Case Studies on ECA-Based Flaw Acceptance Criteria for Pipe Girth Welds Using BS 7910:2005

Abstract

The use of an engineering critical assessment (ECA) approach to derive flaw acceptance criteria for pipe girth welds has become common practice. It allows the maximum tolerable size of weld flaws to be determined on a fitness-for-purpose basis, offering substantial advantages over the conventional workmanship approach. BS 7910:2005 is widely used to derive ECA-based flaw acceptance criteria for pipe girth welds. It offers a flexible assessment framework within the context of the well-established failure assessment diagram (FAD) approach. However, it can be relatively complex to apply and it may lead to assessments that are more conservative than codified pipeline-specific procedures. This paper illustrates, through practical case studies on assessing the significance of circumferential girth weld flaws, some of the options available to the user of BS 7910. The case studies cover the selection of the FAD (generalised or material-specific, with and without yield discontinuity), tensile properties (specified minimum or actual values); fracture toughness properties (single point CTOD values including δ0.2BL and δm, or full CTOD resistance R-curve), and welding residual stress (assumed to be uniform through the pipe wall with a yield strength magnitude, or considered to have a through-wall distribution associated with a specific level of welding heat input).