A rupture and an explosion occurred on a 12.75 inch OD high pressure gas pipeline after 40 years in service. The force of the explosion broke the riser off and sent two large pieces of the riser flying into the surrounding forest.
The failure occurred as a result of the simultaneous action of several contributing factors:
• The weld had a non-specified profile (a step) and contained a large slag inclusion at the location of fracture initiation.
• Corrosion pits were growing from the internal surface close to the weld root.
• Dewpoint corrosion took place on the internal surface of the riser close to massive flanges.
• The dehydrator at the compressor station was not removing the target amount of moisture.
• Low temperatures contributed to the failure by decreasing material fracture toughness.
• Ground movement could have created additional stress required for the failure to occur.
Several of the above listed factors (pitting corrosion, ground movement, malfunctioning of the dehydrator) developed with time, which explains the delayed mode of failure.
The conclusions were supported by Finite Element Analysis and Fracture Mechanics calculations.