Five environmental groups hoping to halt a controversial oil exploration project want to block a plan to send “highly corrosive” crude oil from western Canada to Maine.

The groups are asking the National Energy Board of Canada to deny a special exemption request by a Canadian company to reverse the flow of a portion of a pipeline that would eventually deliver oil from Alberta to South Portland.

Denial of the request could disrupt Portland Pipe Line Corp.’s plan to reverse the flow of oil through one of its two 236-mile-long pipelines that run underground between South Portland and Montreal.

The company in 2008 began studying the possibility of spending $100 million on the project. But the project was put on hold in 2009 when the recession reduced demand for oil.

The project calls for transporting crude oil extracted from “tar sands” in Alberta to Montreal, then to Portland Harbor for shipping to refineries on the East Coast.

David Cyr, treasurer of Portland Pipe Line, said the company is in discussions with the oil industry to determine the level of interest in the project.

The project would boost the number of oil tankers visiting Portland Harbor. Although the port remains among the busiest oil ports on the East Coast, the number of tankers has declined in recent years.

The tankers now bring conventional crude oil from overseas to South Portland, where it is stored in tanks and then piped underground to refineries in Montreal.

Portland Pipe Line is using only one of its two pipelines. The plan calls for using one to maintain the flow of oil from South Portland to Montreal and using the other to send oil from Montreal to South Portland. The project already has permits.

The pipeline was built in 1941 to safely transport oil to Quebec at a time when German U-boats were patrolling the western Atlantic. It now pumps about eight million gallons of oil daily. In recent years, as oil fields in Alberta have increased production, Canada has become less dependent on foreign oil.

Three Canadian environmental groups, plus the Natural Resources Council of Maine and the Natural Resources Council of Vermont contend that the Canadian company Enbridge plans to split its pipeline project into pieces — across Canada and then through the pipeline from Quebec — to bypass regulatory and public scrutiny.

Enbridge operates Canada’s longest and most complex crude oil pipeline system. Cyr said Portland Pipe Line’s project can’t happen unless Enbridge reverses the flow of oil in a portion of its system.

According to The Gazette, a newspaper in Montreal, Enbridge sought an exemption this month for a $17 million project to reverse the direction of crude-oil flow in the section of pipeline in Ontario.

Documents say the project is necessary “to meet business demands of shippers.”

Pete Didisheim of the Natural Resources Council of Maine said the company’s request is the first step in bringing highly corrosive tar sands oil to Maine.

Critics say huge amounts of energy are needed to extract tar sands oil, which could cause an ecological disaster in a spill.

Didisheim said his group is trying to prevent Portland Harbor from becoming the “tar sands capital of the eastern United States.”

The issue has been controversial nationally. The Obama administration upset environmentalists last week when it approved a separate, 1,700-mile pipeline that will send oil from the tar sands of Alberta to the Texas coast.

Tar sands crude is thicker, more corrosive and abrasive and less stable than conventional crude oil, and requires higher pressure for transport through pipelines, increasing the chance for leaks or ruptures, Didisheim said.

But Cyr said the oil flowing from Canada to Portland would be similar to oil that has been flowing through the system safely for many years.

Portland Mayor Nicholas Mavodones said the project would boost Portland Harbor — more ship traffic would create more work for the harbor’s pilots, ship chandlers, tugboats and repair facilities.

“Having more ships coming and going in the harbor would be very good for the harbor,” Mavodones said. “It would create more jobs.”

The tankers are critical to the harbor because they cover the fixed costs for the infrastructure that supports other maritime industries, said Jack Humeniuk, business agent for the International Longshoremen’s Association Local 861.

Staff Writer Tom Bell can be contacted at 791-6369 or at:

tbell@mainetoday.com