VANCOUVER - The National Energy Board has asked Calgary-based Enbridge (TSX:ENB) to provide documentation of improvements it has made since a massive oil spill in Marshall, Michigan two years ago.

Board inspectors visited the company's control centre in Edmonton a week ago as part of an increase in inspections the board announced following a damning report by U.S. authorities on the spill in the Kalamazoo River in July 2010.

In a letter sent Friday, the board asked for a copy of Enbridge's internal investigation into the pipeline rupture and documentation of the corrective actions it has taken.

The board also wants to know what improvements have been made to the company's in-line inspection program since the incident and it wants a copy of the control room management plan.

The investigation by the U.S. National Transportation Safety Board found a litany of errors in Enbridge's control centre that led to a spill of more than 20,000 barrels of oil into the Kalamazoo River.

According to the report, a segment of Line 6B ruptured during the final stages of a scheduled pipeline shutdown on July 25, 2010.

Even though multiple alarms were set off at the control centre in Edmonton, staff believed it was because of a change in pressure due to the shutdown and not because of a rupture. After the 10-hour shutdown was finished, pipeline operations resumed.

"Leak-detection alarms were generated, but Enbridge staff continued to believe the alarms were the result of column separation, even though the Marshall area was relatively flat, without significant changes," the report said.

More than 17 hours after the rupture, a gas utility worker finally notified the Enbridge control centre about oil on the ground.

The U.S. report concluded that, among other things, deficiencies in Enbridge's pipeline integrity and inadequate training of control centre personnel were to blame for the spill that affected more than 50 kilometres of waterways and wetlands and cost $800 million to clean up.

Since the Michigan spill, Enbridge says it has made multiple improvements to its pipeline integrity management, leak detection and control centre operations.

In an email sent to The Canadian Press last month, Northern Gateway President John Carruthers said Enbridge has now doubled the number of employees and contractors to detect leaks, added more control centre staff, improved training and technical support, and opened up a new control centre in Edmonton.

The oil giant has "revised and enhanced all procedures pertaining to decision making, handling pipeline startups and shutdowns, leak detection system alarms, communication protocols, and suspected column separations," the email says.

The company's proposed Northern Gateway pipeline from the Alberta oil sands to a tanker port on the B.C. coast is currently under review by a federal environmental assessment panel.

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  • June 18, 2012 -- Elk Point

    Enbridge Inc.'s <a href="" target="_hplink">Athabasca pipeline leaked an estimated 230,000 litres of oil</a> about 24 kilometres southeast of Elk Point, Alberta. <br></br> A member of Greenpeace cleans up a mock oil spill outside the Enbridge Northern Gateway pipeline office in downtown Vancouver, Wednesday, June 13, 2012. The mock spill was set up by Greenpeace to show the risks of spills similar to the recent one outside of Red Deer, Alberta. THE CANADIAN PRESS/Jonathan Hayward

  • June 18, 2012 -- Elk Point

    Although the spill didn't leak into any waterways, Energy Resources Conservation Board's Darin Barter said the<a href="" target="_hplink"> spill was considered "significant" in size</a>.<br></br> "Any amount of crude oil out of a pipeline is significant to us. Obviously we've had a number of pipeline incidents in the past short while and we're monitoring cleanup on them and we have a number of investigations underway."

  • June 7, 2012 -- Red Deer River

    An estimated 475,000 litres of oil <a href="" target="_hplink">spilled from a Plains Midstream Canada pipeline</a> and proceeded to leak into the Red Deer River. <br></br> Oil from a pipeline leak coats a pond near Sundre, Alta., Friday, June 8, 2012. Plains Midstream Canada says one of their non-functioning pipeline leaked between 1,000-3,000 barrels of oil. THE CANADIAN PRESS/Jeff McIntosh

  • June 7, 2012 -- Red Deer River

    Some of the oil <a href="" target="_hplink">seeped into the Gleniffer reservoir</a>, which some Albertans rely on for drinking water. Plains Midstream Canada <a href="" target="_hplink">trucked in drinking water</a> for those residing near the area.

  • May 19, 2012 -- Northwest Alberta

    Pace Oil and Gas's waste disposal line <a href=" Lake spill pegged at 22,000 barrels/6683338/story.html" target="_hplink">leaked about 22,000 barrels of a mixture of oil and water</a> 20 kilometres southeast of Rainbow Lake. The spill was discovered on May 19 by another oil and gas company.

  • May 19, 2012 -- Northwest Alberta

    The oil spill "<a href="" target="_hplink">ranks among the largest in North America in recent years</a>," the Globe and Mail wrote.

  • June 26, 2011 -- Swan Hills

    A pipeline explosion and oil leak at a Pengrowth Energy facility caused a pipeline to leak <a href="" target="_hplink">500 barrels of light, sweet crude oil into Judy Creek</a> near Swan Hills, Alberta.

  • June 26, 2011 -- Swan Hills

    Energy Resources Conservation Board spokesman Darin Barter said the <a href="" target="_hplink">leak was relatively small</a>. <br></br> "It's what we would consider a minor spill with 95 per cent of the product coming out of the pipeline being water and five per cent oil," he told CBC. "However, we're taking it very seriously, as is the company."

  • April 29, 2011 -- Little Buffalo First Nation

    Plains Midstream Canada's 45-year-old Rainbow pipeline<a href="" target="_hplink"> spilled roughly 28,000 barrels of light crude oil</a> near Little Buffalo First Nation.

  • April 29, 2011 -- Little Buffalo First Nation

    Residents, including children, <a href="" target="_hplink">reported incidents of burning eyes, stomach pains, disorientation, nausea and headaches</a>, according to the Assembly of First Nations.