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Alyeska issues TAPS low flow report Trans-Alaska pipeline heading for point at which mitigation measures will be needed to deal with several low flow rate issues By Alan Bailey Petroleum News
People have understood for some time that action will be needed to deal with various issues arising from the gradually slowing flow of oil through the trans-Alaska oil pipeline, as production declines from Alaska’s North Slope oil fields. But a report published on June 15 by pipeline operator Alyeska Pipeline Service Co. brings into sharp focus the nature of the challenges involved in keeping oil flowing south to market from northern Alaska, and the steps required to head off major operational problems on the line.
Mitigation needed Problems may start arising at flow rates below 600,000 barrels per day, with low-flow mitigation measures being essential to maintain reliable pipeline operation below about 550,000 barrels per day, the report says. The report makes various recommendations for possible mitigation measures but says that it has not addressed the problems that would likely arise at flow rates below 350,000 barrels per day, thus presumably making this throughput level a current lower limit for practical pipeline operation.
“The study findings make it clear that the technical challenges compound and increase as throughput declines,” said Alyeska President Tom Barrett in announcing the report. “The simplest, most direct and cost effective path to dealing with these challenges is to stop the decline by adding more oil.”
Current average throughput is around 600,000 barrels per day. Alaska North Slope crude oil production averaged 604,508 barrels per day in May, down 4.66 percent from an April average of 634,028 bpd. And on more than a dozen days during the month, beginning May 13 and including May 31, production dropped below the 600,000-bpd mark.
Water separation likely Essentially, oil cools as it travels south from Prudhoe Bay, especially during the winter, albeit with a boost to temperatures when unused fluids from the North Pole refinery enter the line at around the halfway point to the Valdez Marine Terminal. As the oil flow slows, the oil becomes progressively colder before it reaches North Pole or Valdez. If the oil cools excessively, water will separate from the oil and form ice inside the line. Slowing oil flow will also increase the tendency for sludge, otherwise suspended in the oil, to drop out. Line-clogging wax will also increasingly tend to separate from the oil at lower temperatures.
Water suspended in the oil as small droplets will start separating out to form a flowing layer at the bottom of the pipe when the flow rate drops below about 500,000 barrels per day, the report says. Engineering analysis and tests indicate that when this drop out happens the remaining water is very likely to freeze, potentially disabling check valves and causing ice accumulations at certain points in the pipeline system, including pipeline bends and inside pipeline valve bodies. The dropping out of water also increases the potential for pipeline corrosion, the report says.
Wax build up increasing Wax deposition already occurs in the line, but at lower flow rates the settlement of wax particles inside the line will exacerbate problems associated with wax clogging. The buildup of wax will present problems with the operation of pigs, the torpedo-shaped devices that scrape clean the pipeline interior. Pig operation will become particularly problematic at flow rates below 350,000 barrels per day, as the differential fluid pressure that drives a pig down the line drops, the report says.
And if the temperature of the oil in the line drops below the freezing point of water, the soil surrounding buried portions of the pipeline could freeze, causing movement of the line as a result of frost heaves. Unless the oil is heated, this problem is likely to start occurring at a flow rate of 350,000 barrels per day, with unacceptable pipeline displacements and overstressing occurring at 300,000 barrels per day, the report says.
The slowing flow of oil may also reduce the efficiency of the pipeline leak detection system, with the possibility of leak detection capabilities dropping below regulatory requirements, the report says.
The formation of ice in the line during any pipeline shutdown during the winter poses particular risks for the system. And any interruption to the operation of the oil refinery at North Pole would significantly increase the cooling of the oil in the southern portion of the line, causing the freezing of water in the line at flow rates below 780,000 barrels per day unless the oil is artificially heated, the report says.
Mitigation measures The report presents a shopping list of mitigation measures that could potentially be implemented to keep the pipeline operating at flow rates down to 350,000 barrels per day. These measures include the heating of the oil at points upstream of locations subject to particularly low oil temperatures; introducing contingency measures and equipment for handling ice; reducing the amount of water allowed in oil accepted into the pipeline; enhancing the insulation of the pipeline at certain critical points; the injection of corrosion inhibitors and biocides into the oil; installing additional pig launching points in the line; and implementing a new pig technology development program for addressing water and wax issues.
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