ECAs, FE and Bi-Axial Loading: A Critique of DNV-OS-F101, Appendix A

Abstract

Controlled lateral buckling in offshore pipelines typically gives rise to the combination of internal over-pressure and high longitudinal strains (possibly exceeding 0.4 percent). Engineering critical assessments (ECAs) are commonly conducted during design to determine tolerable sizes for girth weld flaws. ECAs are primarily conducted in accordance with BS 7910, often supplemented by guidance given in DNV-OS-F101 and DNV-FP-F108. DNV-OS-F101 requires that finite element (FE) analysis is conducted when, in the presence of internal over-pressure, the nominal longitudinal strain exceeds 0.4 percent. It recommends a crack driving force assessment, rather than one based on the failure assessment diagram. FE analysis is complicated, time consuming and costly. ECAs are, necessarily, conducted towards the end of the design process, at which point the design loads have been defined, the welding procedures qualified and the material properties quantified. In this context, ECAs and FE are not an ideal combination for the pipeline operator, the designer or the installation contractor. A pipeline subject to internal over-pressure is in a state of bi-axial loading. The combination of internal over-pressure and longitudinal strain appears to become more complicated as the longitudinal strain increases, because of the effect of bi-axial loading on the stress-strain response. An analysis of a relatively simple case, a fully-circumferential, external crack in a cylinder subject to internal over-pressure and longitudinal strain, is presented in order to illustrate the issues with the assessment. Finite element analysis, with and without internal over-pressure, are used to determine the plastic limit load, the crack driving force, and the Option 3 failure assessment curve. The results of the assessment are then compared with an assessment using the Option 2 curve. It is shown that an assessment based Option 2, which does not require FE analysis, can potentially give comparable results to the more detailed assessments, when more accurate stress intensity factor and reference stress (plastic limit load) solutions are used. Finally, the results of the illustrative analysis are used to present an outline of suggested revisions to the guidance in DNV-OS-F101, to reduce the need for FE analysis.