Enbridge says an optimization program on its Mainline system — which accounts for about 70 per cent of Canadian oil exports to America — is expected to add 100,000 barrels per day of additional capacity by year-end.

Line 3, which was built in the 1960s and is increasingly subject to corrosion and cracking, runs from Hardisty, Alta. to Enbridge's terminal in Superior, Wisconsin, near Lake Superior.

 A Native American tribe in Wisconsin is suing Enbridge Inc in a bid to shut down an aging oil pipeline that runs across its tribal land, the latest in a string of challenges against the Canadian company's pipelines in the U.S. Midwest.

Enbridge insists the Line 5 dual pipes, which have been in place since 1953, are in sound condition and could operate indefinitely.