ECAs: Lifting the Lid of the Black Box

Abstract

An engineering critical assessment (ECA) is commonly conducted during the design of a pipeline in order to determine the tolerable sizes for flaws in the girth welds. An ECA is a method for assessing the acceptability of a flaw in a structure, i.e. to demonstrate fitness-for-service. API 579-1/ASME FFS-1 2016 and BS 7910:2013+A1:2015 Incorporating Corrigenda Nos. 1 and 2 give guidance for conducting fitness-for-service assessments of cracks and crack-like flaws. Appendix A of DNV-OS-F101, 2013 provides detailed procedures for evaluation of the fracture limit state of girth welds, based on BS 7910:2005, but prescribes a number of amendments and modifications. API 579-1/ASME FFS-1 and BS 7910:2013 will give similar, but slightly different, results. An ECA can be something of a black box because of the use of software to conduct the calculations. Software can have the unfortunate side-effect of obscuring the understanding of the calculations. ECAs are also seen as a thing that is very complicated. In an attempt to lift the lid on the black box that is, or is perceived to be, the ECA, the assessment of circumferentially orientated, surface-breaking crack-like flaw (a planar flaw) in a girth weld is illustrated through a comparison of the relevant stress intensity factor and reference stress solutions in API 579-1/ASME FFS-1 and BS 7910:2013; and through illustrating the effect of the choice of the Level or Option of the assessment, and assumptions made in the assessment with respect to constraint, misalignment, residual stresses, etc. Also illustrated are the implications of defining the fracture toughness in terms of the crack tip opening displacement or J-integral, and of using a single value of fracture toughness, based on the initiation of stable tearing or the first attainment of a maximum force plateau (maximum load), or of using tearing resistance data. A comparison is also made with the notionally simplified fitness-for-service procedures given in Annex A of API Standard 1104, Annex K of CSA Z662-15, and the EPRG guidelines on the assessment of defects in transmission pipeline girth welds–Revision 2014. The result is an outline of what might be considered to be good practice for when conducting an ECA during the design of a pipeline.