Permafrost presents one of the key challenges to development of oil and gas reserves in both onshore and near-shore Arctic fields. The settlement due to permafrost thawing has been observed for warm foundations and pipelines. Such settlement is considered a severe threat to the integrity of these foundations and pipelines. Further, large-scale permafrost degradation due to the thermal interference with warm foundations and pipelines may potentially lead to major environmental issues, which may take hundreds of years to recover.

As one of the most effective approachs to protect the permafrost from thawing, two-phase closed thermosyphons have been employed in many Arctic projects. The thermosyphons' response as a "thermo-diode" is the key to this technology. However, due to the complex nature of the fluid flow and heat transfer processes within the thermosyphon, simulating a thermosyphon with CFD methods in the engineering design could be overwhelming and unpractical.

This paper presents a finite element (FE) model that can be used in the geothermal analysis of the permafrost, taking into account the external thermal interference and the effect of thermosyphons. An anisotropic conduction model of the thermosyphon is implemented to simplify the thermal-fluid processes within the thermosyphon, and reduce the computational cost. The developed model can be used to optimize the design of new infrastructures and pipelines in the permafrost, as well as to assess how thermosyphons work as a mitigation method in existing projects that are affected by permafrost thawing.

Introduction to permafrost

Permafrost is ground at or below the freezing point of water (0°C or 32°F) for two or more successive years. It is a very common geographic phenomenon in the North. Most permafrost of the Northern Hemisphere is distributed in Russia (80% of Siberia), Alaska (80%), Canada (50%), Greenland (81%), China (22%) and northern Europe.

Recently, there is an increasing interest in the research of Arctic offshore permafrost because of the difficulties it poses for offshore oil development. For Arctic offshore projects, permafrost typically exists in shallow waters and at shore crossing. The challenge of constructing a pipeline in the permafrost is two-fold. One is to design the pipeline in a way that is affordable and stable over its working period; the other is to eliminate or limit the impact of the pipeline to the fragile arctic environment.

Thaw settlement is considered the most direct problem related to warm pipeline buried in permafrost. With the heat released from the pipeline, the surrounding permafrost may gradually thaw over years of operation. Depending on the amount of heat released from the pipe, the soil type and the distribution of massive ice in the permafrost, differential settlement is likely to occur. In areas, the pipeline may be no longer supported vertically and may be, in effect, bearing the weight of soil on its top. This causes the pipeline to deflect into the void created by settlement and induces strains in the pipe wall, which could result in over stress and even damage of the pipeline.

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