EFFECT OF GAS-TEMPERATURE REGIME ON PIPELINE CONDITION IN PERMAFROST

摘要

Regulatory documents are currently devoid of analytical procedure for determination of heaving forces that act on "cold" underground gas pipelines when soils freeze through into its bed. Measures employed in practice to ensure the design position of gas pipelines, which had been developed on the basis of standard procedures that take into account only the effect of thawing forces caused by their floating on the pipeline, are little effective. Vertical displacements of underground pipelines, which are caused by the heaving of freezing soils, give rise to considerable stresses in the walls of the pipes; these stresses will depend on the nonuni-formity of heaving of the soil along the route of the pipeline. Under natural conditions, this nonunifor-mity occurs as a result of the heterogeneity of the soils over the route, and nonuniform distribution of their moisture content and density, nonuniform freezing conditions, and the influx of water to the freezing front. Moreover, the rate of displacement of the pipeline will depend not only on heaving of the bed soil, but also on the operating conditions of the "pipeline-soil" system, and can be determined from the equilibrium of the forces that cause and prevent displacement of the pipe. Let us examine the action of these forces as a function of the "effective" thickness of the soil that freezes from the top, as determined by the difference in the frost depths at which displacement of the pipeline begins and ceases. Displacement is initiated where the thickness of the frost is such that forces contributing to displacement exceed the forces that resist the movement. Displacement of the soil ceases either with cessation of freezing, or when the forces resisting the displacement exceed those causing this displacement [2]. Let us examine three basic cases of the laying of a pipeline.