Investigation of subsea oil pipeline rupture

•A full-bore rupture in the form of a fast-running ductile fracture has occurred at an 18-inch offshore pipeline in the subsea portion.•The visual inspection and ROV survey both revealed a conspicuous dent on the outer surface of the pipeline.•The visual examination revealed that the crack originated at a sharp weld toe angle (149°) at the outer surface.•The rupture origin was a fatigue pre-crack of 0.078-inch depth and 6.7-inch length that propagated at a sharp weld toe angle during the pipeline service life.•The stress analysis revealed that the burst was not due to the operation pressure, but rather due to stresses introduced by a sharp tool dragging over the bare pipeline surface. In the current work, a full-bore rupture in the form of a fast-running ductile fracture has occurred at an 18-inch offshore pipeline in the subsea portion. The pipeline was constructed from longitudinal seam welded pipe conforming to API 5L Grade X42. A subsea survey using the remote operation vehicle showed a pipeline rupture at a depth of 2756 in. from the sea surface. Shiny scratch marks on the outer surface of the pipeline at the rupture location were observed as well. The visual examination revealed that the crack originated at a sharp weld toe angle (149°) at the outer surface. The crack originated in the HAZ region and propagated along the longitudinal seam weld. The rupture origin was a fatigue pre-crack of 0.078-inch depth and 6.7-inch length that propagated at a sharp weld toe angle during the pipeline service life. The stress analysis revealed that the burst was not due to the operation pressure, but rather due to stresses introduced by a sharp tool dragging over the bare pipeline surface. The tool introduced huge external stresses to the pipeline. Both of the internal pressure and the external stresses contributed to the crack propagation during the rupture.