TC Energy Corp., the operator of the Keystone pipeline, raised the volume of oil flowing through it before the system suffered its worst leak on record.

Crude flows on the conduit carrying heavy crude from Western Canada to the U.S. Midwest reached nearly 650,000 barrels a day on the night of the spill, according to data from WoodMackenzie. That’s an increase from a baseline of about 622,000 barrels a day. TC Energy said in October it would temporarily raise flows in November and December to test the system’s operational efficiency. 

The company declined to comment on specific flow rates at the time of the spill, saying in a statement: “At the time of the incident, the pipeline was operating within its design and regulatory approval requirements.” 

Keystone has spilled more oil than any other US pipeline since 2010, when the system began operation, according to preliminary data from the Pipeline and Hazardous Materials Safety Administration. The severity of its spills has worsened in recent years, with construction issues being a key contributor, the Government Accountability Office found in a 2021 study.

The company continued to boosts flows on the line following at least six leaks between 2016 and 2020 that released more than 11,000 barrels of crude. In September, the Keystone system transported 639,000 barrels a day, Canada Energy Regulator data show.

Keystone also operates at a higher pressure rate than other U.S. liquid pipelines, having obtained a special waiver to operate above the federal maximum. 

A 2019 spill in North Dakota “was caused by flaws in the pipeline’s original construction,” specifically defective pipe, David Vanderpool, president of Vanderpool Pipeline Engineers Inc., an engineering consulting firm, said in testimony before the Canada Energy Regulator in January. TC Energy also uses drag-reducing agents to raise capacity on the pipeline, which wasn’t designed to carry the volumes its transporting, he said.