Pipeline anchors shift south of Atigun Pass

Cause of incident not determined; force significant enough to shear bolts, move support shoes, crush expendable aluminum honeycomb

Petroleum News Alaska

Pipeline supports on the trans-Alaska pipeline south of Atigun pass were damaged when the pipeline shifted due to unknown causes over the winter.

Alyeska Pipeline Service Co. said May 16 that six pipeline anchors near milepost 170 south of Atigun Pass moved causing some pipeline support shoes to move from their normal position on the horizontal cross beam which is part of the above-ground support system.

The displaced anchors were discovered May 15 by Alyeska maintenance crews conducting a routine visual inspection of the pipeline. The company conducts ground and aerial surveillance on the pipeline regularly. No abnormalities were discovered during the prior ground surveillance Feb. 26.

The Joint Pipeline Office said May 18 that the cause of the incident has not been determined but the force was significant enough to shear bolts, move the support shoes and crush the expendable aluminum honeycomb (energy absorbers) pipe protectors inside several pipe anchors. JPO said that the anchors and supports constrained the pipe. No pipeline damage has been found and realignment of the pipe can be done quickly.

A JPO field representative said that there was still about five feet of snow in the area but it was being cleared for wider examination of the pipe and support members; there were about 30 workers on site. The JPO representative examined all the culverts in the area and found no evidence of oil leakage.

Alyeska said temporary cribbing would be constructed to protect the pipe until the anchors are reset and the shoes returned to their correct position.

Alyeska spokesman Tim Woolston said the pipeline moved between 3 and 6 inches.

As designed, the bolts that hold the pipe to supports sheared off, allowing the pipe to move without breaking.

"Everything worked as designed," Woolston said. "Now, we're all very curious about why this happened."

The pipeline shift happened on the south side of Atigun Pass, where the pipeline levels off after the steep descent from the 4,739-foot pass that cuts through the Brooks Range.

Engineers have not determined what caused the shift. But unique conditions created by oil falling steeply from the pass have caused the pipe to shudder and vibrate in the past and will be examined, Woolston said.

The shift poses no immediate risk to the strength of the pipe and the flow has not been affected, Woolston said.

Alyeska says the pipe has never shifted off its supports since start-up in 1977. However, a similar incident happened during a test before start-up, Woolston said.

The pipeline is buried for about half its length, and 78,000 supports prop up the pipe where it runs above ground. The pipeline is designed to adsorb shifts due to earthquakes and other forces.

—The Associated Press contributed to this story