Heat pipes cool the ground under the pipeline

Each winter, more than 124,000 sealed tubes embedded in the supports of the trans-Alaska pipeline absorb heat from the ground and release it into the cold air. They have no pump, no motor, and no electricity.

How the heat pipes work

The system relies on anhydrous ammonia, a fluid that circulates without a pump due to gravity. When the air is colder than the ground, the ammonia evaporates in the buried part and rises toward the part exposed to the air, where it condenses and then descends.

The pipeline's route and challenges

The 800-mile pipeline runs from the Prudhoe Bay fields in the north to the port of Valdez, crossing permafrost. Engineers elevated parts of the line on 78,000 vertical supports to prevent the ground from melting and sinking under the pipe.

Heat pipes prevent support failure

The steel supports are driven into the same permafrost ground. Heat from the pipeline could melt the ice, causing the supports to lose anchoring. Heat pipes cool the ground directly under the pipeline to prevent this.

Design and efficiency of the heat pipes

A heat pipe is a closed tube filled with anhydrous ammonia. The system only works in winter, when the air is colder than the ground. The exposed part of the tube has fins to increase the surface area for heat exchange.

Limitations and maintenance

The system stops in spring when the air warms up. However, hydrogen gas, which does not condense, can block the tops of the tubes, reducing their effectiveness. Some heat pipes have malfunctioned over the years, requiring repairs.

Contract and construction timeline

The heat pipes, called Cryo-Anchor, were manufactured by McDonnell Douglas under a 13 million dollar contract. The pipeline was constructed from March 1975 to May 1977, with the first oil flowing in June 1977.

Ongoing monitoring and adaptations

Nearly 50 years after the pipeline's completion, engineers continue to monitor the supports. Some are sinking due to warming, but no collapse is imminent. In one case, 100 autonomous thermosiphons were installed to stabilize a slope by freezing the ground.