Stress Analysis of Pipeline Subjected to Differential Frost Heave

Frost heave must be considered in cases where pipelines are laid in permafrost in order to protect the pipelines from overstress and to maintain the safe operation. In this paper, a finite element model for stress/strain analysis in a pipeline subjected to differential frost heave was presented, in which the amount of frost heave is calculated using a segregation potential model and considering creep effects of the frozen soil. In addition, a computational method for the temperature field around a pipeline was proposed so that the frozen depth and temperature variation gradient could be obtained. Using the procedure proposed in this paper, stress/strain can be calculated according to the temperature on the surface of soil and in a pipeline. The result shows the characteristics of deformation and loading of a pipeline subjected to differential frost heave. In general, the methods and results in this paper can provide a reference for the design, construction and operation of pipelines in permafrost areas.

Assessing the role of differential frost heave in the origin of non-sorted circles

A. L. Washburn famously proposed and reviewed 19 hypotheses for the origin of patterned ground in periglacial environments over 50 years ago (Washburn, 1956). Of these 19 mechanisms, only a few have been analyzed starting from a fundamental description of the physics to assess their potential contribution to the initiation of patterned ground. Here, differential frost heave (DFH) is investigated as the origin of non-sorted circles in periglacial landscapes. Model results indicating that DFH can spontaneously lead to patterned ground are compared to measurements of patterned ground in the Canadian Arctic Archipelago. The characteristic size of the predicted emerging pattern depends strongly on the maximum depth of freezing but is only weakly dependent on the soil composition. The predicted emerging patterns may also be dramatically different in size with a small change in active layer when a relatively thin (~ 10 cm) insulating snow covers the ground during freezing. The model predicted trends agree with field observations of pattern size and active layer depth at two distinct sites. Although two data points cannot conclusively indicate a trend, this correlation gives support for the possibility of determining some subsurface properties using remote sensing images of surface patterned ground.

Modelling of pipeline under differential frost heave considering post-peak reduction of uplift resistance in frozen soil

The interaction between buried chilled gas pipelines and the surrounding frozen soil subjected to differential frost heave displacements has been investigated. A simplified semi-analytical solution has been developed considering the post-peak reduction of uplift resistance in frozen soil as observed in laboratory tests. The nonlinear stress–strain behaviour of the pipeline at large strains has been incorporated in the analysis using an equivalent bending stiffness. The predicted results agree well with our finite element analysis and also with numerical predictions available in the literature, hence the simple semi-analytical solution can be considered as an alternative to numerical techniques. A parametric study has been carried out to identify the influence of key factors that can modify the uplift resistance in frozen soil. Among them, the residual uplift resistance has been found to be the important parameter for the development of stresses and strains in the pipeline.Key words: pipeline, frost heave, discontinuous permafrost, semi-analytical solution, uplift resistance, frozen soil.

An Integrated Engineering Model for Prediction of Strain Demands in Pipelines Subject to Frost Heave

Long distance pipelines are actively pursued by the industry to transport natural gas from remote arctic regions to markets. A chilled gas pipeline is one of the options to minimize the environmental impact resulting from operation of such pipelines. When a chilled gas pipeline crosses discontinuous permafrost areas, differential frost heave can occur. The result is pipe being subjected to potentially high strains, primarily in the axial direction. Reliable prediction of strain demands is one of the key components for a strain-based design process and it is essential for both ensuring pipeline integrity and facilitating life-cycle cost optimization for the design and maintenance of pipelines. The prediction of strain demands resulting from frost heave of chilled gas pipelines involves three fundamental engineering analysis processes. They are gas hydraulic analysis, geothermal analysis and pipeline structural analysis. Not only are these three processes complex, they are also mutually interdependent. To reliably predict strain demands and fully capture the interactions among these processes, TransCanada Pipelines Ltd. (TransCanada) and its partners developed an integrated engineering model on the basis of three well established programs for the three individual engineering processes. This paper will briefly review the integrated model for strain demand prediction.

Aspects of the geomorphology, genesis and environmental significance of earth hummocks (thúfur, pounus): miniature cryogenic mounds

Miniature varieties of cryogenic mounds that are capable of forming in seasonally frozen ground are commonly referred to as earth hummocks (e.g., North America), thúfur (e.g., Greenland and Iceland) and pounus (Fennoscandia). Over the past few decades there has been a consistent interest to study earth hummocks from a variety of environmental settings. This review summarizes the current knowledge of earth hummocks, highlighting aspects on the external and internal morphology, and thermal characteristics, which may assist to explain hummock formation. Several hypotheses have been proposed for the genesis of earth hummocks, including the ‘cryoexpulsion’ of clasts, hydrostatic and cryostatic pressure, cellular circulation, and differential frost heave. These hypotheses are critically evaluated and some research gaps identified. It emerges that considerable advances have been made towards an improved understanding of earth hummock development, modification and disintegration. Much progress has been made in the application of earth hummock studies to a variety of environmental research approaches such as palaeoenvironmental reconstructions and assessing their impact on hillslope drainage.