TWI Industrial Member Report Summary 1084/2017
By Yan-Hui Zhang, Tyler London and Damaso De Bono
Engineering critical assessment is widely 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 at the weld toe on the stress intensity factor range, ΔK, a magnification factor, Mk, is used. For single-sided girth welds, fatigue cracking often initiates from weld toes on the root side, rather than on the weld cap side. 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.
Mk solutions and parametric equations for three different defect models were developed and established. It was found that:
- The weld toe angle at the weld root has a negligible effect on the Mk.
- For Type I defects (weld root hi-lo=0), the BS 7910 2D solution provides an upper bound for Mk.
- For Type II (weld root bead height = hi-lo) and Type III (weld root bead height≠hi-lo) defects, Mk values are significantly greater than for Type I defects, and increase sharply with increasing hi-lo.
- The accuracy of the Mk solution developed was confirmed by experimental data in terms of both fatigue crack growth and S-N curve approaches.
- For girth weld roots with hi-lo>0.25mm, the guidance given in DNV-OS-F101:2013 is non-conservative.