Numerical evaluation of the effectiveness of frost heave mitigation strategies for chilled arctic gas pipelines

Abstract

To prevent the thawing of ice-rich permafrost, it is suggested that gas should be transported in a chilled state (below the freezing temperature) in pipelines buried in permafrost. However, frost heave occurs when water migrates towards the chilled pipeline and ice lenses grow underneath the pipe. This might endanger the integrity of the pipeline and the environment as well. Therefore, innovative frost heave mitigation measures are required when designing the pipeline, especially those sections in discontinuous permafrost or near the compressor stations. The ground temperature field in response to the operation of a proposed chilled gas pipeline traversing permafrost regions in Alaska was simulated by a pipe-soil thermal interaction geothermal model. Frost heave mitigation measures, including insulation around the pipe, flat slab insulation under the pipe, and heating cables combined with slab insulation, were evaluated for chilled pipeline operation in seasonally varying ambient temperatures. The numerical results show that the minimum temperature of the observation point at 2.5 m below the pipe bottom increases by 17%, 29%, and 48% when the thermal conductivity of the outer insulation layer is 0.1, 0.05, and 0.02 W/(m·K), respectively. For flat slab insulation, the thermal field is less sensitive to varying slab thicknesses than to varying thermal conductivity, implying the thermal conductivity, not the thickness, is the crucial factor. Additionally, the heat flow could be redirected from vertical to horizontal by flat slab insulation. The electrical heating cables could be regarded as a new heat source to balance the heat removal rate of the soil around the chilled pipe. The minimum temperature of the observation point at 1.1 m below the bottom of the pipe increases from −15.2 °C to −3.0, 1.5, and 7.5 °C, corresponding to the heating cable power of 20, 30, and 40 W, respectively, with the power of 30 W deemed appropriate for the study case. It is concluded that heating cables in combination with insulation slabs could be adopted to regulate the temperature field around the chilled pipeline efficiently and economically. The advantages of this combination include redirecting the heat flow and eliminating frost in the soil underlying the pipe. These approaches could be considered for applications in gas pipeline projects in arctic and alpine/high-plateau permafrost regions.