Apr. 30—MACKINAW CITY — Canadian pipeline operator Enbridge Inc. is another step closer to building a tunnel beneath the Straits of Mackinac as legal battles over its construction rage on.

The Calgary, Alberta-based company announced Tuesday that it has inked a joint venture partnership with Barnard Construction Company, plus Civil and Building North America.

Together, they plan to bore a roughly 25-foot diameter, 4.5-mile-long concrete lined tunnel through the rock beneath the Straits of Mackinac. Those figures are according to Enbridge and a consultant for the Mackinac Straits Corridor Authority, which would own the tunnel and lease space within for an oil and natural gas liquids pipeline.

Tom Schwartz, senior vice president of Enbridge Strategic Projects and Partnerships' Liquids Pipelines, in a written statement called the agreement a "milestone for this historic project."

"The selection of these two world-class companies reinforces our commitment to safely building the Great Lakes Tunnel," Schwartz said in the statement.

Both companies sport tunneling experience among their respective portfolios of big civil engineering projects — CBNA was the design-build contractor for the 3-mile-long Tuen Mun Chek Lap Kok Tunnel in China, and Barnard Construction Company bored the Central Subways Tunnel Project beneath San Francisco.

The U.S. Army Corps of Engineers is reviewing the project, and Enbridge in its release stated it anticipates a decision in early 2026.

That gives the companies time to finalize design and engineering, among other construction prep work, Enbridge spokesman Ryan Duffy said. The biggest element of that would be selecting and ordering a 300-foot-long tunnel boring machine.

"And so that's a key piece of the whole project, is that tunnel-boring machine," he said.

Duffy acknowledged a $500 million estimate for the project cost is likely outdated, but the company didn't have a recent estimate just yet.

That number dates well before the pandemic — 2019 filings to the Canada Energy Regulator show the company couldn't provide an updated number then — and, since then, construction costs for major infrastructure projects have spiraled.

What Enbridge bills as a crucial piece of energy infrastructure and a safer alternative to Line 5's current Straits crossing — a twin pipeline on the lakebed built in 1953 — project opponents cite as a dangerous undertaking through methane-laced shale, and a conduit for fossil fuels that pollute and create greenhouse gases when burned.

For Love of Water Executive Director Liz Kirkwood pointed to the outdated cost estimate as one of many questions calling the project's feasibility into question. The company, in a Federal Energy Regulatory Commission filing, acknowledged the pipeline would face future competition and regulatory pressure, including heightened scrutiny of carbon emissions from fossil fuels that could effect demand.

Kirkwood said this latest announcement looks like more public relations amid a blitz of advertising from the company. Yet Enbridge undercut its public argument that shuttering Line 5 would cause price spikes and energy shortages in court filings for a different segment of the line, she said.

That federal lawsuit involves part of the Superior, Wisconsin-to-Sarnia, Ontario, pipeline that runs through Bad River Band of the Lake Superior Tribe of Chippewa Indians' reservation in Wisconsin. The tribe accused the company of trespassing on reservation lands after pipeline easements there expired in 2013. A federal judge ordered Enbridge in 2023 to leave tribal lands within three years, a decision the company told an appeals court violates a 1977 treaty between the U.S. and Canada.

"Enbridge is taking the position that it should dictate where it can locate its pipelines, even if it trespasses on state sovereignty or tribal sovereignty, and that's not OK," Kirkwood said.

Traverse City-based For Love of Water argued for years that Line 5 violates the public trust doctrine through the dangers it poses to the Straits of Mackinac and the publicly-owned bottomlands — not to mention tribal fishing rights in the Straits, among other issues of tribal sovereignty.

While Kirkwood said expanding the company's pipeline network elsewhere would give Enbridge what it needs, Duffy said that's not so.

A pipeline linking Superior to Chicago is at or near capacity, and building a twin stretch linking the Wisconsin town to Sarnia via Illinois would come with its own high costs and permitting difficulties, Duffy said. Plus, closing Line 5 would cut off propane to the company's Cut River facility in the Upper Peninsula, and a crude oil injection station in the northern Lower Peninsula.

Duffy dismissed concerns over bedrock geology, methane and other potential dangers of drilling under the Straits, noting it's an issue the company studied extensively. The Michigan Public Service Commission also reviewed the information before granting Enbridge's request to build the tunnel.

"So we've studied all of those things, and we're confident that the tunnel can be built safely," he said.

Should the tunnel be built, that project should be a boon to union construction workers, according to Geno Alessandrini, business manager for the Michigan Laborers District Council.

"This announcement is big news for the tunnel project and it's big news for Michigan laborers," he said in a Great Lakes Michigan Jobs Coalition release.

The Michigan Laborers District Council calls itself the parent organization for seven local labor unions in the state.

Meanwhile, the Michigan Climate Action Network will lead some other organizations and the Little Traverse Bands of Odawa Indians in an appeal of the MPSC's decision. That filing is set for May 9.

Another lawsuit filed in 2019 by Michigan Attorney General Dana Nessel pushing for a shutdown of the line continues. The state recently argued in the U.S. Sixth Circuit Court of Appeals that the case belongs in a state — not a federal — court. Enbridge, meanwhile, argued the state has ignored the federal interests in international trade at hand.

The Associated Press contributed to this report.

The Associated Press contributed to this report.