A court case has erupted over the future of Enbridge Inc.’s 540,000 b/d oil export Line 5 after negotiations failed to settle a Michigan dispute about its four-mile underwater leg across the Straits of Mackinac.

Enbridge filed a request in the Michigan Court of Claims to enforce a 2018 agreement with the state on burying the line, as a safety measure, in a tunnel below the bottom of the channel between Lake Huron and Lake Michigan.

The Calgary company took the legal initiative rather than wait for Michigan Attorney General Dana Nessel to carry out a threat to launch a state lawsuit demanding closure of the pipeline’s underwater section.

Nessel has written an official opinion that the tunnel agreement is unconstitutional. Michigan Governor Gretchen Whitmer acted on the opinion by ordering state agencies to stop granting pipeline tunnel construction permits.

As Democrats, Nessel and Whitmer each won office last fall on campaign platforms that called for shutting down the Enbridge Line 5 Mackinac Straits crossing within two years.

Michigan’s former Republican administration made the tunnel agreement. The majority Republican legislature ratified the deal and continues to support it.

Before filing its lawsuit, Enbridge gave the Michigan governor an offer to complete the $500-million tunnel project in 2024 on an accelerated construction schedule. Whitmer’s office accused the company of walking away from the negotiations.

“We require a court review of the enforceability [of the contested agreement] in order to remove obstacles to building the tunnel as quickly as possible and ensure energy security and environmental protection for Michigan,” said Enbridge Vice President Guy Jarvis.