In the near future, hydrogen will be transported from producers to consumers on the large scale exclusively by long-distance transmission pipelines, whereas the vast majority will be converted NG pipelines. Like in the NG grid, welding work on/onto pipelines as part of the “hot-tapping” is unavoidable e.g. for maintenance or in case of grid expansion. Meanwhile, the mere compatibility of low-alloyed pipeline steels under pressurized hydrogen was proven. But for repurposed NG pipelines (in use for decades), the transferability of in-service welding concepts is partially discussed. The reason is that NG does not penetrate the pipeline, whereas hydrogen can show the embrittling effects, which strongly interfere with possible defects in the legacy pipeline. The inner pipeline surface undergoes multiple short-term heating to high temperatures, which accelerates the hydrogen diffusion and increases the solubility. In particular for pipes with small wall thickness, the first welding passes can reach an austenitic transformation close temperature. Hence, the pipeline integrity vs. hydrogen embrittlement (HE) susceptibility is of vast interest, despite testing of the diverse material and thickness combinations is challenging. For the first time, welding experiments on pressurized DN300 pipeline-like demonstrators were conducted at approx. 85 bar hydrogen. For wall thicknesses between 5.6 and 6.3 mm this allowed realistic hoop stresses during welding of approx. 50 % of the individual SMYS. Using newly developed sample extraction routines, it was possible to quantify the hydrogen ingress in the material for both, the weld metal and the HAZ. It turned out that the existing surface oxides effectively limited the unavoidable hydrogen uptake during welding (compared to thermodynamic-based calculations). Hence, HE was unlikely to occur and confirmed by comprehensive NDT during and after welding. Accompanying numerical calculations suggest that the gas flow speed must be decreased during welding to limit the internal cooling effect of the gas