WALHALLA (AP) - North Dakota's state forester says state officials made it clear to the company planning a crude oil pipeline through eastern North Dakota that the Pembina Gorge area should be protected.

"We were concerned that anything that would be done wouldn't have an effect on the forest," Larry Kotchman said. "We have to protect that resource."

The North Dakota State Forest Service acquired the 432 acres of Pembina Gorge land in 1970 that now is Tetrault Woods State Forest.

In May 2006, the Forest Service started to negotiate with representatives of TransCanada Keystone Pipeline, Kotchman said.

"We made it clear early on that we wanted this horizontal, directional drilling," he said. "We're pleased that the Keystone folks are moving in this direction."

TransCanada plans to build a 218-mile stretch of pipeline through eight North Dakota counties. The pipeline would carry an estimated 590,000 barrels of crude oil a day from Alberta to refineries in Illinois and Oklahoma. In North Dakota, it would run from near Walhalla to near Cogswell.

Kotchman said the proposed site of the river crossing is in the narrowest area of the valley. "So you can cross the valley fairly quickly," he said.

L.A. "Buster" Gray, the TransCanada Keystone Pipeline project director, said horizontal drilling technology evolved from the oil industry in the early 1980s. In Pembina County, it involves setting up a drill rig on the south side of the Pembina River.

A 4- to 6-inch-diameter pilot hole is drilled at an inclined angle, 25 to 30 feet below the surface of the ground and river. The drill has a guidance system similar to a GPS navigational system that monitors and corrects the course as it proceeds to the north side of the river.

"These tools have become extremely accurate over the years," Gray said.

Sections of 30-inch pipe are hooked to the drill head and pulled on rollers through the hole, under the river, to the other side a distance of 3,410 feet. Bentonite clay is used to fill the hole around the pipeline.

Gray said horizontal drilling is commonly used on a much smaller scale by city utility departments to pull utility lines under streets and sidewalks.

He anticipates about 15 workers will be involved in making the hole to feed the pipe under the Pembina Gorge.

The pipeline construction resembles a 50-mile-long assembly line that moves about 1.25 mile per day.

"If you start at the Canadian border, the last one doesn't start until the first one is almost 50 miles away," Gray said. "You kick off one crew. Then, maybe 3 or 4 days later, you start the next crew. You get some flavor for the duration you'd be in the county."

The Keystone Pipeline will be buried 4 feet underground through most of its 2,848-mile route from Hardisty, Alta., to Illinois and Oklahoma. Conventional construction requires digging trenches in which to place the pipeline.

TransCanada considers a variety of factors in determining whether to use conventional or horizontal drilling methods of building a pipeline.

"In our industry, we have typically used these techniques for very large rivers and streams like the Missouri River, scenic areas and environmentally sensitive areas," Gray said.

The cost of horizontal drilling is estimated at about $500 a foot for a conventional 30-inch pipeline, buried 4 feet underground, compared to about $100 a foot for the conventional digging method. Soil conditions do not always allow horizontal drilling.

The North Dakota Public Service Commission and the state Health Department requested that TransCanada use horizontal directional drilling to cross the Sheyenne River near Fort Ransom State Park, as well as through the Pembina Gorge area.

"We're doing the geotechnical work at the river, to assess whether the work can be done that way. If it turns out that we can, we would do it," Gray said.

Gray estimates it will cost about $500 million to build the pipeline through North Dakota. That is $200 million more than initial estimates, which were done in 2005.

Construction could begin in May if the Public Service Commission approves a route permit.