Study On Failure Mechanism Of Buried Pipelines With Internal Corrosion Defects In Permafrost Regions

The most economical way of transporting oil and gas energy is pipeline transportation.China’s oil and gas dependence on foreign countries is relatively high.The dependence on foreign oil is as high as 70%,and crude oil imported from Russia through pipelines is as high as 30 million tons per year.The Moda line of China-Russia crude oil pipeline enters from Xing’an Town of Mohe River in China,stops at Linyuan Oil Station of Daqing,and passes through the alpine region.The special climate in these areas makes the pipeline project inevitably pass through the frozen soil area.Some of the frozen soil is unstable and frost heave and thaw settlement occur,which threatens the safe operation of the pipeline.The harsh environment in permafrost regions and the corrosiveness of the input material make the pipeline inevitably corroded.The wall thickness of the pipeline becomes thinner due to corrosion,and the bearing capacity decreases accordingly.Corrosion defects are an important factor threatening the safety of pipelines in permafrost regions.At the same time,China is also in an earthquake-prone area.Buried pipelines in permafrost regions may also be damaged by extreme load earthquakes,and the long-term safe operation of pipelines is seriously threatened.If the buried pipeline is damaged by these disasters,failure or even leakage accidents will not only cause economic losses,but also cause great damage to the surrounding natural ecological environment.Therefore,it is of great theoretical significance and engineering practical value to study the failure mechanism of buried pipelines with corrosion defects in permafrost regions for pipeline design,operation monitoring,maintenance and replacement.In this thesis,the Mohe-Daqing section of the Sino-Russian crude oil pipeline was taken as the research object.The mechanical properties of the internally corroded pipeline under frost heave,and the mechanical properties of the internally corroded pipeline under the combined action of earthquake and frost heave were analyzed by theoretical analysis,numerical simulation and experimental data verification.The main research elements of this thesis are as follows:According to the theory of frozen soil mechanics and heat transfer,the heat balance control differential equation of temperature field was established.According to the basic theory of elastic-plastic mechanics of solid,the stress field equation of pipeline was established.The thermal-mechanical coupling model of buried pipeline in permafrost regions was established by numerical simulation method.Combined with the field monitoring data,the temperature field model was verified and simulated,and the temperature field distribution law around the buried pipeline was obtained.The elastic foundation beam theory was applied to solve and analyze the stress and deformation law of the pipeline under the condition of differential frost heave,and the stress field model of the pipeline was verified.The indirect coupling simulation of buried pipeline in permafrost regions was carried out to explore the influence of different land surface temperatures,different pipeline internal pressures and different transportation temperatures on the stress characteristics of the inner wall of the pipeline.The results show that the influence of transport temperature on the maximum stress position of the inner wall of the pipeline is significantly less than that of internal pressure.The maximum equivalent stress position of the inner wall of the non-destructive pipeline is near the junction of the transition section and the frost heave section.A three-dimensional nonlinear finite element model of buried pipeline with a single corrosion defect was established by taking the unfavorable position determined by non-destructive pipeline as the location of corrosion defect.The specific working conditions were determined by orthogonal test method,and the influence of various factors on the mechanical properties of corroded pipeline was discussed.The results show that the corrosion depth is the main factor affecting the failure pressure of the pipeline,followed by the corrosion length and the corrosion width.According to the numerical simulation results,the prediction formula of the failure pressure of single corrosion pipeline in permafrost regions was fitted,and compared with the standard evaluation results and the pipeline volume corrosion blasting test data,the applicability and accuracy of the formula were verified.The thermal-mechanical coupling analysis of buried axial double corroded pipelines in permafrost regions was carried out,and the failure pressure loss ratewas introduced to explore the influence of various factors on the mechanical properties of corroded pipelines.The results show that the failure pressure of corroded pipeline is positively correlated with corrosion length and corrosion depth,and negatively correlated with corrosion spacing.Whether the pipeline is damaged is mainly determined by the bearing capacity of the more serious corrosion defect,and the influence of other corrosion defect on the failure pressure is relatively small.A neural network model was developed to predict the failure pressure of axial double-point internal corrosion pipelines in permafrost regions,and compared with the specification and blasting test data.The results show that the neural network model is more accurate for the prediction of pipeline failure pressure and can be used as an effective prediction tool.A three-dimensional thermal-mechanical coupling model under seismic action was established by using viscoelastic boundary for buried single corrosion defect pipeline under differential frost heave.The seismic time history response analysis of two natural waves and an artificial wave was carried out,and the influence of various factors on the mechanical properties of the pipeline was discussed.The results show that the influence of corrosion depth on the axial velocity response of the center of the buried pipeline corrosion zone is greater than that of the vertical velocity response.When the corrosion width and corrosion length are constant,with the increase of corrosion depth,the vertical and axial maximum displacement of the center point of the pipeline corrosion area gradually increases.The influence of corrosion depth on the maximum vertical and axial displacement of the center point of the pipeline corrosion area is greater than that of the corrosion length.Under the action of three different seismic waves（Kern County wave,Borrego Mtn wave and Artificial wave）,the Artificial wave has the greatest damage effect on the buried single corrosion pipeline.