Abstract
An engineering critical assessment (ECA) is commonly conducted during the design of an offshore pipeline in order to determine the tolerable size of flaws in the girth welds. API 579-1/ASME FFS-1 2016 and BS 7910:2013+A1:2015 Incorporating Corrigenda Nos. 1 and 2 give guidance on conducting fitness-for-service assessments of cracks and crack-like flaws. DNVGL-RP-F108, 2017 Assessment of flaws in pipeline and riser girth welds describes a methodology to satisfy the fracture and fatigue limit states in DNVGL-ST-F101, 2017 based on Option 2 with ductile tearing in BS 7910:2013. It requires that the stress-strain curve used in a strain-based fracture mechanics analysis should represent a high yield strength combined with low strain-hardening properties (a characteristic high stress-strain curve with low strain hardening), and that used in a stress-based fracture mechanics assessment should represent a low yield strength.
A pipeline operating at high temperatures and/or high pressures is subject to high compressive axial forces. The pipeline might then relieve these forces by buckling. A design that incorporates controlled lateral buckling is an efficient solution to the problem of high compressive axial stresses. Lateral buckling does, however, give rise to relatively high tensile axial strains (possibly exceeding 0.4 percent) in the pipe wall, and, relatively high fatigue loading associated with movement of the buckle under start-up and shut-down cycles.
The calculated tensile axial strain in the pipe wall in a lateral buckle depends on the assumed stress-strain curve. It tends to be higher if a low yield strength combined with low strain-hardening properties is assumed. There is then an apparent inconsistency between the two sets of assumptions.
A deterministic assessment of a circumferentially-orientated, internal surface crack-like flaw in a girth weld in a lateral buckle is used to investigate the significance of this apparent inconsistency.
