Numerical Study of Thermosyphon Protection for Frost Heave

Abstract

Thaw subsidence and frost heave are two different hazards to pipelines in arctic regions. The former is due to the thawing of permafrost induced by a warm pipeline, while the latter is resulted from a cold buried pipeline that causes ice lens growth upon freezing in the direction of heat loss. Some pipelines may be operated in a wide temperature range and thus subjected to both types of threats. Two-phase closed thermosyphons have been employed extensively in Arctic projects to protect the permafrost from thawing. The thermosyphons’ response as a “thermo-diode” is the key to this technology. This paper presents a finite element analysis (FEA) based feasibility study for using thermosyphons with pipelines in arctic regions to reduce the potential for frost heave. There are two major challenges in the numerical simulation. One is the efficient modeling of a thermosyphons which works as a heat pump in winter and stops working in summer. This study proposes an anisotropic conduction model that simplifies the thermal-fluid processes within the thermosyphon without overwhelming computational cost. The other challenge is the frost heave modeling, which was recently achieved based on the framework of the porosity rate function. New developments involved in this paper include the extended application to permafrost and transient temperature boundary conditions. The outcome of this work proves the value of using thermosyphons with pipelines that transfer both cold product. The method introduced here can also be used to optimize the design of new infrastructure and pipelines in permafrost, as well as to assess how thermosyphons work as a mitigation method in existing projects that are affected by frost heave.