the basicconcepts of ISO-7 is to specify a minimum fatigue life. If theservice life has not been defined by the purchaser, a minimumof 5 years shall be used.

For fatigue loading, the target component safety level isachieved by setting limits on the service life to a fraction of thecalculated fatigue life as follows:

FD

lifeFatiguelifeService (2)

DF is the design fatigue factor,Table 2.Fatigue life or fatiguecapacity is the number cycles of stress that produces fatiguefailure. Service life is the period (number of cycles) in whichthe component is required to perform safely with an acceptableprobability. Design fatigue factors are applied to reduce theprobability of fatigue failures.

Table 2 Fatigue design factors

Analysis method Pfa DF

Method based on stress-life (S-N)10-5

10

Method based on fracture mechanics (FM) 5ba The failure probability cannot be interpreted as an expected

frequency of failure.bAssumes no flaws are detected. The initial flaw size to be usedis the size which is reliably-detectable by the applied NDTmethod, see API Std 2RD.

Careful design of details and stringent quality requirementsfor fabrication are essential in achieving acceptable fatiguecapacity. Fatigue analysis results are used for identifyingfatigue sensitive regions for inspection planning both duringmanufacturing/welding and during service.

The fatigue capacity may be determined by calculationstests or both. Calculation methods include the stress life (S-Nmethod and the fracture mechanics (FM) method. The S-N

method is the primary design tool in ISO-7 due to its simplicityand efficiency. Fatigue life extension may be performed by theFM method. The number of cycles to fatigue fracture failure inthe design S-N curve in DNV-RP-C203 represents 97,7 %probability of survival (2,3 % probability of failure) with a75 % confidence interval, Lotsberg and Ronold (2011).

The fatigue life of a component is defined as the time todevelop a though-thickness crack of the component. Fracturecriterion is not explicitly accounted when using stress life (S-N)data. Probabilistic fatigue crack growth and fracture analysisusing FAD method indicates that the failure probability isrelatively insensitive to the failure criteria (attainment of a finacrack depth equal to 10 % of wall thickness, 20% of wal

thickness and through-thickness crack) when considering thetotal fatigue life, see Maljaars et al (2012). This also justifiesthe present industry practice of using the S-N approach.

ISO-7 fatigue design is based on safe life design, i.e. leak-before-break is not required. DNV requires a fatigue designfactor of 10 for the S-N method if leak-before-failure cannot beproven, Andreassen and Valsgrd (2002). For S-N fatiguemethod a DFof 10 is applied, which implies that the probabilityof generating a through the thickness fatigue crack during theservice life becomes very small, i.e. less than 10-5, seeTable 2


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DNV-OS-F101/F201, DNV-RP-C203 and API Std 2RD appliesa fatigue design factor of 10 for critical components. The factorof 10 seems high, but a small change in stress range gives alarge change in fatigue life. Typical a change in stress range bya factor of 1.81 gives a change in fatigue life by a factor of 10.Note that gross (human) errors are not included in the failureprobabilities inTable 2.

Fracture mechanics may be used to extend the predictedfatigue life. The fracture mechanics fatigue design factor inTable 2 is then applied to determine the next inspection intervalin cases where no cracks are found. For more details oninspection planning and use of probabilistic analysis methods,see Kirkemo (1988).

Figure 5 Definition of S-N design curve

At present ISO-7 includes fatigue analysis due to cyclicexternal loads, however, 17G is considering to include fatigueanalysis requirements due to cyclic internal pressure, especially

for higher pressure rating as required in Div.3, see Wormsen etal (2014). Fatigue analysis has traditionally not been part of thedesign for equipment covered by 6A/17D equipment neither forcyclic external loads nor for cyclic pressure.

Loads, load effects and failure modesLoad and load effects that shall be covered are given in

section 6.3 in general and in section 6.6.2 for connectors. Theintent is to require considerations of all loadings that can causesignificant stresses in the components. Typical loads whichshall be considered are pressure, temperature, and external loadeffects (bending moment, axial load and torque). Forconnectors and connections additional loads apply, i.e. make-

up/break-out loads (torque, bending and tension) and remainingpreload after make-up have to be considered.

External loads are typically environmental (waves, currentand vessel offset/motions) and functional (weight, buoyancy,contents and applied top tension). External load effects aredetermined by global riser analysis. Accidental loads may becaused by abnormal environmental induced load effects, loss ofvessel station keeping, anchor line failure or motioncompensator lock-up.

ISO-7 requires that all relevant failure modes shall beinvestigated. A failure mode is relevant if its occurrence isphysically possible and the anticipated consequence(s) of thefailure cannot be disregarded. Failure mode, effects, andcriticality analysis (FMECA) is a useful tool to identify relevanfailure modes.

The possible ways in which components can fail areclassified into static short term, static long term and cyclic typefailure modes, or a combination of these. The following failuremodes are typically addressed:

Static failure modes that lead to immediate failure like:o Structural failure modes: bursting, plastic collapse

local strain, brittle fracture, local buckling (wrinkling)ratcheting (progressive plastic deformation), threadunzipping (jump out), dog disengagement, backdriving of locking mechanism.

o Functional failure modes: seal/gasket leakage (smalvolume leakage), preload exceedance, loss of preloadgalling, failure to unlock, loss of multi make/break

performance, loss of locking mechanism performanceloss of interchangeability, failure to operatemechanism to seal the well bore.

Static failure modes that lead to delayed failure:o Structural failure mode: environmental assisted

cracking (service environment).o Functional failure mode: erosion, corrosion.

Cyclic failure modes that lead to delayed failure like:o Structural failure mode: fatigue failure, back driving of

locking mechanism, unscrewing.o Functional failure mode: seal/gasket leakage due to

fretting.

Large volume leakage shall be considered as structurafailure mode. Static failure modes that lead to delayed failureare normally avoided by:

Material selection to avoid failure due to environmentaassisted cracking.

Using corrosion/erosion allowance in combination withcoating/cathodic protection is used to avoid failure due toerosion and corrosion.

Many of these failure modes are addressed by calculations andothers by material selection. However, some of the failuremodes have to be addressed by testing. Seal/gasket leakage due

to fretting is normally avoided by using preloaded connectorsand gaskets which are kept in position during cyclic loading.

Compared to 17D and Div.3, ISO-7 includes failure modeswhich also are applicable to connectors. 17D considers onlyexplicit plastic collapse for pressure and rated external loadsDiv. 3 includes plastic collapse, local failure, buckling andfatigue for pressure vessels. The vessels involved are generallyan internally pressurized thick-walled cylinder. Little guidanceis given for external loads and connectors.


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Global riser analysisThe aim of the global riser analysis is to establish the

operating limitations of the riser system for all operating modesas well as calculate the fatigue life of the system componentstogether with recommended inspection intervals. The C/WOriser load effects are mainly caused by vessel motions anddirect environmental loads are required, see 6.3.4 ISO-7.Global riser analysis is carried out using a non-linear approach.One or combinations of the following methods may be appliedto establish load effects to determine operation limitations:

Regular (design) wave analysis in time domain;

Irregular wave analysis in frequency domain;

Irregular wave analysis in time domain.

The irregular wave analysis term refers to modelling ofwater particle motions and vessel motions. Irregular waveanalysis in time domain is the preferred method. If regularwave analysis or frequency domain analysis is used, validationagainst irregular wave should be carried out.

The C/WO riser system global analysis models include thesubsea tree and wellhead primary load bearing components inorder to accurately predict load effects of the subsea wellintervention system.

The performance and operating limitations of the wellintervention system are established by global system loadanalyses with all relevant design load conditions to determineload effects and utilization of component capacities in additionto vessel response limitations, e.g. top compensator stroke.Extreme and accidental loads may be caused by abnormalenvironmental induced load effects, loss of vessel stationkeeping, anchor line failure or motion compensator lock-up.

The external load effects determined by global riser

analysis are given as bending moment, effective tension andtorque. This forms the basis for requiring component capacitiesas closed form formulas using effective tension and bendingmoment to simplify code checks.

There is a link between system requirements throughglobal load and load effect analysis (Clause 6), Annex B(Operational modes and global riser analysis) and systemintegration testing (validation and verification). The global riseranalysis covered in Clause 6 and Annex B is a systemverification and validation activity and is independent of thedetermination of component structural and functionalcapacities. Global riser analysis is a system engineering activityperformed for new, used and rental C/WO riser systems.

There has been some discussion in how to combine anddetermine design load effects. The following is proposed in17G. Design load effects, Ftot, may be combined taking intoaccount their probability of simultaneous occurrence. Static(mean) load components, Fi,mean, and dynamic (varying) loadcomponents, Fi,amp, which are statically independent may becombined according the following formulae, DNV-OS-H102:

, ,== (3)

Dynamic load components may include load effects fromwaves/swell, vessel motions and top tension (hysteresis). Staticload components may include loads from pressure, temperaturecurrents, tide, surge, top tension, weight, buoyancy, vesseoffset, and wellhead inclination.

MATERIAL AND FABRICATION

GeneralClause 7 in ISO-7 includes requirements and guidelines for

material selection, manufacture, testing, corrosion protectionfabrication and documentation. ISO-7 is restricted to seamlesspipes, forged components, fasteners and seal rings (metallic andnon-metallic). For welding, ISS-7 is limited to girth welds andoverlay welds.

Materials for high pressure components in the primary loadpath of the riser require a combination of high structuralstrength, high fatigue strength, high toughness and large sectionthickness. Thicker wall components are more difficult to hea

treat and can have lower toughness. Higher strength materialsare more susceptible to environmental cracking. Wave inducedcyclic external loads require material with high fatigue qualityespecially for shallow water applications. Pressure inducedstresses at the bore (inside) surfaces will increase withincreasing pressure rating which reduces the cyclic pressure lifeand increases the susceptibility to environmental crackingThese demands require robust material selection, manufactureprocess, and quality control.

Material selectionSection 7.2.1 gives the material selection requirements

Materials are to be selected with due consideration to

internal/external media, loads, temperature, possible failuremodes and maintenance. The selection of materials is to ensurethat the requirements to materials are considered for alcomponents in the C/WO riser system. The following materiacharacteristics are considered:

Mechanical and physical properties

Weldability

Environmental assisted cracking

Corrosion resistance

Wear resistance

A robust material selection includes a careful selection of

the chemical composition to ensure weldability (carbonequivalent), avoid environmental assisted cracking, prevenbrittle fracture (toughness) and ensure sufficient hardenabilityto meet the minimum mechanical property requirementsthrough the critical cross sections, see sections 7.2.7, 7.2.8 and7.2.9 in ISO-7.


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Environmental assisted cracking (service environment)Section 7.2.4 and 7.2.9 give requirements to reduce the risk

for environmental assisted cracking. Environmental assistedcracking (e.g. hydrogen induced stress cracking, hydrogenembrittlement and sulfide-stress cracking) is a non-ductilepremature failure that is caused by:

Tensile stress (applied and residual stress) applied over acertain time;

Susceptible material, e.g. microstructure and inclusiondistribution;

Presence of hydrogen (H+) in the material, e.g. frommanufacture and/or charged from external source like sourservice (H2S) or cathodic protection (CP).

For environmental assisted cracking to take place all threeconditions must be met simultaneously. The component needsto be in a particular crack promoting environment, thecomponent must be made of a susceptible material, and theremust be sustained tensile stresses above some minimumthreshold value. An externally applied load is not required asthe tensile stresses may be due to residual stresses in thematerial or preload stresses. The threshold stresses aresometimes below the yield strength of the material.

All applied loads expect momentary loads are normallyconsidered. It has been an industry practice to neglect loads thatact on a component for less than a couple of minutes orsometimes less than to 1 hour. Accidental loads may inducepermanent deformations and build residual stresses and strainsinto the components. These permanent, residual stresses andstrains are then considered.

All metallic materials exposed to or likely to be exposed towell-bore fluids that contain H2S shall be selected and qualified

in accordance with ANSI/NACE MR0175/ISO 15156, 7.2.4 inISO-7.

Table 3 Environmental assisted cracking limitations

Environment Requirement

Cathodic protection in seawater

Actual yield strength1, max 950 MPa (140 ksi)

Hardness, max 35 HRC/330 HBW

H2S service , hardness, max

Forging 22 HRC/237 HBW

Bolting A320 L7M 99 HRB/228 HBW

Tubulars T95 API 5CT 25,4 HRC/255 HBW

Pipes X70QS API 5L 22 HRC/250 HV101Proposed for 17G.2ANSI/NACE MR0175/ISO 15156

Present industry practice is to avoid environmental assistedcracking by selecting materials with limited hardness and yieldstrength, see Table 3, which also is given in ISO-7.Environmental effects on mechanical properties are normallyneglected when these limitations are fulfilled in CPenvironments.

Post weld heat treatment is performed of all welds exposedto well bore fluids to reduce residual stresses, hardness andhydrogen content and hence reduce the risk for environmentalassisted cracking, see 7.4.1 in ISO-7. Repair by welding is noacceptable for seamless pipes, forgings, fasteners and seal ringsas sealing and fatigue performance will be degraded.

Div.3 does not include environmental assisted cracking dueto H2S and CP. 17D refers to ANSI/NACE MR0175/ISO 15156and restricts hardness due to CP, however, no guidance onallowable tensile stress for H2S service is not given.

Materials and manufacturing specificationsSection 7.2.2, 7.2.3, 7.3.3.1 and 7.3.3.2 gives requirements

to materials and manufacturing speculations. Steel makingchemical composition, manufacture process and associatedquality are key elements to obtain high quality materials, seeFigure 3.A material specification is to be prepared stating thematerial requirements. Manufacture is to be performed inaccordance to a qualified manufacturing procedurespecification of a prequalified manufacturer. The manufacture

procedure specification shall address the following to obtainhigh strength, high ductility, high toughness and high qualityCALAS:

Steel making and refining practiceo Allowable melting practice(s)o Fully killed, fine grain and clean steelo Ingot type/size

Chemical composition with toleranceso Reduce impurity elements like sulfur and phosphorus

with increasing strengtho Increase hardenability elements with thickness

Manufacture process and quality controlo Forging techniques, hot work ratios and forged to

shape as close as possibleo Heat treatment, e.g. furnace loading/ unloading, time

temperature control, quenchingo Critical regions/sections identified for

Mechanical properties (significant loading) Surface NDT (fatigue quality of notches)

o Representative mechanical testing from Sacrificial forging Qualified prolongation

All components shall be within specified dimensions andsurface roughness (sealing surfaces and fatigue hot spots).

Full traceability from heat to heat treatment lot and final

product. Material certification (material certificates or test reports)

to comply with ISO 10474 3.1.

ISO-7 uses generally more stringent material requirements than6A/17D. 6A uses a constant safety factor of 1.50 (allowablestress of 67 % of yield strength) independent of PSL level. 17Duses 6A PSL3. 6A/17D does not include extreme and accidentaload conditions which ISO-7 considers. The next sections will


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give more details with respect to ductility requirement,toughness requirements and representative mechanical testingin ISO-7.

Representative material testing of forgingsForgings in ISO-7 use the same design factors as pipes;

hence, the forgings should have the similar confidence inmechanical properties during production as for pipes, see DNVF101/F201. A pipe has always a prolongation and the shape isknown. This forms the background for requiring high qualitymechanical testing during production, see 7.2.8 and 7.3.3.3 inISO-7.

Hardenability is the main challenge with CALAS forgingswhere the key essential variables for production are chemistryand size (thickness) at the time of heat treatment. Selection ofCALAS forging materials (F22, 8630, 4340, 4130) shall bebased upon verified hardenability through the critical crosssections of the forged components in the final condition.Verification shall include a review of mechanical properties andmicrostructure.

For critical cross section representative testing means thatminimum mechanical requirements are required throughout theentire critical cross-section of the component. This may not bethe thickest section of the component where the sizes aregoverned by functional requirements, e.g. elastic deformations(stiffness) or start of yielding, and hence relatively low appliedstresses. For thicker non critical sections (low utilization)fracture mechanics (FM) testing and an engineering criticalassessment is performed to document that the component is fitfor purpose.

Production testing of mechanical properties in ISO-7 isperformed per heat and heat treatment load and is performed onmaterial taken from:

a qualified integral prolongation of the component or

a sacrificial production forging.

The mechanical properties at the prolongation duringproduction shall be representative the critical cross section ofthe component. In other words, the mechanical propertiesdetermined at the prolongation shall ensure that the minimumrequirements are met at the critical cross section. Note thatprolongations may affect the mechanical properties of forgingcompared to a forging without a prolongation. Figure 6 showssome examples of sampling for mechanical testing.

API 6A allows for use of separate qualified test coupons

(QTCs) for checking mechanical properties of pressure-containing components. The results from a separate QTC mightnot always correspond with the properties of the actualcomponents. Experience has shown that mechanical properties,especially CVN impact energy values vary significantly fromthe material properties of the actual components. Reduction intensile strength in the range of 20-30 % and reduction in CVNvalues more than 50 % are not uncommon. Brittle failure hasoccurred due to improper testing after manufacturing by use ofseparate QTCs, Koneti et al (2013). This may result in a lower

structural safety margin than intended. It should be noted tha6A require prolongation testing for tools, see Annex H in 6A17D refers to ISO-7 for extreme and accidental load conditionswithout specifying use of prolongation or sacrificial forgings toensure representative mechanical testing.

Figure 6 Representative sampling for mechanical testing

Ductile failure mode

GeneralYield before fracture is the prevailing philosophy in the

design of critical pressure-containing components in theprimary load path in ISO-7. It is assumed that the componentshave sufficient ductility to accommodate the required plasticflow and deformation without premature failure, seeFigure 7.

Figure 7 Load-deformation characteristics of components

Plastic collapse analysis implicitly assumes that thematerial possesses sufficient post-yield ductility and toughnessto ensure that the plastic collapse analysis is appropriate for thecomponent. It is assumed that the components fail in a ductile

manner and not in a brittle manner. Brittle manner

includes unstable fracture, buckling and thread unzippingDuctile manner means that the component has a


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strength/deformation reserve beyond the plastic collapsecapacity (limit load).

Ductile structural designA component may be brittle even if it is made of ductile

and tough materials, e.g. when there are sudden changes insection properties which give high constraint (triaxiality).Ductile design, e.g. low triaxiality/constraint, implies use ofsmooth transitions/notches/grooves and rounded thread rootswith radius or elliptical transitions, seeFigure 8.

Figure 8 Examples of brittle (3) and ductile designs (1,2)

Further design considerations are given in ISO-7 to ensureductile failure modes are:

Girth welds shall be stronger than base material to protect(shield) welds from plastic deformation.

Welds shall be post weld heat treated (stress relieved).

Connectors should be stronger than the connecting pipe.

Block bodies (e.g. EDP, LRP, SFT) have a capacity greaterthan the end connectors.

Ductility and toughnessBecause of the use of high strength materials, it is essential

that all components and welds have sufficient ductility andtoughness to avoid unstable fracture. Mechanical propertyrequirements for CALAS to ensure failure in a ductile mannerare given inTable 4,which includes the following parameters:

Elongation is given to ensure sufficient uniaxial straincapacity.

Reduction in area which is an indicator of forging ratio andinfluence the multi-axial strain capacity.

Maximum yield strength to tensile strength ratio is given toensure minimum strain hardening to ensure reserve

strength beyond yield strength, energy absorption andallow for some shake-down in the component.

Charpy V-notch impact energy or fracture toughness (i.eresistance to the extension of a crack) is given to preventunstable (brittle) fracture for small defects, i.e. damagetolerant design and is also used as a quality measure fomanufacture.

Table 4 Ductility and CVN requirements for CALAS

Property Requirement

Ductility

Minimum elongation after fracture 16 %1

Minimum reduction in area 35 %

Maximum yield/tensile ratio 0.92

Impact energy requirements3

Minimum average CVN impact energy 40 J (30 ft lb)1Proposed to increase to 18 % in API 17G 3rded.2Proposed to be 0.90 for forgings and fasteners in API 17G 3 rded.3 Full size specimens at minimum design temperature in transverse direction.

Fracture toughness is preferably quantified by fracturemechanics testing and determination of crack tip openingdisplacement (CTOD) or J-integral. However, due to itssimplicity, low cost and vast amount of existing data, CVNtesting is generally applied for production testing. CVN impacttests are required for all pressure-containing components andfor all components in the primary load path. Specifyingmaterial Charpy impact requirements has historically beenapplied to help prevent brittle fracture. ISO-7 requiresminimum average CVN impact energy of 40 J (30 ft lb) atminimum design temperature in transverse or critical directionwith limitations regarding yield strength and thickness. Fracturemechanics testing is part of the qualification for high strength

thick materials. For example fracture mechanics testing andanalysis are required when the thickness exceeds 40 mm (1,57in) for yield strength 560 MPa (81 ksi). See Annex A for

CVN requirements proposed for 17G.6A/17D PSL3 requires minimum CVN energy 20 J (15 ft

lb) independent of wall thickness and yield strength. It shouldbe noted that 17D refers to ISO-7 regarding extreme andaccidental load conditions, i.e. for 1.2 and 1.5 times rated loadwithout increasing the CVN energy requirement. Div.3 andB31.3 IX require 40J (30 ft lb) impact energy. Div.2 hassimilar requirements as in Annex A; however, minimum impactenergy is 27 J (20 ft lbf).

Non-destructive testing at fatigue sensitive locationsNDT requirements are given in 7.2.17, 7.3.3.4, 7.4.3 and

7.5 of ISO-7. NDT methods selected shall be suitable fordetection of the type of defect considered detrimental to thesafety and integrity of the components. All components shall be100 % surface and volume tested as far as practical for bothsurface and volumetric (internal) imperfections.

Personnel responsible for all NDT activities shall bequalified and certified (3rd party) to Level 3 according to ISO9712 or equivalent. NDT personnel shall be qualified and


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certified (3rd party) to Level 2 according to ISO 9712 orequivalent. API 6A allows use of the recommended practiceASNT-TC-1A with manufacturer (2nd party) certification. AsNDT quality control is an important part of the quality control,ISO-7 therefore refers to a standard instead of a recommendedpractice.

NDT detection capability and acceptance criteria shallreflect design requirements for the actual component andapplication. NDT shall be performed with qualified procedures.The selected NDT method should demonstrate its capability todetect the defined flaw reliably. Annex E of BS7910 indicatesdetection capabilities of different NDT methods.

Ultrasonic testing is selected as the primary method forvolumetric testing. Ultrasonic testing is preferred for detectinglinear defects in comparison to radiographic testing. Ultrasonicscan plans shall be developed for ultrasonic testing as part ofthe procedure to demonstrate volumetric coverage.

Fatigue failure often initiates from surface breakingdefects, either at outer or inner (back) surfaces, hence highlysensitive NDT methods such as wet fluorescent magnetic

particle testing (WFMT) are used in the final machinedcondition of forged components and welds. For girth weldsultrasonic testing may be performed by semi-automated phasedarray and time of flight diffraction (TOFD) to improvedetection capability compared to manual ultrasonic testing.

The NDT requirements in ISO-7 are similar to PSL 3 inAPI 6A for forgings, but there is increased emphasis on surfaceNDT to detect small crack like flaws that may be initiation sitesfor fatigue cracks. No crack like defects is acceptable at fatiguesensitive locations (hot spots). Fatigue hot spots shall beidentified on design drawings. Crack like defects found atfatigue hot spots are normally repaired by blend grindingwithout weld repair for ISO-7 components.

TESTING

Clause 8 of ISO-7 establishes minimum requirements forpressure testing, qualification testing, function testing andsystem integration tests.

All pressure-containing components and welds are requiredto be subjected to a full hydrostatic pressure testing as part ofthe factory acceptance test. The full pressure test is 1.5 timesthe RWP which is the same as 6A/17D. The hydrostatic testpressure for Div.3 is 1.25 times the RWP.

This is equivalent to 17D requirements. It should be notedthat the test pressure is governing the wall thickness in 17D/6A,

and not the design pressure. In ISO-7 the design pressure isgoverning the pressure based wall thickness.Div.3 requires use of elastic-plastic finite element analysis

without strain hardening for hydrostatic test conditions and notwith strain hardening as for the other load conditions.

STATIC ANALYSIS AND CAPACITY

General

Annex D contains requirements for calculating the staticcapacities of components and connectors. ISO-7 is primarily adesign-by-analysis code, i.e. finite element analysis is appliedto determine the static capacity a component. The yield strengthused in structural capacity calculations are the minimum oyield strength and 90% of tensile strength. Design-by-formulaeis also allowed for simple geometries like cylinders/pipes. ISO7 covers only load controlled design methods, i.e. where limitsare given to the allowable applied load and not, as for strainbased design methods as used for pipelines for reeling, wherelimits are given to allowable applied strain.

Material strength derating starts at 50C (120 F) andfollows Div.3 and DNV F101/F201. Any corrosion allowanceand non-lead bearing weld overlays are not included in thecapacity calculations. API 6A/6X starts derating when theinternal fluid temperature exceeds 121 C (250 F).

Definition of plastic collapse capacityPlastic collapse load in ISO-7 is defined as the limit load

based on hand calculations or elastic-plastic analysis withou

material strain hardening and with large deformation theoryISO-7 uses the same method as used in Div.3 (before 2007)For internal pressure and tension this means through thethickness yielding and for bending full cross section yielding oplastic hinge. The basis for this selection was to:

Use same definition as used in elastic analysis for pressureand axial loads;

Use analytical based limit loads solutions available, seeGerdeen et al (1979), Kirkemo (2001) and Kalnins (2003);

The limit load for most common high strength materialswill not be excessively conservative because of the yield totensile strength ration is high (e.g. 0.8-0.9);

Loads above limit load (100 % yield strength) can result inexcessive deformations and strains;

Have a slight reserve between limit load and true plasticcollapse load (e.g. 10-15 %).

Analysis (calculation) methods

GeneralSince the introduction of elastic analysis in Div. 2 in 1968

non-linear finite element analysis (FEA) codes have developedThe elastic stress analysis method provides an approximationfor determination of the limit load. However, since it has beenused for many years, it is still permitted for simple geometriesElastic-plastic methods provide a more accurate assessment of

the plastic collapse load of a component relative to the elasticmethod because the actual structural behavior is more closelyapproximated. The redistribution of stress that occurs as aresult of inelastic deformation (plasticity) and deformationcharacteristics of the component are considered directly in theanalysis. This is reflected in ISO-7 which recommends use onon-linear FEA of components as Div.2/Div.3. Two alternativeanalysis (numerical calculation) methods are provided toestablish plastic collapse capacities, seeFigure 9:


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1. Elastic stress analysis method2. Elastic-plastic stress analysis method

It is seen from Figure 9 that elastic-plastic analysis stressmethods compares better with actual physical testing thanelastic stress analysis methods. This is of importance whencomparisons are made to actual physical tests and calculateaccurate capacities.

Design by analysis, i.e. using elastic finite element analysisin design, was introduced by ASME for nuclear applications in1964 and for pressure vessels and boilers by Div.2 in 1968. Allthese routes lead to the same well-known problems, especiallythe stress categorization problem and 3D stress fields, Hechmerand Hollinger (1988) and Kalnins and Updike (1991), and allare now out-of-step with the continuing development ofcomputer software and hardware. For preloaded connections itis not possible to separate applied stresses into primary andsecondary with elastic analysis. Elastic analysis cannot be usedto calculate bending moment capacities due to the inherentlimitation of checking thickness stresses and not section

stresses. Div.3 indicates limitations to elastic methods to shellswith D/t ratio less 10. Functional capacities have to bedetermined by elastic-plastic analysis methods as linear elasticanalysis methods cannot simulate local yielding which affectsfunctionality.

Figure 9 Load against deformation

Pressure and tension capacities can be determined by 2Dmodels with axisymmetric geometry. For bending and non-axisymmetric geometry (e.g. bolt holes, bolts, and latch dogs)3D FEA is performed. In case of compressive stresses and riskfor buckling, imperfections shall be explicitly considered in the

analysis model geometry.Both elastic and elastic-plastic analysis includes contact

elements between contacting bodies and large deformations.Unfavorable combinations of dimensions and coefficient offriction are used in the analysis. The inherent safety is reducedwith higher material grades; hence the yield strength to beapplied in analysis has been limited to 90 % of the tensilestrength.

Von Mises yield criterion is recommended, however, theTresca yield criterion (stress intensity) may also be applied. TheTresca yield criterion ignores the third (intermediate) principastress. The more accurate, but more complex, von Mises yieldcriterion considers all three principal stresses. The Trescacriterion is a little (0% to 15 %) more conservative than MisesThe effect of non-linear geometry (large displacement theory)and nonlinear contact is applied in the analysis to improve theaccuracy of the predicted plastic collapse load, e.g. unzippingcombined with ovalisation for threaded connectors.

When selecting analysis methods it is important that thecalculated plastic collapse load compares well with testing andgives consistent safety margins independent of pressure rating.

Elastic stress analysis methodStresses are computed using an elastic analysis, linearized

at governing cross sections of the component and classified intocategories, and limited to allowable values such that the plasticcollapse load can be established. This requires knowledge ogoverning sections of the component and extensive post-

processing, involving linearization of the stresses into primarysecondary and peak, as well as membrane and bending, seeFigure 10. For complex components, especially 3D stressfields, and/or bending moments this method requires significantknowledge and judgment as part of the analysis. Elastic stressanalysis can validate through-thickness yielding for pressureand tension loads, however, not directly applicable forestablishing bending capacity where plastic hinge of thecomponent is the failure mode.

Figure 10 Illustration of through wall stress linearization

Elastic-plastic stress analysis methodElastic-plastic stress analysis does not require

categorization into primary and secondary stresses as elasticstress analysis requires. The elastic-plastic analysis includestrue stress-true strain curve and isotropic strain hardening. Theload is applied in increments. Locations within the componenwhich start to yield are identified. Iterations are necessary a


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each load increment to satisfy equilibrium due to non-lineargeometry and non-linear material behavior. A plastic collapseload is derived from an elastic-plastic analysis considering boththe applied loading and deformation characteristics of thecomponent.

For elastic-plastic analysis without strain hardening thetrue stress-strain curve shall use the model given in Div.3 KD-231.4 with a cut off at the yield strength, seeFigure 11. It isimportant to model stress-strain curve with yield at 0,2 %plastic strain when considering functional failure modes, i.e.loss of preload due to local yielding. The curves given Div.3KD-231.4 may be used in cases where the actual stress-straincurves are not available. For steels which exhibit a yielddiscontinuity (Lders strain or yield plateau) in the stress-straincurve, the yield discontinuity should be included in the stress-strain curve. BS 7910 (2013) gives some guidance onmodelling yield discontinuity. Actual stress-strain curves haveto be corrected to represent minimum specified values ifapplied.

Figure 11 True stress-true total strain curves

Elastic failure criteria

Plastic collapse capacity criteriaPlastic collapse load in ISO-7 is defined as the limit load

based on hand calculations or use of non-linear finite elementanalysis. First, stresses are categorized into two types withdifferent stress limits. The two categories of stress are primaryand secondary. Primary stresses are load controlled; secondarystresses are displacement controlled. Primary and secondarystresses can be membrane or bending. The stresses are thenlinearized over the wall thickness in the principal directions.The stresses are linearized into two elements; membrane stress

and bending stress. The membrane stress is a uniform stressthat is in force equilibrium with the actual stress distribution.The bending stress is a linear bending stress that is in momentequilibrium with the actual stress distribution (through-thickness). The peak stress is the maximum stress of the actualstress distribution. This is illustrated inFigure 10.At the plasticcollapse load the primary membrane is limited to 100 % ofyield strength and the primary membrane bending is limited to

150 % of yield strength. This is consistent with present Div.3requirements.

Local stress criteriaISO-7 does not include any triaxial stress limitations. Div.3

limits the algebraic sum of the three principal stressesincluding the primary, secondary and peak stresses to 2.5 timesthe yield strength. 17G proposes to use the Div.3 requirements.

Functional criteriaIn a linear-elastic analysis, the material is assumed to be

linear elastic although the stresses are higher than the yieldstrength. This means that elastic analysis is not applicable forvalidating functional performance of components like boltedflanged connectors and threaded connectors where locayielding will occur and affect functionality, i.e. leak tightnessand loss of preload.

Elastic-plastic failure criteria

Global criteria

In ISO-7 the plastic collapse load is taken as the load thacauses 2% total primary membrane strain across the loadcarrying cross section for a material model that include strainhardening, seeFigure 11.This strain limit is based on the Div.3strain limit used for determination of collapse pressure bytesting (KD-1253). The collapse load is evaluated with the loadthat causes yielding through the entire thickness of a crosssection or the load generating a plastic hinge. Furthermore, loadsharing between parts in a component/connector are evaluatedat the plastic collapse load. Large deformations are applied toimprove accuracy where finite deformations influence theresults, e.g. ovalisation due to bending and unzipping ofthreaded connectors.

For some components, the 2 % strain criteria have beentime consuming to apply, e.g. for threaded connector. Based onthis experience it is proposed for 17G to allow for the use oelastic-plastic stress-strain curve without strain hardening, seeFigure 11.The plastic collapse capacity is taken as the load thatcauses structural instability. Div.3 (post 2007) allows for thismethod.

Before 2007 Div.3 used ideal elastic, perfectly plastic (non-strain hardening), and required a margin of minimum 2.0against the calculated collapse load. After 2007 Div.3 useelastic-plastic stress strain curve with strain hardening andrequires a margin of 1.8 against the calculated collapse load.

Local failure criteriaIt is necessary to limit local strains in areas with highstress/strain concentration, such as notches and thread roots, toavoid local rupture before plastic collapse. A local straincriterion is given in ISO-7 which limits the equivalent vonMises plastic strain at any point in a component as follows:

.;. (x)


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This typically will limit the local plastic strain to be limited to5% to 10 % depending on the yield to tensile strength ratio.Weld overmatch is assumed to shield the welds, i.e. weld rootsand caps/undercuts are not included as part of the local straincriteria. Local strain limits also apply to geometricstrengthening components, i.e. snap through problems where

bending stress is transformed to a membrane stress. The localstrain criterion was inspired by the 5 % local criteria in Div.3(1997). It is well known that local strain limits depend ontriaxiality (constraint). The plastic strain limit is reduced withincreased stress tension triaxiality or constraint and increasedwith increased stress compression triaxiality or constraint.When ISO-7 was written the local criterion was believed to be asimple but conservative local strain criterion for design withlow constraint.

In 2007 Div.3 introduced a new local strain criterion whichdepends on local constraint or local stress triaxiality in additionto yield strength, tensile strength, elongation to failure andreduction of area. Failure is defined as when the material beginsto show micro-void formation or cracking at the stress

concentrations with high triaxial stresses (constraint). Thiscriterion is based on testing of notched tensile specimens. Div.3puts a design margin of 1.8 (Fd=0.55) on the load causingglobal failure, but uses only a design margin of 1.28 (Fd=0.78)on load causing local failure in a FEA model. ISO-7 limitsplastic strains at the limit load or plastic collapse load(Fd=1.00).

The yield-before-break approach in 17G is to avoid localsharp notches (ductile design) and to use tough materials toavoid premature failure from small flaws. Figure 12 showsshallow notch/crack type geometries with increasing constraint.Cracks will always have the higher triaxial tensile stresses thanrounded notches with equal depth. Experience from full scale

testing to failure of notched components, e.g. such as threadroots with small radii (less than 0.05 mm/0.002 in), have notshown any local failure. However, 17G is evaluating the ASMEcriterion in the updating process.

Crack depthNotch depth

Local ductule failure Unstable/stable crack growth

Notch

r

Increasing degree of constraint

Figure 12 Notches and cracks with increasing constraint

Functional criteriaThe design of components shall be such that any

permanent deformations caused by local yielding whensubjected to hydrostatic test pressure and other relevantexternal load conditions shall not impair the functionality of thecomponent. Examples of local plastic strain which affectfunctionality are connector loss of preload and gasket leak

tightness performance. The plastic collapse criteria and locastrain criteria may be satisfied, but the functionality is notsatisfactory. The manufacturers have to set these local straincriteria applicable for the component, see D.2.4 in ISO-7.

Pipe/cylinder capacity equationsLimit load based pipe/cylinder capacity equations for thin

walled pipes subjected to pressure and bending are given byRodabaugh (1979). Analytically based, closed form limit loadpipe capacity equations, independent of pressure and diameterto wall thickness ratio, were presented by presented byKirkemo (2001) and are given as follows for p i po:

|| , ( , ) (4)||

, (5)

, (6) (7), (8) (9)

These equations were validated against tests and nonlinearfinite element analysis, Kirkemo (2001), and are valid for bothlow and high pressure, i.e. independent of diameter to thicknessratio. The limit loads for combined loading are found by settingFd=1.00. The plastic moment capacity Mc corresponds toyielding of the cross section, i.e. plastic hinge. The plastictension capacity Tc corresponds is the load which producesyield over the cross section.

The fundamental failure mode of concern for a pressure-containing component is burst. When increasing pressure in acylinder, there are three points which are reached as thepressure is increased from ambient to final rupture:

Initial bore yield

Through-thickness yield

Ultimate rupture

The internal bore pressure which cause initial yielding inthe bore is called the initial yield pressure or yield pressureInitial bore yield pressure is a functional concern more than apressure boundary concern. As pressure increases, yieldingprogress though the wall until the fibers at the outside surfacebegin to yield. This is referred to as through-thickness yield


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pressure or plastic pressure capacity, see Equation (8). Equation(8) applies for a capped end pipe or for an axially restrainedpipe. Since most of the materials are ductile and will strainharden, the pressure can be increased beyond the through thethickness yield pressure before bursting of the pipe/cylinderoccurs.

Equations (4), (5) and (6) limit resulting axial(longitudinal) loads (e.g. bending and effective tension) of thepipe/cylinder when subjected to internal pressure.

It is proposed in 17G to change the burst capacity equationin ISO-7 to limit any permanent deformations during FATpressure testing for cylinders with low yield to tensile ratiogiven. Present pipe/cylinder burst capacity equation in ISO-7 is

, , ( ) (10)Equation (4) is slightly modified in 17G compared to ISO-7 inorder to be consistent with API Spec 2RD (2013).

Component/connector load capacity envelopes (charts)The load capacity envelopes are determined by single loadcapacity analysis and some combined load capacity analysis tovalidate the combined load interaction relationship. Thecomponent or connector is subject to make-up, hydrostatic testand then loaded to structural failure. The plastic collapsecapacity is then determined. It must be shown that shakedownoccurs within two RWL cycles.

The structural and functional load capacities are presentedas closed formed equations like bore pressure vs. bendingmoment for different tension levels. For flanges where thebolting is governing the closed form may be presented aseffective tension vs. moment. Note that sealing diameters aregiven in order to calculate capacities for different pressures.

Figure 13 Connector load capacity envelopes

Figure 13 illustrates a linear capacity chart. As analternative to this capacity chart, the connector capacity may begiven by effective tension-moment-pressure (Te-M-p)interaction equations in the format of:

. (11) . (12)The line with arrow against the origin indicates the change

in capacity with increasing tension. Note the maximumpressure is limited to design pressure. Closed form capacityequations are determined for both structural and functionafailure modes.

Several interaction equations may be provided for oneconnector in case a simple linear relation as suggested abovegives sufficient accuracy. Internal pressure may give globaexpansion, hence, increase thread engagement for threadedconnectors, and thereby preventing unzipping and non-conservative tension/bending capacities. This non-conservativetension/bending capacity shall not be used, hence, T c/Mc, shalin these cases only be based on tension only without anyinternal pressure.

FATIGUE ANALYSIS AND CAPACITY

GeneralAnnex C contains requirements for the calculation of the

fatigue capacities of components. It should be noted thatconnectors may leak or unscrew during cyclic loadinghowever, these failure modes are not covered by Annex CLimitations with respect to material strength, environment, andtemperature are given in the referred codes like DNV-RP-C203BS 7608, BS 7910 and ASME FFS-1. ISO-7 covers only highcycle fatigue, i.e. stress-life, and not low cycle fatigue, i.estrain-life.

Methods for calculating fatigue capacityFatigue capacities may be performed by the following:

Methods based upon fatigue tests (S-N curves for normallysound connections) and estimation of cumulative damage(Palmgren-Miner rule);

Methods based upon fracture mechanics (fatigue crackgrowth predictions of flawed components);

Direct experimental approach by fatigue testing ocomponents representing given manufacturer methods ofproduction;

Combination of calculations and full scale fatigue testinge.g. for connectors.

The fatigue capacity should in general be based on S-Ndata, determined by fatigue testing of a representativecomponent and the linear damage hypothesis (Palmgren-Miner). The S-N method of fatigue analysis is the primarydesign tool used to assess fatigue resistance of C/WO risercomponents due to its simplicity and efficiency. As discussedearlier in this paper, the S-N approach may be used due to thesafe life design philosophy applied in ISO-7. Annex D


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compares S-N and FM calculation methods for a simpleexample.

The S-N method may be applied welds, seamless pipe,machined components and fasteners, see Table 5. The S-Nrelationships for the various structural detail classes as shownby some examples in Table 5, have been based on statisticalanalysis of available experimental data. In general fracturemechanics is not a suitable design method, as the results aredependent to a very large extent, upon the assumptions whichare made regarding the size of the initial flaw(s), the shape ofthe resulting fatigue crack, and the local stress fields normal tothe assumed crack growth plane. All this information is notusually available at the design stage for all components, e.g.welds, fasteners, connectors. Therefore, if fracture mechanics isused at the design stage, assumptions have to be conservative.

Table 5 S-N curves and fatigue livesDNV-RP-C203

DescriptionS-N curve Life/W31

Weld/detail Geometry/hotspot

Single-sidedgirth weld

F1 5,40

F3 3,80

D 16

Seamlesspipe

B1 111

Machinedcomponent

B1SCF=1 111

SCF=3 34

HSSCF=1 921

SCF=3 4

FastenerF1 (rolled) 5,4

W3 (cut) 1,0

Fatigue life normalised with respect to W3 S-N curve at a stress range of 200

MPa (29 ksi) for seawater with cathodic protection

Fracture mechanics can, however, be a useful method forcarrying out parametric studies, where the objective is to definethe relative influence of a particular set of variables. Ifappropriate, the fatigue analysis may alternatively be based onor supplemented by fracture mechanics based fatigueassessment. This may be relevant if the remaining life of acracked component is sought or if crack detection limits forfabrication and in-service inspection NDT are of interest.

If no representative fatigue resistance data are available, adirect experimental approach by fatigue testing of componentsshould be applied. This may be relevant in cases wherelimitations on fatigue strength data are for temperature, materialstrength and fluids to which the material is exposed.

However, fracture mechanics based fatigue analysis is usedin the following cases:

Assessing the fitness for purpose of components withknown flaws/cracks;

Extend fatigue life for components being in service.

The recommended design method or method to determine thefatigue capacity is the S-N approach due to its simplicity

To extend the fatigue life of components in service, theupdated fatigue capacities may be determined by fracturemechanics methods where applied initial flaw size is based onthe NDT inspection detection capability applied duringinspection to extend the fatigue life.

Typical environments include, air or non-corrosiveenvironment, seawater environment with cathodic protectionand free corrosion and sour service. Sour service reduces thefatigue capacity. Present practice in North Sea has been toassume free corrosion environment S-N curves for well fluidwetted fatigue hot spots (bore) in cases with sour serviceBuitrago et al (2008) and Hudak et al (2012) report adegradation factors on fatigue life between 10 and 100 whencompared to fatigue life in air. Free corrosion typically gives adegradation factor on fatigue life in the range of 3 (high stressrange) to 20 (low stress range) when compared to fatigue life inair, see DNV-RP-C203. However, it should be noted thaC/WO risers normally are exposed to well fluids only a small

fraction of the connected time.

Fatigue capacity formatFatigue capacities are established for all significant cyclic

loads, e.g. cyclic bending moment, cyclic internal pressure, andcyclic tension. The fatigue capacities should be given as closedform equations as follows for cyclic bending moment, seeFigure 14,

(13) (14)

The fatigue capacities are presented as closed form asmoment range (M) vs. number of cycles to failure (N), i.eM-N curves, tension range vs. number of cycles to failure andpressure range vs. number of cycles to failure.

Figure 14 ComponentM-N curve

The M-N curve plot is a straight line on log-log paper, seeFigure 14.The cyclic capacity can also be presented as bilinear


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(two-slope) or multi-linear M-N curves. Similar capacitycurves shall be established for cyclic pressure, i.e. P-N, whereP is the pressure range and N is the number of pressure cyclesto failure and for cyclic effective tension, i.e. Te-N, where Teis the effective tension range and N is the number of cycles tofailure for cyclic effective tension.

By using elastic-plastic material stress-strain curves forfatigue capacity calculations, local yielding in areas of hightensile stress during make-up, pressure testing and first loadcyclic loading is accounted for. This improves the accuracy ofthe calculated mean stress and cyclic stress range. Residualcompressive stresses are imposed on these areas, reducing themean stress on subsequent cyclic loads. For threadedconnections, linear elastic analysis often predicts the firstthreads to have the highest stress concentration factors. Elastic-plastic analysis has shown to predict other threads (e.g. threadnumber five to seven) to be critical. Fatigue testing confirmedthese calculations.

FEA analysis and models are similar to the static capacity3D models, contact elements between contacting bodies. If

stress ranges are in the elastic range, linear elastic stress-straincurve may be used. For local yielding, an elastic-plastic stressstrain curve with isotropic strain hardening is used to allow forlocal yielding during make-up, pressure testing and forsimulating the first load cycle. A fine mesh is required at stressconcentrations to allow for local yielding. The calculated meanstress and stress range during a load cycle (bending, tension, orpressure) are then entered into the S-N curve and the number ofload ranges (M, p, Te) to failure can be calculated.Different cyclic load levels are analysed and the M-N curvesare determined. The curves are then representing the region(s)where the fatigue capacity is lowest in theconnection/component. Industry practise has been to calculate

SCFs or transfer functions which are combined with theappropriate S-N curve to calculate the fatigue life. By using M-N curves, the interface and the format of capacities becomesimpler and the possibilities for misunderstandings betweenglobal analysis and component performance are reduced.

Alternatively a fracture mechanics approach may be usedassuming crack growth from an initial crack size to the finalcrack size using the results from the elastic or elastic-plasticcalculations.

QUALIFICATION OF CONNECTORS

Historically, many riser connectors, particular those with

complex geometries, have been successfully qualified forservice by performance testing, e.g. ISO 13625 (API 16R), ISO21329, ISO 13679 (API 5C5), and API 17D. These codes formthe basis for the qualification testing requirements in ISO-7.ISO-7 test to failure to confirm the failure mechanism and themargin between design capacities and actual failureloads/fa

structural design of metallic components

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