(Adds stress factor, interview with investigator)
By Rod Nickel
WINNIPEG, Manitoba, March 4 (Reuters) - An explosion and fire in 2018 along an Enbridge Inc natural gas pipeline in northern British Columbia was caused by normal stress that, coupled with corrosion, created cracks, Canada's Transportation Safety Board (TSB) said on Wednesday.
The pipeline operated by Enbridge subsidiary Westcoast Energy Inc ruptured on Oct. 9, 2018, in a forested area near Prince George. No one was injured, but the blast led to the evacuation of 125 people, including from the Lheidli T'enneh First Nation.
Police photos from a helicopter showed a nine-meter-deep crater and dozens of scorched evergreen trees at the site.
The temporary shutdown of the line, which carries gas destined for the Pacific Northwest, led to disruptions for refineries owned by Royal Dutch Shell and Phillips 66 in the U.S. state of Washington.
The TSB said in a report that polyethylene tape used to protect the pipeline's surface had deteriorated, allowing moisture underneath and corrosion to develop. Stress on the pipeline that was caused by normal operations, combined with the corrosion, created the cracks, in a phenomenon known as "stress corrosion cracking," said TSB senior investigator Jennifer Philopoulos.
The board, an independent agency that reports to Parliament, said it had investigated three other ruptures since 2002 with a similar cause, all on TC Energy lines.
That section of line was last inspected 10 years before the blast, and was scheduled for inspection the week the incident occurred, said Michele Harradence, Enbridge's senior vice-president of gas transmission and midstream.
"These are very stressful things to occur in your community and we apologize for that," Harradence said in an interview from Houston, Texas.
Enbridge has since begun integrity assessments of all its gas pipelines in North America, adopted newer tools to detect problems, expanded inspection criteria and increased scheduled excavations to examine pipelines, she said.
The Canada Energy Regulator did not lay charges against Enbridge. (Reporting by Rod Nickel in Winnipeg, Manitoba; Editing by Matthew Lewis and Grant McCool)