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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

ECA: Engineering critical assessment, usually used to denote a fitness-for-service analysis of a cracked body
LTA: Locally thinned area
RS: Residual stress
FFS/FFP: Fitness-for-service/fitness-for-purpose, ie an analysis based on integrity rather than code compliance
LBB: Leak before break
CTOD: Crack tip opening displacement
FAD: Failure analysis diagram
LEFM: Linear elastic fracture mechanics

 

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.

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

  1. Glynn, E, Woods, and Ron, B, Baguley, 1996: 'Practical guide book series', Volume 3: ASME B31.3 process piping, pub CASTI.
  2. Smith, T A, and Warwick, R G, 1983, '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,11, 127-166.
  3. Davenport, T J, 1991 '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.
  4. Harrop, L P, 1983 'The integrity of pressure vessels', Science Progress, 68, 423-457.
  5. Bush, S H, 1988, 'Statistics of pressure vessel and piping failures', '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 vesseltechnology, 110, 225-233.
  6. Engel, J R (ed), 1974, 'Pressure vessel failure statistics and probabilities', Nuclear Safety, 15(4), 387-399.
  7. Sooby, W, and Tolchard, J M, 1993, 'Estimation of cold failure frequency of LPG tanks in Europe'. Proceedings of conference 'Risk and Safety Management in the Gas Industry, Hong Kong, October.
  8. European Standard for Unfired Pressure Vessels EN 13445, 2002.
  9. ASME Boiler and Pressure Vessel code. 2007.
  10. BS 7910:2005 (incorporating Amendment 1, published in 2007); 'Guide to methods for assessing the acceptability of flaws in metallic structures', BSI, 2005.
  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.
  12. R6: Assessment of the Integrity of Structures containing Defects, Revision 4.
  13. FITNET Fitness-for-Service (FFS) - Procedure (Volume 1) ISBN 978-3-940923-00-4, Koçak, M, Webster, S, Janosch, J J, Ainsworth, R A, Koers, R and FITNET Fitness-for-Service (FFS) - Annex (Volume 2) ISBN978-3-940923-01-1, Koçak, M, Hadley, I, Szavai, S, Tkach, Y, Taylor, N, both printed by GKSS Research Center, Geesthacht, 2008.
  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.
  16. API: 579-1/ASME FFS-1 2007, 'Fitness-for-service'.
  17. Hobbacher, A, 1996, 'Fatigue design of welded joints and components', IIW, Abington Publishing, Abington, Cambridge.
  18. BS 7608: 1993: Code of practice for fatigue design and assessment of steel structures.
  19. R5: Assessment procedure for the high temperature response of structures, issue 3, 2003.