A branched pipe joint has been employed to execute the pressure control, condition check, purgation, and distribution of the gas in the natural gas facilities. Installation of branch pipes is generally done through the welding work, and as a welding process, the weldolet and the sockolet are used. During the maintenance working of in-service natural gas pipeline, there was gas leakage in sockolet weldment. The causes of incident were investigated with various tests. We found the wrong pipe material, the weld defect and the non-destructive test limitation of fillet weldment as the reasons of gas leakage. As the follow-up measures, it was done to assess the soundness depending upon the configuration of the weld zone, a change in the welding process and a change in the pipe diameter by assessing the mechanical properties of the sockolet weld zone and further to assess comparatively the mechanical performance of the sockolet weld zone and that of the weldolet weld zone. In the sockolet weld, the tensile strength showed no difference and the fatigue strength showed a difference depending upon a change in the welding process. In the case that the leg length of the weld zone was made lengthwise in the direction of the branch pipe, the SMAW welding work compare with the GTAW, the sectional area of the weld zone was more increased, and the pipe diameter was more increased, the fatigue strength was increased.

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