Alyeska pipeline milepost 687 is in the Copper River Basin of Alaska approximately 3 miles south of Glennallen. The project site is on the north facing slope of the Tazlina River valley; the slope is steep at approximately 30%. Soils in the project area primarily consist of perennially frozen glaciolacustrine deposits of clay and silt underlain by sequenced deposits of silt, sand, and gravel. The pipeline is supported above ground on passively-cooled thermal vertical support members. Active, bimodal, retrogressive landslides/flows are common along the south slope of the valley.
Long term temperature and ground movement monitoring shows relatively slow downhill ground movement is occurring near the top of permafrost and within active layer soils across the slope. Slow downhill movement is causing VSM’s to translate and tilt, requiring continued shoe adjustments, load redistribution, and split ring adjustments. The goal of this project was to mitigate slope movement, stop progressive tilting and translating VSM’s, reduce long term maintenance efforts, and reduce long-term degradation of permafrost.
The mitigation strategy included increasing soil resistance to slow downhill creep by freezing active layer and thawed soils above permafrost. This concept included installing a strategically placed two-phase, closed loop passive refrigeration system and woodchip insulation blanket to freeze active layer and thawed soils above permafrost. In-stock standard Alyeska heat pipes were used to provide the passive refrigeration system. The system includes a woodchip insulation blanket across the ground surface encompassing existing VSM’s and new passive refrigeration free standing heat pipes.
The passive refrigeration system utilized inclined heat pipe configurations. Inclined loop heat pipes were installed in shallow trenches across the pipeline and workpad elements. To function properly, inclined heat loops require gravity-driven movement of liquid phase operating fluids from the finned condenser section down through the length of the buried evaporator section. Trenches were aligned cross slope to establish a relatively uniform embedment depth and required slope for the inclined heat loops. Vertical helical piers were design to support the aboveground potion of the heat pipes and provide frost-jacking resistance.