The Theoretical Analysis And Numerical Simulation Of Pressure-bearing Capacity During In-service Welding Of Oil/gas Pipelines

In-service welding plays an increasingly important role with the improvement of pipeline network.Therefore,reasonable predictions for the maximum pressure-bearing capacity of pipeline become an crucial prerequisite to ensure the safety of in-service welding.This paper stated with radical deformation of pipe wall,and then studied the effect of various parameters such as wall thickness,diameter,medium type and heat input on weldable pressure.At the same time,the more appropriate formula used for weldable pressure prediction were ascertained by modifying the existing formula.By comparing the predictive value of weldable pressure gotten by each formula under different pipeline parameters,we found that the weldable pressure gradually increased with growing wall thickness,decreasing diameter and increasing material strength.The influence of wall thickness on weldable pressure was greater than that of diameter and material strength.The wall thickness of pipe was the only factor that affected the relative size of the predictive value of each formula.the predicted value in GB/T28055 gradually became more conservative for the pipeline with thickness from 3.4 to 6.4 mm.The ASME B31.8 was not suitable for the prediction of weldable pressure when the welding heat input was relatively large.Combined with the actual welding process,the heat transfer coefficient of pipe with different medium was corrected.And the radical deformation of pipeline was analyzed with1/2 pipeline model by using SYSWELD.The results showed that the maximum radical deformation of pipeline could be found in the vicinity of welding pool.The radical deformation gradually grew with the increase of medium pressure.Also,The medium pressure which corresponded to the yield point of maximum inner wall radial deformation curve was the maximum weldable pressure.Under the same condition,the pressure-bearing capacity of blank pipe was greater than that of gas pipeline and oil pipeline.Compared to the first weld bead,the second weld bead increased the radical deformation of pipe,but have no influence on pressure-bearing capacity.Based on those results above,the maximum weldable pressure needs be determined by maximum radial deformation curve of inner wall with relatively larger heat input during the process of single pass welding without medium.The pressure-bearing capacity of in-service welding pipe according to the radical deformation was compared to the theoretical results,which showed that the GB/T28055 and ASME B31.8 had a wider application in the four formulas.Confined to the effect of pipe wall thickness on the safety factor and temperature reduction coefficient,GB/T28055 was more suitable for weldable pressure prediction of thin-wall pipe.Once the pipe wall thickness exceeded the critical value t?which was gradually increased as the welding heat input increasing,ASME B31.8 would predicted the weldable pressure more accurately.Considered the practical weldable pressure,the temperature correction factor k which depended on the inner wall temperature level was introduced in the modification of GB/T28055 when the wall thickness was larger than t?.When the ratio of welding heat input to the square of wall thicknessand the maximum temperature of inner wall of the pipe were determined during in-service welding,the value of k could be defined by the SYSWELD analysis.Therefore,k was a function of E/t₂.