Alyeska studies how to operate TAPS at low flow
Alyeska Pipeline Service Co. has launched studies on ways it can operate the Trans-Alaska Pipeline System at lower oil throughput levels and at colder temperatures.
As production drops in North Slope fields, the company is working on ways to mitigate potential problems with wax and the freezing of water in winter.
Dan Roberts, Alyeska’s flow assurance manager, said the pipeline company is now completing a test loop facility at the University of Tulsa, a small-scale model of the 48-inch TAPS, to simulate pipeline operating pipeline conditions at low levels of oil flow. Results are expected in late 2014, Roberts said in an interview.
The pipeline is now moving about 540,000 barrels per day, or b/d, on average and Roberts said it is now equipped to handle a flow down to about 500,000 b/d without further modifications.
That threshold is not that far off, however. With North Slope fields currently declining at about 6 percent yearly, production could be averaging 500,000 b/d in two years, according to estimates by the Alaska Department of Revenue.
Alyeska spokesman Michelle Egan said the pipeline company is pursuing two paths to deal with lower flow: one being to add more heat to the pipeline in winter, in addition to heat now being added, and the second is to test whether removing water from the crude will allow TAPS to operate at colder temperatures and lower flow rates.
Alyeska is already adding heat to the oil at four pump stations by recirculating oil through pipes. This was a step taken after an unexpected midwinter shutdown in 2011. The company is now considering a plan to add additional heat at Pump Station 5, Roberts said. Adding heat is only needed in winter, from October to April, Roberts said.
Alaska winter temperatures can drop to 60 degrees below zero Fahrenheit in the state’s Interior region. Alyeska works to keep the average temperature of the crude in TAPS at 40 degrees Fahrenheit or above, to prevent water dropping out and forming ice. Oil now enters TAPS at 105 degrees Fahrenheit at Pump Station 1 on the North Slope but the temperature drops rapidly because of the slow velocity of the oil.
TAPS was designed with insulation to retain heat in the oil, but while it once took only four days for crude to get from the North Slope to Valdez, it now takes two weeks, Roberts said.
Heat is now added at Pump Stations 3 and 4 south of Prudhoe Bay on the slope and at Pump Stations 7 and 9 in Interior Alaska. Heat is also added at the Flint Hills refinery near Fairbanks, which takes crude oil from TAPS and returns hot residual oil to the pipeline. If the project is done to add more heat at Pump Station 5 it would have to be done through typical combustion heaters using diesel fuel, Roberts said.
Meanwhile, the studies on water removal could point to a way TAPS could be operated at temperatures below 32 degrees Fahrenheit.
Alyeska’s specification for water in its crude is now 0.35 percent and the actual average is about 0.15 percent, Roberts said. There can be slugs of water coming in from North Slope field pipelines, however, with water sometimes 1 percent or higher in the oil, he said.
“If we can get enough of the water out we may be able to let the pipeline run colder,” Roberts said. “The flow loop testing at the University of Tulsa facility will identify how low the water content must be.
“Our modeling shows we could operate at much lower temperatures,” but there are other problems that compound the issue, like the wax.
It’s not now known just how cold the crude could be and still flow, and the tests in Tulsa will help answer the question. In theory, North Slope crude oil would not “jell up” and become immovable until the oil temperature is about minus-20 degrees Fahrenheit, but in reality problems would develop before it reached that level, Roberts said.
Wax is a big problem looming for the future, Roberts said. As velocity of the oil in the pipe continues to slow more wax drops out of the crude. Some of it coats the inside of the pipe and has to be removed by pigging, which requires more maintenance pigging procedures.
A “pig” is a device that is inserted into the pipeline to move along, pushed by the oil. Pigs are used for wax removal and also, with special “instrument pigs,” to do inspections of the pipe walls.
Alyeska must now launch a pig for wax removal from the pipe walls every six days, so at a given time there are three pigs in the pipeline, Roberts said.
However, other wax drops out of the oil and can accumulate as sludge at low points in the pipeline, Roberts said.
At some point the sludge could accumulate to a point where it could be difficult to clean out with pigs moving at slower velocity. Alyeska’s studies show that problems with managing the wax accelerate when the oil velocity drops to about 1 foot per second, which would occur if throughput drops to about 200,000 b/d.
“The ability to pig the pipeline becomes the ultimate factor,” Roberts said.
North Slope crude now contains about 2 percent wax and at any given time there are 9 million barrels of crude moving in TAPS, so if a large portion of the wax in the oil were to drop out it could be a problem, he said.
“Our preliminary modeling shows we could operate at lower temperatures and throughput, but there are other problems that compound the issue, like the wax,” Roberts said.
If the challenges with water and wax can be resolved, at some point in the future Alyeska could also operate in “batch” mode at lower flow volumes, but that would take additional facilities like more storage tanks on the North Slope, he said.
Tim Bradner can be reached at firstname.lastname@example.org.