Alyeska Pipeline Service Co. is looking at ways to keep oil flowing through the trans-Alaska pipeline system as the amount of oil continues to drop.
Alyeska is now studying a “cold dry flow” procedure to remove water from crude oil shipped through TAPS to help manage a decline in throughput, the pipeline company’s president told a state legislative committee Feb 5.
TAPS is experiencing operating problems because of cooling of the crude oil due to the low throughput, Alyeska President Tom Barrett said. The decline is averaging about 6 percent a year because oil production on the North Slope is falling.
“We receive oil at 105 degrees (at TAPS’ pump station No. 1) but because half of the pipeline is built above ground and the transit time to Valdez is so much longer the oil is more exposed to winter temperatures,” Barrett told the Senate’s Special Committee on TAPS Throughput.
TAPS was designed to operate as a hot-oil pipeline moving 1.5 million barrels per day, Barrett said, although peak throughput actually reached 2.1 million b/d in 1988.
At high throughput rates, heat was retained in the oil by the friction of liquid against the pipe wall. With throughput now averaging about 550,000 b/d there is less volume and velocity, less friction and less heat.
At the current throughput, segments of the crude oil in the pipe can be below 32 degrees Fahrenheit during the winter months, which can cause water in the oil to freeze, Barrett told the legislators.
Alyeska is now adding heat to the oil at certain pump stations but if throughput continues to decline, adding heat may start to create other problems, he said.
“At some point, and teams are researching this now, it appears the most cost-effective approach will be to operate the line in a ‘cold dry flow’ state where water is removed from the crude before it enters the pipeline and the system runs much cooler,” he said.
Crude oil entering TAPS now contains about 2 percent water.
“Since the purpose of the heat is mainly to prevent ice formation, eliminating most of the water eliminates the need for elaborate heating systems,” Barrett said.
TAPS is now being given heat by re-circulating oil at pump stations, but the procedure costs about $80,000 per day to boost temperatures 8 degrees to 10 degrees at each pump station where it is done.
The pipeline also benefits from warm residual oil injected back to TAPS from two refineries near Fairbanks owned by Flint Hills Resources and PetroStar Inc.
The refineries take crude oil and extract parts of the oil to make jet fuel, diesel and gasoline, returning the unused residual.
Barrett said Alyeska believes it will be able to operate TAPS at throughput levels down to about 300,000 b/d by heat addition and other steps, but by eliminating water and the need to heat the reduction can be managed in a safer, more reliable manner.
In 1977, when TAPS began operating, it took four days for oil to move from Prudhoe Bay to Valdez. By 2014 it will take 18 days, Barrett said.
As the crude oil cools at a higher rate, wax accumulation in TAPS has also become a problem, he said. To deal with this Alyeska is having to increase its use of scraper pigs to one every 6 to 10 days, Barrett said.