ANCHORAGE - The sharp reduction of crude flowing through the trans-Alaska oil pipeline after a partial shutdown of the nation's largest oil field caused a steep section of the line to vibrate for weeks, the operator said Monday.
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The sporadic vibrations at Isabel Pass in the Alaska Range were prompted when daily North Slope production dipped below 600,000 barrels a day after the August shutdown of eastern Prudhoe Bay, said Mike Heatwole, a spokesman for Alyeska Pipeline Service Co. The pulsation in the line is caused when the reduced volume of crude rushes down a steep incline.
"The vibrations there were very minimal," Heatwole said.
North Slope production is currently at 750,000 barrels a day, of which 350,000 barrels are from Prudhoe Bay. Heatwole said crews actively monitored Isabel Pass when the flow was below 600,000 and the last visual inspection Oct. 1 showed no activity.
"There does remain a potential for very infrequent and minor vibrations," he said.
The eastern side of the gigantic field had been producing a daily average of 200,000 barrels until a leak and corrosion were found in a transit line that carries market ready oil to the 800-mile trans-Alaska pipeline. The entire Prudhoe field, operated by BP PLC, normally produces up to 450,000 barrels of petroleum products a day, slightly more than half the North Slope production.
Heatwole said vibrations are nothing new as oil reserves diminish on the North Slope. They've been noted on steeper descents of the 48-inch diameter pipe.
"It's part of a condition called slackline, where the oil comes up and over the pass and the pipeline is not full at that junction," he said. "As the oil goes down the pass it speeds up a little. Vibrations are caused when this faster moving oil catches up with the oil" farther down the pipeline.
The problem was first noted in the mid-1990s when daily production fell below 1.4 million barrels. The vibrations were discovered at a section running through the Chugach Range at Thompson Pass, the steepest drop in the entire line. In response, Alyeska installed a system that essentially slowed the rate of flow in that section, mimicking a full line, Heatwole said.
When production dipped below 1 million barrels in recent years, the vibrations began occurring in the second steepest section, at Atigun Pass in the Brooks Range. Heatwole said Alyeska began actively monitoring a 360-foot section there last year and pipeline engineers are analyzing data to determine a fix, such as a system like the one employed at Thompson Pass.
Vibrations were among concerns Alyeska immediately addressed when BP first announced it planned to shut down all of Prudhoe Bay while it corrected the pipeline corrosion.
BP ultimately decided to keep the western side of the field open and in late September restarted partial production on the eastern side. By that time, Alyeska already had considered the impact a greatly reduced flow would have on mechanics of the line as well as the crude, which moves more slowly in smaller quantities, dropping in temperature and leaving a paraffin buildup.
"We always knew there would be reduced throughput long term," Heatwole said. "The shutdown announcement accelerated a lot of our thinking, but we're not starting at ground zero."
The vibrations at Atigun Pass have jostled the line as much as a half inch, but closer to a quarter inch on average, he said. The effect was similar at Isabel Pass, but occurred less frequently.
There's no risk for damaging pipeline supports and beams, according to Heatwole. The pipeline was designed to absorb energy and move under conditions including earthquakes.
"It's important to stress that this is something our engineering staff was well-aware of and predicted," Heatwole said. "There's no immediate threat to the line. It's a long-term challenge."
Meanwhile, BP workers continued to scrape and clean Prudhoe's east side transit line with devices called maintenance pigs, a company spokesman said Monday. 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.
"We're getting very close," spokesman Daren Beaudo said. "We're not going to tip our hand as to when it'll happen. We'll be glad to talk about it when it does."