Stability Analysis Of Downslope Oil Pipeline In Xing'an Permafrost Regions

Winding across 441km in permafrost zones in the northern Da Xing'anling Mountains in Northeast China,the CRCOP was built on the rugged terrains of varied complexity.Usually,there are two ways for pipeline to travel across mountain slopes:down-slope and skirting-slope.The thaw collapsing and slope stability of slope along slope may pose great difficulties to pipeline engineering construction and operation.Based on the CRCOP project.Thus,the slope stability of the Xing'an permafrost zones along the slope pipeline is analyzed.Additionally,the thermal effect of the insulation layer is simulated and calculated for selecting the appropriate thickness of the insulation layerin order to provide some references and theoretical guidance for pipelining across the similar permafrost zones along the slope.The main achievement of the research are as follows:First,based on the Harlan hydrothermal coupling model,the water-heat-force coupling model is established by taking into account of the influences of ice-water phase transition and water migration on the distribution of force(strain and stress).By the secondary development of multi-physical field coupling analysis software COMSOL Multiphysics,the PDE module in the mathematical module is coupled with the solid mechanics module to realize the three-field coupling of hydrothermal force.The temperature field of permafrost around a bare pipeline at the Jiagdaqi section of CRCOP is simulated on the basis of the water-heat-force coupling model established by the COMSOL.The ground temperature distribution of the simulation results is consistent with the measured data,proving the applicability of the numerical simulation and the reliability of the software.Second,by analyzing the stability of slope,it is found that the moisture,heat and stress fields are interdependent and change concordantly.The change in soil temperature field drives the changes in moisture field and the unfrozen water content in pipeline foundation soils,which further lead to reduction in the cohesion and internal friction angle of foundation soil and the pipelined stability of slope.Through the study of different influencing factors,it is found that the higher the oil temperature and the greater the burial depth,the greater the temperature field disturbance to the surrounding soil.The greater the thaw depth of the soil at the bottom of the pipeline,the decreases in the slope stability of the soil.The higher the water content of the slope soils,the greater the thaw settlement coefficient,the greater the thaw settlement amount at the bottom of the pipeline,the more prone to thaw settlement failure.The higher the total water content,the lower the slope safety coefficient.The larger the slope,the larger the gravity component of soil,the smaller the safety factor of slope.With the increase of freeze-thaw cycles,the cohesion and internal friction angle decrease,and the slope safety weakens.Finally,through the change of temperature field of foundation soils with different thickness insulation layer.It is found that with the increase of the thickness of the insulation layer,the melting depth of the soil at the bottom of the pipeline decreases gradually,but after reaching a certain thickness,the insulation effect does not increase significantly.Through the analysis of the parameters of the insulation material,it is found that the thermal conductivity has the greatest thermal effect while the thermal effect of density and specific heat of insulation materials is relatively small.The protective measures of insulation layer are adopted for oil temperature 14 ?,buried depth of pipe top 3 m,initial water content of pipe-based soil is 55% and high and steep slope is 60.The insulation effect is compared to select the appropriate thickness insulation layer.