Study On Residual Strength Of Buried Pipeline With Corrosion Defects In Seasonal Island Shaped Frozen Soil Region

With the successful completion of pipeline projects in the north of China and the increase in service life,certainly buried pipelines in the seasonally cold areas of the northeast have experienced varying degrees of corrosion and may also be threatened by frozen soil frost heave.In particular,the soil in sporadic island-type frozen areas is susceptible to differential freezing and swelling due to cyclic temperatures,which can cause buried defective pipes to be squeezed horizontally to form ellipses or lifted vertically to bend and deform,so this will further increase the risk of defective pipes.Therefore,in order to grasp the stress changes of buried X70 X65and X80 pipelines with corrosion defects in service and accurately predict their residual strength,this article conducted the following research:(1)Based on the theory of thermal mechanical coupling,a three-dimensional solid model of pipe soil thermal mechanical coupling with corrosion defects was established using finite element software.Sequential coupling analysis method was used,and combined with measured data,the temperature field,vertical displacement,and stress field of the model were verified.At the same time,the location of the high stress corrosion zone of the buried pipeline was determined.(2)A rectangular volume corrosion defect with chamfers was installed at the dangerous location of the buried X70 pipeline mentioned above.Considering the influence of different defect parameters(corrosion depth,length,width),pipeline burial depth,length of frost heave section,and frost heave rate of frozen soil,the residual strength and stress distribution of the buried X70 pipeline with single corrosion were investigated.It was found that the influence of corrosion depth on the residual strength of the pipeline was dominant,followed by corrosion length,The impact of corrosion width is minimal;As the length of the frost heave section and the burial depth of the pipeline increase,the residual strength of the buried pipeline gradually decreases,but the opposite is true when the burial depth exceeds 2.2 meters;As the frost heave rate of the soil increases to 0.18,the residual strength of the buried pipeline gradually increases,but when the frost heave rate of the frozen soil exceeds 0.18,the opposite is true;In addition,based on the residual strength data of buried X70 pipeline with single corrosion defect,a multivariate linear fitting was performed to obtain a prediction equation with good fitting effect(R~2=0.947),and it was verified with the DNV RP-F101 evaluation method.A residual strength prediction formula suitable for buried X70 pipeline with single corrosion defect in the island-type frozen region was proposed.(3)On the basis of the above model,the strength level of the pipeline was changed to explore the residual strength and mechanical characteristics of buried X65 and X80 pipelines with single corrosion.Through comparative analysis with the X70 pipeline mentioned above,it was found that the Mises stress in the corrosion area of the outer wall of buried X65,X70,and X80 pipelines is concentrated along the axial direction of the pipeline towards the corrosion center,and gradually increases towards the edge of the corrosion area along the circumferential direction of the pipeline;And the influence of corrosion defect parameters(corrosion depth,length,and width)on the residual strength of buried X65 and X80 pipelines is consistent with that of X70 pipelines;When the relative corrosion depth is the same,the safety of buried pipelines is X70,X65,and X80 from high to low;In addition,based on the residual strength data of X65 and X80 pipelines,compared with the proposed residual strength prediction formula and the predicted values evaluated by DNV RP-F101,it was found that the former had smaller prediction errors and was more suitable for predicting the residual strength of buried medium and high grade pipelines with single corrosion in island type permafrost differential frost heave areas.Further verified the accuracy and applicability of the residual strength prediction formula mentioned above.The residual strength prediction formula proposed in this study provides certain reference value for the service,maintenance,and monitoring of buried pipelines in island type permafrost areas after corrosion defects occur.