At around 2:30 a.m. (local time), a rupture occurred on an elbow of a gas pipeline located in a wooded area before the pipeline passes under the Wabasca River. A total of 16.5 million m³ of natural gas leaked out, but did not ignite. The force of the rupture created a 15-m-wide crater and sent five pipe fragments flying up to 130 m away from the crater.

Response to the accident

The drop in pressure caused the block valves to isolate the failed section. A technician was dispatched to the scene to survey the damage.

Initial findings

The failure was caused by a fracture that had formed on the weld seam of an elbow and which subsequently spread through the thickness of the pipe wall. The temperature of the gas was higher than usual (58 °C instead of the design basis temperature of 45 °C) in order to meet demand. This high temperature resulted in thermal expansion that applied additional mechanical stresses to the weld seam.

Root causes

Following an investigation conducted by the Canadian authorities, it appears that:

• The elbow’s operating pressure had been overestimated in relation to the actual thickness of its wall.
• The maximum possible discharge temperatures at the compressor station upstream of the accident had not been effectively communicated between the contractor that had built the gas pipeline and the pipeline’s operator.
• The temperature profile used to design the pipeline was different from the profile used during operation.
• The pipeline’s operator implemented a quality assurance system (QAS) for the purchase and acceptance of elbows. However, this system did not detect the aforementioned failures.

According to the Canadian press, the metallurgical properties of more than 1400 components were lower than initially expected. The National Energy Board issued an opinion on the accident.

Corrective measures

The operator inspected and reinforced similar elbows on the pipeline with a composite repair system that improves the yield strength of steel. A parametric model was developed to examine the threat of thermal expansion on the pipelines.

The elbow manufacturer improved the heat treatment process and particularly the furnace temperatures. The quality assurance and control process was amended to require that the thickness of every steel plate be measured.