Study On Thermo-hydro-mechanical Coupling Of Frozen Soil Around Oil Pipeline Under Freeze-thaw Cycles

Since 2011,the China-Russian crude oil pipeline has been put into operation successively.The pipeline started in Skowalodino and ended in Daqing.The domestic section passes through the permafrost region,which is the first long-distance and large-caliber crude oil pipeline in China.Although the influence of temperature has been considered in the design,the seasonal frost heaving and sedimentation of the soil in the permafrost region inevitably has a large extrusion and pulling effect on the pipeline,resulting in deformation,floating and even fracture of the pipeline.Based on the actual situation of marsh permafrost and slope permafrost in the Tahe area of CRCOP,the paper is studied the influence of pipeline operation on the thermo-hydro-mechanical behaviour of the adjacent soil and the stability and safety conditions of the pipeline under freezing and thawing.Based on the Harlan hydrothermal coupling theory,the hydrothermal coupling model of saturated frozen soil with porosity and temperature as variables is established.Based on the relationship between porosity and strain,the relationship between hydrothermal field and mechanical field is established.The governing equation is discretized and written into a coefficient partial differential equation module in COMSOL Multiphysics.Referring to available unidirectional freezing experiment,the applicability of the model and related parameters are verified from three aspects:temperature field,water field and frost heave.Based on the data of Sino-Russian crude oil pipeline(CRCOP),a two-dimensional pipe-soil finite element model in permafrost region with high water content is established.The changing laws of the freezing and thawing of soil around pipeline,distribution of soil moisture field,distribution of soil mechanical field,depth of soil melting,amount of soil freezing and thawing,and mechanical component of pipeline are analyzed with the change of season,number of freeze-thaw cycles,buried depth of pipeline and thickness of insulation layer during pipeline operation.According to the above analysis,to ensure the safe operation of pipelines,the buried depth should be more than 2.1m～2.3m without thermal insulation layer,and the thermal insulation layer depth of 50mm～60mm or more is required at the buried depth of 1.8m～2.1m.Based on the strength reduction method and the Drucker-Prager elastoplastic constitutive model,a three-dimensional finite element model of slope-pipeline is established.The variation of stability of permafrost slopes and seasonal frozen soil slopes with different water content and angle under the influence of pipeline is studied.The safety coefficient of the cross section of the permafrost slope which is 0.4065m away from the center of the pipeline decreases by 61%as compared with the natural state,and the seasonal permafrost slope decreases by 16%.For the section that 5 m away from the center of pipeline,the safety coefficient of permafrost slope decreases by 46%and the seasonal permafrost slope decreases by 12%;When the permafrost slope angle is greater than 30°,or the seasonal frozen soil slope angle is greater than 25°,the cross section that 0.4065m away from the center of the pipeline is in an unstable state.Therefore,when the pipeline passes through the slope of permafrost and seasonal permafrost areas without protective measures,the safe operation of the pipeline is ensured with the slope angle less than 30 degrees and 25 degrees,and for more than 30 degrees and 25 degrees,protective measures is required.The influence of water content on seasonal slope is greater than that on permafrost slope.When the water content increases from 15%to 25%,the safety coefficient of permafrost slope decreases by 12%～36%,and the seasonal permafrost slope decreases by 20.7%～40.39%.The research results are of great significance to grasp the variation of thermo-hydro-mechanical field and slope stability of pipeline and adjacent soil under the influence of pipeline operation,and to guide the design,construction and maintenance of pipeline better in special section of permafrost region.