In a Nutshell:

The challenges associated with converting an existing natural gas pipeline infrastructure can be summarized in two simple questions for pipeline operators: Can my existing natural gas pipeline be converted to transport a different fluid (hydrogen) instead? And, equally important: How can I manage the integrity of a hydrogen pipeline? In search for an answer to those questions, ROSEN experts Neil Gallon and Roy van Elteren discuss the newest developments in the transition to hydrogen in this article.

The climate emergency is one of the biggest challenges humanity must face in the 21st century. We all need to be involved in the process of moving toward a decarbonized economy. At the same time, the advancing global energy transition faces many challenges in ensuring a sustainable, reliable and affordable energy supply. Economic realities dictate that the existing gas infrastructure will play a role in the future energy system for many years to come. Decarbonization of this infrastructure will inevitably lead to the greater utilization of greener fuels, such as hydrogen.

Challenges Associated with the Transition to Hydrogen

This hydrogen reality has already been recognized by governments and industries across the world, with many initiatives such as the “European Hydrogen Backbone” gaining increasing focus and investment. There are two obvious major challenges associated with the transition to hydrogen: the first is how to generate the hydrogen, and the second is whether the existing pipeline infrastructure can cope with the change in duty from natural gas to hydrogen. If not, what needs to be done to convert existing, potentially aged pipelines? There are options for the generation of hydrogen, ranging from “green” hydrogen generated entirely by renewable energy sources to “gray” hydrogen, which uses natural gas as the primary energy source. Interestingly, “blue” hydrogen, where natural gas is used but carbon capture and storage technology is utilized to minimize greenhouse gas emissions, is increasingly considered a viable solution. There are already many aspects to overcome when looking at how to address these generation challenges; however, regardless of the source of the hydrogen, the challenges associated with transport remain the same.

The challenges associated with converting an existing natural gas pipeline infrastructure can be summarized in two simple questions for pipeline operators:

  • Can my existing natural gas pipeline be converted to transport a different fluid (hydrogen) instead?
  • How can I manage the integrity of a hydrogen pipeline?

As with so many simple questions, there are no simple answers, and these are no exception because the effects of hydrogen are still being extensively researched. ROSEN is currently engaged in EPRG and PRCI activities and has joined various partners in the HYREADY JIP, which aims to provide guidelines for the transition to hydrogen. In parallel with this work, ROSEN is developing a “Hydrogen Integrity Framework” aimed at providing a systematic approach for the safe and economic conversion of existing pipelines to hydrogen. ROSEN’s Hydrogen Integrity Framework is introduced below.

History of Hydrogen

It is often thought that hydrogen is in some way “new” and that the concepts of using hydrogen as an energy source and transporting hydrogen in pipelines are novel. While the conversion of the natural gas infrastructure to hydrogen is new, hydrogen itself is anything but.

There are currently over 4,500 kilometers of hydrogen pipelines in operation throughout the world. These pipelines are almost all made out of carbon steel and transport gaseous hydrogen, although, as far as the authors are aware, these pipelines were all designed and built for hydrogen transport, with only one exception. At a wider level, gaseous hydrogen has been industrially manufactured, stored and transported using carbon steel vessels, pipework and storage bottles for over 100 years. For many years prior to the introduction of natural gas, “town gas,” which contained roughly 50% hydrogen, was widely used for domestic heating and lighting in the UK. Indeed, some of the existing UK urban gas distribution infrastructure was originally designed for town gas.

While the replacement of natural gas with hydrogen and the introduction of hydrogen into modern natural gas transmission and distribution networks introduces challenges, there is nothing new or inherently impossible about the concept of hydrogen pipelines.

Understanding Hydrogen Challenges

As with so many things, to address the challenges introduced by hydrogen, it is first necessary to understand them. This means understanding what makes hydrogen pipelines different, and what the threats associated with hydrogen are.

The integrity challenges and damage mechanisms involved in hydrogen transport can be split into two main areas. Hydrogen can directly cause defects (e.g. cracking), or it can act in a more insidious manner and affect the mechanical properties of the material. Both damage mechanisms result from the dissociation of gaseous hydrogen at the internal surface of the pipeline via Sieverts’ law, leading to the adsorption of hydrogen into the pipe wall. The amount of dissociation and adsorption will largely depend on the hydrogen partial pressure and temperature, while the susceptibility to damage will depend on the amount of absorbed hydrogen, the material microstructure and the stress present. Existing hydrogen codes control these factors by generally being more restrictive than their natural gas equivalents in terms of allowable pipeline materials (often restricting to grade X52 or below) and by operating at lower pressures to reduce the stresses present. These restrictions can be difficult to follow if converting an existing pipeline. In this case, the construction materials are already present, and existing operating pressure can be higher than allowable under hydrogen-specific codes.

It is therefore necessary to understand the challenges and damage mechanisms, and the rationale behind hydrogen-specific restrictions, to identify how to convert pipelines. It is becoming generally understood that direct hydrogen cracking is unlikely as a result of exposure to gaseous hydrogen (there is simply not enough hydrogen present). That said, the effects on mechanical properties can be significant.

The most important mechanical properties of line pipe steel are the strength (yield strength/Rt0.5 and ultimate tensile strength/UTS), ductility, fracture toughness and resistance to fatigue cracking. In general terms, the effect of hydrogen on these properties can be summarized as follows:

Table – Mechanical Properties

The magnitude of these effects, and their implications for pipeline operation, will depend on various factors. Some of these factors will be operational (for example the applied stress); however, some will be microstructure dependent. When converting existing pipelines, which can often be 40 or 50 years old, understanding the microstructures involved (both parent and weld) is both challenging and potentially critically important.

The authors believe that the introduction of hydrogen, while undoubtedly an integrity challenge, is one that can and should be managed within an integrity framework. As with all pipeline-anomaly management activities, the first step should be focused on understanding the pipeline; then, the damage mechanism(s), their morphologies and the associated susceptibility of the pipeline system to them should be thoroughly classified.

Only then can effective inspection and monitoring regimes be specified to accurately target the threat(s). While in-line inspection (ILI) of hydrogen pipelines is challenging, with hydrogen ingress into the tool sensors’ electronics being an elevated risk, it is possible to successfully inspect hydrogen pipelines, as ROSEN has proven in the past.

Hydrogen Integrity Management

ROSEN believes that the best approach to addressing the transition to hydrogen is in the form of a Hydrogen Integrity Framework.

This approach has been proven in related fields – in ROSEN’s Crack Management Framework, for example – and in addressing related threats such as sour gas or CO2. ROSEN’s team in Newcastle, UK, supported by ROSEN’s unique global network, has a proven track record in delivering these frameworks. The approach is founded on the extensive research that has already been completed on issues such as material susceptibility to hydrogen-induced embrittlement or accelerated fatigue cracking, and on the technologies that are already available to map microstructures, material properties, geometry and deformation features where stress levels are elevated, and to detect features that may be starting points for fatigue cracks.

Figure 1 – A systematic approach for de-risking the introduction of hydrogen into existing (natural gas) pipelines

Figure 1 – A systematic approach for de-risking the introduction of hydrogen into existing (natural gas) pipelines

Current conversion feasibility studies and initiatives are primarily focused on the left part of the framework, looking at identifying threats, material compatibility, code compliance or code amendment, and operational compatibility. ROSEN believes that a holistic approach adds more value; integrity management is an ongoing process that needs constant attention and management.

A robust knowledge of material properties forms the basis of any “Fitness-for-Hydrogen” assessment. In recent years, the ROSEN Group has introduced a holistic approach to pipeline material verification, incorporating review and alignment of existing records, ILI data, in-situ field examinations, material testing and industry expertise to ultimately establish a complete and thorough knowledge of pipeline material properties. At the core of this service are ROSEN’s RoMat technologies and the pipeline DNA process, which provides a comprehensive view of the pipeline makeup. The process combines multiple ILI datasets, such as Magnetic Flux Leakage (MFL), geometry, mapping, material properties and other pertinent information in order to establish “populations” of pipe within a pipeline. Traditionally, ILI has not been able to provide strength data, but with the addition of ROSEN’s Pipe Grade Sensor (PGS) technology, a strength grade is assigned to each pipe population.

Figure 2 – With ROSEN’s RoMat technologies including a Pipe Grade Sensor (PGS) technology, a strength grade is assigned to each pipe population

Figure 2 – With ROSEN’s RoMat technologies including a Pipe Grade Sensor (PGS) technology, a strength grade is assigned to each pipe population

For pipeline systems that already contain hydrogen, ROSEN has tools tested and approved up to 100% hydrogen and 100 bar at ambient temperature, using a special tool setup regarding sealing, material of discs and cups, and H2-proofed electronic components, alloys and magnets.

Conclusion

Although the use and transport of hydrogen has been around for many decades, there are some specific areas and integrity challenges involved in transitioning pipelines from natural gas to hydrogen that require further research. This might lead to changes or additions to current integrity management (and potentially operating) practices that are needed to monitor and mitigate the potential new threats.

A holistic approach, like the presented Hydrogen Integrity Framework involving identification and quantification of the threats, together with appropriate mitigation and management, will enable the safe, economic and successful introduction of hydrogen into the natural gas network.