Isabel Hadley, TWI Ltd
Proceedings of the PVP2011
Pressure Vessels and Piping Division Conference
17-21 July 2011, Baltimore, Maryland, USA
BS 7910, the UK procedure for the assessment of flaws in metallic structures, is being revised with a view to publication in 2012. Like the existing procedure, the new procedure will address all major failure/damage mechanisms, namely fracture, fatigue, creep and corrosion, and is intended to be used across a range of industry sectors and component types. There are several major proposed changes, which draw mainly on the existing BS 7910 procedures, the UK nuclear industry’s R6 document and the European FITNET procedure. The most far-reaching changes are in Section 7 (fracture) and related annexes. Here, the modifications include:
- a re-structuring of the fracture assessment procedures from their present form (Levels 1-3) to a new hierarchy based on Options 1-3, which are more compatible with the current R6 and FITNET approaches
- revised treatment of flaw interaction,
- a new annex (Annex N) permitting analysis under conditions of reduced crack tip constraint,
- a new annex (I) addressing analysis of weld strength mismatch,
- a revised residual stress compendium (Annex Q).
As part of the revision, all annexes will be reviewed and edited where necessary, and a new annex on non-destructive examination (NDE) will be included for the first time.
In view of the fact that many of the major changes concern the fracture assessment clauses, this paper presents a case study based on the analysis of a fully-circumferential flaw in a pipeline girth weld. The basic assessment Options (1 and 2) given in the new procedure are used to analyse the flaw, and three more advanced techniques (constraint-based assessment, assessment using an idealised residual stress distribution and analysis based on weld strength mismatch) are also applied.
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