Energy firm Enbridge and Michigan's governor have been trying to negotiate a deal to shut down the company’s aging Line 5 that runs under the Straits of Mackinac. The state's governor wants a deal within a week while Enbridge reaffirmed plans for a $500 million tunnel to house a new pipeline.

Line 5 carries around 540,000 barrels per day of liquified natural gas and crude oil from Superior to Sarnia, Ontario, through the Straits. The Detroit News reported this week that Democratic Gov. Gretchen Whitmer wrote a letter to Enbridge CEO Al Monaco voicing concerns about the company’s five-year timeline for completing the project. She told Enbridge she’d like to reach an agreement within the next week.

The Detroit News reported Whitmer wrote in the letter that the project would likely see delays and take longer to build than Enbridge’s proposed timeline. Enbridge said last week that it could have the tunnel in operation by early 2024.

Michigan’s governor told reporters last week she didn’t want to see the more than six-decade-old pipeline remain in service that long. She and environmental groups argue the Great Lakes are at too great a risk in the event of a spill from the aging pipeline.

Michigan Attorney General Dana Nessel also threatened last week to pursue legal action within a month if the governor and Enbridge are unable to reach a deal.

On Wednesday, Enbridge said it’s moving forward on engineering details and other aspects to prepare for the project.

"As previously announced, Enbridge believes the tunnel can be under construction in 2021 and in service as soon as early 2024, assuming no delays in the permitting process," said the company in a statement. "Enbridge has further committed that operation of the existing Straits Line 5 crossing would cease immediately following the placement into service of the replacement pipeline in the tunnel."

The energy firm said the company’s plan would avoid disruptions in the state’s energy supply. Enbridge said Line 5 serves around 55 percent of Michigan’s energy needs. The company also said it would take more than 800 rail cars each day to deliver the crude oil and natural gas liquids carried by Line 5.

Enbridge said it looks forward to working with the state on additional safety measures for Line 5 in the interim and ways to shorten the timeline for its tunnel project.

Whitmer’s office did not immediately respond to requests for comment Wednesday afternoon

Wisconsin Public Radio can be heard in the Twin Ports at 91.3 FM or online at wpr.org/news.