For Petroleum News
With the onset of winter, the operator of the trans-Alaska pipeline system enters another season of challenges to prevent catastrophe due to potential freezing in the line.
Alyeska Pipeline Service Co. is using a range of tactics to avoid a freeze-up, including operations to add heat to the crude oil as it makes the 800-mile journey south from the North Slope.
Another tactic is minimizing the amount of water that’s mixed in with the oil.
In fact, Alyeska “appears to be concentrating on the option of water removal,” said the newly released 2013 annual report from the State Pipeline Coordinator’s Office.
Potential for calamity
In recent years, Alyeska has faced a mounting problem — the decline in the volume of oil moving daily on the trans-Alaska pipeline system, or TAPS.
The pipeline is oversized, having been designed to ship three or even four times the current throughput of around 550,000 barrels per day.
The low flow means the oil moves slower to the pipeline terminus in Valdez.
This means the warm oil is exposed longer to arctic weather conditions in winter. About half the line is above ground.
If for some reason the pipeline must shut down for an extended period, and the oil chills too much, freezing and other problems could develop. Restarting the pipeline could become difficult, if not impossible.
To date, Alyeska has always managed to restart the line promptly after winter shutdowns. But multiday outages following a January 2011 spill at Pump Station 1 caused serious worry about potential freezing and wax buildup severe enough to idle the pipeline until the summer thaw.
Obviously, a shutdown of that duration would be an economic calamity for the state, and a technical nightmare for Alyeska and the North Slope oil producers.
Declining water content
It’s the small amount of water mixed in with the oil that poses much of the freeze-up threat.
Water and natural gas are found naturally with the oil in North Slope reservoirs. Companies also inject water underground to enhance oil recovery.
“Processing plants remove the majority of produced water,” the pipeline coordinator’s annual report said. However, a significant fraction remains. And a bit of sediment, too.
By policy, the TAPS owners aim to limit water and sediment content to no more than 0.35 percent of the crude oil delivered to Pump Station 1.
“However, in recent years the North Slope oil fields have averaged water contents below this limit,” the annual report said. “Reports indicate that the average ... is typically in the range of 0.10 to 0.21 percent.”
In addition, operators have cut the magnitude and number of water pulses, up to 2.5 percent, than can sometimes occur, the report said.
Still, water remains a concern when coupled with the low oil throughput.
As flow decreases and becomes laminar, or less turbulent, water can drop out as the oil and water separate. This can increase internal corrosion, especially at the bottom of the pipe.
Free or fixed ice has potential to cause myriad problems: disabled instrumentation, plugged pump screens, frozen valves and so forth.
Getting it out
So, how can the troublesome water be wrung out of the oil?
“This could involve something as simple as a large tank at PS 1 that allows water to settle to the bottom, where it can be drained,” the pipeline coordinator’s report said. “Reduction of water and sediment content below the current standard might reduce problems caused by ice formation.”
Alyeska spokeswoman Katie Pesznecker told Petroleum News that several technologies for removing water from crude oil are being considered.
“We have conducted tests with static separation in tanks and expect to do so again next summer,” she said.
Pesznecker defined static separation as letting the crude oil “rest” in a tank so separation of oil and water occurs, with the water falling to the tank bottom.
The testing was conducted in existing tanks at Pump Station 1, she said.
The state pipeline coordinator’s annual report said Alyeska would conduct ice studies at the University of Oklahoma.
“Very few facilities have the capability of performing flowing ice studies of hydrocarbon mixtures,” the report said. “The primary focus of this set of investigations is to characterize the rate and volume of ice formation at various water concentrations and the conditions under which ice forms.”