Enbridge Inc. is offering to speed up work on a planned tunnel for the four-mile underwater leg of its Line 5 oil pipeline across the Straits of Mackinac in response to Michigan state demands for swift action.

Construction could begin in 2021 and the environmental safety improvement for the 540,000-b/d line would be completed in early 2024 if the state cooperates by granting timely project approvals, Enbridge said.

The Calgary firm’s statement followed discussions with Michigan Governor Gretchen Whitmer and a threat by Attorney-General Dana Nessel to launch a lawsuit with potential to shut down the 65-year-old pipeline.

Enbridge made a tunnel agreement with Michigan’s former Republican executive branch that the state legislature’s Republican majority ratified last year. But at the same time, Whitmer and Nessel won power as Democrats on election platforms critical of the deal.

The original schedule allowed a leisurely project pace, with construction forecast to take up to 10 years and cost $500 million for a large all-purpose utility tunnel to accommodate other public services such as power lines as well as the oil conduit.

Nessel issued a formal attorney-general opinion that the legislature’s enactment of the tunnel deal was unconstitutional. Whitmer handed down a governor order for state agencies to withhold construction permits.

Whitmer opened discussions with Enbridge. She also gave a further order for ships to stop dropping anchor near the pipeline after underwater photos of lake-floor accident damage were publicly released.

After talks with the Michigan governor Enbridge said, “The tunnel represents a practical approach to meeting the state’s energy needs while providing the people of Michigan with even greater protection of this crossing to as near zero risk as humanly possible.”