Alyeska prepares for next big one

Petroleum News

Alyeska Pipeline Service Co. has completed a $6.5 million project to prepare the trans-Alaska pipeline to ride out another major earthquake, after it survived “the biggest slip-fault earthquake in a hundred years” on Nov. 3, 2002.

In a story in the September issue of Alyeska Monthly, the company said the pipeline’s builders anticipated the earthquake risk and built the pipeline to withstand a magnitude 8.0 earthquake in the Denali fault corridor.

The pipeline survived the 7.9 magnitude earthquake because a 2,000-foot section is on “shoes” on top of long slider beams so ground movement does not overstress the pipeline. The 2002 earthquake shifted the ground beneath the pipeline 14 feet horizontally and 2.5 feet vertically.

But, the company said, many of the slider beams moved so much in that quake that they would not protect the pipeline from another major earthquake on the Denali fault.

And even though such a magnitude quake is believed to be a 500-year event, Alyeska “reset” the pipeline because it isn’t possible, Alyeska civil project manager Tom Coghill told Alyeska Monthly, “to predict when the next big earthquake will hit. A hurricane can be predicted, but not an earthquake. We can’t let our guard down because earthquakes can come at anytime.”

Studies predict future ground movement around the pipeline will be to the west, so this summer Alyeska crews reset the fault crossing, moving 16 beams westward and extending nine of them to accommodate possible ground movement. A vertical support member that bent during the earthquake was replaced with an at-grade slider beam support, the company said.

“We did not change the original design basis of the pipeline,” said Coghill. “Since the pipeline performed so well, we didn’t want to threaten its integrity by making drastically changes.”

Alyeska said the work completed this summer should allow the trans-Alaska pipeline to move 18 feet horizontally and five feet vertically.