Study On Evaluation Method Of Residual Strength Of Pipeline Under The Coupling Effect Of Internal And External Defects

As a special equipment for conveying fluid media,defects will inevitably occur in the process of operation,resulting in thinning of the pipelines in the direction of wall thickness,reduction of the residual strength of the pipeline and stress concentration.In serious cases,it may even lead to leakage or explosion of the pipeline.At present,the evaluation methods used in the research on the failure mechanism and failure pressure of pipelines with multiple defects at home and abroad consider the influence of simple external defects or internal defects.There is a lack of research on the evaluation method of pipeline residual strength considering the joint action of internal and external defects at the same time.Therefore,the research on the evaluation method of pipeline residual strength under the coupling of internal and external defects has practical engineering significance and value.Based on the finite element method,this paper analyzes the buried pipe section with one-sided defects and internal and external defects,and probes into the influence of the size and relative position of multiple defects on the residual strength of the pipeline.On the basis of this,the influence of the size and relative position of internal and external defects on the residual strength of pipeline is deeply explored,the interaction criterion between internal and external defects is established,and the calculation method of equivalent parameters of internal and external defects is given.A model for calculating the residual strength of the pipeline under the coupling of internal and external defects is established to realize a more reliable and reasonable evaluation of the residual strength of the pipeline.The main achievements are as follows:（1）The pipe segment models with different sizes and different positions of ellipsoidal defects are established by using ABAQUS finite element simulation software,and the effectiveness and feasibility of the finite element modeling method is verified,on the basis of which the related research work is carried out.（2）Based on the simulation data of failure pressure of pipelines with different sizes of unilateral defects under internal pressure,the influence of defect size parameters on pipeline failure pressure is obtained,and the key defect characteristic parameters affecting the residual strength of pipelines are identified.The results show that the residual strength of the pipeline is mainly affected by the length and depth of the defect,while the influence of the width and the angle between the long axis is negligible.When the axial length and depth of the defect increase,the pipeline failure pressure decreases gradually.For the pressure value of pipeline failure,the influence of defect depth is greater than that of defect axial length.（3）Based on the simulation data of failure pressure of single-sided multi-defect pipelines with different sizes and different positions under internal pressure,the variation law of residual strength of single-sided double-defect pipelines under internal pressure is analyzed,the interaction criterion between single-sided double defects is determined,and it is concluded that the limit action distance L_lim between single-sided multi-defects is related to the depth and length of defects.The interaction criterion between one-sided defects based on defect depth is established:when d≤0.6,the limit action distance of the axial distance between adjacent defects is L_lim=1.5Dt^1/2;when d>0.6,the limit action distance is L_lim=2.0Dt^1/2.And through the comparison with the interaction criterion in the existing standard,the accuracy of the finite element calculation result is verified,and the finite element analysis of the interaction between the defects is reliable.（4）By establishing the finite element model of the pipeline with internal and external defects,the difference of the influence of the defect location on the residual strength of the pipeline is analyzed.he results show that the internal defects have a greater adverse effect on the residual strength of the pipeline,and the difference between the internal and external defects decreases with the increase of the length and depth of the internal and external defects.（5）Based on the simulation data of pipeline failure pressure under different sizes and relative positions of internal and external defects,the influence of internal and external defects on pipeline residual strength is analyzed,and the simulated failure pressure is taken as the basis for analysis.5%criteria are used to determine the interaction between internal and external defects,and the interaction criterion between internal and external corrosion defects is established.It is concluded that the limit influence distance between internal and external defects is related to the depth and length of internal and external defects.The depth of internal and external defects increases,the limit influence distance between internal and external defects increases,and the increasing trend decreases.With the increase of the length of internal and external defects,the limit action distance of internal and external defects increases,but the increasing trend decreases.The limit action distance of inner and outer defects based on depth and length is L_lim=1.6Dt^1/2.（6）Based on the simulation data of residual strength of internal and external defect pipeline and the interaction criterion of internal and external defects,the influence weight of key characteristic parameters of internal and external defects on failure pressure is determined by nonlinear fitting.Combined with the key characteristic parameters of internal and external defects,the equivalent characteristic parameters of internal and external defects are calculated.Finally,the residual strength calculation model of internal and external defects is established to evaluate the residual strength of pipelines.And establish a complete set of pipeline residual strength evaluation methods under the coupling effect of internal and external defects to evaluate the residual strength of pipelines.