Operators of twin oil pipelines beneath the swirling waterway where Lakes Huron and Michigan converge insist they remain structurally sound even though an outer layer of protective covering has worn away in some places.

Skeptics say the deterioration is further evidence the lines should be shut down.

Officials with Enbridge Inc. told the Michigan Pipeline Safety Board Monday the exterior material isn't essential to the pipes' integrity and there's no reason for concern about its absence in spots along the surface of the lines in the Straits of Mackinac.

But opponents said recent disclosures about the missing covering, and a report from an independent engineer warning of wear to the pipelines from the waterway's whipsaw currents, should heighten concerns about a potential rupture that could devastate the area.

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