Ultimate Bearing Capacity Study Of Large Diameter Bend With Ellipsoid Drop Pit

Corrosion pit is a common flaw of oil and gas pipelines.Having corrosion pits does not mean losing bearing capacity.Repairing or replacing with carrying capacity will cause unnecessary economic losses.In order to develop the potential of the material and reduce the repair frequency,careful mechanical analysis of pipeline with corrosion pits need to be carried out and the safety range of ultimate bearing capacity should be figured out.Bend pipe as one of the most essential parts of long distance pipeline is suffering from more complicated stress meanwhile being a probable of high stress concentration having pits will add risks to the safety of pipe.In order to guarantee pipeline safety and reduce economic loss,it is of great importance to study on the ultimate bearing capacity of bend pipes.The 90° bend pipes made of X80 with radius of curvature in 6d and diameters in 1016mm and 1219mm are taken as objects in this paper.According to the theories of pipeline stress under internal pressure and combined with finite element simulation software ABAQUS,ultimate bearing capacity of bend pipes with different pit types has been studied.Circumferential stress and axial stress of bend pipe under internal pressure has been analysed and the plastic failure criteria and twice elastic slope method are chosen as the standards of confirming the ultimate load.Modeling analysis of bend pipes without pits is completed by ABAQUS and compared with the theoretical calculation value,the reliability of boundary conditions can be confirmed.Modeling analysis of bend pipes with 25 defect locations,6 defect depths,5 defect lengths,4 defect widths and 3 wall thicknesses under the same boundary condition has been done and 118 ultimate load values of different defect parameters have been obtained.The change rule of the ultimate load under each defect parameters and numerical fitting of each defect parameter have been studied.Nonlinear regression of fitting formula and ultimate load data is done by MATLAB,and ultimate load fonnula of bend pipes with pits is obtained and its rationality is confirmed.Finally,the integration formula is compiled into a ultimate load calculation software of bend pipes with or without pits by Visual Basic.This provides references to safety assessment of large diameter bend under actual working condition and gives suggestions to timely repair and replacement of the pipes.The research results of the thesis can provide scientific guidance to operational decision and dispose of pipeline under the same working condition.On the premise of safety,potential of the pipe material has been developed and unnecessary economic losses have been reduced.Meanwhile,the research results provide guidance and reference to integrality assessment and residual strength assessment of large diameter pipe with defects.