Trans-Alaska Pipeline System operators are taking new steps to keep North Slope crude oil warm enough to flow through the 800-mile pipeline during cold months of the Alaskan winter.
In 2011, a mid-winter disruption in operations almost resulted in oil congealing into sludge, to a point where the pipeline would be difficult to restart. Since then, operators have been adding heat during the winter by recirculating oil through pipe loops at pump stations.
In 2015 they added a plug-in heating unit at a remote gate valve in Interior Alaska, where winter temperatures drop below minus 60 degrees Fahrenheit, said Alyeska Pipeline Service Co. spokeswoman Michelle Egan.
The pipeline company is battling a gradual, long-term cooling of the oil temperature as production from the North Slope drops and as lower volumes reduce natural mechanisms that previously warmed the oil, such as the friction of fluids against pipe walls.
During winter, oil that now enters the pipeline at 104 degrees on the North Slope drops to 40 degrees by the time it reaches the Valdez Marine Terminal in southern Alaska.
Egan said TAPS operators now work to keep the temperature above 32 degrees, the point at which ice can form at low points in the pipeline and wax can build up and impede the flow of oil.
TAPS, built in 1977, now operates at about 25% of its 2 million b/d design capacity.
“Our operations in winter are becoming increasingly complex,” Alyeska CEO Thomas Barrett said in a briefing.
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The heat is on to keep Alaskan crude from freezing
Barrett said it is also taking longer for oil to reach Valdez — 15 days now compared with four days in 1977. This means the oil is exposed to winter cold for much longer on above-ground sections of the pipeline, about 400 miles or half its length.
“The real danger for us is if there is an unexpected winter shutdown. There could be significant problems,” Barrett said.
This happened in 2011 when a small oil leak at Pump Station 1 caused TAPS to shut for several days during cold weather. The crude temperature dropped below freezing at several points along the pipeline in Interior Alaska.
When regulators gave approval to restart, Alyeska was able get the congealed oil moving again, but with difficulty. Had the shutdown lasted a day or two longer, the freeze-up could have lasted for several months.
Since then, as a contingency, Alyeska has been adding heat at four locations by circulating crude through loops of piping, with the friction adding the heat. At Pump Stations 3, 4, 7 and 9, heat is added by running oil through a recycle loop at a rate of up to 25,000 b/hour.
This helps, Egan said. Last January, for example, oil left Pump Station 1 at a temperature of 106 degrees, with an ambient air temperature of minus 17 degrees. By the time the oil traveled 100 mile to Pump Station 3, its temperature had dropped 51 degrees. Recirculating the oil at Pump Station 3 added 15 degrees of heat back, she said.
A new heat source was added in 2015, with a diesel-fueled heat skid put at Remote Gate Valve 65, 17 miles north of Pump Station 7, a point where cold winter temperatures are common and the pipeline could be vulnerable to ice
Alyeska is considering adding similar heat skids at various points. TAPS also gets a bump in heat by the return of residual oil from a small refinery near Fairbanks owned by Petro Star Inc. It takes crude from TAPS to make jet fuel and diesel, returning unused portions of the crude at a high temperature to the pipeline.
The pipeline company’s main strategy is to protect Pump Station 9, near Delta, southeast of Fairbanks, where oil must be warmed enough to get the rest of the way to Valdez and over two mountain passes.
One experiment by Alyeska was unsuccessful. In a special test facility built at the University of Tulsa, the company experimented with removing water from North Slope crude, from its ambient content of 0.2% water to 0.02%. But ice still formed even at the lower water content.
TAPS is averaging about 550,000 b/d this winter, but its yearly average is about 500,000 b/d, according to Alaska’s Department of Revenue. Throughput is continuing to decline at an average or 5% a year, and Alyeska is concerned the wax buildup will become a real problem when throughput reaches 400,000 b/d or 350,000 b/d, which it will if the trend continues.
“The real solution for us is finding more oil on the North Slope and adding new production and throughput,” Barrett said. — Tim Bradner