Perhaps the most talked about and least understood technologies in the transportation sector of the oil industry are in-line inspection devices known as “pigs.”
These oinkers, like their namesakes, are built to thrive in messes. But unlike the barnyard animals, oil industry pigs are lean, industrious machines designed to wallow inside operating pipelines, where operators cannot.
Industry pigs, an acronym for pipeline inspection gauges and sometimes as big as automobiles, are periodically inserted into pipelines by operators to perform numerous and increasingly sophisticated tasks. There are two basic types, mechanical and instrument.
Mechanical pigs do everything from sweep trash out of a pipeline to scrape waxy buildup off its walls.
These are the most frequently used pigs on the 800-mile trans-Alaska oil pipeline. Alyeska Pipeline Service Co., the pipeline’s operator, once sent cleaning pigs through the pipeline about once a month. But as Alaska North Slope crude flow has declined in the pipeline to about 800,000 barrels per day, operators are cleaning out the line as often as every week.
“From a pig’s eye-view, the pipeline is really two pipelines,” said Dave Hackney, program engineer in charge of pigging at Alyeska.
The section from Pump Station 1 to Pump Station 4 is typically cleaned by a pig every two weeks, but the longer section from Pump Station 4 to Valdez is quite a bit cooler. “So we run a pig on that section every week,” Hackney said.
Instrument pigs, or so-called “smart” pigs, have high-tech circuitry that enables them to record images of the pipe using ultrasonic and magnetic sensors. Operators check these images for signs of corrosion, stress and bending in pipe walls.
First pigs down line in 1978
But the story of pigs in Alaska’s oil industry begins and ends at Alyeska Pipeline Service Co.
The company sent its first corrosion pig and caliper pig through the pipeline to look for dents in 1978, less than a year after the pipeline’s startup.
“Neither (of the pigs) was very sophisticated, but they got better as time went on,” said Hackney.
Among the company’s early experiences was the time it sent a curvature, or deformation, pig through the pipeline in 1979 and it got stuck at check valve 29. Alyeska ended up opening the check valve, removing the pig and installing a stopple and bypass at the location.
Alyeska engineers had been advised by expert consultants that the pipeline would have little trouble with corrosion. Still, the company chose to commit substantial time and resources to developing better instrument pigs to make doubly sure.
In time, it became clear that the pigs were returning data to the company that left huge gaps in information. Alyeska began working with NKK, a Japanese vendor, to develop an ultrasonic transduction, or UT, pig that could produce more reliable data.
A UT pig sends sound waves through the pipe and compares the speed of their propagation with what would be expected in a pipe of proper thickness.
“NKK used Alyeska as a test bed for a better ultrasonic pig and when they were successful, we paid them for the data,” Hackney said.
Alyeska also worked with a Canadian company, International Pipeline Engineering Ltd., to develop a magnetic flux leakage, or MFL, pig.
An MFL pig uses magnets to saturate the pipe walls with magnetic lines of force or flux, and if the lines are disturbed and leak out, the pig can sense the leakage and deduce how much metal has been lost from the pipe’s walls.
“We did all this at a time when we thought we wouldn’t have a problem with corrosion,” Hackney said.
The new instrument pigs took their first maiden trip down the pipeline in 1987 and encountered significant signs of external corrosion on the buried pipe near the Chandalar River.
“The first anomaly we dug up had enough corrosion that we had to put a sleeve on it,” Hackney said. “We thought we wouldn’t have a problem with corrosion, but our world changed when the pigs told us otherwise.”
Alyeska ended up putting so many sleeves on pipe in the Atigun Pass area in the late 1980s and 1990 that the company decided in 1991 to dig up and replace nine miles of pipe on the north side of the pass.
Meanwhile, concern arose in the early 1980s about the pipeline moving, Hackney recalled.
A deformation pig, which detects dents and bends in the pipe, began to return disturbing data.
“One of the company’s greatest successes with this instrument happened when it detected wrinkling of pipe under the Dietrich River near Milepost 200 in 1985,” Hackney said. “We did a reroute in February when it was 68 degrees below zero Fahrenheit. It was a success story because we intervened before oil got out of the pipe.”
Pigs grow smarter
The pipeline was settling as it melted ice and permafrost below buried sections. This was another phenomenon that Alyeska engineers needed to carefully watch, so they urged instrument pig manufacturers to develop an even smarter device called a “Geo” pig.
“The Geo pig has the brains of a guidance missile,” Hackney said. “It gives its position in three-dimensional space every 2 inches.”
Using data from different Geo pig runs, Alyeska engineers can plot the position of the pipeline within millimeters on a computer and determine conclusively if the line has moved and if that movement poses a threat to the pipeline’s integrity.
“The pipeline moved quite a bit as it settled, but it’s settled in now and moves less and less,” Hackney said. “Most of the buried pipe is now a foot (deeper) than where we put it, but now the pipeline hardly moves at all.”
Over the years, pig vendors have made more technological advances.
For example, Alyeska is now using second-generation ultrasonic pigs, instruments that operate 512 transducers, with each recording 625 readings a second, Hackney said. An on-board computer stores all the data and keeps track of where readings are taken. This helps operators pinpoint places quickly where the pipe wall is thinner.
By comparison, the ultrasound instrument in the doctor’s office uses a single transducer to register and record images of the human body.
Alyeska also encouraged vendors to make improvements in the magnetic pigs.
“They’ve done a pretty good job keeping up with technology,” said Hackney. “As a result, we have a pretty good idea of what the inside of the pipe looks like.”
He likened Alyeska’s new pigs to race cars. “Pigs come in different sizes, shapes and configurations, and other companies buy the taxicab models,” he said.
Most of the pipeline’s corrosion has stemmed from external forces such as water over the years. At 30, Hackney said the line is still relatively young by industry standards.
“Age is less important than maintenance and the characteristics of the contents of the pipeline,” he explained. “For example, the guys who manufacture our magnetic pigs run a pipeline in northern Canada that has to be replaced every two years.”
Hackney said that pipeline transports a highly corrosive product.
Smaller models coming
More advances in pig technology also may be under way.
Hackney said an instrument that can detect cracks in the pipeline, ostensibly called a “crack” pig is currently being developed, but Alyeska does not anticipate having a big need for it.
“We have really good steel, and our pipe isn’t prone to cracking,” he said. “We have no evidence that we are susceptible to cracks. But we look for it every time we dig up the pipe.”
The last time Alyeska took a look? In June, said Hackney.
Despite their increasing sophistication, smart pigs do have limitations. Current models can be too big to navigate many pipes. To remedy this, the industry envisions smaller, svelter robots that move under their own power and go wherever an operator directs them, no matter the direction of the flow of oil (or natural gas in gas pipelines). These robots will require much lighter sensors, and researchers are looking at a number of techniques.
J. Bruce Nestleroth and Richard J. Davis, of Battelle, based in Columbus, Ohio, describe one sensing method in an article published on Aug. 30 in the journal Nondestructive Testing and Evaluation International. They use a device that moves through a pipeline while rotating pairs of permanent magnets around a central axis, stirring up powerful “eddy currents” in the surrounding metal. Variations in these currents can paint a detailed picture of the pipeline’s walls.
—Petroleum News staff writer Alan Bailey contributed to this article.
