Numerical Analysis Of Repair Welding And Bearing Capacity Of X80 Pipeline With Defects

The failure and cracking cases of X80 pipeline in service in recent years indicate that repair welding has an important impact on the safety of the pipeline.Due to the high weld restraint of local repair welding,it is highly possible to generate high residual stresses in the welding area.At the same time,its harsh construction conditions pose a challenge to the precise implementation of the welding process.Therefore,it is important to clarify the impact of the residual stresses generated during the repair welding process and the bearing capacity of the repaired welded pipeline on ensuring pipeline safety.In this paper,the residual stress and joint bearing capacity of X80 pipeline repair welding were studied by combining finite element method and experimental method.After the X80 pipeline butt welding experiment,the macro metallography of the joint was prepared,and the thermal cycle curve of the welding process was obtained through the thermal cycle test.The residual stress of the butt weld was obtained using the blind hole method test.A finite element model of butt welding was established based on the macroscopic metallography of the joint.The accuracy of the heat source was verified by comparing the morphology of the weld pool,the accuracy of the model temperature field was verified by comparing the welding thermal cycle,and the accuracy of the model stress field was verified by comparing the stress amplitude and distribution.Finally,applying different internal pressure to the joint,the stress distribution and bearing capacity of the joint were analyzed.The results show that the peak equivalent stress of X80 pipe joint is 395 MPa.When the bearing internal pressure reaches 14 MPa,the peak equivalent stress of the joint reaches the yield strength;When the internal pressure reaches 23 MPa,the stress level at the weld toe first reaches the tensile strength,and the joint fails.After butt welding,local repair welding was carried out on the joint cover,and the characteristics of residual stress redistribution in joints with different repair welding sizes were studied.The influence of repair welding size on bearing capacity was analyzed.The results show that when the repair welding lengths are 25 mm,40mm,and 55 mm,the optimal repair welding length is 55 mm,and the ultimate internal pressure is only reduced by 7%;When the repair welding widths are 15 mm,18.4mm,and 22 mm,the optimal repair welding width is15 mm,and the ultimate internal pressure is only reduced by 22.6%;When the repair welding depth is 5mm,7.5mm,and 9.2mm,the optimal repair welding depth is 5mm,and the ultimate internal pressure is only reduced by 21.8%.The longer the repair welding length,the shallower the depth,the narrower the repair welding width,and the better the bearing capacity.Two common situations in the process of butt welding are analyzed.One is the occurrence of defects at the root after the completion of butt welding,and the impact of root cracks and porosity defects on the residual stress and bearing capacity of the pipeline is studied;The other type is that after the completion of butt welding,there are defects in both the cover position and the root.The impact of the cover repair welding process on the root defects is studied,including the stress distribution at the defects and the changes in the bearing capacity of the defective pipeline.The results show that the ultimate bearing internal pressure of the joint containing only root pores decreases by 22.6%,and the cover repair welding reduces by 29%;The ultimate internal pressure of the joint containing only root cracks decreased by 34%,and the cap repair welding decreased by 41.3%.Finally,the accuracy of the finite element model was verified according to the DNVRP-F101 standard.The ultimate internal pressure of defect-free pipes,unrepaired pipes with root weld cracks,and repaired pipes after welding were 23.5MPa,15.8MPa,and 14.2MPa,with errors of 2.1%,3.9%,and5.1% compared to the simulation results.