Two oil pipelines at the bottom the waterway linking Lakes Huron and Michigan will get additional support structures to help prevent potentially devastating spills, officials said Thursday.

Michigan Attorney General Bill Schuette and Dan Wyant, director of the Department of Environmental Quality, said they had put Enbridge Energy Partners LP on notice following the company's acknowledgement it was partly out of compliance with an agreement dating to 1953, when the pipelines were laid in the Straits of Mackinac.

As a condition of an easement granted by the state, Enbridge agreed that support anchors would be placed at least every 75 feet. In a response last month to a lengthy series of questions about the condition of the lines from Schuette and Wyant, the Canadian company acknowledged some sections don't meet the requirement, although the average distance between supports is 54 feet.

"We will insist that Enbridge fully comply with the conditions of the Straits Pipeline Easement to protect our precious environmental and economic resources and limit the risk of disaster threatening our waters," Schuette said.

Enbridge spokeswoman Terri Larson said the company had agreed to add more supports, even though engineering analyses peer-reviewed by experts at Columbia University and the University of Michigan concluded previously that gaps of up to 140 feet between supports would be safe. The work will begin in early August and be completed within 90 days, she said. Afterward, the average distance between supports will be 50 feet.

"The Straits of Mackinac crossing has been incident-free since it was constructed in 1953," Larson said. "Through even greater oversight, the use of new technology and ensuring all risks are monitored and where necessary mitigated, Enbridge is committed to maintaining this incident-free record into the future."

The two pipelines are part of the 1,900-mile Lakehead network, which originates in North Dakota near the Canadian border. A segment known as Line 5 runs through northern Wisconsin and Michigan's Upper Peninsula before ducking beneath the Straits of Mackinac, then continuing to Sarnia, Ontario.

The line divides into two 20-inch pipes beneath the straits at depths reaching 270 feet and carries nearly 23 million gallons of crude oil daily. The 5-mile-wide straits area is ecologically sensitive and a major tourist draw.

A June report by hydrodynamics specialist David Schwab of the University of Michigan Water Center concluded that because of strong currents, a rupture of the pipeline would quickly foul shorelines miles away in Lakes Huron and Michigan.

Larson said Enbridge began installing steel anchors for the underwater lines in 2002, replacing sandbag supports. They consist of 10-foot-long screws augured into the lakebed on either side of the pipes, holding a steel saddle that provides support. No washouts have been seen during inspections since then, she said.

Schuette and Wyant said their staffs are still reviewing Enbridge's responses to other questions about the pipelines.

Enbridge Energy Partners is a unit of Calgary, Alberta-based Enbridge Inc.