TUESDAY, JUNE 19, 2018
Enbridge Energy released its report to the state of Michigan on possible replacement of its Line 5 pipeline Friday (June 15), concluding that a tunnel or an open cut with secondary containment would be feasible methods for replacing the 65-year-old liquids pipeline.
The Canadian firm was tasked late last year with preparing an analysis of options for the eventual replacement of Line 5 under the Straits of Mackinac, which has grown increasingly controversial over the past year due to reports of holes in its protective coating. While the agreement the company reached with the state in November did not mandate the shutdown of Line 5, it moved the conversation toward replacement.
The Tunnel Approach
In the new report, Enbridge says its study, performed by independent consultants, showed a tunnel created by a tunnel-boring machine could be a feasible method of installing a new pipeline to replace Line 5. Such a tunnel would have a 10-foot inside diameter and would stretch four miles long, 100 feet below the deepest part of the Straits.
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| Renderings: Enbridge promotional video |
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The first feasible method of replacing Line 5 would be a tunnel under the Straits, built to contain the new 30-inch pipeline and stop any potential future release from reaching the water. |
Enbridge says, given that any release from the pipeline itself would be contained by the concrete tunnel, there is “no credible scenario” that would result in a release of oil or other product into the Straits.
A recent proposal by Michigan Technological University engineering students called for a tunnel that would contain other utilities aside from Line 5, including a natural gas line and electric transmission cables. The Enbridge study does not specifically look at including other utilities in the tunnel, but notes that the tunnel size could be increased to accommodate other utilities if that were deemed appropriate.
A tunnel would cost $350-500 million, Enbridge says, and if this alternative were chosen, the dual 20-inch pipelines comprising the current Line 5 would be replaced with one 30-inch line. The concrete tunnel would be lined with precast concrete tunnel lining; the annulus outside the tunnel lining would be filled with cement grout.
Secondary Containment Method
The other alternative the company says could work would be an open cut with a secondary-containment pipeline around a new 30-inch Line 5. This approach would cost $250-300 million and would involve trenching the pipeline for the first half-mile or so from the shoreline, then laying the majority of the line on the lake bed with an engineered protective cover.
The pipe-in-pipe system would involve a 30-inch steel pipeline surrounded by a 36-inch secondary-containment line with low-friction spacer rings in the space between. The outer pipeline would be engineered to withstand pressures greater than the line’s operating maximum.
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A pipe-in-pipe secondary-containment system would greatly reduce the probability of a release of oil into the Straits. |
The design, proposed by engineering firm INTECSEA, would involve coating the inner pipeline with fusion-bonded epoxy and covering the outer pipeline with a three-layer polypropylene coating system for protection against corrosion and against abrasion. The outer pipeline would also have an impressed-current cathodic protection system.
While the open-cut method would come at a lesser cost and the secondary containment system would reduce the risk of a release to a miniscule level, the more expensive tunnel method would make maintenance and repairs easier if needed. An open-cut installation would take about four years to finish, Enbridge says, while the tunnel would take six, but because the tunnel method would have less of an environmental impact, it could take less time to secure the needed permits.
Enbridge’s analysis found that the other approach studied, horizontal directional drilling, would not be feasible for the Straits.
Line 5 Controversy
Enbridge has held that Line 5, which has never leaked, is in good condition and can continue to operate safely, but some nearby residents and environmental groups have expressed concerns about the consequences if a breach occurred, as it is the only liquids pipeline running under the Great Lakes.
Over the course of last year, Enbridge admitted there were areas of coating loss on the submerged pipeline, and that they were larger than the company first said. Still, the company holds that its integrity has not been compromised. Line 5 passed a high-pressure hydrotest in June 2017 as part of a consent decree the energy company entered into with the U.S. government after a 2010 spill on its Line 6B in Michigan.
While the new report outlines feasible methods of replacing Line 5, there is no concrete plan in place to do so as yet.
Editor's note: A previous version of this article incorrectly identified Michigan Technological University.
Tagged categories: Government; Oil and Gas; Pipelines; Program/Project Management; Tunnel
