MACKINAW CITY, MI -- The U.S. Coast Guard has proposed an indefinite ban on anchoring in the Straits of Mackinac in an effort to prevent further anchor strikes on lake-bottom utility lines.

If implemented, the ban would replace the need for Gov. Rick Snyder to renew the "no-anchor zone" emergency rule he signed in late May.

The proposal is open for public comment until Sept. 4.

On April 1, a tug and barge dragged its anchor across the Straits floor, denting underwater petroleum lines and rupturing transmission cables.

The ruptured cables owned by American Transmission Company spilled an estimated 550 gallons of toxic coolant oil into the Straits. The dented Line 5 oil and gas twin pipelines didn't spill.

At the time, there were no restrictions to dropping or dragging anchor in the Straits, only an advisory.

The anchor strike renewed calls to shut down the controversial Line 5. Enbridge Energy, the Canadian company which owns Line 5, floated the idea of piling rocks onto the 65-year-old pipeline to protect it from another anchor blow.

The Coast Guard's proposed ban on certain ships anchoring and "loitering" encompasses the same area of the Straits as Snyder's current emergency rule, according to Jerome Popiel, the Coast Guard's incident management advisor for the Great Lakes region.

Snyder's no-anchor zone is roughly two miles wide and terminates on the western side of the Mackinac Bridge. It runs from shore to shore between the peninsulas.

The no-anchor zone established by Gov. Rick Snyder.

The Coast Guard's proposal does not state what punitive measures a company dragging its anchor in the ban zone would face. Popiel said he did not know and would return comment later.

The proposed ban applies only to the following vessels:

Ships 131 feet or longer while navigating

Towing vessels 65 feet or longer engaged in towing

Ships engaged in trade which are certified to carry 50 or more workers

Any dredge or floating plant

These vessels can anchor or loiter in the Straits only with permission from Coast Guard leadership.

Unlike the governor's ban, the Coast Guard proposal does not include language allowing anchoring in emergency situations. However, Coast Guard officials can implement temporary exceptions to the rules in situations that "include but are not limited to channel obstructions, winter navigation, unusual weather conditions, or unusual water levels," the proposal states.

Line 5 transports up to 540,000 barrels of light crude oil and natural gas liquids per day. The anchor strike dented the twin lines in three places but, according to Enbridge Energy officials, line integrity wasn't compromised.

Composite sleeves were wrapped about the dented portions to strengthen the lines. That work finished the week of July 30, according to Enbridge officials.

In a worst-case scenario Straits rupture, Line 5 would gush 2,436,000 gallons of oil, slick 437 miles of Great Lakes shoreline and cost $1.86 billion in damages, according to a study spearheaded by Michigan Technological University.

The study, released in July, is the last of five analyses requested by Snyder as he deliberates decommissioning and possible alternatives for the twin oil and gas pipelines under the Straits of Mackinac.

Snyder has set a deadline of Sept. 30 to formalize an agreement with Enbridge on the future of Line 5.