Research On Interaction Of Buried Oil Pipeline-soil In Seasonally Cold Regions Based On Multi-field Coupling

Pipeline transmission is one of the most important ways of oil and gas transportation.During the pipeline operation,owing to the coupling effect of external load,frost-heaving and thawing-settlement,environmental corrosion,material aging,vibration fatigue and other complex factors,the mechanical performance of the structure degrades gradually,the bearing capacity decreases,and even the phenomenon of pipeline damage and leakage occurs,which seriously threatens the national economy and the safety of people’s lives and property.Typical West-East Gas Pipeline and China-Russia Oil Pipeline located in seasonally cold regions are long-distance buried pipelines.The temperature variation can cause serious frost heaving and thawing settlement of soil around the pipeline.The frost heaving and thawing settlement of soil in seasonally cold regions is a complex physical process,involving multi-field coupling of moisture field,temperature field and stress field.The excessive frost heaving and thawing settlement will make buried pipeline deviate from the original laying path,causing bending and cracking of pipelines,and eventually leading to pipeline failure.Therefore,research on the interaction between buried pipeline and soil under the action of frost heaving and thawing settlement is of great theoretical significance and engineering practical value for the rational design of buried pipeline structure and the guarantee of pipeline transmission safety.In this paper,China-Russia buried oil pipeline is taken as the research object.The frozen soil-pipeline interaction test and numerical simulation based on multi-field coupling are carried out by combining theoretical,experimental and finite element numerical analysis.The theoretical model of pipeline-soil interaction is established to study the law of pipeline-soil interaction and analyze the influence of various parameters on the stress-strain of buried pipelines under frost heaving and thawing settlement.The main work and results are as follows:Based on the basic theories of heat transfer and frozen soil mechanics,considering the volumetric strain heat conduction term under the temperature gradient of frozen soil,the heat conduction equation including water migration,latent heat of ice-water phase change and volumetric strain effect is established.Based on volume ice content,hydraulic diffusion coefficient and permeability coefficient of frozen soil,the moisture migration equation is given by using fluid dynamics theory.And the stress field equations are established based on the strain caused by ice-water phase change and the strain caused by temperature change.By studying the theoretical model of buried pipeline-frozen soil interaction and analyzing the mechanism of pipe-soil interaction in detail,the differential equations of temperature field,moisture field and stress field are established.And the influence factors of internal and external load of pipeline-soil interaction and the numerical analysis model of pipeline-soil interaction are given.The pipe-soil interaction model test method is proposed,and the freeze-thaw cycle model test under open system conditions is designed.The pre-experimental soil and pipeline preparation methods,test principles and test device settings,temperature,pipeline displacement,pipeline stress measurement methods,and measurement point layout are given.The precious stress-strain data of buried pipelines under the action of freeze-thaw cycles are obtained,which verify the "ratchet effect" predicted by freeze-thaw cycle theory.The general law of pipeline stress changes under the action of freeze-thaw cycles is obtained,which proves the feasibility of the experimental scheme.The freeze-thaw cycles test of buried pipeline in Daqing section of China-Russian crude oil pipeline under open system is carried out.Considering the influence factors such as burial depth,moisture content and soil properties,the change laws of soil temperature around the pipeline and of pipeline stress-strain caused by frost heaving and thawing settlement of soil are obtained respectively.The results show that the amplitude of soil temperature curve decreases with the depth increase,and the peak value has a certain lag in time.Under the condition of freeze-thaw cycles,the combined effect of gravity and frost heaving force is superimposed on the pipeline,and the stress variation trend at the pipeline top and bottom is consistent.The distribution of soil frost heaving force is closely related to soil moisture content,which means the higher the moisture content,the greater the frost heaving force is.Pipeline stress is affected by complex stress,such as gravity,frost heaving force,extrusion pressure on the upper,middle and lower parts of the pipeline in the oval thawing area,etc.Based on water-heat-force coupling theory,the three-dimensional finite element model of water-heat-force(H-T-M)three-field coupling of buried oil pipeline in seasonally cold regions is established by using COMSOL Multi physics.The finite element simulation analysis of buried oil pipeline under multi-physical field coupling is made with seasonal temperature change,moisture migration,ice-water phase change,soil stress field and deformation field,temperature change along the pipeline,and pipeline displacement as the main parameters to be investigated.The distribution law of moisture field,temperature field and stress field of buried oil pipeline is obtained.The results show that the pipeline stress fluctuated greatly due to the influence of temperature in the early stage of freeze-thaw cycle,and weakly in the later stage.Moisture migration in soil causes the gradual decrease of pipeline pressure.During freeze-thaw cycles,the change of soil moisture content and temperature are the main factors affecting the stress of pipeline.In this paper,a theoretical model of pipeline-soil interaction is established,the law of pipeline-soil interaction based on multi-field coupling is explored,and the influence laws of various parameters on stress-strain under frost heaving and thawing settlement of buried pipelines are obtained,which provide strong support for the rational design and safe transportation of buried pipelines in seasonally cold regions.