Numerical Simulation Of X80 Pipeline Girth Welding And Limit Load Analysis Of Pipeline With Welding Defects

The rapid development of economy is inseparable from the rational use of energy.As the carrier of long distance transportation of oil and natural gas,pipeline plays an important role in energy transportation.Long distance oil and gas pipelines are connected by welding.The welding residual stress caused by local uneven heat input in the welding process will reduce the bearing capacity of welded components.Long distance oil and gas pipelines are connected by welding.The welding residual stress caused by local uneven heat input in the welding process will reduce the bearing capacity of welded components.Welding pipeline also has many welding quality problems,which affect the safe operation of pipeline.It is of great significance to study the residual stress distribution of pipeline welding joint and the influence of welding defects on the bearing capacity of pipeline for guiding pipeline welding process and improving welding quality.This paper first describes the theory of welding finite element analysis,and introduces the mechanism of welding residual stress formation.Taking the China Myanmar natural gas pipeline as the research object,based on the thermal elastoplastic finite element analysis theory,the finite element model of X80 pipeline girth weld is established,and the transition mesh of pipeline welded joint is divided.The DFLUX subroutine of welding moving double ellipsoid heat source is written in Fortran language,and the shape parameters of double ellipsoid heat source are adjusted according to the shape of pipe welding pool.Based on the single pass multi-layer welding process,the appropriate welding voltage,welding current,heat source moving speed and other parameters are selected.The dynamic process of multi-layer girth welding of pipeline is simulated by birth and death element technique.The temperature gradient distribution of the moving heat source in each layer of the pipeline was observed and compared with the double ellipsoid heat source parameters.Finally,the thermal cycle of each point of the pipeline welded joint was analyzed.Secondly,according to the calculation results of welding temperature field,taking the transient temperature value of the pipe model node during welding as the load,the welding residual stress of the pipe welding joint is calculated by using the sequential coupling method.Considering the influence of post weld heat treatment on residual stress,the distribution of circumferential and axial welding residual stress on the inner and outer surface of pipeline before and after heat treatment is summarized.In order to verify the reliability of the simulation results,the blind hole method was used to test the welding residual stress.Finally,focusing on the welding quality defects,the finite element model of the pipe with porosity and incomplete penetration defects is established.For the inhomogeneity of mechanical properties of the welded joint,Ramberg-Osgood constitutive model is used to define the material parameters of the weld,heat affected zone and base metal respectively.The influence of pore diameter and location,circumferential length and depth of incomplete penetration defect are considered.Based on the plastic failure criterion,the bearing capacity of pipeline under internal pressure and bending moment is studied,and the prediction formula of ultimate bearing capacity of pipeline with welding defects is proposed.