Fatigue Crack Growth of NPS20 Steel Oil Pipe at Varied Orientations
Very few studies concerning fatigue of oil pipe have been conducted using full-scale tests. Further, no research is found in open literature concerning fatigue cracks in the longitudinal direction of pipes, despite being deemed more critical. Fatigue crack growth was investigated in NPS 20, API 5L X-70 grade, straight-seam steel oil pipes in the base metal and at the welded joint for various orientations. Specifically, the effect of the welded joint and crack orientation on the fatigue crack growth behaviour relative to a longitudinal crack in the base metal was determined. The fatigue crack growth behaviour of a longitudinal crack in the base metal was fully characterized and compared with existing fracture mechanics and fatigue theories. Data from all full-scale pipe tests is provided to be used to enhance existing fatigue life prediction models.