Sinkholes surface along Keystone route
PEMBINA - A small series of sinkholes -- some 30 to 40 feet deep that have swallowed a handful of 20- to 30-foot pine trees -- developed this spring in the sandy soil of the Pembina Escarpment along the TransCanada Keystone Pipeline route, limiting access to a spectacular panoramic view of the Pembina Gorge from a North Dakota Forest Service lookout.
The first sinkhole was discovered in March on the pipeline right-of-way along the Cavalier-Pembina county line.
Keystone crews are cleaning the site now and an assessment team is working on an environmental restoration plan to repair and reclaim the environmentally sensitive area, which includes the Tetrault Woods State Forest, according to State Forester Larry Kotchman.
The North Dakota State Forest Service acquired the 432 acres of Pembina Gorge land in 1970 that now is Tetrault Woods State Forest.
So, when the Forest Service learned that TransCanada wanted to build its pipeline through the gorge and through a portion of Tetrault Woods, Kotchman and others insisted that the pipeline be built beneath the Pembina River without disturbing it.
Horizontal drilling was the answer, even though it is much more expensive than conventional pipeline building.
Last year, company officials estimated the cost of digging the 30-inch pipeline at $100 per foot, increasing to $500 per foot for horizontal, directional drilling that was done under the Pembina River and under the Sheyenne River near Fort Ransom State Park.
"This is something that absolutely has to be fixed, and it'll be done safely and so that it is environmentally sound," Kotchman said.
A portion of the site is on a 15-acre pine tree plantation the State Forest Service planted in the mid-1970s. Fire burned an area adjacent to the plantation last year.
One sinkhole stretches across the main dirt access road to the Forest Service outlook, which is slated to be developed as a scenic overlook by an economic group in nearby Walhalla, a town of 1,100.
Officials have identified seven sinkhole areas along the north escarpment, high above the Pembina River, where Keystone Pipeline crews used the horizontal drilling method last year to bury its 30-inch pipeline a minimum of 4 feet in the ground.
"This wasn't totally unexpected," Kotchman said. "That area has some pretty sandy soils, and this can happen with horizontal drilling. It's something of concern, but we're not overly worried."
The Keystone Pipeline is being built through 218 miles of North Dakota soil, along a 2,148-mile route from Hardisty, Alta., to U.S. markets at Wood River and Patoka, Ill., and to Cushing, Okla.
Much of the construction through North Dakota and South Dakota was completed in 2008.
Besides the environmental reclamation at the Pembina Gorge, crews will spend this summer testing tie-in welds and hydrostatic tests, as well as cleanup and other reclamation work, according to Jim Prescott, a Keystone representative. He added that all work in North Dakota is expected to be completed by late August or early September.
When completed late this year or in 2010, the pipeline will move about 435,000 barrels of heavy crude oil daily, increasing to 590,000 barrels.
Here's how horizontal drilling works and how it was applied in the Pembina Gorge:
Crews set up a drill rig on the south escarpment above the Pembina River.
A small, 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 contains 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.
It may take several attempts, widening the hole each time, until it is 38 to 42 inches in diameter. Other steps may be taken to ensure that the hole is solid, with no obstructions.
Once it is completed, the 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.
Officials believe the problem began when drilling fluids used in this method mixed with the unstable, sandy soil in the escarpment.
The Pembina Gorge area, like many areas of North Dakota, had an extremely wet spring. In Walhalla, the Pembina River hit its second-highest crest in history. The record flood was in 1950.
One of the challenges with reclamation is that the land in the escarpment contains several layers of different types of sandy soil.
"They have to find fill that's exactly the same, to match the layers the best they can," said David Nowatzki, Forest Service riparian forester based in Walhalla.
The sinkhole area is fenced off and restricted. Crews are working on both state-owned and privately owned land.
"They're working real well with us," Kotchman said. "We're confident this will be taken care of and there won't be any environmental problems."