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The difficulties have led to dramatic price swings for Canadian crude as production outstrips the ability of producers to ship it to market, contributing to high levels of space rationing known as apportionment on the country’s export pipelines.

Enbridge is also seeking clearance from the U.S. State Department to nearly double capacity on its Alberta Clipper line from Alberta to Wisconsin to 890,000 barrels a day. But the process, which requires amending an existing permit, has been delayed past a previous target of mid-2014, Mr. Monaco said last month.

The Line 3 replacement requires approvals from the National Energy Board and the U.S. Federal Energy Regulatory Commission.

Work on the Canadian side will cost $4.2-billion, while Enbridge’s U.S. affiliate will spend US$2.6-billion. A final capital cost is expected by April.

Enbridge commissioned Line 3 in 1968. Between 1974 and 1979, a succession of ruptures was blamed on pipe-related manufacturing defects. A 2007 rupture on the line spilled nearly 1,000 cubic metres of crude into a wetland near Glenavon, Sask., according to a Transportation Safety Board investigation into the incident.

Mr. Monaco said the pipe was identified for replacement as part of an ongoing maintenance program. The switch shaves $1.1-billion from the $6.6-billion the company had budgeted for maintenance capital between 2013 and 2017.

Companies have boosted maintenance spending on older pipelines amid a rash of high-profile ruptures. Cross-town rival TransCanada spent $376-million last year on pipeline integrity due to increased levels of inspections and pipe replacements, it said in regulatory filings. That’s up $67-million from a year ago.

“There’s been concern about pipeline leaks,” said Bob Schulz, a professor at the University of Calgary’s Haskayne School of Business. “So if I were Enbridge I would probably do very careful maintenance of the mainline to make sure I didn’t have any leaks, because that would undermine my other applications.”