Research On Safety Analysis Of In-service Pressure Piplines Containing Incomplete Welding Defects Based On Limit Load

The defects are inevitable since pressure pipes are usually adopting manual arc welding whose quality is difficult to control, and incomplete welding defects are in the majority, which is a potential threat of the safety. Therefore, reasonable evaluation of incomplete welding defects is significant to ensure the safety of pressure pipes. The failure model of pipes, made of low and medium strength steels, is frequently dominated by plastic collapse in our country.By means of the theoretical analysis, non-linear FEA and experiments, the limit loads and safety evaluation of the incomplete welding defect of pressure piping under combined loads of internal pressure, bending and torsion moment was systematic researched in this paper. The main research achievements and creative ideas are as follows:(1) Theoretical correlative formulas of plastic limit load for pressure piping with incomplete welding defects under combined bending, torsion and internal pressure loadings are deduced. The loading range of Net Section Collapse Criteria (NSC) is extended from combined tension and bending loadings to combined bending, torsion and internal pressure loadings. Using the non-linear FEA to verify the theoretical correlative formulas is accuracy.(2) Engineering estimation formulas of plastic limit load for pressure piping with incomplete welding defects under pure internal pressure, pure bending moment or pure torsion moment are deduced.The influences of pipe curvature, circumferential length and depth of incomplete welding defects are considered.(3) An experimental four-point bending test device used for testing the limit load of pressure piping under combined loads was designed. Using the self-designed limit load test device, the experiments of pressure piping containing incomplete welding defect are carried out. The experimental results compared to those from FEA, which indicate that the calculation of FEA and engineering estimation formulas are reliable.(4) Safety assessment method for pressure piping with incomplete welding defects under combined bending, torsion and internal pressure loadings are deduced. Two in-service industrial pipelines containing incomplete welding defects scored 4 degree by Regular inspect criterion are assessed by the safety assessment method.