Installing infrastructure on Alaska’s North Slope is anything but simple. The ground consists of permafrost that extends 500 to 2,000 feet below the surface. Permafrost is a mix of soil, water and/or rock that remains frozen for at least two consecutive years with the exception of the top level, referred to as the “active layer,” which thaws to a depth of 1 to 3 feet each summer. As the climate warms, we maintain resilience to the dynamics of degrading permafrost and changes to the active layer by using various engineering tools.

One important mitigation tool is the use of passive thermosyphons to stabilize areas of thaw adjacent to infrastructure. Passive thermosyphons are not "chillers." They are simple devices, usually vertical sealed pipes that are partially embedded in the permafrost. Pressurized two-phase gas (typically natural refrigerants such as CO₂ or NH₃) moves through the sealed closed-loop system, driven by the difference in temperature between the cold winter air and the warmer ground temperature. As the vapor/condensate moves, heat is transferred out of the permafrost. They require no external power supply. They are often seen in Arctic structures and are used at ConocoPhillips facilities.

“Passive thermosyphons have been commonly used in arctic engineering for over 50 years and are a best practice for sustainable infrastructure constructed on permafrost. They are just one of many tools used by engineers to ensure stable foundations on permafrost and have allowed for establishment of critical infrastructure in the Arctic region,” said Senior Environmental Coordinator Sarah Kenshalo.

Most structures are constructed on piles and elevated above ground, so heat from structures is not transmitted to the permafrost and cold air can circulate beneath. Thermosyphons are not generally needed for these raised structures. Structures that cannot be elevated are built on an engineered insulated base in conjunction with passive thermosyphons to mitigate heat transmission from these structures into the frozen soils below. Permafrost temperatures and maximum summer thaw depth (active layer thickness) are two of the many environmental conditions ConocoPhillips monitors in our North Slope development work.

The first known use of passive thermosyphons in Alaska was in the 1960s by the U.S. Army Corps of Engineers for the construction of a telecommunications tower in central Alaska that is still in service today. Thermosyphons were used in construction of the Trans-Alaska Pipeline (TAPS) in the 1970s and have been instrumental to its longevity. Passive thermosyphons are commonly used in the Arctic and are accepted as a modern engineering best practice for infrastructure including:

• Facilities such as schools, hospitals, recreation centers, museums, washeterias and water treatment plants
• Communications towers and power poles
• Railroads and bridges
• Above-ground pipelines
• Traditional ice cellars (underground spaces used to store and cure subsistence foods)