A U.S. regulator has ordered TC Energy Corp. to lower the operating pressure on the entire Keystone pipeline system.

The Pipeline and Hazardous Materials Safety Administration issued the ruling Tuesday as a result of its ongoing investigation into a Dec. 7 leak that saw around 13,000 barrels of oil spill into a creek in Washington County, Kansas.

Pipeline operator TC Energy was able to restart most of the 4,324-kilometre pipeline a week after the spill, but a 154-kilometre section running from just south of Steele City, Nebraska to Cushing, Oklahoma remained shut down until the end of December.

That smaller section has been operating at a reduced pressure since it reopened. But Tuesday's regulatory order means the entire Keystone pipeline system must now operate at a stress level of no more than 72 per cent of its specified minimum yield strength.

Keystone is the only crude oil pipeline in the U.S. to have been granted a special permit from the PHMSA to operate certain portions of the pipeline at a higher stress level, 80 per cent of its specified maximum yield strength, than is allowed under existing regulations.

As a result, since 2017, TC Energy had been operating Keystone at a higher operating pressure than would normally be allowed under U.S. rules.

This report by The Canadian Press was first published March 8, 2023.