How existing pipelines can be used for the hydrogen economy

What was the starting point for the publication?

Dr. Robert Fussik: The transport of hydrogen is a key issue in the transition to the hydrogen economy. To ensure the fastest and most cost-effective transition possible, we examined the possibility of converting existing natural gas pipelines for hydrogen transport.

The challenge is that hydrogen has different product properties and changes material behavior. Only in recent years have pipeline steels been qualified for their mechanical properties under pressurized hydrogen, with the majority of the results relating to American pipeline steels.

In order to take these changes in mechanical properties into account for use in German pipelines, we applied fracture mechanics methods in our joint study based on the DVGW (German Association for Gas and Water) regulations and ASME B31.12 in order to be able to make a well-founded statement on the safety and service life of existing pipelines.

What exactly was investigated?

Julius Langenberg: In the DVGW project SyWeSt H2, various pipeline steels from German long-distance network operators were tested for fracture toughness and crack growth in relation to the material requirements of ASME B31.12. Our test data for the two materials used, which correspond to the American steel grade API 5L X70, were generated within this project and include longitudinal and circumferential welds. The samples were tested in air and under pressurized hydrogen to analyze possible degradation under different pressure levels and cyclic loads (R-value).

The fracture mechanics calculations for the pipeline evaluation were based on AMSE B31.12 and DVGW G 464. For this purpose, we used real loads from three years of natural gas operation for the cyclic crack propagation calculation, including an assumed full-load cycle every 30 years for maintenance purposes. In addition, the actual pipe geometry and an error assumption of five percent of the wall thickness derived from non-destructive material testing were taken into account. The operating pressure corresponded to the previous maximum permissible operating pressure of the pipeline, and we considered weld residual stresses of 100 MPa in addition to existing regulations.

What was the most important result?

Julius Langenberg: With material X70, it is clear that hydrogen has a particularly strong influence on the crack propagation rate. Although toughness is significantly affected, it remains at a high level. We were also able to show that the base material and welds exhibit very similar properties under hydrogen exposure.

Both materials tested meet the requirements of ASME B 31.12. The fracture toughness is even more than twice the required minimum value. Based on this, we were able to conduct fracture mechanics calculations for the conversion of an existing natural gas pipeline. Even with conservative assumptions, such as a nominal fracture toughness of 55 MPa√m and residual stresses of 100 MPa, this results in a calculated service life of over 500 years.

What do the results mean for a network operator like GASCADE Gastransport GmbH?

Dr. Robert Fussik: There are several aspects. First of all, the results showed me that we can proceed with the project implementation with confidence.

The conversion of our first existing pipeline was not dependent on the publication, as it had already taken place. Nevertheless, the positive results were highly relevant for validating our fracture mechanics assessment during the conversion.

The fracture mechanics assessment in the publication was also conducted by IWT-Solutions AG, thus by independent third parties. Finally, I consider it a milestone that the study results have been assessed and accepted by international experts as part of a peer review process. This means that the approach and results are recognized and reliable. We can refer to this publication in future conversion projects.

The project was implemented jointly by IWT-Solutions AG and GASCADE Gastransport GmbH. How was the collaboration?

Julius Langenberg: We complemented each other very well. GASCADE, as an experienced long-distance pipeline operator, was able to provide us with the necessary information on pipe loads, pipe dimensions, and material properties. Through this collaboration, we were able to implement the assessment for a real pipeline and thus create added value for the transition to the hydrogen economy.

Dr. Robert Fussik: The collaboration was constructive and brought us added value. This meant that I was always aware of the specific results, regardless of the publication date. This accelerated our internal process for the fracture mechanics assessment for the specific conversion of one of our pipelines.

What role does the hydrogen economy play for GASCADE Gastransport GmbH?

Dr. Robert Fussik: The study is part of our “Flow – making hydrogen happen” program, which envisions the world’s first conversion of a pipeline with a diameter of 1.4 m. Fundamentally, we believe in the successful ramp-up of the hydrogen market and are working intensively to ensure its success. In October 2024, the German transmission system operators received approval for the German Hydrogen Core Network from the Federal Network Agency. This provides us as a network operator with a robust legal framework to advance the transport infrastructure and thus create planning security for the upstream and downstream value creation stages.

What do you hope for in practice?

Julius Langenberg: More courage to bring together science and practical application. If we take hydrogen seriously as an energy carrier, we need not only innovation, but also safety and efficiency. Fracture mechanics is key to combining these goals. With an individual assessment, very good, reliable statements can be made about whether an existing infrastructure can continue to be used. The key is to analyze the real loads, the material properties, and the boundary conditions of the hydrogen and the pipe. We have laid the methodological foundation for this.

The full publication can be found here: asmedigitalcollection.asme.org/pressurevesseltech/article-abstract/Evaluation-of-Existing-Natural-Gas-Pipelines-for-Repurposing-to-Hydrogen-Transport-by-Means-of-Fracture-Mechanics