Trans-Alaska oil pipeline withstands big quake

Posted: Sunday, November 10, 2002

ANCHORAGE, Alaska (AP) -- 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.

Sunday's 7.9 magnitude earthquake -- the worst ever recorded on the Denali fault in Alaska's Interior -- struck in a sparcely-populated area 90 miles south of Fairbanks, the state's second largest city.

The earthquake -- considered by seismologists as 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.

''I think what it tells you we had a super group of people 30 years ago ... that did an absolutely wonderful job of designing it,'' said Alyeska CEO and President David Wight.

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

''We can talk about a crack, opening up part way or a total guillotine break. The objective is not to spill oil,'' Nyman said. ''I'm very, very pleased that it really worked as it should have.''

If anything, Sunday's powerful earthquake shows 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 -- the site of Sunday's earthquake -- and near Valdez, the end of the 800-mile pipeline where tankers are loaded for the Lower 48.

The fault near Valdez is the one that caused the March 27, 1964, 9.2 magnitude quake that was the strongest earthquake ever recorded in North America.

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.

Sunday's quake moved the pipeline 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.

The Denali fault is one of the longest strike-slip faults in the world, similar in size to California's San Andreas fault, according to the U.S. Geological Survey. Strike-slip faults produce shallow earthquakes as one plate moves horizontally against another.

Before Sunday, seismologists didn't think the Denali fault could produce a really big earthquake, even though the fault had historically been very active, said Roger Hansen, state seismologist for the Geophysical Institute of the University of Alaska.

''Indeed it was a sleeping giant so far,'' he said.

And it hasn't been quiet since. Seismologists have recorded more than 550 aftershocks, the strongest being a 5.8 magnitude quake 20 minutes after the initial jolt.

Engineers found the right kind of 48-inch diameter pipe in Japan. Nyman said they were looking for pipe that had good elastic properties so it could expand under pressure and return to its original form when released. And it had to have the capacity to be deformed and not break.

Some 70,000 welds, all of which were x-rayed, also had to be strong enough to work with the properties of the pipe.

The pipe was tested at the University of California at Berkeley in 1972. Huge frames were set up where eight sections of pipe were laid in vertically and loads were applied to the ends and sides.

Enough pressure was applied so that the pipe wrinkled, buckled and then folded on itself, and it still held pressure, Nyman said.

Damage to the pipeline from Sunday's quake 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 that support the pipeline and at least two vertical support members. Nine anchors that help restrict the horizontal movement of the pipeline were tripped and a honeycomb of insulating material was crushed in several places, but the pipeline was not dented, Lusher said.

''Generally speaking, the pipeline is in a cocoon of energy absorption components,'' Lusher said. ''I'm pleased the system functioned as designed.''

In the next couple of weeks, Alyeska will run a device called a ''pig'' through almost all the pipeline to check for any dents, cracks, gouges and small underground leaks.

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 at about 2 a.m.

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On the Net: www.aeic.alaska.edu

www.alyeska-pipe.com

www.jpo.doi.gov/index.htm



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