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

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

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

The spill occurred on farmland in Kansas on Dec. 7, 2022, with most of the oil entering a creek, the worst of several such incidents since the pipeline began operating in 2010. A Bloomberg investigation last year revealed that PHMSA had notified TC Energy Corp., which operated the line at the time, made at least five times as many parts of Keystone’s construction and operating practices safety hazards.

TC Energy spun off its pipeline business this year, and Keystone went to the new company, called South Bow Corp. TC Energy referred questions about the report to South Bow.

South Bow has conducted 68 exploratory excavations on the pipeline, it said in a statement, adding that it had “not detected conditions similar to Milepost 14” where the incident occurred. The company “will continue long-term monitoring and recovery activities,” it said.

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

“The pre-fail stress analysis for increased flow rates did not consider the effect these combined increased cyclic stresses may have on circumferential weld failures that are already subjected to excessive bending stress,” PHMSA said.

PHMSA did not identify flow rate as a contributing factor, nor did the third-party investigation, South Bow said. “The system was operating within its design and operational limits at the time of the incident,” it said.

(Adding South Bow comments throughout)

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