ANCHORAGE (AP) -- The trans-Alaska pipeline shifted more than a foot toward Prudhoe Bay in October at a site south of the Brooks Range after Alyeska Pipeline Service Co. took steps to stop the flow from a bullet hole.
Company officials say they have determined that the pipeline shift discovered in January occurred after the flow of oil in the line was reversed. The high rate of back flow tripped a check valve clapper, a dome-shape plug that swings down like a door on a hinge, and the pressure pushed the pipeline north.
The shift did not damage the pipeline and it remained operational. Anchor assemblies along aboveground portions of the pipeline are designed to absorb movement vertically, laterally and longitudinally.
Alyeska discovered the pipeline shift Jan. 4 during quarterly ground surveillance, spokesman Mike Heatwole said Friday.
''The amount of movement is pretty difficult to see from the air,'' Heatwole said.
The shift tripped seven anchors along a mile-and-a-half section of the line. Repairs to the anchors were completed six days later and crews used hydraulic jacks to put the pipeline back in place.
Company officials at the time said they thought the pipeline had shifted one day before it was discovered. But engineers since then determined that the shift occurred as workers prepared to plug a hole from a bullet fired Oct. 4.
An estimated 285,600 gallons of oil spewed onto the ground 75 miles north of Fairbanks before Alyeska plugged the line three days later.
The shift came about as oil flowed north and hit the clapper of Check Valve 50, located south of Pump Station 5 between Mile 275 and 280 of the pipeline.
Check valves operate one-way and prevent reverse flow of oil. Check valves are designed to be held open by flowing oil and to close automatically when oil flow stops or is reversed.
As part of the bullet hole repair, Alyeska relieved pressure on the line. The clapper on Check Valve 50 was cranked to the ''up'' position so that oil could be pumped around a remote gate valve to a relief tank at Pump Station 5.
The day after the shooting, workers opened four relief valves to create a path for the oil to the relief tank. However, during the procedure, when crude oil flowed north from Gobbler's Knob, a landmark higher than the check valve, the flow rate exceeded 1.2 million barrels a day at Check Valve 50.
The rate was so high, it sheared an actuator shear key, part of the assembly that holds the clapper in place. That caused the clapper from Check Valve 50 to drop, and the pressure shifted the pipeline 13 inches.
''That flow rate is what our engineers determined is what caused the clapper to come down,'' Heatwole said.
The check valve functioned properly, he said.
''From our perspective, all the pieces worked as they were supposed to,'' Heatwole said.
The pipeline has moved on its supports before. In April 2000 a vapor pocket in the pipeline collapsed at Atigun Pass in the Brooks Range, causing a wave of pressure that moved the pipeline.
Daniel Carson Lewis, 37, faces state felony charges of criminal mischief, assault and drunken driving as well as misdemeanor charges of oil pollution and weapons misconduct in connection with the pipeline shooting. He is accused of firing at the pipeline multiple times with a .338-caliber rifle.
Cleanup crews continue to work at the site and expect to have all contaminated material picked up in about a month, Heatwole said. They have already removed an estimated 1,300 yards of contaminated trees, vegetative material and soil for storage at Moose Creek south of Fairbanks.
More than 175,000 gallons of spilled oil has been reinjected into the pipeline.
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