Developing Mk Solutions for Fatigue Crack Growth Assessment of Flaws at Weld Root Toes in Girth Welds

Abstract

Engineering critical assessment (ECA) is increasingly being used in the offshore industry to determine the maximum tolerable initial flaw size in girth welds for pipelines and risers. To account for the effect of the stress concentration factor (SCF) at the weld toe on the stress intensity factor range, ΔK, a magnification factor, Mk, is used. The existing Mk solutions given in BS 7910 were developed for fatigue assessment of flaws at the toes of fillet and butt welds and may not be suitable for assessing flaws at girth weld root toes, where the weld width is relatively small. On the other hand, for single-sided girth welds, fatigue cracking often initiates from weld toes on the root side, rather than on the weld cap side. Finite element (FE) modelling was performed to determine a 2D Mk solution for ECA of a flaw at the weld root bead toe. The weld root bead profile was uniquely characterised by five variables including weld root bead width, weld root bead height, hi-lo, weld root bead angle and weld root bead radius. Following a parametric sensitivity study, defect size, weld root bead height and hi-lo were identified as the governing parameters. A total of 6,000 FE simulations was performed and three types of defect models, which covered different combinations of weld root bead height and hi-lo, were generated and analysed. A series of automation scripts were developed in the Python programming language and the Mk solution for each type of defect model was developed and provided in a parametric equation. The accuracy of the 2D Mk solutions was confirmed by the experimental data, in terms of both fatigue crack growth and S-N curves. It was found that the methods and Mk solutions currently recommended in BS 7910 and DNV OS-F101 are inappropriate for assessing a flaw at a girth weld root toe.