Stress Prediction And Fitness Accessment Of Girth Weld Of Pipeline Loaded By Thawing Settlement Displacement Of Frozen Soil

The China-Russia crude oil pipeline(Mohe-Daqing line)which was put into operation in2011 in the alpine region of Northeast China,is the first large-diameter crude oil pipeline in China that passes through a large area of unstable permafrost.The positive temperature operation of buried hot oil pipeline makes the soil around the pipeline freeze-thaw changes,resulting soil thawing deformation.The commissioning of the second Mohe-Daqing Pipeline,which is laid in parallel with the first line in 2017,makes the temperature fields around two pipelines overlap and interact,making the pipeline face greater risk of thawing settlement.If there is a certain crack defect in the inner girth weld position of the pipeline,the additional axial stress caused by permafrost thawing may induce the fracture failure of weld crack,which poses a serious threat to the supply guarantee of this strategic energy channel and the environmental protection of the original forest in the Greater Khingan Mountains.However,due to the insufficient research on the law of thawing consolidation settlement and the relationship between pipe-soil interaction of frozen soil around the large-diameter thermal transmission pipeline under the multi-field coupling effect of temperature,soil stress,pore water at different temperatures and different phases,pipeline self-weight and rigidity in China,it is difficult to accurately analyze and predict the axial additional stress of pipeline induced by thawing settlement.Furthermore,it is impossible to carry out accurate applicability analysis and evaluation of the pipeline based on the real additional stress of the girth weld to ensure the instrinsic safety of the pipeline.In view of this,this paper focuses on the following four aspects of research on the strength prediction of Mohe-Daqing line weld under the action of thaw settlement displacement.Based on the similarity equivalent theory,a centrifuge test scheme was designed to simulate the frozen soil around the pipe and the interaction relationship during the 20-year service period of Mohe-Daqing line.Centrifuge test equipment was used to carry out centrifuge physical model test research for the typical thawing hazard scene along MoheDaqing line.The law of soil thawing settlement deformation around the pipeline under the influence of heat transfer pipeline is directly obtained,and the distribution characteristics of thermal strain and bending strain of the pipeline under action of thawing settlement are revealed.The effectiveness of thawing settlement protection measures such as thermosiphon and soil replacement are verified.Based on the heat conduction theory and the thawing consolidation settlement theory of frozen soil,the finite element numerical calculation method is used to establish the moistureheat-stress three-field coupling numerical calculation model of frozen soil around the pipe,which accurately considers the sequential coupling relationship between the soil temperature field around the pipe and the consolidation settlement displacement field of the thawing soil.Taking centrifuge test as the inversion object,the accuracy of the numerical calculation model is verified from three aspects: the distribution law of temperature field,the variation law of thaw settlement and the distribution state of pipeline stress and displacement.Based on the three-field coupling numerical model of moisture-heat-stress,a batch analysis model is established for the special working conditions of double-pipe parallel laying of China-Russian crude oil pipelines by using parametric programming method.The effects of single-tube operation and double-tube parallel operation on the temperature field and thaw settlement displacement field in permafrost region were discussed.The influence of key factors such as wall thickness,insulation layer thickness,buried depth of pipe roof and length of thaw settlement area on axial stress-strain state of pipeline under the operation of double pipelines in Mohe-Daqing line was systematically studied.A database of pipeline axial stress(2816 sets of stress results)was formed,and a data-driven support vector regression prediction method for axial stress of the pipeline in Mohe-Daqing line under the action of thaw settlement displacement load was proposed.The engineering applicability of the applicability evaluation methods of pipeline girth weld based on failure assessment diagram and finite element crack driving force calculation and evaluation were compared.The full-scale bending test of girth weld pipe with crack defects was carried out to verify the applicability of the failure assessment diagram method for the evaluation of girth weld of pipeline.Based on the evaluation method of failure assessment diagram,the ultimate axial stress of girth weld of pipeline with crack defects under different service conditions was determined.The influence of material yield-tensile ratio,misalignment,wall thickness and crack size on the limit axial stress of girth weld of pipeline with crack defects in Mohe-Daqing line was studied.On this basis,the database of limit axial stress of girth weld of pipeline with crack defects containing 4320 sets of data is established.A prediction model of limit axial stress of girth welds of Mohe-Daqing line based on support vector machine(SVR)was proposed.