Kjell Erxleben

In-service welding of pipelines without interrupting the internal gas flow is nowadays state of the art for the maintenance and extension of the grid. However, due to access restrictions, it is quite challenging or even impossible to measure local temperatures during welding and subsequent cooling. To address this issue, in the present work welding experiments were performed that consider representative weld configurations and process parameters relevant for in-service welding of gas pipelines. A flat steel sheet was multi-pass welded onto the outer surface of a straight steel pipe segment by manual shielded metal arc welding (SMAW) or by manual gas tungsten arc welding (GTAW). Thermocouples were used for measuring local temperatures at defined positions next to the weld seam. The influences of joint type, wall thickness of the sheet and pipe and heat input on the temperature distribution and, in particular, on the local peak temperatures and cooling rates were studied. Moreover, a finite element (FE) model of a selected weld configuration was created and validated for calculating the temperatures at positions that were inaccessible for direct measurements. The results of the measurements showed that the peak temperature at the inner surface of the pipe decreases with increasing wall thickness. However, independently from the wall thickness, the results of the simulations indicated austenitization next to the weld seam. The low cooling rates did not promote any martensite transformation, as confirmed by the moderate increase of the hardness and by microstructure characterization. The results are of practical interest for pipelines used to transport hydrogen gas, as the temperature-dependent composition of the microstructure and the temperature itself influence the absorption, diffusion and solubility of hydrogen in steel.

Hydrogen is the energy carrier for a sustainable future without fossil fuels. As this requires a reliable transportation infrastructure, the conversion of existing natural gas (NG) grids is an essential part of the worldwide individual national hydrogen strategies, in addition to newly erected pipelines. In view of the known effect of hydrogen embrittlement, the compatibility of the materials already in use (typically low-alloy steels in a wide range of strengths and thicknesses) must be investigated. Initial comprehensive studies on the hydrogen compatibility of pipeline materials indicate that these materials can be used to a certain extent. Nevertheless, the material compatibility for hydrogen service is currently of great importance. However, pipelines require frequent maintenance and repair work. In some cases, it is necessary to carry out welding work on pipelines while they are under pressure, e.g., the well-known tapping of NG grids. This in-service welding brings additional challenges for hydrogen operations in terms of additional hydrogen absorption during welding and material compatibility. The challenge can be roughly divided into two parts: (1) the possible austenitization of the inner piping material exposed to hydrogen, which can lead to additional hydrogen absorption, and (2) the welding itself causes an increased temperature range. Both lead to a significantly increased hydrogen solubility in the respective materials compared to room temperature. In that connection, the knowledge on hot tapping on hydrogen pipelines is rare so far due to the missing service experiences. Fundamental experimental investigations are required to investigate the possible transferability of the state-of-the-art concepts from NG to hydrogen pipeline grids. This is necessary to ensure that no critical material degradation occurs due to the potentially increased hydrogen uptake. For this reason, the paper introduces the state of the art in pipeline hot tapping, encompassing current research projects and their individual solution strategies for the problems that may arise for future hydrogen service. Methods of material testing, their limitations, and possible solutions will be presented and discussed.