There have been several recent weld failures either during the initial post construction hydrostatic tests, or immediately following construction. Girth welds typically do not fail as a result of internal hoop loads without the contribution of loads due to out side forces. External loading should be considered during design, welding procedure development, construction, and pipeline operations. This paper presents one example where a girth weld failed as a result of preexisting 1940’s weld imperfections and recent, 1980’s, external loading. This analysis of the girth weld failure in the 30-inch pipeline included an initial failure analysis, a fracture mechanics analysis, and a finite element analysis that integrated the pipe-soil interaction, as well as localized stresses associated with weld imperfections. A critical part of this study was to evaluate how changes in soil conditions associated with a drought followed by soil saturation associated with rainfall, contributed to lack of local support and increased overburden loads associated with the saturated soil. The failure analysis of the ruptured girth weld and surrounding pipe concluded that the failure of the girth weld was caused by increased bending loads imposed on the pipeline after recent construction activities, and that the fracture initiated at a lack-of-penetration/fusion imperfection that was 20 1/4-inches long and 0.110 inches deep. A coupled investigation using finite element and fracture mechanics analyses verified numerically that with reduced-strength soil, stresses were generated in the girth weld of sufficient magnitude to cause a fracture. Temperature, terrain, and fatigue were considered, but were not deemed to significant enough to affect the stresses or other conditions that resulted in the failure. The overriding observation of this study is that no single factor contributed to the failure that occurred. Rather, the girth weld failure was the result of weld imperfections that generated elevated stresses due to excessive loads imparted to the pipe due to settlement associated with non-compact backfill associated with excavation work. Had the pipe not displaced vertically due to localized soil conditions, it is unlikely that the pipeline would have failed. The recent excavation activities were adequate for normal soil conditions; however, dry soil at the time of construction resulted in lack of compaction and excessive moisture just prior to the failure that generated in differential settlement and heavy overburden, combined with lack of penetration imperfection in the girth weld in question, resulted in generating excessive bending stresses that contributed to the eventual failure of the pipeline.
Case study on advanced 3D finite element limit analysis of counter-acts installed at Ormen Lange
