Enbridge Inc.'s Line 5 pipeline tunnel agreement with the previous state government is unconstitutional, Michigan maintained in its latest response in the continuing court battle over the Calgary-based company’s crude oil pipeline through the Straits of Mackinac.

Michigan argued the agreement didn’t have a title that adequately reflected its contents, which isn’t allowed.

“First, the title did not provide fair notice to legislators and the public of substance of the bill. Second, it improperly combined two unrelated purposes: the Mackinac Bridge and a proposed ‘utility tunnel’ meant to house a new Enbridge oil pipeline,” Attorney-General Dana Nessel’s office said in a statement Thursday.

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Michigan also asserted the energy company used different language in its latest court reply compared to the wording of the original agreement.

This is the last step before Michigan’s Court of Claims either makes a decision or proceeds to spoken arguments.

The state’s new Democratic government is trying to get the underwater section of the pipeline through the Straits of Mackinac removed within two years, whether or not a replacement tunnel beneath the straits is complete, because the state doesn’t want to risk an oil spill in the Great Lakes.

The original tunnel plan didn’t contain a time limit for when Enbridge needed to take the existing crude oil pipeline out of the water, which the state’s administration took issue with. If the court sides with Michigan, a new tunnel agreement would likely need to be negotiated, which could include a hard time limit for removing the existing pipeline.

Title-body rules are meant to ensure legislation is easy for the public to understand, said James Coleman, a fellow at the University of Calgary Faculty of public policy and an energy law professor at the Southern Methodist University of Texas.

Challenges about a law’s title and contents not matching happen occasionally, but they’re not usually arguments that win, he added.

While Enbridge executives told investors Aug. 2 that shutting down the Straits of Mackinac pipeline section before a replacement is complete is a “low probability" scenario, the outcome of two continuing court battles with the state of Michigan could be anyone’s guess, Mr. Coleman said. Enbridge’s Line 5 is also involved in a second lawsuit in which Michigan is trying to have the land-use agreement in the Straits of Mackinac from the 1950s overturned by arguing the pipeline is a public nuisance.

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It would be “foolhardy to predict the outcome of any pipeline lawsuit,” especially one as highly politicized as the future of Line 5, he said.

In Michigan, some people are concerned about the existing pipeline in the water – especially following an anchor strike that damaged it last year. But there are others who oppose oil extraction and pipelines more generally. Enbridge’s proposed tunnel solution would only satisfy the former, Mr. Coleman said.

Enbridge is reviewing Michigan’s latest filing, and it’s still focused on pre-construction tunnel work, company spokesperson Ryan Duffy said.

Canadian energy producers and refiners should be concerned about what’s going on with Line 5 in Michigan, Mr. Coleman said, because the country is in such a difficult position with its oil’s market access. Any further impediments would “hurt.”

Line 5 transports 540,000 barrels a day of crude oil from Alberta to refineries in Sarnia, Ont. It also delivers fuel to the Detroit airport, picks up oil from northern Michigan producers and carries propane used to heat homes in Michigan’s Upper Peninsula.

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