Research On Buckling Instability Of Subsea Pipelines With Corrosion Defects Under External Pressure And Combined Loads

Subsea pipeline is among the most important equipment in subsea oil and gas exploration,and corrosion defect is one of the most common defects in subsea pipeline,which would seriously reduce the resistance to local buckling of pipeline.Once the local buckling occurs,the pipeline would collapse and buckle deformation would propagate along the pipeline,which seriously threaten the safety operation of subsea pipeline.Hence it is of great importance to study the buckling and post-buckling behavior of corroded subsea pipeline and investigate the influence of corrosion defect on the problem.This paper studied the behavior of corroded subsea pipeline and rings under pure external pressure or combined external pressure and axial loads through finite element method and theoretical method.The main contents and results of this paper are as follows:(1)By using finite element methods,the buckling behavior of corroded subsea pipeline under combined external pressure and axial tension was studied.The influence on the collapse pressure of pipeline's diameter-to-thickness ratio,initial ovality imperfection,depth of corrosion defect,angle of corrosion defect,the angle between the symmetric plane of corrosion defect and initial ovality imperfection,anisotropic yielding,loading path,the length of initial ovality imperfection or corrosion defect and the combination of local and run through corrosion defects had been investigated.Based on the parametric studies an empirical equation for predicting the collapse pressure had been given and good agreement had been found between the results of empirical equation and finite element analyses.Besides,the buckling behavior of corroded subsea pipeline under external pressure and axial compression had also been studied and the influence of axial compression,initial ovality and initial wrinkle imperfection in the axial direction on the collapse pressure was investigated.(2)The large deflection post-buckling behavior of intact ring with strain-hardening under external pressure was studied through curvature model and curved bar model.The ring was studied in a step by step manner through three strain-hardening models.For each strain-hardening ring model,the problem was studied by the proposed two theoretical methods and the influence of ring's diameter-to-thickness ratio and material properties on the post-buckling response was investigated theoretically.Besides,the validity of the theoretical methods is verified through finite element analyses.(3)The theoretical methods used in solving the post-buckling behavior of intact ring were extended to solve the problem of corroded ring.By assuming that the strain-hardening region of corroded region would not extend into the intact region,the post-buckling behavior of corroded ring was studied theoretically and the influence of corrosion defects on the response of deformation was also analyzed.Finally the validity of the theoretical methods and the assumptions were verified by finite element analyses.(4)The quasi-static propagation of corroded subsea pipeline was studied through finite element method and the effects of corrosion defects,diameter-to-thickness ratio,initial ovality imperfection and axial loads on the propagation pressure and deforming configuration was studied.Then the problem of dynamic propagation of corroded pipeline was studied on the basis of the analysis of quasi-static propagation.It was found that the external pressure,corrosion depth,corrosion angle and axial loads could affect the velocity of propagation.