(Reuters) - Enbridge Inc on Friday endorsed either an underwater tunnel or trench to replace a section of its 65-year-old energy pipeline that travels under a stretch of the U.S. Great Lakes in Michigan.
Some local officials want the line removed to prevent the potential for a spill into lake waters. The state of Michigan proposed moving the line into a tunnel under the lake bed and asked the company to evaluate it and other alternatives.
The Canadian pipeline operator said a tunnel would cost between $350 million and $500 million and take five to six years to assemble.
A concrete-lined tunnel built as much as 100 feet below the lake bed and 350 feet below the surface of the lake would prevent an oil spill from reaching the water, the company said.
The alternative, an open-cut trench, would cost between $250 million and $300 million and take four to five years to build, the company said.
That method would prevent spills by constructing a second 36-inch outer pipe around a 30-inch energy pipeline. The outer pipe would have a leak detection system, Enbridge said.
Building either a tunnel and a trench would require 15 state and federal permits, with oversight by the Army Corps of Engineers and Michigan environmental agencies.
In April, the pipeline was damaged by a boat anchor, raising fears of a potential spill in an environmentally sensitive area.
The 20-inch Line 5 pipeline carries crude oil and natural gas liquids on twin lines running from Superior, Wisconsin, to Sarnia, Ontario. A portion of it runs 4.5 miles (7.2 km) under the Straits of Mackinac, which links Lake Michigan and Lake Huron.
Enbridge concluded that one proposed option, using horizontal directional drilling to push a pipe underground, was not technically feasible.
Reporting by Collin Eaton; Editing by Bill Berkrot and Cynthia Osterman