Enbridge Inc. has agreed to cover the costs of an underwater tunnel that would accommodate a replacement for its 65 year-old oil and natural gas liquids pipeline beneath the Straits of Mackinac.

An agreement with Michigan Governor Rick Snyder commits the Calgary firm and the state government to cooperate on the tunnel. The company pledged to post a $1.87 billion bond to cover damages if the conduit leaks before the project is completed. No target date is set for construction, which is forecast to take seven to 10 years and cost $300-650 million.

The four-mile crossing of the Straits between Lake Michigan and Lake Huron is a vital spot for Enbridge — but also an environmentally sensitive flashpoint for public concern — on a conduit the company designates Line 5. The pipe carries 540,000 b/d 547 miles from Wisconsin through Michigan to Ontario.

Work is under way on replacing a similar but smaller underwater leg of Line 5 that crosses the international border into Canada on the floor of the St. Clair River to reach Sarnia, a refining and petrochemical industry center in southwestern Ontario.

The new agreement calls for Enbridge to bore a shaft under the Straits on a big enough scale to contain other utility lines. The project would be a public-private partnership with Michigan’s Mackinac Bridge Authority. The state agency would own the completed tunnel and lease it to Enbridge as the operating partner.

The agreement allows state representatives to audit Enbridge’s $1.87 billion safety bond, a figure determined sufficient to cover potential leak damages and emergency response costs.

The agreement also commits Enbridge to honor a previous pledge to shut down the pipe on the bottom of the Straits during severe storms, with the safety precaution enhanced by the company paying $200,000 for new weather observation video cameras.

Michigan authorities called the Enbridge agreement “a reasonable, thoughtful, pragmatic solution.”

“It’s protective, it’s comprehensive, and it’s spurred by safety,” Enbridge added. But environmental and native groups protested that the deal makes the state take big risks without setting a date to end hazards posed by the Canadian-owned pipeline.