R&D helping to repurpose infrastructure to transport hydrogen

Fit for purpose

At present various strategic projects have been initiated at high-level assessment levels on the potential for repurposing the existing infrastructure, but with very little shared technical data or analysis. Consequently, Wood launched our own R&D initiative to review the available international onshore and offshore pipeline codes and standards to understand their applicability for H₂ delivery and assessing the real feasibility of repurposing these pipelines. The transportation of H₂ via pipelines is not new, however compared to natural gas and liquids, there is less data available on H₂. Although guidelines related to H₂ do exist, they are either conservative in their approach to materials and safety issues or neglect them altogether. Similarities are drawn to the carbon dioxide pipeline design initiatives in the early 2000’s, for which specific codes have only recently been produced or updated.

To date, only one code (ASME B31.12) addresses H₂ specific material requirements such as embrittlement and degradation of fracture toughness. As such, it is currently considered the governing code within the H₂ pipeline industry. This code allows for a flexible design approach in which both new and repurposed pipelines of various steel grades may be designed.

The ASME B31.12 code provides two options; option A, which applies penalising design restrictions, whereas option B allows for high grades of material and increased design factors (only where proven through testing in a H₂ environment).

Through our work we have identified where international codes and standards require updating to be applicable for both new and repurposed H₂ pipelines, as their current form doesn’t allow best practice design.

Feasibility

There are material and testing challenges when designing pipelines to ASME B31.12 option B, high design factors and high material grades. This applies to the construction of new pipelines, but even more so, to repurposing of existing lines as they have been constructed from materials not specified with the necessary material requirements needed to withstand the H₂ challenges.

In the repurposing of natural gas pipelines to make them suitable for H₂, it is anticipated the redesign is potentially restricted to option A. The acceptability of the existing pipeline and welds is assessed by reviewing existing documentation, in-line inspection, and testing, as necessary, of representative pipe and welds to confirm compliance with the hardness limits of the code. The availability of existing infrastructure welding and materials data will be challenging putting a greater emphasis on destructive testing and in-line inspections. It should be noted that even when applying option A, fracture mechanics testing in hydrogen environments may still be required to confirm acceptability of any defects detected or hardness deviations above the code limit before a pipeline can be repurposed for H₂ service.

Showing the value

Case studies have been investigated for the repurposing of steel onshore and offshore pipelines for H₂ service.  The study topics included the code minimum wall thickness requirements for natural gas and H₂ and the reduction in maximum allowable operating pressure (MAOP) associated with change of service to H₂ (ASME B31.12 option A). The potential drop in MAOP for onshore and offshore pipelines for a change in service from natural gas (NG) to H2 could result in a de-rating in range of 29% to 38% and 37% to 54% respectively based on code compliance to ASME B31.12 option A.

Typical offshore wall thickness is not governed by pressure containment and this accounts for the increased MAOP when repurposing. Additional considerations when repurposing not covered within the case study include longitudinal stress, free spans and fatigue risk, notably for offshore pipelines.  It should be noted that if repurposing can be conducted to ASME B31.12 option B, there is little to no reduction in MAOP.

Pipeline derating on energy capacity

For practical reasons, it is essential to assess the impact of repurposing on the total energy which can be provided to the end user by replacing natural gas with H₂. Hydrogen as an energy carrier has by far the highest energy density by mass; the mass-based energy density of H₂ is roughly two to three times higher than that of methane or natural gas, however the volumetric energy density of hydrogen is comparatively low. Therefore, for practical transport purposes, the flowrate and density of H₂ should be maintained as high as possible providing the end user with the most energy. De-rating the MAOP results not only in a lower available pressure (and hence flowrate), but in lower mass density of transported fluid. This creates less energy transfer through the pipeline for the end user and potentially not meeting demand.

Wood performed a range of calculations, comparing both energy carriers using rudimentary models for different scenarios. Based on the comparisons of energy flow and pipelines capacity between natural gas and H2, pipelines repurposed can be capable of achieving circa 70-80% of the energy flow rate if MAOP can be maintained. However, the de-rating in the MAOP can reduce the achievable energy content to 50-60%. If other operational restrictions are considered, such as flow velocity (e.g. due to the history of the pipeline, residue of solid/liquid from past operation and system / equipment integrity) it can potentially have additional knock-on effect on the achievable flowrate and consequently to energy ratio. This should be assessed on case-by-case basis.

The possibility of adding/blending H₂ to the gas network also comes with huge opportunities and additional challenges. This would require a detailed understanding of blending threshold during normal and transient operations in to eliminate the potential material, integrity, and operational issues as well the pipeline design implications. This can be studied from a design perspective and achieved by applying advanced hydrogen tracking tools e.g. Virtuoso®.

Roadmap to repurpose

It’s clear that existing natural gas lines can be repurposed for hydrogen, however, limitations may apply in their design and operation. As such, a road map to repurpose pipeline systems for future H₂ has been developed to identify infrastructure for repurposing and to maximise the design pressure and energy flow for hydrogen.

In terms of organisations, this work has relevance to gas transmissions and distribution operations along with hydrogen producers. We’re seeing the major transmission system operators and distribution system operators opening tenders for repurposing pure H₂ and NG/H₂ blending projects in Europe, the Middle East and Africa as well as midstream operators in the United States. For projects as part of a wider scope, developers in Asia-Pacific are looking to produce H₂ for export to the Asian market, as well as large scale green H₂ projects which will need connections to grid systems across the global market. The market is also seeing conventional energy operators looking at integrating green or blue H₂ into existing operations.

The common thread being that many of our clients are actively pursuing H₂ projects and will need some method of moving it. This means that a range of existing pipelines will need to be assessed individually considering the limitations, requirements and potential benefits of repurposing the lines, noting that in some cases even where a pipeline already exists, a newer pipeline may offer the best solution. Through this work we can provide solutions to operators that are facing major challenges and risks around code compliance, material compatibility and H₂ flow assurance.

Although this article focuses on transportation of H₂ via carbon steel pipelines, Wood’s Hydrogen Pipeline Taskforce has conducted a parallel study into the use of non-metallic pipelines such composites and liners which play an important role in the intermediate and low-pressure segments of gas networks.

Future of hydrogen networks

Hydrogen is expected to play a critical role in the future energy transition, but cannot solely be produced at the point of use. To move H₂ over distances, users either must ship it, build new pipeline infrastructure or repurpose existing gas networks. Every time hydrogen is converted between energy vectors along the chain from production, through transportation and on to storage and use, this results in efficiency losses.

The selection of H₂ transportation method and vector is multi-faceted and requires early evaluation to ensure the process is optimised. For short to mid-distance transmission, for example, linking industrial hydrogen producers with residential users, gas networks can be used to minimise losses, as no conversion is required.

Hydrogen transportation by pipeline is a complex process with additional challenges when infrastructure repurposing is the goal. Updates to design codes and standards, joint industry practices (such as the DNV H2Pipe where Wood participates) and further collaboration with operators, technology IP owners and pipe mills, particularly in relation to testing and operational data, are crucial to unlocking H₂ transportation through existing pipelines. Our early evaluation of existing infrastructure fast tracks client’s energy transition needs and reduces their overall capital expenditures. This work puts us at the forefront of this rapidly developing technology, allowing clients to maximise the hydrogen transportation potential of new and existing infrastructure.