Enbridge Inc.'s failure to consider several uncertainties during a geohazard analysis was the probable cause of a natural gas pipeline rupture in Hillsboro, Ky., according to the U.S. National Transportation Safety Board.

The May 2020 incident on Line 10 of Enbridge's Texas Eastern Transmission LP system did not kill or injure anyone but caused a fire across five acres and released about 148 MMcf of gas. More broadly, the incident renewed concerns about the threats that landslides and other earth movements present to pipelines, as well as the way operators manage those risks.

The U.S. Pipeline and Hazardous Materials Safety Administration recently issued a safety bulletin reminding operators of the risk of geohazards and outlining safety actions to mitigate them and cited the Hillsboro incident in the bulletin.

In the years prior to the rupture, several indications showed that Line 10 was coming under external load, according to a May 31 report from the National Transportation Safety Board, or NTSB. Integrity tests in 2018 and 2019 showed pipeline movement of 4.2-5.2 feet, and patrols identified erosion and features typical of landslides.

Enbridge could have foreseen threat, NTSB says

Enbridge assessed the site in October 2019 by comparing tensile strain demand, the amount of strain exerted on Line 10, to the line's tensile strain capacity, or the amount of strain it can withstand. Afterward, Enbridge resolved to monitor the site and mitigate threats but decided urgent action was not necessary.

The company developed a mitigation and monitoring plan in February 2020, but the pipeline ruptured before the company could perform stress relief work planned for the summer.

Following the incident, an NTSB contractor who assessed the incident site found that it was "highly susceptible to landslides," sat in past landslide deposits and was located in part of an active landslide. The landslide accelerated in the six months prior to the incident, rapidly increasing strain on Line 10.

Other NTSB contractors identified two defects along the weld where the line ruptured. They also excavated and evaluated similar girth welds, which connect pipes. Post-accident analysis found that the tensile strain demand on the girth weld was at least three times higher than indicated by Enbridge's pre-rupture analysis.

"The post-rupture analysis demonstrated that in April 2018 or earlier, Enbridge could have foreseen the likelihood that the tensile strain demand would exceed the strain capacity due to documented land movement at the site," the NTSB said.

Further, Enbridge's pre-rupture calculations did not appropriately consider uncertainties, including pipeline defects and changes in landslide conditions that could increase the chance of a weld fracturing, according to the NTSB. Any pipeline operator must take into account such assumptions and uncertainties in order to make decisions based on test results, the NTSB said.

Enbridge takes steps to mitigate risk

Following the incident, Enbridge implemented a number of new procedures, including reducing the tensile strain capacity threshold on Line 10 in the incident area. Had that threshold been in place at the time of the incident, it would have prompted a high-priority response, Enbridge told the NTSB.

The company said it would also work with contractors to determine if it should adopt a new tensile strain demand methodology, acknowledging that its methodology may have underestimated the strain on Line 10 prior to the incident.

The Pipeline and Hazardous Materials Safety Administration has ordered Enbridge to take corrective actions with respect to Texas Eastern lines 10, 15 and 25, which run parallel along the same right-of-way.

An August 2019 explosion on Line 15 in Danville, Ky., killed one person and injured six others. The Pipeline and Hazardous Materials Safety Administration also traced a January 2019 rupture on Line 10 in Summerfield, Ohio, to ground movement that overstressed a girth weld.

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