Alyeska's pump station project will permit lower flows ________________________________________________
alaskajournal.com
Web posted Sunday, June 3, 2007
By Tim Bradner
Alaska Journal of Commerce
A $430 million pump station reconfiguration project now being implemented will allow the trans-Alaska oil pipeline to operate mechanically at throughput rates as low as 300,000 barrels per day, Alyeska Pipeline Service Co. president Kevin Hostler told a contractors' association in Anchorage May 24.
The project will also allow pipeline throughput to increase to 1.1 million barrels per day without substantial modifications, Hostler said. “This gives us the capability of operating across a tremendous range,” he said.
Alyeska brought a revamped Pump Station 9 into operation last February, with new electric-driven pumps designed for lower throughput rates, replacing 1970s-era diesel-powered pumps that are not only aged, but also not efficient at the volumes of oil now being moved.
Additional pump stations will be changed over the next two years. The project, which also involves automation of the pump stations, will be completed in 2010.
The pipeline now moves about 780,000 barrels daily, but declining production from North Slope oil fields could see throughput falling to 500,000 barrels/day over the next few years, Hostler told the Alaska Support Industry Alliance.
At the peak of North Slope production in 1988, the pipeline moved 2.1 million barrels a day, but rates have now declined to less than half of that.
In planning for lower throughput, Alyeska is focusing on technical issues created by fewer barrels of oil moving at slower speeds through the line, Hostler said. Wax buildup has already become a problem, and Alyeska is changing its pigging procedures to deal with it, but there is also an increasing amount of free wax flowing through to the Valdez terminus in the crude oil, Hostler said.
As rates drop to 600,000 barrels per day and crude temperatures drop, there will be challenges with water dropped out of the crude oil and accumulating in the pipeline.
The leak-detection system used with the pipeline, which relies on detection of changes in crude oil pressure and volumes, will also become less effective with fewer barrels moving through the system, Hostler said. Wax deposits will become an even greater problem.
Alyeska now generates power for the Valdez Marine Terminal with vapors recovered from crude oil storage tanks, and at 600,000 barrels per day throughput there might not be enough vapors. An alternate source of power for the terminal be needed, he said.
At 500,000 barrels per day, vibration could become a serious issue, particularly at Isabell Pass where the line crosses the Alaska Range. Vibrations became a problem at Thompson Pass, north of Valdez, several years ago when pipeline throughput fell below 1.4 million barrels per day. The pipeline descends a steep slope from Thompson Pass, and vibrations developed when liquids in a partially filled pipeline accelerated and then collided with slower-moving fluids at the bottom of the slope, a condition called “slackline interface” by engineers.
Alyeska was able to solve the problem with back-pressure created in the pipeline from the Valdez terminal, which has the effect of slowing the liquid velocity on the downslope.
Hostler said similar problems developed at Atigun Pass, in the Brooks Range, when crude throughput fell below 1 million barrels per day. Alyeska is now monitoring these, Hostler said.
Vibrations could pose a threat to the mechanical integrity of the pipeline, he said.
At 500,000 barrels per day, ice could also form inside the pipeline during winter. At higher volumes, heat from friction keeps the pipeline warm; but at lower volumes and slower velocity, the crude temperature will decrease. At the lower rate, it will take 22 days for crude oil to transit the pipeline from the North Slope to Valdez.
At some point, accumulated water in the pipeline could freeze, Hostler said. There could also be ice buildup outside the pipeline, which could cause damage. |
