Alyeska continues to evaluate maintenance "pigging" practices

In late December, Alyeska Pipeline Service Co. sent a scraper pig down the 800-mile-long trans-Alaska pipeline to remove the wax that builds up on the inside walls of the pipe. During the course of transit, the pig encountered resistance that caused it to come apart as it is designed.

Some of the parts of the December pig run were captured at Pump Station 7 and other parts were carried downstream. A subsequent scraper pig was launched and pushed the remaining portions of the pig, along with 40 barrels of wax to the receiver at the Valdez Marine Terminal.

All of the parts have since been accounted for with the exception of a retainer ring used to keep the pig assembled. The stainless steel retainer ring is approximately 20” in diameter, 1” wide, and 1/8” thick.

Alyeska formed an investigation team to evaluate this recent incident and determined the missing retainer ring to be a low integrity risk to pipeline operations. The investigation did find that there is a slight possibility that the ring could interfere with a mainline check valve.

On February 4, 2007, a disc pig – a more aggressive cleaning device – was sent down the 48-inch diameter pipeline in attempt to clean the pipe’s walls and extract the retainer ring. When it arrived at the Valdez Marine Terminal, it was not apparent that the disc pig pushed the ring into the pig trap. The disc pig did push some wax down the pipeline and that material and the wax collected from previous cleaning pig runs will be sent to a processing facility in Seattle, WA where it will be strained to determine if the ring is in the wax.

Alyeska routinely sends a scraper pig down the pipeline every 7-14 days as part of its maintenance practices. The recent performance of the pig runs has led Alyeska to reassess the effectiveness of its pigging program to determine what can be learned from this incident. Part of this evaluation includes analyzing the frequency of our pig runs, the effectiveness of current practices, and the overall maintenance procedures. The current plan is to run cleaning pigs every five days, including additional disc pigs, in advance of a smart pig scheduled to run in early March. The amount of solids received in the Valdez pig trap after each pig run will be measured to further evaluate the effectiveness of the maintenance practices. After all of this data is compiled, Alyeska will make prudent adjustments to its pigging operations.

Alyeska is planning to run an in-line-inspection device or smart pig down the pipeline in the next few weeks. The data gathered from this smart pig run will allow engineers to identify any wall loss anomalies and to assess any types of corrosion threats. The smart pig runs are generally run every three years and on occasion more frequently.

To learn more about Alyeska's pigging program, click here.

Background Information on Pigs

Two basic types or classes of pigs are run through the pipeline: cleaning and instrumented or "smart". These devices help clean and inspect the pipeline to prevent and detect problems. Routine pigging cleans the pipeline of wax and other deposits every 7 to 14 days and "smart pigs" deliver data on wall thickness, alerting engineers to any abnormalities that may have developed in the pipe. We have run 60 smart pigs through the line since start up almost 30 years ago. Smart pigs are run every three years.

SMART PIG: There are two smart pigs used to detect and measure corrosion and metal loss internally and externally on the pipe wall: Ultrasonic Transducer (UT) Pig and Magnetic Flux Leakage (MFL) Pig. The MFL Pig below uses powerful magnets to saturate the pipe wall with magnetism. Sensors between the poles of the magnets detect disturbances caused by metal loss due to corrosion or other mechanical damage.

SCRAPER PIG: These pigs sweep the pipe of built up wax, or other solids that precipitate out of the oil stream. They prevent the buildup of a corrosive environment. Their use also makes the oil easier to pump.

DISC PIG: Aggressive cleaning pig typically only used immediately in advance of a smart pig.

The route a pig takes from Pump Station 1 in Prudhoe Bay to the Valdez Marine Terminal in Prince William Sound: