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Alaska Pipeline Passed Big Test

It was damaged but not ruptured in last week's 7.9-magnitude quake, to engineers' delight.

November 10, 2002|From Associated Press

ANCHORAGE — The trans-Alaska oil pipeline was designed to withstand an 8.5-magnitude earthquake, but could it?

When a team of structural and geo-technical engineers came up with the pipeline design in the early 1970s, they didn't expect it to be tested in their lifetimes. They were wrong.

The 7.9-magnitude earthquake on Nov. 3 -- the worst ever recorded on the Denali fault in Alaska's interior -- struck in a sparsely populated area 90 miles south of Fairbanks.

The earthquake -- considered a once-every-600-years event -- left a 145-mile crack across the landscape and was the first significant quake to test the pipeline's mettle.

The pipeline withstood the powerful quake just as designed -- damaged but not ruptured, said Doug Nyman, a consulting engineer in Houston who from 1973 to 1977 was the pipeline's seismic design coordinator for Alyeska Pipeline Service Co., which operates the pipeline.

The $8-billion pipeline, completed on May 31, 1977, delivers approximately 17% of the nation's domestic oil. Nearly 14 billion barrels of oil have moved through the line.

If anything, last week's powerful earthquake shows that the pipeline could have withstood more, Nyman said.

One of the earliest tasks was to map the potential for seismic activity along the pipeline route, which crosses three mountain ranges, 800 rivers and streams, and three active faults.

Nyman said some of the country's top seismologists and geologists were brought together. They determined that the greatest potential for earthquake damage was where the pipeline crossed the Denali fault and near Valdez, the end of the 800-mile pipeline.

At the Denali fault, engineers designed the pipeline to move if the earth moved. It was laid close to the ground on a gravel berm and is supported by shoes that slide on beams. It allows movement of 20 feet horizontally and 5 feet vertically.

"It allows for the fault to rupture. The pipe is floating on these beams. Those shoes accommodate the new geometry to the ground," Nyman said.

The pipeline moved 7 1/2 feet horizontally and 2 1/2 feet vertically. "Compared to what happened, we still have a lot of capacity there," Nyman said.

Quake damage to the pipeline was right in line with what was expected, said Jim Lusher, engineering manager for the Joint Pipeline Office in Anchorage, a consortium of 13 federal and state agencies that oversees the pipeline.

The shaking broke five aboveground cross beams and at least two vertical support members. Nine anchors that help restrict horizontal movement were tripped and a honeycomb of insulating material was crushed in several places, but the pipeline was not dented, Lusher said.

Alyeska pressure-tested the line for big leaks before restarting it Wednesday morning after a 66-hour shutdown. The first tankers were loaded with oil early Friday morning.

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