Refrigerating the Pipeline

For almost every inch of the 800-mile pipeline, design engineers who designed and built this system faced a different environmental challenge. There were more than 800 river and stream crossings. The migration of indigenous animals could not be interfered with. The pipeline had to traverse three mountain ranges. And for most of the year, construction was taking place in one of the harshest climates on earth.

One of the more daunting challenges was figuring out a way to avoid burying the line in areas dominated by permafrost, where the ground is permanently frozen, and the warm crude oil posed a risk to frozen soil and pipeline stability. To avoid this, 420 miles was elevated above the surface of the ground. For three segments—totaling about four miles—this was not possible and these sections had to be buried in the permafrost to accommodate caribou migrations and road crossings. One of the sections, located at Milepost 647, is about two miles long. The other sections are located at Mileposts 652 and 684, and are each about one mile long.

The solution? Cooling systems were installed to keep the permafrost from thawing. In these areas, the pipe is insulated with three inches of polyurethane foam covered with a resin-reinforced fiberglass jacket. Pipes adjacent to the mainline carry refrigerated brine to keep the surrounding soil frozen.

To maintain pipeline integrity and protect the environment, Alyeska monitors the pipeline, including its belowground sections, with “smart pigs”—inspection gauges that run inside the pipeline and to aid in identifying any anomalies, using ultrasonic, high-resolution imaging.

In 2004, a Magnetic Flux Leakage (MFL) corrosion pig was sent through the pipeline. This state-of-the-art pig uses a magnet and sensor to record pipe wall thickness. After analyzing data from this pig run, Alyeska identified locations where wall loss had occurred around some girth welds in heat-affected zones and along the axis where insulation pieces meet and the original pipe coating had failed. Where necessary, repairs will be made by installing reinforcement sleeves.

After the sleeves are installed, a quality check will be performed to verify that the welds meet Alyeska standards. Under the sleeves, areas of corrosion-related wall loss will also be filled with putty or an epoxy. Monitoring rods will be mounted on the top of the sleeves to check for settling of the pipe after the repair sites have been backfilled.