Permafrost is ground that remains below 0°C for more than two years. Design of pipelines across permafrost terrain differs from that of pipelines where there is no permafrost. A buried pipeline is subject to a variety of internal and external loads such as frost heave and thaw load induced by relative movements between the pipeline and the surrounding soils. Frost heave occurs as fine-grained soil (i.e., fine silts and clays) freezes. Thaw settlement occurs when soil with excess ice (i.e., ice volume is greater than the pore volume) thaws. Differential heave/settlement occurs due to changes in thermal state (i.e., frozen/unfrozen interfaces) and due to changes in soil type within unfrozen soil. Both differential heave and differential settlement can lead to large pipeline strain. Strain demand prediction is required to develop a strain based pipeline design approach. Proper design and construction of such pipelines poses numerous special challenges and requires consideration of some important processes that govern the behavior of soils. Numerical modeling of the pipeline strain demand prediction considering the condition change of the permafrost due to heave/settlement is a challenging problem due to the complexity of the soil/pipe interaction. In addition, the prediction requires integration of multiple analysis including pipeline hydraulics, soil heat transfer, soil heave/settlement and pipeline deformation. This paper will describe an integrated finite element analysis methodology to estimate pipeline strain demand due to permafrost frost heave.
Chilled gas buried pipelines have been used for transporting natural gas from the arctic region over the discontinuous permafrost region to the warmer populated regions of the world. The design of a buried gas pipeline in an arctic environment poses numerous special challenges. Proper design and construction of such pipelines require consideration of some important processes that govern the behavior of the permafrost soils.
Stress based or elastic design has been extensively used in pipeline design. However, the need to use strain-based design is growing due to potential pipeline projects in harsh environments that include permafrost. Soil loads are generally displacement controlled loads, so strain-based design is the preferred approach. A stress-based design for these soil loadings would be overly conservative and impractically expensive.