Enbridge presents Line 5 Straits of Mackinac replacement alternatives to State of Michigan
Study IDs tunnel and open-cut secondary containment installations as feasible options
Trails. Parks. Island nature preserves. Picturesque ports of call.
The Straits of Mackinac, where Lake Huron and Lake Michigan meet, support recreation, commerce and a Michigan way of life. They’re also a natural treasure—a place worth protecting.
Back in November, Enbridge entered into a formal agreement with the State of Michigan to continue protecting the Great Lakes. Part of that pact included exploring options for the long-term future of our Line 5 pipeline, a light crude oil and natural gas liquids (NGL) conduit that crosses the Straits of Mackinac.
And today, we marked an important milestone in that agreement—by delivering a report to the State that includes feasible methods for replacing the dual Line 5 pipelines that travel across the bottom of the Straits.
“Enbridge worked with teams of engineering, construction and environmental consultants, all of whom are renowned for their expertise and are considered leaders in their respective fields,” says Guy Jarvis, Enbridge’s Executive Vice President of Liquids Pipelines. “The State of Michigan also engaged subject-matter experts who reviewed and verified the data, and participated fully in all aspects of the study.
“This study determined that two alternatives—a tunnel bored under the Straits, and an open-cut installation with pipe-in-pipe secondary containment—are technically feasible,” adds Mr. Jarvis. “In other words, they can be carried out with existing equipment and technology.”
Line 5, and Enbridge's other energy pipelines, deliver the products that heat homes and businesses, fuel vehicles, and power industry across the state.
As part of the tunneling alternative, the study found:
- A Tunnel Boring Machine (TBM) would be used to excavate a 12-foot-diameter tunnel through bedrock as much as 100 feet beneath the lakebed;
- The tunnel would be lined with a precast reinforced concrete lining;
- The tunnel would contain a 30-inch-diameter oil and NGL pipeline, in addition to lighting and ventilation systems; and
- The tunnel would be open and accessible for inspection and maintenance.
As part of the open-cut alternative, the study found:
- A 30-inch-diameter product pipeline, placed inside a 36-inch-diameter secondary containment pipe, would be trenched to a depth of 30 feet, and then laid on the lakebed;
- The pipe-in-pipe system would be covered with six to eight feet of gravel and cobble, or small rocks, to protect against accidental anchor strikes; and
- Low-friction spacer rings would provide separation between the pipes.
“Both of these alternatives would need considerably more detailed design and engineering, should one of them be chosen as an eventual replacement for the Line 5 Straits crossing,” says Mr. Jarvis.
(TOP PHOTO: A large Tunnel Boring Machine (TBM) prepares to begin construction of a rail tunnel in Germany. Source: Herrenknecht)