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As more long-distance oil and gas pipelines with high pressure, large diameter, high steel grade and large wall thickness are constructed, the wall of the fittings such as the hot tapping tees and the repair sleeves that are involved in the in-service welding is thickened increasingly. Thereby, the pipeline maintenance and repair by manual arc welding is at low efficiency, and the welding quality is difficult to be guaranteed. Hence, it is urgent to realize the mechanical construction of in-service welding of oil and gas pipelines. In terms of the selection of welding process, welding test process, the selection of welding materials, the influence of protective gas, as well as the welding procedure qualification test, research was conducted for the automatic welding of oil and gas pipelines. Through the field engineering application of in-service automatic welding of type-B sleeves for the high-grade large-diameter gas pipelines in China, the feasibility of in-service automatic welding process was demonstrated. Finally, some suggestions were put forward for the promotion of in-service automatic welding technology.