The rupture and subsequent explosion on TransCanada PipeLines’ mainline system in Canada in early 2002 was due to a host of factors that came together in a near “perfect storm,” causing “stress corrosion cracking to initiate and grow to failure,” according to a report by the Transportation Safety Board of Canada (TSB) on the accident.

The report, which was released last Tuesday, came more than two years after the April 2002 rupture of TransCanada’s Line 100-3 near Brookdale, MB, which sent sweet natural gas into the atmosphere that burned for three hours. One hundred people were evacuated from their homes for a day, but no one was injured.

A report by the TSB Engineering Laboratory concluded that the pipeline ruptured due to over-stress extension of pre-existing cracks, according to the board. “The cracks had initiated on the outside surface of the pipe and progressed in a mode of failure identified as near-neutral (low) pH stress corrosion cracking,” the report noted.

“Although pitting corrosion was present in the failure initiation area, it was superficial and had not significantly reduced the wall thickness of the pipe. However, the corrosion pits did provide potential stress concentrator sites [where] cracks could have initiated. The presence of minor corrosion pits is indicative that the cathodic protection (CP) was locally ineffective for some time during the operation of the pipeline.”

The TSB said a combination of factors caused a “zone of near-neutral stress corrosion cracking to initiate and grow to failure” of the 1970s-era TransCanada pipeline. The factors included: disbonded exterior coating, fluctuations in the environmental conditions surrounding the pipe, the presence of anaerobic bacteria, a susceptible high-strength steel pipe, and the existence of atomic hydrogen (probably from the cathodic protection reaction), and sustained tensile stress due to the internal operating pressure of the pipeline.

Corrosion pitting “may have facilitated the growth of the cracks,” the report said, adding that the line ruptured as a result of the “extension in an over-stress mode within this pre-cracked region.”

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