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ANCHORAGE - Operators of the trans-Alaska pipeline hope a scraping device will push out a 20-inch diameter piece of metal that's been missing in the pipeline since December.
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A pipeline cleaning device knows as a scraper or paraffin pig broke apart inside the pipeline in December between pump stations just north of Fairbanks and near Delta Junction about 100 miles to the south.
Most of the pieces have been recovered but a stainless steel ring that holds other pieces in place has not been found.
Mike Heatwole, spokesman for Alyeska Pipeline Service Co., which operates the 800-mile pipeline, downplayed the risk to the pipeline from the ring, which has been missing for two months. He called it "not a big deal."
"From our view, there's no integrity risk associated with this," he said Saturday. "It's a concern, yes. We'd like to locate it."
An Alyeska engineer described it as losing the pop top off of a soda can down the drain of a house, Heatwole said.
The pipeline runs from Prudhoe Bay on Alaska's North Slope to Valdez on Prince William Sound, where oil tankers collect crude oil and deliver it to West Coast refineries.
At its peak in 1988, the pipeline carried 2.1 million barrels per day. With diminished production on the North Slope, the pipeline averaged 809,412 barrels per day in December.
With the lower amount of crude in the pipeline, the oil cools and paraffin drops out. Over time, the wax builds up on pipeline's 48-inch diameter walls, slowing flow.
Alyeska inserts paraffin pigs - a sort of tubes with fins - inside the pipe to scrape the walls of the pipe about every seven to 14 days. Other pigs perform other pipeline maintenance jobs, such as the detection of abnormalities in metal.
The missing 20-inch ring is an inch wide and one-eighth inch thick, Heatwole said. It's positioned in the back of the pig and holds in place the fins that scrape the pipe's walls.
Scraper pigs pushed along by the flow of oil are designed to come apart if they encounter obstacles so that the pipeline will not be plugged or oil flow slowed to a trickle.
After the scraper pig broke apart in December, Alyeska sent another one in roughly 25 to 30 says after the previous scraping. It pushed out 40 barrels of wax and most remnants of the broken pig. A barrel equals 42 gallons.
A second scraper pig sent down 15 days later pushed out 15 more barrels of wax.
A third pushed out less than one barrel, about the usual amount of wax extracted by a scraper pig, Heatwole said.
The paraffin, almost the consistency of candle wax, was loaded into 55-gallon drums and sent south by barge for processing, Heatwole said. Alyeska did not initially realize the 20-inch ring was missing.
A more aggressive cleaning tool, a disc scraper, was working its way through the line Saturday and was expected in Valdez, the pipeline terminus, on Saturday night.
"When that arrives, we'll see what it pushes in front of it," Heatwole said.
Alyeska officials hope the missing ring will be there. If not, they will strain the paraffin from the drums now moving south by barge to see if it's there.
He said the ring may no longer look like it did when it was part of the pig.
"Who knows what condition it's in?" he said. The pressure of the oil could have collapsed it, he said.
A worst-case scenario could have the missing piece of metal interfering with the performance of a check valve that stops the flow of oil in the case of a leak.
"We view that as an extremely low-risk probability item," Heatwole said.
Heatwole said the likely location for the missing ring is the lower end of the line, making it unlikely a valve would be damaged.
A safety team is trying to determine why the pig came apart in December. Its report is due later this month.