State regulators decided Thursday to take a deeper look at the environmental effects of crude oil pipelines planned across northern Minnesota.

In procedural decisions on a pair of pipelines — one to deliver North Dakota crude oil, another to import Canadian crude — the Minnesota Public Utilities Commission set the stage for an expansive environmental analysis that supporters fear will delay the projects.

"We already know it is going to be gigantic," commission Chairwoman Beverly Jones Heydinger said of the planned environmental study.

The effort was compared to PolyMet Mining Corp.'s proposed northern Minnesota copper mine. A 3,500-page environmental report on that project was released in November — after 10 years of work.

Calgary-based Enbridge Energy, which proposes the two pipelines, wants the reviews done in time to begin construction in 2017. Company officials are concerned that delays in the studies could wreck that schedule.

"We intend to do extensive modeling of spill deposition and what happens at key, representative points — what would happen with a small- or medium-sized leak or a large leak," said Bill Grant, deputy commission for energy in the state Commerce Department, which will oversee the environmental studies.

Grant told regulators that the department will seek expert help from state environmental agencies and outside consultants. He defended the department's earlier environmental studies after pipeline opponents raised doubts about its expertise. "You are talking about people who have decades of experience in environmental review matters," he said.

For the North Dakota pipeline, known as Sandpiper, as well as the replacement of a line from Canada, called Line 3, the commission authorized full Environmental Impact Statements (EIS). The process requires draft and final versions of the study — steps that will be layered on top of the usual regulatory review for energy projects.

Although Enbridge supported the basic approach, delays of even a few months could be a problem, the company's attorney said. The commission declined to set a completion deadline of January 2017 that Enbridge requested.

"[I]n the Minnesota construction season you are talking about potentially the difference between being able to be in the field for the construction season in 2017 — or missing it," said Christina Brusven, an attorney for Enbridge, who noted that Enbridge applied to build Sandpiper two years ago.

Regulators had little choice but to do an EIS, at least for the Sandpiper project. The state Appeals Court in September threw out the PUC's initial approach as a violation of state law. On Tuesday, the Minnesota Supreme Court declined to review that decision calling for an EIS.

"We got here because we short-circuited the process," said Kathryn Hoffman, an environmental attorney who had urged the PUC to do an EIS. "An EIS should have been ordered, and it was not — and we lost a year."

Pipeline critics, including the White Earth Nation, Friends of the Headwaters, a Park Rapids-based group, the Sierra Club and Honor the Earth, founded by activist Winona LaDuke, are fighting both pipelines, fearful of the risks to pristine northern waters and wild rice lakes.

Some groups have proposed entirely different routes for the line — paths that Enbridge has said are longer, more expensive and have their own environmental issues. The extent of the environmental review, including what alternate routes should be examined, will be the subject of a "scoping" process.

"What we are talking about are two large energy pipelines that are going to go through a relatively new corridor," said Joseph Plummer, attorney for the White Earth, which has asked to participate in the environmental study. "We need to take a serious look at this because this is bound to become the new main line."

Construction unions opposed environmentalists' study demands.

"This is about delay," said Ellen Boardman, an attorney for United Association of Journeymen and Apprentices of the Plumbing and Pipe Fitting Industry.

The $2.6 billion Sandpiper project would carry North Dakota crude oil to Superior, Wis., where Enbridge has a terminal and links to other pipelines serving refineries in the Midwest and elsewhere. The 210-mile path through Minnesota would go to Clearbrook, Minn., where Enbridge has another oil terminal, then cut south toward Park Rapids before turning east partly along a power line.

Separately, Enbridge has proposed a $7.5 billion project to replace the nearly 50-year-old Line 3 from Alberta to Superior, including a 364-mile U.S. segment mostly in Minnesota, estimated to cost $2.6 billion alone. From Clearbrook to Superior, it's proposed on the same route as Sandpiper.

That 36-inch-diameter line — even larger than Sandpiper — has a history of leaks and currently operates about half its capacity as a precaution. The replacement project would restore the line to its 760,000-barrel-per-day capacity.

Brusven said Line 3 suffers from "very significant corrosion and long-seam cracking." If the PUC process drags out, Minnesota could be left with the only remaining old sections of Line 3 because other parts are to be completed in 2017, she said.