The headache of low oil flow in TAPS
Alyeska’s approach to dealing with wax and avoiding ice formation in the line is evolving as the flow rate gradually declines
In addition to responding to declining oil production from Alaska’s North Slope by installing new electrically powered pumping systems along the trans-Alaska oil pipeline, Alyeska Pipeline Service Co., the pipeline operator, has for several years been contending with some significant technical issues associated with low oil flow rates through the line. In particular, as the oil flow slows, the oil becomes colder as it travels the 800 miles or so between the North Slope and the Valdez Marine Terminal for transfer onto oil tankers. As the oil cools, equipment-clogging wax tends to separate out. And, if the oil temperature falls below 32 F, water in the oil will start to freeze, leading to the formation of ice that can damage pumping equipment.
Over the years, Alyeska’s approach to this low-flow challenge has evolved.
Recycling the oilFor some time the company has been warming the oil by recycling oil at the pump stations along the pipeline’s route - the pumping action in the recycling has the effect of heating the oil. And, in addition to using the active pump stations for this purpose, Alyeska can use pump station 7 near Fairbanks, a pump station that has been de-activated, as a recycling station, Alyeska engineering manager Rob Annett told Petroleum News.
As an alternative approach to the low flow problem, Alyeska has investigated the removal of water from the oil entering the pipeline system on the North Slope, and then allowing the oil to flow cold through the pipeline. In fact, the company conducted some tests, to assess the viability of this cold, dry flow approach, Annett said. But, unfortunately, it turned out that, despite best efforts at removing water from the oil, ice still tended to form when the oil temperature dropped below the freezing point of water, he said.
Temperature maintenanceAnd, so, the company has concluded that cold, dry flow is impractical and, instead, is following a strategy of ensuring that the oil temperature remains above 32 F along the entire length of the pipeline, Annett said. Alyeska spokeswoman Katie Pesznecker said that the target minimum temperature along the line is 40 F, to allow a margin above the freezing point level.
Recent data from a day towards the end of January, with ambient air temperatures below freezing along the entire pipeline route, show oil departing pump station 1 at a temperature of about 110 F. Despite a steady decline in the oil temperature as the oil travelled south, upwards jolts in the temperature at the three operational pump stations ensured that the oil arrived in Valdez at a temperature a little above that 40 F target.
In addition to oil recycling at the pump stations, Alyeska is now installing what are referred to as slipstream heaters at strategic points along the pipeline, upstream of critical equipment. A slipstream heater takes a portion of the oil from the line, heats it through a direct-fire heat exchanger, and then returns it to the line, Annett explained. Trying to heat the entire oil flow, rather than a portion of it, would be much more complex, he said.
In addition, as a contingency against a winter shutdown of the pipeline system, Alyeska has been establishing the capability to inject methanol into the pipeline at some pump stations as a freeze suppressant, Annett said.
WaxWax deposition creates another set of issues, especially since wax tends to start separating from crude oil at significantly higher temperatures than the freezing point of water. Thus, with wax deposition inevitable in a low flow situation, Alyeska has to resort to the used of pigs, torpedo shaped devices that pass down the inside of the pipeline, to clear wax from the line. But, as the oil flow has continued to slow, a new problem has started to emerge in that, when the flow rate drops below a certain level, wax starts to drop out of solution as particles in the oil, rather than be deposited on the interior wall of the pipeline, Annett said.
So, Alyeska is now faced with the specter of a pig becoming overwhelmed by a growing pile of wax debris accumulating in front of it as it traverses the pipeline, rather like a snow plow with no means of disposing of snow when clearing a path across snow covered ground.
Alyeska is in the process of installing optical devices that can measure the size, shape and amount of wax particles in the oil at certain points in the line, to enable the monitoring of wax throughput, Annett said. The company is also assessing the possible use of a new tool that acts like a power washing pig, inside the pipeline, he said.
Wax collected by pipeline pigs has to be removed from the pipeline at pig receiving and launching stations at certain pump stations along the line. As the flow slows and the wax deposition increases, it may be necessary to add more of these stations, to accommodate the increasing quantities of wax collected, Annett commented.
Throughput neededOverall, actions taken to address low flow issues add cost to the operation of the pipeline. And, since the overall operating costs are divided across ever fewer barrels of oil as throughput drops, the transportation cost per barrel of oil will tend to increase.
“Throughput is the answer for a lot of this,” Annett commented.
Data from Alyeska show that, on average, daily throughput declined from 513,441 barrels per day in 2014 to 508,446 barrels per day in 2015. However, throughput actually increased compared with 2014 levels for five months of 2015: July, August, October, November and December. At its peak in the late 1980s the line shipped about 2 million barrels of oil per day.