Hydrogen is the energy carrier of tomorrow for a fossil-free future. This requires a reliable transport infrastructure capable of transporting large quantities of hydrogen. In addition to the construction of new pipelines, the conversion of existing natural gas (NG) networks is an essential part of global hydrogen strategies. The transport of hydrogen is fundamentally different from that of NG, as hydrogen can be absorbed into the pipeline material. However, pipelines require frequent maintenance, repair, or the need to install additional outlets. In some cases, it is necessary to perform welding on or to the pipelines while they are still in service (i.e. under pressurized hydrogen gas flow). This in-service welding poses challenges in terms of hydrogen absorption during welding. The challenge can be roughly divided into the possible austenitization of the inner pipe material exposed to hydrogen, which can lead to sufficient hydrogen absorption, and the welding itself, which causes an increased temperature range. Both lead to a significant increase in hydrogen solubility and diffusivity of the respective materials compared to room temperature. In this context, knowledge about hot tapping on hydrogen pipelines is scarce due to the lack of operational experience. Fundamental experimental investigations are required to investigate the transferability from natural gas to hydrogen pipeline networks. For this reason, the present study presents a specially designed mock-up / demonstrator concept for the realistic assessment of the welding process conditions. The mock-up was designed to allow in-situ temperature measurement during the welding process as well as ex-post sample extraction for quantification of the absorbed hydrogen concentration. Welding experiments on the DN60 and DN200 pressurized mock-ups showed that the austenitizing temperature can be reached on the inner surface of the pipeline, especially on thinner-walled pipelines, using current welding practices. This corresponds to an increased hydrogen uptake in the welded area of several ppm.