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Lansing — Enbridge Inc. said Friday it would be feasible but costly to replace its controversial Line 5 pipeline with a tunneled version and “virtually” eliminate any risk of an oil spill in the Straits of Mackinac.

A 30-inch pipeline could be housed in a roughly four-mile tunnel lined with concrete and buried 100 feet below the lake bed, the Canadian energy company told the state of Michigan in a required report. The projected cost would be $350 million to $500 million over six years of planning and construction.

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Enbridge Inc. animation of a potential alternative to Line 5: A tunnel beneath the Straits of Mackinac. Enbridge

Enbridge could also “significantly” reduce the probability of a Great Lakes oil spill by replacing Line 5 with a trenched and rock-covered version inside a second containment pipeline that could cost up to $300 million, according to the 100-page report.

The company agreed to assess multiple Line 5 alternatives under a November 2017 deal with Gov. Rick Snyder, who wants Enbridge and other companies to move underwater infrastructure into a shared utility tunnel beneath the Straits, a concept environmental groups have already criticized.

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This Enbridge Inc. animation shows a Line 5 alternative to build a trench and rock-covered pipe-in-pipe system in the Straits of Mackinac. Enbridge

Enbridge isn’t committed to building a tunnel or trenched pipeline, but the report is “a starting point” that will help the state and company “determine a possible path forward for Line 5,” said spokesman Ryan Duffy.

“Both of these alternatives would enhance safety and ensure the continued protection of the Great Lakes.”

Snyder said the report will help the state "define a comprehensive solution for all utility crossings.”

“Line 5 cannot remain in the Straits indefinitely," the governor said in a statement. "We need a concrete strategy and time line to expedite its replacement."

The aging dual pipelines, built in 1953, transport up to 540,000 barrels a day of light crude oil and natural gas liquids through the Straits, a turbulent waterway that connects Lake Michigan and Lake Huron.

Enbridge determined a third potential alternative — using horizontal directional drilling to construct a new pipeline beneath the lakebed — is “not technically feasible” because of rock characteristics and the length of the crossing. It would be more than twice the lengthy of any comparable and completed project, the company said.

Environmental activists have long called for a Line 5 shutdown, warning an accident like Enbridge’s massive 2010 inland Kalamazoo River oil spill could devastate the Great Lakes. But state and industry officials have warned that a complete shutdown would cut off a supply of propane that many Upper Peninsula residents rely on for heat.

Line 5 scrutiny intensified in early April after an anchor strike ruptured underwater transmission cables and dented Enbridge’s 65-year-old dual pipeline. Environmental groups have criticized Snyder’s call for a tunnel replacement.

“A tunnel will only continue to keep our Great Lakes at risk of a disastrous oil spill,” said Lisa Wozniak, executive director of the Michigan League of Conservation Voters. Enbridge "has a vested interest in continuing to pump oil through the Straits, which explains why the company failed to adequately consider alternatives that keep oil pipelines out of our waters."

Liz Kirkwood, an environmental attorney and FLOW executive director, said in a Friday statement that “in a time of water shortages and changing climate in this century, it doesn’t make sense to even contemplate constructing Canadian oil pipelines in a tunnel under the world’s largest supply of fresh surface water.”

But the probability of an in-tunnel spill contaminating the Great Lakes is negligible, according to Enbridge. A reliability assessment conducted for the company “demonstrated there is no credible scenario that would result in a release of product from the tunnel into the Straits.”

Enbridge said it hired three engineering consultant companies to assess the alternatives, then used three separate teams of engineers and construction reviewers to verify conclusions. Two environmental firms assessed and verified potential impacts and evaluated needed permits.

A tunnel would act as a secondary containment system, according to the Enbridge report. The 10-foot diameter interior would be lined with a “precast reinforced concrete lining” with high-strength gaskets, and the space outside the lining would be filled with cement grout.

Securing at least 15 required state and federal permits, gathering materials and constructing the tunnel could take about five to six years, according to company estimates.

The tunnel would be "well within the size and length range” of others constructed around the world, Enbridge said. “Many tunnels have been completed under lakes, rivers and seas; numerous energy pipeline tunnels have been constructed to date, particularly in the last five to 10 years.”

Replacing Line 5 with a pipe-in-pipe system trenched underground up to a half-mile from shore and then placed on the lake bed would cost $260 million to $300 million, according to Enbridge estimates. Securing permits and building the system could take about four years.

A 30-inch pipeline carrying crude oil and natural gas liquids would be placed inside a 36-inch outer pipe that would include a real-time leak detection system and provide a form of “secondary containment.”

To protect the pipeline from an anchor strike or dropped objects, Enbridge said it would be covered with a six- to eight-foot thick protective layer of gravel and “cobble” rocks ranging in size from one to 12 inches.

The alternative would “significantly” reduce the probability of an oil spill or release into the Straits of Mackinac, Enbridge said.

As The Detroit News first reported, the Snyder administration is in talks with Enbridge and other companies about building a shared utility corridor that could also house cables and other natural gas pipelines currently sitting on the bottom of the Straits.  

Snyder said almost two months ago he wants Enbridge to decommission Line 5 and construct a tunneled replacement if studies show the tunnel could be built and wouldn’t cause “significant environmental damage.” 

Enbridge said tunnel construction would have “no impact” on shorelines or the lake bed. Onshore areas "disturbed" during construction would be “reclaimed” when the project was complete, the company said. Long term, a tunnel would require fenced enclosures of up to one acre at the entry and exit points.

The Enbridge report analyzed the potential for a pipeline tunnel, but it could be “scale-able” to include other infrastructure, Duffy said. “But that wasn’t within the scope of this study.”

Enbridge is expected to file additional reports with the state by the end of June evaluating measures to mitigate anchor strikes, and on underwater technologies to enhance leak detection and assess pipeline coating.

As part of the 2017 agreement between Snyder and Enbridge Vice President of U.S. Operations Bradley Shamla, the state and company said they hope to finalize further negotiations by the fall.  

“Now we’re going to be sitting down with the state, using all the information from this report to kind of figure out what the negotiations look like going forward and to figure out the best option,” Duffy said.

joosting@detroitnews.com

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