Vapor pocket cause of pipeline shift

Joint Pipeline Office says anchors, supports constrained pipe, no pipeline damage found, no evidence of leakage

Petroleum News Alaska

Alyeska Pipeline Service Co. said June 20 that its engineers believe they have determined the cause of a shift in a section of above-ground pipe near Atigun Pass. The shift caused several parts of the assembly that anchors the pipe in place to move significantly and affected about a mile-long length of the pipeline.

Alyeska said the company’s engineers believe the pipeline movement occurred as a result of a pressure wave generated by the collapse of a vapor pocket in a low section of the pipe near mile post 170 during a pipeline restart on April 17.

The result of that vapor pocket collapse was discovered May 15 when an Alyeska crew conducting a routine ground reconnaissance discovered seven above-ground anchor assemblies had tripped allowing the pipe to move 21 inches.

“All of the safety mechanisms put in place to keep the pipeline from suffering severe damage worked exactly the way they were supposed to,” said Alyeska Senior Vice President Bill Howitt in a statement. Howitt said though the shift was dramatic and steel bolts were actually sheared off because of the extent of the force on the pipeline, “the support members behaved the way we designed them. They absorbed the impact and kept the pipe itself from suffering any damage.”

Alyeska said crews from Fairbanks, pump station No. 4 and pump station No. 5 were mobilized to work on the problem immediately after it was discovered. Temporary cribbing was put in place to maintain the pipe stability after the anchors were reset and the shoes those anchors sit on were returned to their correct positions. The reposition of the pipe was completed May 26.

Regular ground surveillance has been conducted in the area, Alyeska said, the last on Feb. 26. No abnormalities were discovered at that time. Regular aerial surveillance did not detect the condition.

“This incident taught us a lot,” Howitt said. “We will modify our surveillance procedures by doing everything from lower altitude surveillance flyovers to painting orange alignment stripes on the pipeline anchors to make sure the pipe is positioned where it is supposed to be. We are also in the process of reviewing other pipeline locations where the presence of a vapor pocket could lead to a similar situation.”

“The good think is we learned a lot from this incident,” Howitt said.

“The great thing is the support system for the pipeline worked exactly like it was supposed to.”

The Joint Pipeline Office said May 18 that it was closely monitoring Alyeska’s investigation and correction of the damages to the pipe supports. The force of the incident sheared bolts, moved support shoes and crushed the expendable aluminum honeycomb (energy absorbers) pipe protectors inside several pipe anchors. The JPO said that the anchors and supports constrained the pipe and that no pipeline damage was found. Realignment of the pipe can be done quickly, the agency said.

A JPO field representative dispatched to the site examined all the culverts in the area and found no evidence of oil leakage.