BP PLC said high winds were to blame for a power outage that shut down Prudhoe Bay in northern Alaska. Production fell to about 20,000 barrels Tuesday; about 350,000 barrels were produced Monday.
Flooding near the terminus of the pipeline, caused by heavy rain in south-central Alaska, is suspected of knocking out fiber-optic communication lines along the pipeline, said Mike Heatwole, spokesman for Alyeska Pipeline Service Co.
Operators lost communications to remote valves that can be closed in the event of a spill.
Heatwole said company protocol calls for the pipeline shutdown when valves cannot be closed from long distance. The valves must be staffed by crews that can manually operate the valves, he said.
At Prudhoe Bay, BP spokesman Daren Beaudo said, layers of dust and dirt blown by high winds built up on high voltage insulators on power lines and the field, causing a short just before 3 a.m.
"The whole field came down," Beaudo said.
The power station continued to operate. "It's the distribution system that had the problem," he said.
Beaudo said crews would work Tuesday to wash insulators, restore power and ramp up production. He could not predict whether the work would take more than one day.
Communications are a critical component for operations of the pipeline, which carries nearly 17 percent of the domestic U.S. oil supply daily.
"We lost communication with five of our remote gate valves just north of Valdez at about 4 a.m. Alaska time," Heatwole said.
The remote valves are important when there is a pipeline leak. They are closed to limit the amount of oil dispatched from sections of the line.
"When we lose communication, we shut the pipeline down," he said.
Flooding and mudslides along the Richardson Highway, which parallels the pipeline and is the only roadway out of Valdez, disrupted vehicle traffic.
The Weather Service said 6.5 inches of rain fell Sunday and Monday at Valdez.
High water along other roads in Valdez was hampering Alyeska's ability to staff the Valdez Marine Terminal, where oil is loaded onto tankers. The terminal is across Port Valdez from the city and a road leading to it was affected by flooding.
The entire Prudhoe Bay oil field had produced more than 400,000 barrels a day — or 8 percent of total U.S. output — until leaks and the discovery of pipe corrosion led the company to begin shutting down the eastern half of the field Aug. 6.
The eastern side of the field was restarted late last month as the company began to clean out the eastern transit pipeline.
BP workers this week are continuing to scrape and clean Prudhoe's east side transit line with devices called maintenance pigs. The work follows intensive ultrasonic inspections and other sound wave tests.
Next, workers will put a smart pig that uses ultrasound through the line to check for thin spots. BP officials declined to say exactly when that would occur, only that they were working on a two-week maintenance effort that began Sept. 30.
BP has said it ultimately will replace 16 of 22 miles of transit lines. It expects to get replacement pipe by the end of the year, with construction beginning early next year.