(Bloomberg) — The operator of the Keystone pipeline is partly to blame for failures that contributed to the spilling of nearly 13,000 barrels of crude from the line in 2022, according to a report by US regulators.

The failure occurred at a weld that had been under stress because the soil under the line wasn’t adequately compacted after the pipeline was excavated to replace fittings in 2010, the Pipeline and Hazardous Materials Safety Administration said in the report.

“This stress has been determined sufficient to initiate cracking of the failed girth weld,” PHMSA said.

The spill happened on farmland in Kansas on Dec. 7, 2022, with most of the oil entering a creek, the worst of multiple such incidents that happened since the pipeline began operation in 2010. A Bloomberg investigation last year revealed that PHMSA had notified TC Energy Corp., which operated the line then, at least five times that elements of Keystone’s building and operating practices posed safety risks.

TC Energy spun off its liquids pipeline business this year, with Keystone going to the new company, called South Bow Corp. TC Energy referred questions about the report to South Bow, which did not immediately respond to emails seeking comment.

Two days before the spill, TC Energy increased flow rates on Keystone, a move that would also increase stresses at elbows on the pipeline, PHMSA said in its report.

“The pre-failure stress analysis for increased flow rates did not consider the effect of these combined increased cyclic stresses could have on girth weld flaws already being subjected to excessive bending stress,” PHMSA said.

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