Enbridge Inc. (NYSE: ENB) said it restarted its Line 5 oil pipeline across the Straits of Mackinac in North America's Great Lakes on Dec. 5 after it was shut due to bad weather.

The company temporarily shut down the line due to high winds and nine ft-high (3 m-high) high waves, according to the Michigan Agency for Energy (MAE).

The 540,000 barrel per day pipeline from Wisconsin to Ontario carries Canadian light crude and refined products and is a key link in Enbridge's network, which transports the bulk of Canadian crude exports to the U.S.

Light synthetic Canadian crude prices weakened by 35 cents on Dec. 5, according to Shorcan Energy brokers. One market participant in Calgary, Alberta, said the dip was most likely in relation to the Line 5 outage.

In a statement on its website, the MAE said Enbridge told Michigan that Line 5 was shut down at 10:37 a.m. CST (16:37 GMT) on Dec. 5 and would restart when conditions improve.

Enbridge spokesman Michael Barnes confirmed the shutdown and said the line was restarted at 3:40 p.m. CST Dec. 5 when weather in the Straits of Mackinac improved.

The Straits of Mackinac are a turbulent 5-mile (8 km) wide stretch of water connecting lakes Huron and Michigan. Some local businesses, politicians and environmental groups fear the 64-year-old underwater pipeline crossing the Straits is at risk of leaking and contaminating the Great Lakes.

In response to those concerns, Enbridge signed an agreement with the state of Michigan on Nov. 27 to suspend the pipeline's operations during sustained bad weather in which wave heights reach more than eight feet.

"The purpose of the state's agreement with Enbridge was to find practical solutions to concerns we had about the operation of Line 5 and the safety of the Great Lakes," said Valerie Brader, MAE executive director.

"Enbridge's action today shows the steps outlined in the plan will have immediate and long-term positive outcomes," Brader said.

Under the agreement, Enbridge must also study the feasibility of placing a new pipeline or the existing Line 5 in a tunnel under the Straits, assess installing underwater technology to better monitor Line 5 and look at ways of mitigating the risk of boat anchors damaging the pipeline.

The Calgary-based company must also evaluate ways of minimizing the likelihood of an oil spill at the 245 bodies of water Line 5 crosses in Michigan.