Hydrogen storage and transport are key to the energy transition, with pipeline repurposing offering a lower-carbon, immediately available alternative to new construction. However, integrating hydrogen into existing natural gas grids presents challenges, and industry codes for repurposing remain underdeveloped and have yet to align with evolving research.
A key challenge in repurposing pipelines for hydrogen storage is the impact of fatigue cycling on their long-term integrity. Given the storage function, frequent pressure fluctuations are expected, subjecting the pipeline to repeated loading cycles. In hydrogen service, this can significantly reduce the fatigue life of crack-like flaws and of geometric anomalies such as dents.
This case study presents the process for methodically assessing the technical feasibility of converting a European onshore production gas pipeline to hydrogen storage. The objective was to establish a safe operating envelope, maximize capacity, and ensure long-term integrity under varying operational conditions.
As part of developing a green hydrogen hub to power regional industry and end-consumers, an onshore natural gas transmission pipeline has been targeted to store gaseous hydrogen for peak demand periods and buffer intermittent hydrogen production.
The pipeline was initially constructed in the 1970s. Other segments were added later, resulting in various wall thicknesses, pipe grades, pipe types, specifications, and design factors. Although some documentation had been lost due to the change of ownership, there was clear evidence that some materials might not be ideal for hydrogen service due to low ductility and toughness. Therefore, confirmation of the line pipe and weld populations over the pipeline distance and their mechanical properties in gaseous hydrogen was identified as a key requirement for this project.
Due to process disturbances and short periods of wet operation combined with the product’s nature, sour cracking was identified as a potential threat. This suggests that internal cracks may have been initiated during historical service, which requires further detailed diagnosis and analysis for the safe conversion to hydrogen operations.
Finally, geometry, IMU, and dual magnetization inspections had not been performed in the past, so uncertainties existed over the presence of features such as dents, areas of high bending strain, and hard spots. These features affect hydrogen service tolerance and require detailed analysis to assess suitability for conversion. A comparison with historical inspection data from similar pipelines from the Integrity Data Warehouse assured that these were not likely to be ‘show-stoppers,’ but definite confirmation was needed.
The first step was to identify existing integrity threats that require further assessment for hydrogen service due to its lower defect tolerance, such as mechanical property degradation from hydrogen embrittlement or crack growth under static and dynamic loads, as well as potential escalation risks like near-neutral pH stress corrosion cracking (SCC). A systematic review of design, construction, operating and inspection data, and existing integrity management plans was conducted.
The pipeline’s condition (pre-conversion) was then defined against deleterious anomalies such as volumetric metal loss, geometric defects, crack-like defects, hard spots, and bending strain. A fitness-for-service (FFS) assessment was conducted to determine the acceptance of these defects against the targeted operational envelope (pressure cycle) and other potential principal loads (e.g., axial stresses) in hydrogen.
In the next step, a repurposing strategy was developed to address key assumptions and gaps identified in the initial feasibility assessment. This strategy considered industry knowledge of the challenges of using gaseous hydrogen and ensured compliance with local regulations. This helped to obtain a more rational picture of the pipeline fitness-for-service and risk profile under hydrogen service.
Key aspects of the repurposing strategy included:
The feasibility assessment concluded that Scenario 1 (Base Case) and Scenario 2 posed no significant obstacles under the given assumptions. Scenario 3 appeared feasible but required a reassessment of conservative dent fatigue evaluations. However, Scenario 4 introduced severe risks to safe operation and was not recommended for further consideration.
This structured approach provided a clear framework for evaluating the pipeline’s suitability for hydrogen storage. The project delivered a well-founded feasibility assessment by systematically addressing integrity threats, assessing material performance, and ensuring regulatory compliance. This method mitigated risk and optimized the use of existing infrastructure, supporting a more sustainable energy transition.