Transporting warm crude oil across 800 miles of rugged Alaskan terrain presents a severe environmental and structural hazard: thaw settlement. When oil pumped at temperatures exceeding 100 degrees Fahrenheit passes over permafrost, it threatens to melt the underlying frozen soil, causing the ground to shift, buckle, and potentially rupture the pipeline. To solve this, engineers elevated more than half of the Trans-Alaska Pipeline System (TAPS) on Vertical Support Members (VSMs) equipped with passive refrigeration devices known as thermosyphons.
A thermosyphon is a sealed, hollow steel tube charged with anhydrous ammonia refrigerant, installed deep inside the vertical steel legs supporting the pipe above ground. Thermosyphons operate on a completely passive heat-exchange cycle driven entirely by natural convection and gravity. During the freezing Alaskan winters, the ground beneath the pipeline is warmer than the ambient air above.
Ammonia liquid at the bottom of the pipe absorbs thermal energy from the soil, vaporizing and rising to the exposed radiator fins at the top of the VSM. Outside wind cools the aluminum radiator fins, causing the ammonia vapor to condense back into liquid along the interior walls, where gravity pulls it back down to the subterranean section. This continuous loop passively extracts heat from the soil and radiates it into the air, keeping the permafrost deeply frozen year-round.