The experimental and numerical study has been conducted on the sleeve-repair welding of API 5L X65 pipeline. In order to simulate the in-service condition, the internal pressure of 45kgf/cm2 was applied using a nitrogen gas. SMAW and GTAW were applied to weld the sleeve. The macrostructure and hardness of repair welds have been examined. The finite element analysis of the multi-pass sleeve-fillet welding has been conducted to validate the experiment and investigate the effects of in-service welding conditions. The effect of gas flow rate on the hydrogen cracking was investigated. The effect of internal pressure on residual stresses and plastic strain was investigated. The allowable heat input was predicted considering the maximum temperature of inner surface of pipe and cooling rate at CGHAZ from the simulation of 1-pass sleeve-fillet welding.

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