Local Buckling And Evaluation Criteria Of Subsea Pipelines With Complex Defects

Subsea pipelines are lifelines of offshore oil and gas resources transportation,which play a very important role of the regional allocation of oil and gas resources.With the growth of energy demand and the progress of offshore oil and gas development technology,the development scale of offshore oil and gas is expanding,and the laying mileage of subsea pipelines is also growing rapidly.The environment is complex and changeable.When the pipeline is exposed to the marine environment for a long time,it will often be damaged to different degrees.Defects such as dent and corrosion are inevitable,so as to reduce the residual strength of subsea pipelines;Meanwhile,due to the unpredictable impact of ocean currents and topographic changes,the pipeline will not only bear the internal operation pressure for a long time,but also be affected by various loads such as external pressure,axial force and bending moment,which will threaten the safe operation of subsea pipelines,and even lead to the destruction of subsea pipelines,safety accidents and incalculable natural and property losses.Therefore,it is of great significance to study the local buckling failure mechanism of submarine pipelines with defects under complex loads and put forward the residual strength evaluation method which is generally applicable to the existing pipelines.Based on two conditions of load control and displacement control in the pipeline design and safety verification method of limit state design,the difference of local buckling of pipeline under load control and displacement control is studied by theoretical analysis and numerical simulation respectively;The numerical simulation model of subsea pipelines with dent is established.The reliability of the model is validated by comparing with the code and test results.The local buckling failure mechanism of pipelines with dent under two control conditions is studied,and the ultimate bending capacity of pipelines is calculated.Meanwhile,the failure law of pipelines with defects under combined load is further studied,and the effects of control conditions,load,dent size and dent depth on the ultimate bending capacity of pipelines are analyzed.Finally,the effects of two control conditions on the calculation results of local buckling and ultimate load of pipelines are summarized.The numerical simulation model of subsea pipelines with corrosion-dent combined defects is established.Based on the existing test results,the reliability of the model is validated,the buckling failure mechanism of submarine pipelines with combined defects under bending moment load is studied,and the sensitivity analysis of parameters affecting the ultimate bending capacity of pipelines,such as dent size,corrosion size and dent depth,is analyzed,The changing law of ultimate moment capacity of subsea pipelines with combined defects is obtained.Finally,a nonlinear regression method is used to predict the ultimate bending capacity of subsea pipelines with corrosion-dent combined defects,which provides a certain reference for the safety evaluation of subsea pipelines with complex defects.The numerical simulation model of pipe in pipe with dent defects is established,the eigenvalue buckling analysis of pipe in pipe under different pipe lengths is carried out,the buckling mode,ultimate bending capacity and bending strain under bending moment load are studied,the sensitivity analysis of parameters affecting the ultimate bending capacity of pipe in pipe is analyzed,and the variation law of ultimate bending capacity of pipe in pipe with defects is obtained.Finally,based on the regression of finite element results,the prediction method of ultimate bending capacity of pipe in pipe is proposed,which provides a certain basis for the residual strength evaluation of pipe in pipe.