Fracture assessment methods for welded structures

3

Fracture assessment methods for welded structures

I. H a d l e y, TWI, UK

Abstract: This chapter reviews the use of fracture assessment procedures such as BS 7910, R6, FITNET and API 579-1/ASME FFS-1 to analyse critical engineering structures that contain flaws, such as cracks or fabrication defects. Such procedures, also termed fitness-for-service (FFS) or engineering critical assessment (ECA) procedures, allow the user to demonstrate the safety of a metallic structure or component in which some aspect of design, manufacture or operation may not comply with a recognised engineering code. Each procedure is based on engineering fracture mechanics concepts and validated by testing, analysis and extensive experience. Whilst all of the major procedures have a common underlying technology, each one also has aspects that make it attractive to a particular user group. The background, current structure, status and main application of each procedure is summarised. Key words: fracture, fitness-for-service (FFS), fitness-for- purpose (FFP), engineering critical assessment (ECA), damage-tolerant/defect-tolerant design, failure assessment diagram (FAD), engineering fracture mechanics, flaw assessment.

3.1

Introduction

Fracture from defects in structurally critical welds is normally avoided by close adherence to a recognised construction code. This will typically cover the design, materials requirements, fabrication and inspection of the component, and possibly other aspects affecting integrity, such as pre-service testing (for example hydrotesting of pressure equipment), operational requirements and in-service inspection. This approach is based on many accumulated years of experience, and has been highly effective in avoidance of failure. Where some aspect of design, build quality or materials properties falls outside the experience embedded in the codes, however, an alternative approach may be required, based on engineering fracture mechanics. Here, the possibility of failure (by brittle or ductile fracture, or by plastic collapse of the component) associated with the presence of a real or postulated defect is explicitly modelled. The flawed component can then be conveniently classified as safe or unsafe, typically by using a failure assessment diagram or FAD. The technique is typically known as engineering critical assessment 60 © Woodhead Publishing Limited, 2011

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or ECA, and forms part of the category of methods known as fitness-forservice (FFS), fitness-for-purpose (FFP), or defect-tolerant design. This chapter reviews some of the most widely used methods of fracture assessment of welded structures, such as BS 7910, R6, Structural Integrity Procedures for European Industry (SINTAP), European Fitness-for-service Network (FITNET) and API 579-1/ASME FFS-1. The evolution of the methods and the relationship between rule-based construction codes and ECA methods is described, along with relationships between the different ECA methods, and special features of each. Attention is restricted to procedures that address fracture assessment, i.e. failure under static loading at low or ambient temperature due to the presence of flaws.

3.1.1 Development of design codes for fracture-critical components Pressure equipment, initially in the form of simple boilers, has been used since the early days of the industrial revolution. As reported by Woods and Baguley (1), the design and construction of boilers was left to the individual designer or manufacturer; failures and fatalities were common, peaking at the rate of around one per day in the late 1890s, as shown in Fig. 3.1. In the early years of the 20th century, the American Society of Mechanical Engineers (ASME) issued its first code for power boilers. The dramatic effects of the code can be inferred from Fig. 3.1; the annual number of explosions fell over the remainder of the century, in spite of a rise in mean steam pressure (and a rise in the total population of boilers, which is not considered in the figure). Figures from across the industrialised world (2–7) show the current 5000

300 St

ea

m

e pr

ss

ur

e

4000

Steam pressure (psi)

Boiler explosions in the United States

400

3000

200

2000 100

1000

0 0 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 Year

3.1 Rates of boiler explosions in the USA.

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‘catastrophic’ failure rate of pressure vessels to be approximately 10 –5 to 10–6 per vessel year, although the precise figure varies somewhat among countries and depends on the type of equipment under consideration. The correct application of an appropriate code for design, construction, operation, inspection and maintenance of pressure equipment can therefore be expected to produce equipment with a very low failure rate. This is achieved through a combination of factors: ∑

Control of the operating stresses, and of the stress concentrations associated with changes of section, openings and other discontinuities. ∑ Control of the presence of flaws, especially at welds, by: qualification of both the weld procedure and the welders responsible for applying it, inspection (visual and non-destructive) of the finished product to ensure that it complies with the code (some ‘indications’ such as porosity or inclusions are inevitably associated with fusion welds, and the code will give guidance on what is considered acceptable). ∑ Control of the quality of materials, in terms of their chemical composition, tensile properties and (if the equipment will experience low-temperature operation) toughness. In practice this usually means the use of Charpy testing in order to demonstrate some degree of resistance to low temperature failure, especially if the vessel is constructed from a steel that undergoes a ductile–brittle transition as temperature is reduced. ∑ Application of a preservice pressure test in order to demonstrate the integrity of the equipment. This test, typically carried out using water as a test medium, is highly effective in weeding out potentially dangerous flaws or non-conforming material under conditions of relative safety, if the precautions mentioned earlier (inspection and control of materials qualities) have for some reason proved insufficient. An example of how these principles are implemented in a code is given by the European Pressure Vessel Code (8). Part 2 of the document (Materials) lists acceptable materials and includes an annex specifically addressing avoidance of brittle fracture. Part 3 (Design) gives methods for design by formulae (DBF), including determination of the required minimum thickness, weld joint efficiencies and the requirements for design of heads, stiffeners and openings. An alternative approach, design by analysis (DBA) is described in annexes to this part. Part 4 (Fabrication) covers such issues as welding qualification and manufacturing tolerances, while Part 5 (Inspection and Testing) covers the extent of inspection required and appropriate acceptance criteria. Of course, these principles also apply to other types of fracture-critical structures, including bridges, offshore structures, storage tanks, pipelines and shipping. The ECA techniques described in the remainder of this chapter can be applied to a wide range of welded and non-welded structures at all

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stages of the product life cycle, from design through fabrication, operation, life extension and decommissioning.

3.2

Development of engineering critical assessment (ECA) methods

Although rule-based standards such as the European Pressure Vessel Code have proved highly successful in terms of reducing the number of catastrophic failures in pressure equipment, there are inevitably situations that they do not address, for example cases where some aspect of materials properties, build quality or design fall outside the scope of the standard. Experience from failures shows that it is not the presence of flaws or brittle material per se that leads to failure, rather it is a combination of factors including the stresses on the flawed body and the service temperature, and that these factors can be quantified and related to each other via the discipline of engineering fracture mechanics. Consequently, a shortfall in, say, the toughness of a weldment normally used in the as-welded condition could be offset by the use of post-weld heat treatment to relieve welding residual stresses and reduce the driving force for failure. Fitness-for-service methods can be used in a variety of scenarios: ∑ ∑

∑ ∑ ∑ ∑

To make a decision as to whether a component containing a known fabrication flaw (or flaws) can be safely operated even though the flaws are not permitted by workmanship standards. To interpret the results of in-service inspection, e.g. to decide whether a component containing flaws propagating through fatigue crack growth or some other mechanism can be safely operated until the next shutdown/ inspection. To demonstrate that a particular inspection technique/procedure can safely detect and size flaws that could be critical to the integrity of a structure. To demonstrate whether or not post-weld heat treatment (PWHT) of thick-section components is necessary to ensure the integrity of a component. To support failure analysis by showing which scenarios could credibly lead to failure. To support life extension/change of use studies.

The development of structured procedures for carrying out ECA or FFS can be linked to a desire by industry and regulatory authorities to have a single, agreed and well-validated source of reference that could be used by all parties and would produce consistent results. Features of an ideal FFS document would include:

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∑ ∑ ∑ ∑ ∑ ∑ ∑

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being logical, self-contained and user-friendly (especially where the most conservative options/levels are considered); different options depending on the information available to the user and the level of accuracy required; ability to be used at all stages of life, e.g. design, operation, life extension, failure analysis; fully validated; where possible, consist with historic methods, so that older analyses do not have to be repeated every time a new edition is produced; where possible, taking into account the reliability and accuracy of the result; using terminology and data that are readily available from design documents.

Five such procedures are summarised in Table 3.1 and described in more detail in section 3.4.

3.3

The failure assessment diagram (FAD) concept

The FAD concept is common to all of the fracture analysis methods described in Section 3.5, so is described in general terms here; a comprehensive review is given by Milne et al. (9). It has long been recognised that the static failure of a structure containing defects can fail either by fracture or by collapse. In the case of linear elastic fracture mechanics (LEFM), failure will occur Table 3.1 Comparison between major fracture assessment procedures Method

First published

Main user base

R6 1976 Nuclear power

Current status Continues to be maintained

BS 7910 1980 (as PD6493) General procedure, but Amendment particularly popular published in 2005, with upstream oil, gas revision due c 2012 and pipeline sector SINTAP 1999 General procedure

Now subsumed into FITNET procedure

API 579–1/ 2000 (as API 579) Downstream oil, gas ASME FFS-1 and chemical industry

Joint API/ASME procedure published in 2007

FITNET 2006 General procedure

Published document, will form the basis of future BS 7910 revisions. Publication of weldrelated fracture clauses by IIW also planned

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when the applied stress intensity, KI, attains a critical value known as the fracture toughness, Kmat: K I = s nom p a f (a /W )

3.1

Here, the nominal applied stress is snom, the flaw size is given by a, the structural width by W, and f (a/W) is a geometry function that is tabulated for common structures and specimen types (e.g. (10)). Consequently, the value of nominal stress at failure, sf, is given by:

sf =

K mat p a f (a (a /W )

3.2

at the same time, failure due to collapse is a function of the structural geometry and the ultimate tensile strength of the material, su, and the value of nominal stress at failure, sc, can be expressed as: sc = D(a/W) su

3.3

where D(a/W) is also available from handbooks (e.g. (11)). Both parameters can be normalised and the transition between leFM fracture and plastic collapse represented in the form of a single Fad, which is independent of component geometry and material. Various types of Fad have been adopted over the years, as described in Section 3.4; the version published in the first version of R6 is shown in Fig. 3.2. The proximity to plastic collapse is represented on the horizontal axis in the form of a parameter Sr, that of fracture on the vertical axis, Kr. 1.2 1.0

Potentially unsafe

Kr

0.8 0.6 0.4 0.2 0.0

Safe 0.0

0.2

0.4

0.6 Sr

0.8

1.0

3.2 Strip yield FAD, as given in the first edition of R6.

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The key benefit of the FAD is that it allows the user to apply LEFM concepts to the analysis of a structure in which the development of plasticity affects the failure load. As Sr, and hence plasticity, develops, so the permitted value of Kr reduces until Sr = 1, beyond which failure by plastic collapse is conceded. Consequently, a flaw can be analysed by calculating Kr and Sr independently and plotting the result on a FAD. If the point lies within the failure assessment line (FAL), it can be considered safe, if outside it is potentially unsafe. A point lying on the FAL represents a limiting condition, which could be expressed in terms of flaw size, applied stress, minimum required fracture toughness or some other parameter. In welded structures, the Kr axis needs to take account of all factors influencing crack driving force, namely primary stresses (from self-weight, pressure, live load), secondary stresses (typically welding residual stresses), thermal stresses and local stress concentrations. The horizontal axis of the FAD, which allows for plasticity, is in general affected only by primary stresses. Subsequent developments in EPFM (elastic-plastic fracture mechanics) led to the formulation of the FAD as it is now implemented in several ECA procedures (see Section 3.4). The ‘strip yield’ model shown in Fig. 3.2 has been supplanted by the so-called ‘Option 1’ FAD (in R6 terminology) or ‘Level 2/Level 3 FAD’ (BS 7910), which has the advantage of being independent of geometry and material (although the form of the FAD will depend on the yield behaviour of the material, as outlined later). The Sr axis has been replaced by a new plastic collapse parameter, Lr, but the principle underlying the method has remained the same since the first version of R6 was published. There are, of course, alternatives to the FAD approach, of which the most common are termed crack driving force (CDF) methods. Here, the crack driving force is calculated in terms of KI or equivalent (by means of parametric equations or finite element analysis (FEA), for example) and compared with the resistance of the structure, expressed in terms of fracture toughness. The CDF approach, however, requires a separate calculation of susceptibility to plastic collapse, hence the enduring popularity of FADbased methods. All of the procedures discussed in this chapter make a distinction between primary stresses (such as those due to pressure, self-weight and external loads) and secondary stresses (such as ‘local’ welding residual stress) which are self-balancing across the section thickness and may drive crack propagation, but not plastic collapse. Consequently, the Kr axis reflects the influence of both primary and secondary stresses, the Lr axis primary stresses only. The shape of the FAD, with Kr decreasing as a function of Lr takes account of interaction between crack driving force and plasticity for the case of the primary stresses. So far as secondary stresses are concerned, interaction

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can be accounted for by correcting the value of Kr using a factor designated either V or r. Some procedures allow the use of either parameter.

3.4

Specific engineering critical assessment (ECA) methods: R6

3.4.1 Background The UK nuclear industry’s flaw assessment technique, designated R6 (12), was first issued in 1976, with the aim of ensuring that safety cases concerning power plant (both nuclear and fossil fuel fired) should be carried out in a reproducible and consistent fashion. Although R6 was intended to be applicable in principle to all types of welded or fabricated structures, it gained its early reputation through its application to the analysis of thickwalled pressure vessels, often in the stress-relieved condition and made from ductile materials. The R6 method explicity considered the possibility of failure of structures from both brittle fracture and plastic collapse, and introduced the concept of an FAD to show the interaction between the two. This allowed the user to apply LEFM concepts to calculate KI, with the FAD showing how the permitted level of driving force (designated Kr, the ratio of the crack driving force to the characteristic fracture toughness) reduces as the net section stress or reference stress (normalised to the yield strength of the material), increases. An example of the approach is shown in Fig. 3.3, which shows the simplest 1.2 1.0

Potentially unsafe

Kr

0.8 Approx. Option 2; continuous yielding

0.6 Approx. Option 2; discontinuous yielding

0.4

Option 1 FAD; continuous yielding

0.2 0.0

Safe 0.0

0.2

0.4

0.6

Lr

0.8

1.0

1.2

1.4

3.3 R6 FADs for continuously and discontinuously yielding materials (sY = 400 N/mm2, sUTS = 500 N/mm2).

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(‘Option 1’ or ‘approximate Option 2’) R6 FADs for a continuously yielding material and a discontinuously yielding material, i.e. one showing Lüder’s plateau behaviour. R6 is now in its fourth major revision; selected milestones in the development of the procedure are shown in Table 3.2 (13).

3.4 2 Structure The current edition of R6 is arranged into basic chapters as follows: I Basic procedures II Inputs to basic procedures III Alternative approaches (including mixed-mode loading, constraint, allowance for strength mismatch, use of local approach, probabilistic techniques, leak-before-break, load history effects, calculation of residual stress, displacement-controlled loading) IV Compendia (K-solutions, limit load solutions for homogeneous and strength-mismatched structures, welding residual stress distributions, constraint parameters) V Validation and worked examples Emphasis is given here to the basic procedures of Chapters I and II, as these are widely used and have influenced several other procedures. As well as introducing the concept of the FAD, R6 introduced the notion of carrying out fracture assessment using different ‘options’ of analysis, depending on the nature of the data available to the analyst. The basic hierarchy of FADs in R6 Rev. 4 (discussed in more detail later) is as follows: Table 3.2 Selected milestones in the development of R6 (13) Revision Year of number publication

Features

1

1976

∑ Recognition of fast fracture and plastic collapse as limiting failure modes, use of ‘strip yield’ FAD

2

1980

∑ Methods for the treatment of secondary/residual stresses

3

1986

∑ Plastic collapse axis expressed in terms of Lr instead of Sr (strip yield FAD retained for C-Mn steels) ∑ New FADs ∑ Analysis may be based on tearing resistance curve, and not only on initiation toughness

4 2001

∑ ∑ ∑ ∑ ∑

Restructuring of document New FADs, compatible with SINTAP procedure Consideration of weld strength mismatch Consideration of crack tip constraint Many new appendices

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∑ ∑

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Option 1: does not require detailed stress–strain data; the FAD is defined as a single curve over the range for 0 < Lr < Lr,max, where Lr,max is the ratio of the uniaxial flow strength to the uniaxial yield strength or 0.2% proof strength. The curve is based on a lower bound fit to data for a range of materials and structures. Option 2: the FAD is defined on the basis of detailed stress–strain data for the material of interest, so that yielding in the ‘knee’ region of the FAD (around Lr = 1) can be more precisely defined. Option 3: the Fad is both material- and geometry-dependent, generated from FEA of the cracked structure

In practice, the idea of a single FAD (such as the strip yield FAD shown in Fig. 3.2) representing all possible materials has now been largely superseded, and materials (and thus FADs) are classified according to whether they show (or are likely to show) continuous or discontinuous yielding. Of course, this means that users who already know whether yielding is continuous or discontinuous are likely to have access to a stress–strain curve for the material in question, and could therefore go straight to an Option 2 rather than an Option 1 analysis. Table I.6.2 of R6 gives some advice on this matter; a range of steels are classified in terms of strength, processing route, composition and heat treatment, and information given as to whether or not discontinuous yielding is likely. On the basis of this information, users can adopt the so-called ‘approximate Option 2’ curve, as shown in Fig. 3.3. The simplest analysis available in R6 uses the so-called Option 1 Fad, a successor to the strip yield Fad shown in Fig. 3.2. Here, the relationship between Kr and Lr can be assumed to be independent of materials properties and structural geometry for 0 < Lr < 1, where: f(Lr) = (1 + 0.5L2r)–0.5 [0.3 + 0.7 exp (– 0.6L6r)]

3.4

This originates from the FAD given in R6 Rev.3 (which continues to be used by BS 7910 and API/ASME), but with some slight adjustments to harmonise with the FADs developed under the SINTAP project (see section 3.4.3) and to ensure that the Option 1 Fad lies within the Option 2. For Lr > 1, the form of the curve will depend on whether the yielding behaviour is continuous or discontinuous; for continuously yielding material, the Option 1 Fad follows equation 3.4 up to a ‘cut-off’ at a value Lr,max, which depends on the yield properties of the material. Lr,max =

sf sy

3.5

where sf is the flow strength and sy the yield or proof strength of the material. For Lr > Lr,max, Kr = 0. For materials known to show continuous yielding, but for which a full

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stress–strain curve is not available, the so-called ‘approximate Option 2’ curve is used for Lr values between 1 and Lr,max: f(Lr) = f(1)L(N–1)/2N r

3.6

where N is an estimate of the strain hardening exponent given by N = 0.3 [1 – (sy/sm)]

3.7

For discontinuously yielding materials, either the Option 1 curve (Equation 3.4) can be followed up to Lr = 1, or the ‘approximate Option 2’ Fad is used. This consists of three regions: f (Lr) = (1 + 0.5L2r )–0.5

3.8

up to Lr = 1. This is followed by a rapid drop in Kr at Lr = 1, reflecting the yield plateau: f (1) = Èl + 1 ˘ ÍÎ 2l ˙˚

–0.5

3.9

where

l = 1 + E De sy

3.10

s De = 0.0375 ÈÍ1 – y ˘˙ 1000 Î ˚

3.11

and

followed by the use of equation 3.6 in the range 1 < Lr < Lr,max. These ‘generic’ curves, for which detailed stress–strain data are not required, are illustrated in Fig. 3.3 for a steel with yield/proof strength of 400 N/mm2 and ultimate tensile strength (UTS) of 500 N/mm2. The Option 1 and approximate Option 2 curves are intended to represent a lower bound to fracture/plasticity interactions for a wide range of materials. Consequently, if the user has more detailed materials data available, a more accurate representation of the Fad can be achieved by using Option 2: È Ee L3s ˘ K r = Í ref + r y ˙ ÎLr s y 2Ee ref ˚

–0.5

3.12

Finally, in situations in which detailed elastic and elastic-plastic Fea of the cracked structure is available, it is possible to use the Option 3 curve, defined by the curve: f (Lr ) =

Je J ep

3.13

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for Lr < Lr,max. Here, Je and Jep are the values of J-integral derived from elastic and elastic-plastic analyses respectively at a load corresponding to the value Lr. The advantage of the Option 3 method is that it has the potential to produce a very accurate description of the system, the Fad being dependent on the loading system, flaw size and geometry and materials properties. Conversely, this means that the FAD has to be regenerated for every flaw size and loading condition, so considerable computational effort is required. The interaction between primary and secondary stresses is treated in R6 by the use of either the r or the V parameter, although these are consistent with each other. For example, if the parameter r is chosen, it is added to the Kr arising from both primary and secondary stresses as follows: Kr =

Kl +r K mat

3.14

where r is tabulated in section II.6 of R6 as function of an additional variable p c = KSI (Lr/KI ). Alternatively, the contribution to Kr that arises from secondary s stresses, KI, may be multiplied by a factor V, so that: p

Kr = Kl + VK ls

3.4.3

3.15

Special features

The previous section describes only the ‘basic’ R6 options described in Chapters I and II of R6. There are numerous other special features that make the procedure applicable to a wide range of structural integrity issues. For example, Chapter III includes guidance on mixed-mode loading, the effects of weld strength mis-match on integrity, constraint effects, leak-before-break (LBB) analysis, use of the local approach, calculation of residual stress in weldments, crack arrest, probabilistic fracture mechanics and treatment of displacement-controlled loading. The R6 procedures have been influenced by, and have influenced, several other nuclear standards and procedures such as aSMe XI and the Ge-ePRI procedure.

3.4.4

User group

The main user group for R6 continues to be the power industry, both nuclear and fossil fuel, although the style of the document means that it is not tied to a particular industry sector or type of component.

3.4.5

Status

R6 has continued to be maintained and developed, first by the UK central Electricity Generating Board (CEGB), later by the successor organisation © Woodhead Publishing Limited, 2011

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Nuclear Electric and now by an industrial collaboration managed by British Energy with contributions also from other UK organisations (Serco, RollsRoyce, TWI and Frazer-Nash Consultancy). It is underpinned by a number of UK research projects (e.g. 14–15), and maintains contact with BSI, TAGSI (Technical Advisory Group on Structural Integrity), and a number of European research initiatives and networks. It is now in its fourth major revision, supplied as a subscription document with amendments issued approximately once a year.

3.5

Specific engineering critical assessment (ECA) methods: BS 7910/PD6493

3.5.1 Background The British Standards Institution (BSI) has published a ‘generic’ UK flaw assessment procedure, BS 7910 (16), i.e. one that is not tied to a particular industry or type of structure. The history of the document is described in various publications (17) and (18), and is summarised in Fig. 3.4. The first flaw assessment procedure to be published by the BSI was PD6493:1980 (19), ‘Guidance on some methods for the derivation of acceptance levels for defects in fusion welded joints’. The ‘PD’ (for ‘published document’) designation reflected the fact that the document was not intended to be a standard, but a guidance document. Moreover, it did not lay claim to be the only, or definitive, approach. The intent behind the document was to bring fracture mechanics into more general industrial use and the procedures it contained reflected this (20). Statistical issues CTOD design curve

R6 FAD approach

PD6493: 1980 Paris fatigue law

Load history LBB

PD6539 (creep)

PD6493: 1991 Multilevel analysis

LTA BS 7910: 1999 (+ Amd 1)

Corrections and clarifications

FITNET

BS 7910: 2005 (+ Amd 1)

BS 7910: c2012

Kl/sref library

Mk (weld toe effects)

RS distributions library Mismatch

3.4 Development of BS 7910.

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PD6493:1980 was published at a time when the UK offshore oil and gas industry was enjoying a boom. The construction of offshore jackets, pressure vessels, process piping and pipelines had reached unprecedented levels, and the combination of new construction methods and materials with very short ‘windows’ of weather in which installation could be carried out provided a strong incentive to expedite construction and installation. At the same time, catastrophic failures of offshore structures such as the Sea Gem and the Alexander Kielland had underlined the importance of controlling the quality of materials and fabrication techniques. PD6493 proved invaluable in helping engineers to distinguish between ‘critical’ flaws that could lead to failure and ‘benign’ flaws that were, to a large extent, an inevitable product of welding (21). PD6493 addressed two modes of failure: brittle fracture and fatigue. For the case of components subjected to stresses below the materials yield strength, the fracture assessment method used LEFM to calculate the driving force, KI, for brittle fracture. Simple graphical methods were used to calculate KI as a function of flaw size and shape, component width and thickness, and applied and residual stresses. Through-thickness, surface-breaking and embedded flaws were addressed, but the only geometry explicitly considered was the flat plate. For components subjected to stresses above yield (i.e. when the sum of the ‘primary’ stresses from external loading and the ‘secondary’ stresses such as welding residual stress exceeded the yield strength of the material), the so-called crack tip opening displacement (CTOD) design curve was used (22). This was a relationship between applied strain ratio and critical flaw size, partly empirically based. Failure of the uncracked ligament by plastic collapse was considered separately. The emphasis in PD6493 reflects, of course, the materials and problems of concern at the time, especially in the UK offshore industry, namely fatigue crack growth and brittle fracture of carbon steels at ambient and low temperature. In contrast, R6 (see Section 3.4.1) was aimed at the power industry, where fatigue was less of an issue, but plastic collapse was a credible failure mode, especially at elevated temperatures. Fatigue assessment to PD6493:1980 used the Paris law. Fatigue crack growth constants for air and marine environments were given in the document, along with graphically-based solutions, essential in this era before the widespread use of personal computers. Although the PD6493 and R6 methods (described in more detail in Section 3.4.1) had been developed in parallel, and addressed the needs of different industry sectors, it soon became clear that the underlying technology was virtually identical (23) and the FAD approach to fracture assessment was adopted in the second (1991) edition of PD6493. Users were given the option of using one of three different FADs, depending largely on the materials data available to them. This hierarchical approach to fracture assessment has

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persisted ever since, although the ‘levels’ of analysis in PD6493/BS 7910 do not exactly coincide with the R6 ‘Options’. For simplified calculations, the FAD-based equivalent of PD6493:1980 was adopted. This so-called ‘level 1’ calculation included an inherent safety factor and assumed no interaction between plasticity and crack driving force. Level 2 calculations, used for more critical applications, employed a strip yield model of the relationship between plasticity and crack driving force, while Level 3 calculations, used for assessment of ductile tearing, used a FAD similar to the R6 Option 2 FAD. Although various commercial software programs were available about this time, they were considered insuitable because they either demanded a high level of fracture mechanics expertise or they did not use PD 6493 procedures. The step change in calculation complexity associated with the revised procedures encouraged the development of software tools to act as ‘text animators’ for the calculation procedure (24) and has since developed apace with the evolution of the procedures (25). In 1999, more radical changes took place, a few of which are noted below: ∑

The document was upgraded to become a British Standards Guide, BS 7910. ∑ Creep assessment methods, originally published in PD6539, were incorporated into the document. ∑ Corrosion assessment methods for locally thinned areas (LTAs) in pipelines were proposed, based on a major joint industry project carried out by British Gas. ∑ An expanded library of K-solutions and reference stress solutions was added, allowing analysis of plates, cylinders, round bars, spheres and complex welded joints. ∑ Residual stress (RS) distributions were presented for various common welding processes and geometries. ∑ Load history effects, in particular the role of warm prestressing and prior overload in integrity, were included in the document. ∑ Leak before break (LBB) methods were introduced. Since then, the technical content of BS 7910 has remained stable, although the second (2005) edition provided the committee with the opportunity to act on user feedback by correcting and clarifying selected parts of the procedure. The current version of the procedure includes Amendment 1, published in 2007 (16). A major revision of BS 7910 is currently under preparation (26), further details of which are given in Section 3.5.5.

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3.5.2 Structure The current version of BS 7910 includes methods for assessment of fracture, fatigue, creep and corrosion and is structured as follows: ∑

Sections 1–6 cover the philosophy of FFS and advice on the information required for assessment. ∑ Section 7 covers fracture resistance, the main theme of this chapter. ∑ Section 8 addresses fatigue crack growth. ∑ Section 9 covers creep crack growth. ∑ Section 10 addresses ‘other modes’ of failure, e.g. stress corrosion, buckling.

A series of annexes (A–U) provide essential (‘normative’) information such as K-solutions, residual stress distributions and reference stress solutions, or constitute ‘informative’ annexes with advice on subjects such as fracture toughness testing of welds, effects of weld strength mismatch, mixed mode loading, leak before break, Charpy-fracture toughness correlations and reliability. The structure of the fracture assessment procedures in BS 7910 is described in more detail below. The concept is that three ‘levels’ of analysis are available, the choice of level depending on the information available to the user in terms of stress input and materials properties. The Level 1 procedure has its roots in the CTOD design curve, and treats brittle fracture and plastic collapse as independent, non-interacting events. Consequently, the user does not need to consider primary/secondary stress interaction factors (V or r), and the FAD is a simple rectangle. In order to compensate for these simplifications, the Level 1 procedures include a number of built-in safety factors. When a more precise description of failure conditions is required, the Level 2 or Level 3 procedures can be used. If the ‘generalised FAD’ is chosen, the user has a choice between continuous yielding and discontinuous, i.e. it is discontinuous if the material shows a Lüder’s plateau or a load drop. So far as the FAD for Lr values up to 1 are concerned, BS 7910 treats the two cases (continuous and discontinuous yielding) as identical, using equation (10) of BS 7910, which originates from Revision 3 of R6:

f(Lr) = (1 – 0.14L2r)[0.3 + 0.7 exp (– 0.65L6r)]

3.16

The sharp fall-off in permitted Kr at Lr > 1 for discontinuously yielding materials originates from the SINTAP project (see section 3.4.3), so the BS 7910 curve as a whole is a hybrid of the R6 Rev. 3 and SINTAP FADs. Where detailed stress–strain data are available, a more accurate FAD can be constructed directly from the stress–strain curve:

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È Ee L3s ˘ K r = Í ref + r y ˙ ÎLr s y 2Ee ref ˚

–0.5

3.17

In BS 7910, this is termed the level 2B FAD (if fracture toughness is given in terms of a single value of toughness, such as J0.2Bl, dc, KIc) or the level 3B FAD (if fracture toughness is given in terms of a tearing resistance curve). a comparison of Fads used in Pd6493 and BS 7910 is shown in Fig. 3.5. The x-axis is expressed in terms of the plastic collapse parameter Lr (rather than Sr, the term used in earlier versions of the procedure) in all cases, to facilitate comparison.

3.5.3

Special features

The title of the original PD6493 document (‘Guidance on some methods for the derivation of acceptance levels for defects in fusion welded joints’) indicates that the main emphasis of the document was on welded joints (later, the change from PD6493 to BS 7910 included a change of title to reflect the fact that the methods are applicable to metallic joints in general). Several aspects of BS 7910 (some of which do not appear in other procedures) make it particularly applicable to welded joints, and these are summarised below: Welded joints inevitably introduce some form of discontinuity in a structure, as a result of misalignment. This can be caused by eccentricity (e.g. offset between the two sides of a butt weld), by joining components of different thickness, by angular misalignment, or by some combination PD6493 Level 2 (superseded)

1.0

0.8

Level 1

0.6 Kr

∑

0.4

Level 2a/3a (discontinuous yielding)

Level 2a/3a (continuous yielding)

0.2 Lr,max 0.0 0.00

0.25

0.50

0.75 Lr

1.00

1.25

3.5 Comparison of FADs used in PD6493 and BS 7910.

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∑

of these factors. The result is that there will be local bending stresses that will affect both the stress intensity factor and the susceptibility to plastic collapse. annex d of BS 7910 includes a table of formulae showing how these secondary bending stresses can be calculated as a function of the misalignment, and incorporated into an ECA. Consequently, there is a direct link between the permitted levels of misalignment (which are typically stipulated by construction codes) and the implications of this on structural integrity. In practice, the effects of misalignment can be very significant, both for fracture and fatigue assessment. The weld toe represents a region of local stress concentration, and this should be taken into account in the calculation of stress intensity factor for cracks located at the weld toe. BS 7910 includes parametric formulae for the calculation of a factor Mk, which represents the ratio: Mk =

∑

77

K l for for crack in str structure containing weld eld toe K l for for crack in same st structur cture without weld toe

Solutions for Mk are given in annex M of BS 7910, as a function of the ‘leg length’ of the weld (the distance between the weld toes), the section thickness, loading mode and the position for calculation of Mk (i.e. the distance from the weld toe). In practice, the Mk correction is significant only for crack tips lying relatively close to the weld toe (a < 0.3B, where ‘a’ is crack depth and ‘B’ is section thickness), and is therefore relevant mainly for the analysis of relatively shallow cracks, including fatigue cracks initiating at the weld toe. Some of the solutions given in BS 7910 are based on 2D finite element modelling and therefore strictly applicable only to straight-fronted cracks, while others are based on 3D modelling and can be applied to semi-elliptical surface-breaking cracks at a weld toe. In the fracture assessment clause (clause 7) BS 7910 gives advice on the likely magnitude of residual stress in weldments in the as-welded condition, after post-weld heat-treatment and after application of a high primary stress. The residual stress is usually assumed to act as a uniform tensile membrane stress across the section thickness; the so-called ‘Level 1’ approach to residual stress estimation. although this assumption is not realistic (‘local’ secondary stresses are in general balanced across the section thickness), it is conservative for the purposes of defect assessment and is often the only safe assumption that can be made in many cases, given the likely variations in the residual stress profile from weld to weld, and the uncertainties in locating the exact position of a defect by non-destructive testing (NDT). an alternative to the ‘level 1’ assumptions is given in annex Q, which includes a compendium of residual stress distributions for welds in the as-welded condition, including butt welds in plates, circumferential and

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axial welds in pipes and repair welds. The current Annex M (compendium of K-solutions) does not explicitly support variable stress distributions of the kind shown in Annex Q, the assumption being that they can be handled by stress linearisation, either across the flaw or across the full section thickness. However, Annex Q could be implemented by use of ‘… handbook solutions, numerical modelling or weight function methods…’ provided that the procedure is fully documented. BS 7910 explicitly addresses fabrication flaws, recognising the need to aim for high quality welding. While flaw assessment procedures have sometimes been criticised on the grounds that they condone poor welding, it should be noted that the Introduction to BS 7910 states the opposite:

. . . a proliferation of flaws, even if shown to be acceptable by an ECA, is regarded as indicating that quality is in need of improvement. The use of an ECA can in no circumstances be viewed as an alternative to good workmanship. The response to flaws not conforming to workmanship criteria needs to be the correction of the fault in the process causing the non-conformance. The philosophy that the methods covered by this standard are complementary to, and not a replacement for, good quality workmanship is inherently assumed in this standard.

Of course, this does not preclude the use of BS 7910 for analysis of inservice flaws such as fatigue cracks, or for applications such as failure analysis.

3.5.4 User group Historically, the main user group for BS 7910 and its predecessor PD6493 has been the offshore oil and gas industry. The procedure allows for rapid, robust calculation of defect tolerance, albeit at the expense of excessive conservatism in some cases. The emphasis tends to be on avoidance of failure, rather than prediction of failure conditions. It should be borne in mind that the procedure was developed primarily in order to make rapid decisions about whether or not to repair fabrication flaws, rather than in order to predict precise failure conditions, probability of failure or safety factor against failure. Consequently, a flaw that lies outside the FAL and is originally judged to be ‘unsafe’ may well prove to be safe when re-analysed using more sophisticated methods. BS 7910 is recognised and cited by a number of national and international codes and standards, including codes for pipelines, pressure vessels, gas cylinders and offshore equipment/structures.

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3.5.5 Maintenance BS 7910 is owned and distributed by the BSI, with technical input provided by a permanent committee. This comprises volunteers from groups representing manufacturers, certification bodies, health and safety organisations, academia and end users. The detailed technical content is developed by a series of ‘panels’ (sub committees), covering particular topics such as fracture, fatigue, creep, corrosion, residual stress and materials properties. The committee maintains informal contact with the developers of other procedures, e.g. R6, FITNET and American Petroleum Institute/American Society for Mechanical Engineers (API/ASME). At the time of writing, BS 7910 is being extensively revised. The main source documents for this new edition will be the current edition of BS 7910, plus the R6 and FITNET documents (see Sections 3.4.1 and 3.4.3). The principles behind the new edition, also described in (26) are: ∑

to preserve compatibility with previous editions of BS 7910 (unless the methods are obsolete or there is evidence that they are unsafe); this avoids the need to re-visit analyses carried out with an earlier edition of the procedure, and makes the document more amenable to returning and occasional users; ∑ to harmonise with other procedures (especially R6 and FITNET) wherever possible; ∑ to support the use of the more advanced analysis techniques now available (for example, analyses utilising weld strength mismatch and constraint), while still allowing analyses based on simple, conservative inputs (e.g. for life extension of structures built long ago, for which data may be very sketchy). Some of the main proposed new features can be summarised as: ∑

removal of the current Level 1 procedures and the related ‘manual’ procedure of Annex N; ∑ replacement of the fracture assessment levels with options, designated 1–3 and structured in a similar manner to FITNET and R6; ∑ inclusion of weld metal strength mismatch concepts in the calculation of Lr; ∑ inclusion of a procedure for constraint-based analysis; ∑ expansion and revision of the residual stress annex; ∑ inclusion of an appendix giving advice on NDE (non-destructive examination). The revised edition, which draws extensively on FITNET and R6, is due to be published around 2012. The major changes will be in the area of fracture analysis (clause 7 of the current Bs 7910); Table 3.3 summarises the hierarchy

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Table 3.3 Levels of analysis in the proposed BS 7910:2012 procedures Description BS 7910 2005 FITNET R6

Proposed BS 7910: 2012

Charpy/fracture Annex J Option 0 – toughness correlations

Annex J (use Option 1 FAD)

Simple screening Level 1 – – method

No longer required

Generic FAD, Level 2a Option 1, single-point value of single-point fracture toughness, analysis continuous yielding

Option 1 or ‘approximate Option 2 curve, continuous yielding’

Option 1 (continuous or discontinuous sub-options)

Generic FAD, single- Level 2a Option 1, point value of single-point fracture toughness, analysis discontinuous yielding

Option 1 (up to Lr = 1 only) or ‘approximate Option 2 curve, discontinuous yielding’

Option 1 (continuous or discontinuous sub-options)

Material-specific Level 2b Option 3, Option 2 FAD, single-point single-point value of fracture analysis toughness

Option 2

Generic FAD, Level 3a Option 1, Option 1 fracture toughness tearing expressed as tearing analysis resistance curve

Option 1 (can be continuous or discontinuous)

Material-specific FAD, Level 3b Option 3, fracture toughness tearing expressed as analysis tearing resistance curve

Option 2

Option 2

FEA-based analysis Level 3c Option 4 Option 3 (can include mismatch effects)

Option 3 or 3m

Mismatch analysis, based on tensile properties only

Not Option 2 – considered

Option 1m (Annex I)

Mismatch analysis, Not Option 3m – based on full considered stress-strain curves

Option 2m (Annex I)

Constraint-based analysis

Annex

Not Option 5 considered

–

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of fracture assessments in the new procedure, and how it compares with existing procedures.

3.6

Specific engineering critical assessment (ECA) methods: structural integrity procedures for European industry (SINTAP)/European Fitnessfor-service Network (FITNET)

3.6.1 Background The SINTAP project was a collaborative European R&D project running between 1996 and 1999. The output of the project was a fracture assessment procedure (27), in which the same flawed structure could be analysed using several different options, the choice between them depending on the quality of the data available. The intention was to develop a procedure suitable for a range of different industries and types of analysis. Consequently, a bridge operator faced with a defect in an 80-year-old bridge for which little or no materials data were available might nevertheless be able to assess and ‘pass’ the defect based on a very simple and conservative SINTAP analysis. A more advanced SINTAP option might be used to assess the reasons for failure of a newly constructed welded pressure vessel, taking into account the detailed microstructure (and thus the fracture toughness) of the failed region, the strength mismatch between weld metal and parent metal, the effect of the pre-service hydrotest on failure conditions and the detailed geometry of the vessel. SINTAP was influenced by the R6 and BS 7910 procedures and by the research and development programmes of its participants, who represented a range of European industries, Research and Technology Organisations (RTOs) and universities. It has, in turn, influenced both R6 and BS 7910, in particular in its formulation of the FAD and the distinction between FADs for continuously and discontinuously yielding materials. Subsequently, the main principles of the SINTAP procedure were absorbed into the FITNET procedure, a more broadly based European defect assessment procedure produced as the output of a European thematic network, running between 2002 and 2006.

3.6.2 Structure Like BS 7910, the FITNET procedure (28–29) addresses fatigue, corrosion damage and creep as well as fracture. It is currently available in a two-volume set, the first of which sets out the basic procedures, the second of which is a series of annexes containing essential information such as K-solutions and limit load solutions, plus ‘informative’ annexes on a variety of subjects,

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including advice on NDT and on sources of materials data. A third volume, containing information on the validation of the procedure, case studies and worked examples, is due for publication shortly. A workbook based on the procedure is also available (30), and further background information is given in (31–34). The fracture section of FITNET was built largely on the concepts developed in the SINTAP project, so can be considered to have superseded SINTAP, although the latter is still available, along with the supporting research reports (35). So far as the hierarchy of the FITNET fracture assessment procedure is concerned, there are numerous methods available, as shown in Table 3.4. At one extreme (Option 0), the user is assumed to have only Charpy Table 3.4 Fracture analysis options in FITNET Option no. Type of tensile data required

Type of fracture toughness data required

Other information

0 (basic) YS or SMYS None; Charpy only energy only

Relies on correlations; applicable to ferritic steels only

1 (standard) YS and UTS Single-point fracture toughness data or tearing resistance curves

Based on tensile properties of the weaker material (typically the PM) and the fracture toughness of the material in which the flaw is located

2 (mismatch) YS and UTS of PM and WM

Single-point fracture toughness data or tearing resistance curves

Takes account of strength mismatch; typically worth applying only if M ≥ 1.1 or M < 0.9

3 (stress– Full stress–strain strain) curves for PM and WM

Single-point fracture toughness data or tearing resistance curves

Takes account of material stress–strain behaviour; can also incorporate strength mismatch

4 (J integral) Full stress–strain curves for PM and WM

Single-point fracture toughness data or tearing resistance curves

CDF approach only; elasticplastic FEA is used to calculate the driving force for the cracked body

5 (constraint) Full stress–strain curves for PM and WM

Relationship between fracture toughness and crack-tip constraint, e.g. J as a function of T stress

Can take into account constraint effects, by matching crack tip constraint in the test specimen and the cracked structure

YS, yield (or proof) strength; SMYS, specified minimum yield strength; PM, parent metal; WM, weld metal; M, mismatch ratio (ratio of WM yield strength to PM yield strength).

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energy and yield strength data (or specified minimum values of the same) available. The yield strength is used to estimate UTS, the Charpy energy to estimate Kmat and a simplified analysis can be carried out. The intention is, however, that assessments should be based wherever possible on fracture mechanics data, using one or more of Options 1–5, depending on the nature of the information available. The Option 1 curves in FITNeT are identical to the so-called ‘approximate Option 2’ curves of R6; detailed stress–strain data are assumed not to be available, but the user has some concept of likely yielding behaviour. For materials which show continuous yielding: f(Lr) = (1 + 0.5L2r)–0.5[0.3 + 0.7 exp (– mL6r)]

3.18

for Lr up to 1.0. Unlike the R6 Option 1 curve, this includes a materialdependent term, m, where: Ê ˆ m = min Á 0.001 E ; 0.6˜ Rp Ë ¯

3.19

where E is the elastic modulus of the material and Rp is the proof strength. For discontinuously yielding materials, a discontinuity occurs at Lr = 1, and the Fad is similar to that seen in Fig. 3.3. Option 1 of FITNeT is therefore broadly similar to ‘approximate Option 2’ of R6. a distinction is made between continuously and discontinuously yielding materials, but detailed stress–strain data are not required and the work-hardening behaviour is estimated from the yield and tensile properties. Where users have detailed stress–strain data, Option 3 can be used. The FAD is identical to the R6 Option 2 or BS 7910 level 2b/3b Fads. Of course, one of the features of welded joints is that there are differences in strength, composition and microstructure between the weld metal and the parent metal. There will also be differences in microstructure and strength between the heat affected zone (HAZ) and parent metal. Most structural joints require the weld metal to ‘overmatch’ the parent metal. loading of the joint is then based on the properties of the parent metal, with the result that any flaws in the weld metal are to some extent shielded from the effects of the loading by virtue of the weld metal strength. For situations in which there is significant strength difference (typically more than about 10% difference in yield strength) between the weld metal and parent metal, FITNET includes the option to account for this mismatch, using Option 2. While it is recognised that Lr can be conservatively calculated by assuming the tensile properties of the lower strength material (weld metal or parent metal) in an Option 1 or Option 3 calculation, Option 2 allows the user to benefit from the strength mismatch. The maximum benefit arises for collapse-dominated cases in which there is high strength mismatch, with collapse load potentially increasing by

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the ratio sfs/sfw, where sfs is the flow strength of the stronger component and sfw that of the weaker. Where a full stress–strain curve is available for both the parent and weld metals, mismatch can be incorporated into an Option 3 analysis (by defining a single ‘equivalent’ stress–strain curve), allowing in principle a more accurate assessment than would be possible with Option 2. A demonstration of the potential of Options 2 and 3 is given in Koçak et al. (36), Seib et al. (37) and Hadley and Moore (38), which describe analyses and tests on a range of overmatched and undermatched welded wide plate specimens. The Option 4 procedures of FITNET are broadly similar to those of Option 3 in R6 or Option 3c in BS 7910; elastic-plastic FEA is used to model the cracked structure and the driving force is determined directly from FEA. However, unlike R6 and BS 7910, FITNET Option 4 uses a CDF approach, and has no explicit FAD-based equivalent. Finally, Option 5 gives the user the chance to combine detailed modelling of the driving force with constraintdependent fracture toughness.

3.6.3 Special features FITNET was conceived as a pan-European document for use by a range of industry, and not allied to any particular construction code or component type. Particular features of the fracture clauses include: ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑

a hierarchy of fracture assessment methods from Option 0 to Option 5; clear distinction between FADs on the basis of the yielding behaviour of the material; treatment of weld strength mismatch (under- or over-matching); treatment of crack tip constraint; a choice of driving force parameters; either CTOD (d) or K can be used; a choice between CDF or FAD-based methods; a range of ‘alternative’ fracture assessment techniques, including LBB, crack arrest, fracture under mixed mode loading; a section outlining ‘additional’ fracture assessment techniques (on the whole, these are less well-developed than those classified as ‘alternative’). These include the use of the local approach, treatment of ‘non-sharp’ flaws and advice on dynamic effects on tensile and fracture toughness determination.

3.6.4 User group FITNET is a new document, which will be used as a source document for the forthcoming BS 7910 and IIW procedures (see section 3.5). Examples of

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its potential application to pressure vessels, aerospace components and even medical devices are given in the final conference proceedings (39).

3.6.5 Status FITNET version MK8 is currently available as a stand-alone procedure, but some errors and omissions have inevitably been identified subsequent to publication, and are recorded on the network website (40). Elements of FITNET will be included in future versions of BS 7910 and the IIW procedure, as outlined in Section 3.5.

3.7

Specific engineering critical assessment (ECA) methods: American Petroleum Institute (API)/ American Society for Mechanical Enginners (ASME)

3.7.1 Background In the USA, the development of the API 579 procedure, first published in 2000 (41), was driven principally by the need to address flaws and damage found during the operation of refinery and petrochemical plant, in particular pressure vessels, piping and tanks designed, fabricated, operated and inspected in accordance with ASME and API standards. Of particular concern was the need to maintain the safety of older equipment in which in-service damage could have accumulated. There was also a drive to produce a method that would be compatible with US Occupational Safety and Health (OHSA) legislation and would ensure consistency between different analysts addressing a similar problem. A joint industry project, the Materials Properties Council (MPC), concluded in 1991 that the FFS standards in existence at that time did not adequately cover the many types of flaw and damage found in the refining and petrochemical industries. The response to this was a document organised by damage type (for example, general and local metal loss, pitting corrosion, hydrogen-induced cracking, laminations) and covering the inspection, analysis and future avoidance of such damage in a systematic fashion. The document is also organised by ‘levels’ of analysis, but in contrast with the other FFS methods described in this chapter, the ‘level’ of analysis envisaged by the API procedure is explicitly linked to the skills and qualifications of the analyst, as shown in Table 3.5. Following the publication of the original API 579 procedure in 2000, the work of a joint API/ASME committee led to the revision and expansion of the document in its current form, namely API 579-1/ASME FFS-1 (42).

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Table 3.5 Levels of analysis in the API-579 and API/ASME procedures Level Application

Skills, qualification and experience of analyst

1

Plant inspector or plant engineer

Conservative screening method requiring minimal information from inspection; little computation required (results often tabulated for easy interpretation during plant inspection)

2 Less conservative method than Level 1, but requiring more detailed calculation 3

Plant engineer or specialist engineer (experienced and knowledgeable in FFS methods)

Detailed inspection information required, Specialist engineer (experienced plus details of component geometry, and knowledgeable in stresses and materials properties. FFS methods)

3.7.2 Structure The API/ASME procedure is intimately connected to the ASME codes for boilers, pressure vessels, tanks and piping and to ASTM material grades although it does not preclude the use of other design codes or materials. The document starts with two introductory chapters introducing terms, responsibilities and the principles of FFS procedures. Thereafter, the document is laid out in a consistent manner, with each chapter addressing a particular type of damage, e.g. general metal loss, local metal loss, pitting corrosion, hydrogen blistering, assessment of crack-like flaws. For each type of damage, sub-sections of the chapter address the data requirements, applicability and limitations of the procedure, assessment method (at three different levels), remediation of damage, remaining life analysis and inservice monitoring. The emphasis is on user-friendliness, with flow charts, tables and graphs used wherever possible, with data given in both SI and US customary units.

3.7.3 Special features Two chapters of the API/ASME are of particular interest for fracture assessment: Chapter 3 (Assessment of existing equipment for brittle fracture) and Chapter 9 (Assessment of crack-like flaws). Chapter 3 is essentially a screening technique, and is not intended for use where crack-like flaws are encountered or expected; for this situation, the user needs to consult Chapter 9. The analysis of crack-like flaws at Level 3 (described in Chapter 9 of the procedure) is broadly similar to the approach adopted by BS 7910 and R6, and these methods are in fact explicitly referenced as alternatives. There are,

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however, certain features unique to API 579 and its successor, API 579-1/ ASME FFS-1: ∑

K-solutions for internal and external axial and circumferential flaws in cylinders, and for internal and external surface flaws in spheres, were derived directly from FEA for a wide range of geometries and loading conditions. ∑ Stress intensity can be derived for arbitrary through-wall stress distributions – this represents an advantage compared with the current edition of BS 7910, in which K-solutions are given in terms of membrane and bending stresses only. ∑ Appendix F of the procedure gives advice on sourcing materials data such as fracture toughness and includes Charpy/fracture toughness correlations from Sections III and XI of the ASME Boiler and Pressure Vessel Code. ∑ Appendix B allows the user to define reference stress from the elasticplastic J solution rather than from a limit load solution. This removes the substantial geometry-dependence of FAD and takes weld metal strength mismatch into account. ∑ Appendix E contains a range of residual stress distributions, based mainly on FEA modelling. ∑ The background to the procedure is published in a series of reports by the Welding Research Council (WRC).

3.7.4 User group The procedure is maintained and developed by a joint API/ASME committee, with representatives from a range of industries, both within and outside the USA.

3.7.5 Status The committee is working towards a new edition of the API/ASME flaw assessment procedure (43–44), with a publication date of around 2012.

3.8

Future trends

There is a continuing drive to improve the precision of fracture assessment methods, with the ultimate aim of predicting failure conditions or assigning a probability of failure rather than simply making a run/repair decision. Constraint-based fracture mechanics methods, described in Chapter 1 (O’Dowd) of this volume, have an important role to play here, combined with the constraint matched fracture mechanics testing described in Chapter

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2 (Macdonald, Østby and Nyhus). The use of probabilistic methods, and analysis methods that take weld strength mismatch into account, are also likely to increase. There is also increasing interest in strain-based assessment, as described below.

3.8.1 Strain-based assessment There is increasing interest in situations in which flawed (or potentially flawed) structures are subjected to known displacements or strains, rather than to load-controlled forces. Examples include offshore pipelines installed by reeling, or structure subjected to thermal/seismic loads that induce yielding during operation. BS 7910 does not currently address the analysis of flawed structures subjected to plastic straining in detail, although DNV RP F108 (45) and Appendix A of DNV OS F101 (46) include effectively modified versions of BS 7910 fracture assessment procedures specific to the analysis of pipelines installed using methods involving cyclic plastic straining. Extensive research in this area of strain-based analysis is continuing on a number of fronts, and a new section III.16 of the R6 procedure has been prepared, proposing the use of a strain-based FAD (SB-FAD), analogous to the stress-based FAD currently used by BS 7910 and R6. This concept has been scrutinised by the UK TAGSI committee. There is clearly scope for the development of either an additional annex to BS 7910 or a stand-alone strain-based analysis procedure document. In view of the extensive revisions required for the main stress-based procedure and the fact that strain-based methods are still under active development, there are, however, no plans to include strain-based methods in the immediate next edition of BS 7910.

3.9

References

1. Woods, G.E. and Baguley, R.B., Practical guide to ASME B31.3, Casti Publishing Inc., 1996. 2. Smith, T.A. and Warwick, R.G., ‘A survey of defects in pressure vessels in the UK for the period 1962–1978 and its relevance to nuclear primary circuits’. International Journal of Pressure Vessels and Piping, 1983, 11, 127–166. 3. Davenport, T.J., ‘A further study of pressure vessel failures in the UK’, International Conference on Reliability Techniques and their application, Reliability ’91, London, UK, 10–12 June 1991. 4. Harrop, L.P., ‘The integrity of pressure vessels’, Science Progress, 1983, 68, 423–457. 5. Bush, S.H., ‘Statistics of pressure vessel and piping failures’, in Pressure Vessel and Piping Technology 1985; a decade of progress, 1985, ed Sundarajan, C.R., ASME, New York. Also published in ASME Journal of Pressure Vessel Technology, 1988, 110, 225–233. 6. Engel, J.R., (ed) ‘Pressure vessel failure statistics and probabilities’, Nuclear Safety, 1974, 15(4), 387–399.

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