A proposal by Canadian oil giant Enbridge to construct a new four-mile pipeline beneath the Straits of Mackinac in Michigan has cleared another legal hurdle, with the state Court of Appeals upholding the legality of a 2018 state deal approving the project.

Enbridge claims the $500-million tunnel is needed to replace the existing underwater dual-pipe segment of Line 5, a nearly 70-year-old, 30-in. dia pipeline that transports up to 540,000 barrels per day of crude oil and natural gas liquids 645 miles between Superior, Wis., and Sarnia, Ontario.

Near the end of its 2018 session, the Michigan legislature authorized a deal between the company and outgoing Gov. Rick Snyder (R) to build the tunnel beneath the narrow strait separating the state’s upper and lower peninsulas.

Pipeline opponents critical of the expedited approval received a boost from incoming Gov. Gretchen Whitmer (D), whose administration claimed enabling legislation violated the state constitution because it contained a provision transferring project oversight to another governing authority, which exceeded the scope described in its title.

In its June 11 decision, the three-judge appeals panel upheld a 2019 ruling by Michigan’s Court of Claims that the act properly authorizes the tunnel to proceed; Michigan’s Attorney General plans to appeal the case to the state Supreme Court.

With a busy docket, observers say the court is not likely to hear the appeal until after work has already begun. Public hearings for construction permits start this summer,

The ruling was Line 5’s second legal victory in as many months.

In early May, a lawsuit challenging installation of screw anchors as part of the existing segment’s maintenance program was dismissed when all plaintiffs withdrew from the case. Although Enbridge claims the existing strait crossing is in “excellent” condition with no recorded leaks in more than 60 years of operation, the company scrambled in late May to repair four areas of protective coating that had worn down to bare metal.

According to an Enbridge statement, the worn areas ranged in size from .07 to .43 sq ft. Pending state and federal permit approvals, Enbridge plans to begin tunneling for the new Straits of Mackinac crossing next year, which is set to operate in 2024.

Meanwhile, Wisconsin’s Dept. of Natural Resources is weighing Enbridge’s request to re-route about 42 miles of Line 5 in three counties. Indigenous groups have opposed the project, with the Bad River Band of the Lake Superior Tribe of Chippewa Indians suing successfully in 2019 to remove the pipeline entirely off its reservation.

According to the agency, rerouting the pipeline would affect 109 acres of wetlands and cross 87 waterways via open-cut trenching or dredging.

Jay Dee Contractors Inc., Livonia, Mich. and the U.S. unit of Japanese giant Obayashi Corp. were named in March to build the tunnel through a joint venture called Great Lakes Tunnel Constructors to be the principal construction contractor.

Enbridge also hired London-based Arup as designer.