BS 7910: History and future developments

Isabel Hadley

Paper presented at 2009 ASME Pressure Vessels and Piping Conference, Sustainable Energy for the Third Millennium, Prague, Czech Republic, 26-30 July 2009.

Abstract

BS 7910, the UK procedure for the assessment of flaws in metallic structures, was first published almost 30 years ago in the form of a fracture/fatigue assessment procedure, PD6493. It provided the basis for analysing fabrication flaws and the need for repair in a rational fashion, rather than relying on long-established (and essentially arbitrary) workmanship rules. The UK offshore industry in particular embraced this new approach to flaw assessment, which is now widely recognised by safety authorities and specifically referred to in certain design codes, including codes for pressure equipment.

Since its first publication in 1980, PD6493/BS 7910 has been regularly maintained and expanded, taking in elements of other publications such as the UK power industry's fracture assessment procedure R6 (in particular the Failure Assessment Diagram approach), the creep assessment procedure PD6539 and the gas transmission industry's approach to assessment of locally thinned areas in pipelines.

The FITNET European thematic network, run between 2002 and 2006, has further advanced the state of the art, bringing in assessment methods from SINTAP (an earlier European research project), R6, R5 and elsewhere. In particular, the FITNET fracture assessment methods represent considerable advances over the current BS 7910 methods; for example, weld strength mismatch can be explicitly analysed by using FITNET Option 2, and crack tip constraint through Option 5. Corrosion assessment methods in FITNET are also more versatile than those of BS 7910, and now include methods for vessels and elbows as well as for pipelines.

In view of these recent advances, the BS 7910 committee has decided to incorporate many elements of the FITNET procedure into the next edition of BS 7910, to be published c2012. This paper summarises the history of the development of BS 7910, its relationship with other flaw assessment procedures (in particular FITNET and R6) and its future.

List of abbreviations

Introduction

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, as shown in Fig.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.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 statistics).

Data from across the industrialised world (^[2-7]) show the current 'catastrophic' failure rate of pressure vessels to be approx. 10^-5 to 10^-6 per vessel year, although the precise figure varies somewhat between 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 pre-service 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.

These principles are, of course, implemented in pressure vessel codes, including the European pressure vessel code^[8] and the ASME Boiler and Pressure Vessel code.^[9]

Although design codes have, of course, been extremely effective in bringing failure of pressure equipment under control, there are cases where pressure (and other) equipment does not meet the requirements of the code, examples of which include:

• The design conditions lie outside those envisaged by the code; for example the material may not be covered by the code, or the thickness may lie beyond the values considered by the code.
• Manufacturing faults (for example weld flaws or deviations from intended design) are found before the vessel enters service. These are not acceptable to the code, but perhaps repair would be logistically difficult and liable to introduce new flaws.
• In-service damage (for example fatigue cracking, corrosion or creep damage) has occurred, putting the vessel outside the code standards. Nevertheless the operator wishes to keep it in service until a convenient time, for example the next scheduled shut-down.

These are all cases in which it would be appropriate to use a fitness-for-service (FFS) approach to assess the condition of the vessel, rather than relying on code compliance. The FFS approach is based on a fracture mechanics assessment of the flawed component and requires the user to show that the flaw(s) will not lead to failure during the required lifetime of the component, given the service conditions. In order to ensure that such analyses are conducted in accordance with recognised methods that enable independent verification to be carried out, the FFS approach is codified in several documents, including BS 7910 (the subject of this paper), R6, API/ASME and FITNET.

History of BS 7910

The history of the current UK flaw assessment procedure, BS 7910,^[10] is described below and summarised in Fig.2.

The first procedure to be published by BSI (British Standards Institution) was PD6493:1980. 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.

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.

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 linear elastic fracture mechanics (LEFM) to calculate the driving force, K_I, 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 (ie 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 CTOD design curve was used.^[11] 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. 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.

Meanwhile, the UK nuclear industry had developed its own flaw assessment technique, designated R6.^[12] This was first issued in 1976, with the aim of ensuring that safety cases concerning power plant should be carried out in a reproducible and consistent fashion. Unlike PD6493, the R6 method considered the possibility of failure of structures from both brittle fracture and plastic collapse, and introduced the concept of a Failure Analysis Diagram (FAD) to show the interaction between the two. This allowed the user to apply LEFM concepts to calculate K_I, with the FAD showing how the permitted level of driving force (designated K_r, 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, which shows the simplest ('Option 1') R6 FADs for a continuously-yielding material and a discontinuously-yielding material, ie one showing Lüder's plateau behaviour.

Although the PD6493 and R6 methods had been developed in parallel, and addressed the needs of different industry sectors, it became clear that the underlying technology was virtually identical 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 persisted ever since. 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, whilst Level 3 calculations, used for assessment of ductile tearing, used a FAD similar to the R6 FAD.

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 pipelines were proposed, based on a major 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 complex welded joints
• Residual stress 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.

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 (BSI, 2005).

FITNET

The year 2002 saw the launch of a major European thematic network known as FITNET (fitness-for-service network), with the aim of producing a European consensus document on fitness-for-service. The aim of FITNET was to produce a procedure covering all major failure/damage modes (fracture, fatigue, creep and corrosion) that would be used by a range of industry sectors. To this end, the network comprised representatives of European industry (power, oil and gas, automotive, materials) research institutes and universities, plus some self-funded contributions from Japan, Korea and the USA. A second aim of the network was to promulgate the use of FFS methods throughout Europe, through preparation of appropriate teaching material, presentation of a series of teaching seminars and staging a final international conference.

The FITNET procedure^[13] has been published in two volumes (the procedure itself in Vol I and the supporting annexes in Vol II) and is available from the consortium organiser, GKSS, in Germany. Conference proceedings are also available, and a stand-alone volume covering worked examples, tutorials and the validation of the procedure is under preparation. Details of the individual modules are given below.

The fracture module

The fracture analysis module of FITNET draws on a number of existing sources, including SINTAP^[14], BS 7910, R6 and the GKSS ETM (Engineering Treatment Method) procedure.^[15] The basic concept is that the driving force for the cracked body under load (K_I) is compared with the materials fracture toughness (K_mat), whilst the load on the uncracked ligament (the reference stress) is compared with the limit load for the cracked body. This can be done using either a Crack Driving Force (CDF) curve or a Failure Analysis Diagram (FAD). The two approaches are equivalent, although the FAD approach is perhaps more well-established through its use in other current fracture procedures such as BS 7910, R6 and API 579-1/ASME FFS-1^[16].

The fracture module of FITNET allows users to assess the integrity of metallic structures at several different levels (known as Options), depending on the complexity of the materials property data available to them. On the one hand, they may have very simple information available, such as Charpy impact results and specified values of tensile properties. Under these circumstances, a very simple approach based on empirical correlations between Charpy energy and fracture toughness may be adopted, known as Option 0. This option applies only to ferritic steels, and is in line with current recommendations of BS 7910 Annex J. On the other hand, if detailed stress-strain data for both parent metal and weld metal are available, along with fracture toughness data for the weldment, it may be possible to use higher options, eg Option 3. A list of Options is shown in Table 1 along with the corresponding materials data requirements. A comparison between FITNET Options and the current BS 7910 fracture assessment levels is also shown.

Table 1 Comparison of current BS 7910 fracture assessment levels with FITNET fracture assessment Options

An example of analysis of the same test data using various FITNET Options is given in Fig.4. This shows the analysis of two centre-cracked wide plate test specimens, one loaded uniaxially and the other biaxially These were tested at a temperature of -100°C and failed by brittle fracture. The assessment points associated with the fracture are shown (squares labelled '#40' and '#41' and both points lie outside all of the FADs considered, confirming the ability of all of the Options to predict failure. As the tests are analysed using progressively more advanced techniques (Options 1, 3, 4 and 5), so the FADs change, with the 'safe' area expanding, reflecting a progressively more precise analysis. Note that Option 5 allows the user either to use a 'conventional' (eg Option 3) FAD in conjunction with low-constraint fracture mechanics tests, or to use a 'constraint-corrected' FAD together with conventional high-constraint fracture mechanics test data. The latter option was chosen in this case, so that the analysis points associated with the failure of the specimens are the same regardless of the Option selected, whilst the FADs change according to the Option used. Moreover, it should be noted that Figure 4 presents the results of the Option 4 analysis in the form of FADs, in order to facilitate the comparison of the different Options in a single diagram. In practice, the FITNET document presents Option 4 in terms of Crack Driving Force (CDF) only; the FAD-based and CDF-based methods are, in fact, equivalent, as noted earlier in this paper.

The fatigue module

The fatigue module (Section 7 of FITNET) comprises five different approaches to fatigue analysis, termed Routes 1-5. The first three are essentially fatigue damage assessment approaches, which assume that there is no pre-existing flaw in the structure, whilst Routes 4 and 5 are based on the assumption that a flaw exists. A brief summary of each of the Routes is given below, and summarised in Fig.5.

Route 1 is also known as the nominal stress method and is compatible with other stress-based approaches such as the IIW rules^[17] and BS 7608.^[18] The welded joint is assigned to a particular weld class based on its geometry and the direction of loading. Its fatigue life is then estimated from statistical data on geometrically similar joints, using S-N curves.

Route 2, the structural stress or notch stress method, recognises that fatigue can be associated with very small hot-spot areas of the component. Stresses in the relevant area are calculated, eg by FEA, and the appropriate S-N curve (see Route 1) is then followed.

Route 3 (local stress-strain approach) is applicable mainly to non-welded components, and addresses crack initiation only (subsequent crack growth could be modelled using Route 4 - see below). A relationship between cyclic strain range and the number of cycles to initiation is derived using, for example, the Coffin-Manson law.

Route 4 is based on the assumption of a planar flaw, which can grow on a cycle-by-cycle basis as described by the Paris law; the fatigue crack growth constant and the effects of stress ratio are given in the procedure for a range of materials and environments. As such, Route 4 is very similar to the fatigue crack growth approach used in BS 7910.

Route 5 addresses non-planar flaws in welded joints, and is currently restricted to undercut, slag inclusions and porosity in steel and aluminium butt welds. Non-planar flaws can, of course, be analysed using Route 4 and treating them as planar, but this treatment is likely to be highly conservative. Route 5 therefore gives an empirically-based alternative; the maximum flaw size is tabulated as a function of required fatigue class, flaw type and material.

The creep module

The creep module (Section 8 of FITNET) provides methods for calculating creep crack growth and creep-fatigue interaction. It draws in particular on the R5 high temperature assessment procedure, developed by the UK nuclear industry.^[19] The procedure is presented in the form of 13 steps, some of which are covered by other sections of FITNET.

The corrosion module

Section 9 of the FITNET procedure addresses two main types of analysis:

• Crack propagation by Environmentally Assisted Cracking (EAC), including Stress Corrosion Cracking (SCC) and Corrosion Fatigue (CF)
• Analysis of Locally Thinned Areas (LTAs), in which the most likely failure mechanism is plastic collapse

Analysis of EAC follows the same general principles as those for other types of growing flaw, eg fatigue and creep crack growth. When assessing the integrity of structures with cracks or crack-like defects, it is necessary to consider whether sub-critical crack growth is a potential factor. If so, an estimate of the amount of tolerable growth during the design lifetime or between in-service inspections is required. In that context, structural integrity assessment has to take into account the distinct characteristics of the damage processes associated with EAC. The conditions in which EAC occurs involves a combination of stress, environment and microstructural susceptibility, although it should be emphasised that this interaction occurs at a highly localised level and it is the local characteristics of these variables rather than the nominal bulk values that are critical.

The corrosion module considers subcritical crack growth due to stress corrosion cracking (predominantly static load or slowly rising load) and corrosion fatigue (predominantly cyclic load), with crack growth rate prediction in service based principally on the application of fracture mechanics in terms of either stress intensity factor (K) in the case of stress corrosion cracking, or the range of stress intensity factor (ΔK), in corrosion fatigue. Underlying that assumption is the presumption that the flaws or cracks are of a dimension that allows a description of the mechanical driving force by linear elastic fracture mechanics (LEFM). In practice, for some systems, a significant amount of life may occur in the short crack regime that, if known, should be taken into account in the assessment.

Analysis of LTAs in FITNET goes beyond the analysis of corrosion in straight pipes, as covered by the current version of BS 7910. Methods of analysis for LTAs in spheres, cylinders, elbows, pressure vessel heads (hemispherical, torispherical and elliptical) and integrally reinforced nozzles are included. The principles of the method are that the corrosion-damaged vessel or pipework should be capable of undergoing a hydrotest without failure, and that the stresses in the thinned area of the component should not exceed the yield strength of the material under design pressure. The safe working pressure of the equipment may need to be reduced in order to retain a safety factor similar to that for the undamaged equipment.

Various conditions exclude the use of the LTA approach - for example, there must be no crack-like flaws, no mechanical damage combined with corrosion, no cyclic loading and no flaws with depth greater than 80% of the original wall thickness. These exclusions are intended to ensure that the damage is truly LTA, not crack-like, and that the extent of damage lies within the envelope for which experimental validation exists.

The future of BS 7910

Shortly after the publication of FITNET, the BS 7910 committee decided to adopt many of the features of FITNET into its next edition, to be published around 2012. This process requires national scrutiny of the document, and re-drafting to retain the style and terminology of BS 7910. The current plans for the major failure modes are:

Fracture: the 'levels' of assessment described by the current BS 7910 will be replaced by 'Options' that match the hierarchy of FITNET as far as possible. This will have the added benefit of making the terminology of BS 7910 more compatible with that of R6.

Fatigue: Routes 4 and 5 of FITNET, which address crack propagation from planar and non-planar flaws respectively, will be included in the new edition of BS 7910, in line with the current approach of the 2005 edition. Fatigue design rules, such as are outlined in Route 1 of FITNET, will not be included, since they already exist in a separate document, namely BS 7608.

Creep: the creep clauses given in Section 8 of FITNET will be adapted and incorporated into the new edition of BS 7910.

Corrosion: the FITNET corrosion and EAC assessment methods will be incorporated into BS 7910, after appropriate scrutiny and editing.

In addition, a new sub-committee has been formed to scrutinise the advice on NDE contained in Annex D of FITNET, together with additional information where appropriate, and to prepare an NDE annex for the next edition of BS 7910. This will be the first time that advice on NDE is included in BS 7910 - the main emphasis will be on considering the capability of different NDE techniques to detect, characterise and size the different types of flaw that may need to be considered in an ECA.

A new sub-committee will also be needed in due course to address the important issue of strain-based analysis of structures containing flaws. This is a particularly important issue in the pipeline industry, where plastic strains may be induced during installation (where reeling is used as an installation method), or operation (due to buckling or seismic effects) or both.

Management of BS 7910

The BS 7910 document is owned and distributed by BSI, with technical input provided by a Technical Committee, WEE/37. This comprises volunteers from groups representing manufacturers, certification, health and safety bodies, academia and end users, typically nominated by a trade association. The detailed technical drafting is further delegated to 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, eg R6, FITNET and API/ASME.

Concluding remarks

Since its first publication in 1980 as PD6493, the UK flaw assessment code BS 7910 has undergone a number of changes, the most significant of which are summarised below:

• The CTOD-based approach to determination of crack tip driving force and materials toughness (championed by the offshore industry) has been integrated with the J-based approach preferred by the nuclear industry.
• The scope of the document has steadily increased, and now covers all the main fracture/damage modes (fracture, fatigue, corrosion and creep).
• The methods have become increasing complex, as the widespread use of computers makes the use of more complex equations (eg for K-solutions and residual stress) practicable.

Now, as a result of the FITNET project and other recent initiatives, it is being enlarged once again, to represent a European consensus on structural integrity assessment, that can be adopted by a range of industry sectors.

References

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9. ASME Boiler and Pressure Vessel code. 2007.
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11. Burdekin, F M and Dawes, M G: 'Practical use of yielding and linear elastic fracture mechanics with particular reference to pressure vessels'. Paper C5/71, I Mech E conference on Practical Application ofFracture Mechanics to Pressure Vessel Technology, London, May 1971.
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14. SINTAP: www.eurofitnet.org/sintap_index.html
15. Schwalbe, K-H et al, 'EFAM ETM 97 - The ETM method for assessing the significance of crack-like defects in engineering structures', comprising versions ETM 97/1 and ETM 97/2. GKSS report 98/E/6.
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17. Hobbacher, A, 1996, 'Fatigue design of welded joints and components', IIW, Abington Publishing, Abington, Cambridge.
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19. R5: Assessment procedure for the high temperature response of structures, issue 3, 2003.