By Tim Bradner
Alaska Journal of Commerce

Alyeska Pipeline Service Co. is downplaying reports that recently detected vibrations on the trans-Alaska oil pipeline could cause problems for the 800-mile pipeline. There have been vibrations on the pipeline detected at Atigun Pass and Isabel Pass, spokesman Mike Heatwole said, but the condition has been detected before, is being monitored and is not considered a threat to the integrity of the pipeline.

"It's not a big surprise to us. We've been studying vibration issues since 1997 and did so again two years ago when our throughput dropped below 1 million barrels per day, and in August in connection with the reduction of oil production from the Prudhoe Bay field," Heatwole said. "It's a condition we monitor as to the stress it places on metal, but it's a long-term, 15- to 20-year issue," he said.

Rhea Dobosh, spokeswoman for the federal-state Joint Pipeline Office, said the JPO has had discussions with Alyeska for several years on vibration issues, and asked the pipeline company recently for an analysis of future problems if pipeline throughput continues to decline. The report is due in early December, she said.

Heatwole said the condition is created by the low flow of crude oil through the system. When the pipeline crosses steep mountain passes like Atigun Pass, in the Brooks Range on the northern half of its route, the oil accelerates when the pipeline comes down a steep slope, he said. When it encounters slower-moving oil at the bottom of the slope the impact, a condition Alyeska calls "slackline interface" has caused vibrations of a quarter-inch to a half-inch, Heatwole said. This fall, when BP shut down the Prudhoe field and oil throughput dropped to just more than 500,000 barrels per day, the vibration effect was also noticed at Isabel Pass, on the southern half of the pipeline between Fairbanks and Valdez, the southern terminus.

In 1996 and 1997 Alyeska monitored vibrations on the south side of Thompson Pass, just north of Valdez. The company installed a back pressure control system, which controls the velocity of liquids, to stop the vibrations, Heatwole said.

Tim Bradner can be reached at tim.bradner@alaskajournal.com.