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 800-mile trans-Alaska oil pipeline, as production declines from Alaska’s North Slope oil fields.
But a report published on June 15, 2011, 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, which has been operating for 34 years.
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 bpd, the report says.
The report, which is the result of a $10 million, two-year, low-flow study, 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 bpd, 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,” Alyeska President Tom Barrett said in announcing the results of the study. “The simplest, most direct and cost effective path to dealing with these challenges is to stop the decline by adding more oil.”
Throughput in the Trans-Alaska Pipeline System, or TAPS, peaked in the 1980s at 2.1 million bpd and is currently taking in about 625,000 bpd at Pump Station 1 in the North Slope’s Prudhoe Bay oil field.
According to the Alaska Department of Revenue’s spring 2011 forecast, northern Alaska will be producing only 530,000 bpd in 2020, but in a more recent Revenue forecast due to be released after this part of Pumping up TAPS goes to press, production is rumored to be closer to 486,000 bpd in 2020, and 458,000 bpd in 2021 (check out the On Deadline section on pages 8 and 9 of this issue to find out for sure).
Because of maintenance on the line, on more than a dozen days in the last half of May 2011, throughput dropped below 600,000 bpd.
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 — 110 degrees at Pump Station 1 and, depending on the ambient temperature, about 40 degrees when it reaches the terminus of the pipeline.
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 bpd, 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 bpd, 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 bpd, with unacceptable pipeline displacements and overstressing occurring at 300,000 bpd, 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.
Risk of shutdowns
Barrett said it isn’t the risk of leaking oil, but the risk “that we will be shut down more to address these problems.”
Because of wax due to lower flow, Alyeska runs more pigs to get the wax out.
Alyeska had a shutdown in January and “part of the startup issue is we had pigs in the line,” and when you start up after a length of time, you push wax and ice, whatever is in the line, “towards your strainers and your pumps. And if you take out a mainline pump with that type of stuff you’re going to be down … for a long while,” he said.
So you run more pigs to address the wax issue, “solving one problem and increasing the risk on the other side.”
“Figuring that out, whether we’re adding launchers-receivers, or just the type of pigs we run, is complex — it’s complicated petroleum pipeline engineering. You have people that can do it, but it is a challenge,” Barrett said.
Mitigation measures
The two-year low-flow 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 bpd.
These measures include the heating of the oil at points upstream of locations subject to particularly low oil temperatures (see sidebar); 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.
Increasing the flow
If you increase the flow to the million-barrels-a-day target set by the governor, “a lot of these problems fall off from an operating point of view,” Barrett said.
He said he believes “we’re a couple of years behind.”
When Alyeska started its low-flow study in 2008, the projection for 2011 was 700,000 bpd.
“We’re seeing 600,000 barrels,” Barrett said, adding that he worries that there isn’t the urgency needed to get additional barrels into the line.
The resources are there, the infrastructure is there, so what’s stopping Alaska from moving a million bpd, he asked.
“Two things: political will … political will in Juneau; political will in Washington.”
“I actually think that’s the obstacle to turning this dynamic around and allowing us to operate better in the future.”
