Alyeska’s 40-Plus Years of Experience with Heat Pipes on the Trans-Alaska Pipeline System

Heat pipes (thermosyphons) were installed in the vertical support members (VSMs) of the Trans-Alaska Pipeline System where the pipeline is elevated in warm non-thaw-stable permafrost areas. More than 124,000 heat pipes with pure anhydrous ammonia, NH₃, as the working fluid were installed during pipeline construction in the mid-1970s to thermally stabilize the permafrost surrounding the VSMs. Shortly after pipeline construction, non-condensable-gas, NCG, began to occur in some of the heat pipes, affecting their performance. Alyeska conducted an extensive research effort to identify the root cause for the occurrence of NCG and performed a test program on the degradation in heat pipe performance with the build-up of NCG. Two procedures have been used to repair underperforming heat pipes due to NCG issues: (1) Using “getter devices” and (2) recharging the heat pipes with carbon dioxide, CO₂. Using getter devices was not a permanent solution to the NCG problem, and experience has shown CO₂ recharging to be a successful repair option. This paper describes 40-plus years of heat pipe experience for an aboveground pipeline system in permafrost, several heat pipe options Alyeska considered prior to construction, and the choice and development of the heat pipes that were used on the pipeline. This paper also describes how Alyeska has managed NGC issues that developed after construction to ensure integrity of the aboveground system and the innovative use of heat pipes as thermometers to monitor end-of-thaw-season ground temperatures at the base of thermal VSMs as an additional integrity management tool.