MATERIALS AUSTRALIA Annual Sir Frank Ledger Breakfast Meeting Design, Construction and Operation of Hydrogen Pipelines

Source: Olivier Royet, Dr Sofia Hazarabedian, DNV

Opening the meeting, which attracted an audience of nearly 120, Mike Ledger gave a brief overview of the career of his grandfather, Sir Frank Ledger, noting his role in formation of what was to evolve into the WA Branch of Materials Australia. He then introduced the speakers, Dr Sofia Hazarabedian and Olivier Royet, both from DNV. Sonia is a materials engineer whose PhD focused on mitigating hydrogen embrittlement within the oil and gas sector. Olivier is civil engineer with expertise in materials and welding and 22 years’ international experience in the design, analysis and certification of onshore and offshore steel structures and pipelines.

Starting the presentation, Sofia noted that the future of energy is leaning towards hydrogen as a significant energy carrier and that pipelines have emerged as the most cost-effective solution for large-scale hydrogen transportation. She then outlined a future scenario with large-diameter hydrogen pipelines that can withstand high and cyclic pressures, and extend over long distances, both onshore and offshore. These pipelines are also expected to be used as a hydrogen storage method.

However, there is currently little experience with such pipelines, which will be significantly different from existing hydrogen pipelines, which are predominantly onshore and relatively short, with small diameters (below 20 inches), and are operated at low pressure. Designing these new long high-pressure hydrogen pipelines involves dealing with specific risks and uncertainties, such as the effect of hydrogen on toughness and ductility, static, monotonic increasing, and cyclic loading, and the resulting defect tolerances.

DNV’s corporate goal is to enable its customers and their stakeholders to make critical decisions with confidence. Thus, in response to these challenges DNV launched the H2Pipe Joint Industry Project (JIP), aiming to develop a guideline for the design of and potential re-purposing of pipelines for hydrogen transport. The presentation summarised the findings from Phase 1 of the JIP, which will be consolidated in the recommendation practice DNV-RP-F123. This covers design, construction, requalification and operation of hydrogen pipelines and is expected to be released in December 2024 as a supplement to the existing offshore pipeline standard, DNV-ST-F101.

Olivier, the author or the JIP, continued the presentation with a summary of some of the major issues it covers. The first design issue is the risk posed by escape of hydrogen. Hydrogen is intrinsically less safe than natural gas. Its lower density tends to produce larger clouds of gas, which are flammable over a wider concentration range and require less energy to ignite. The high burning velocity can result in detonation of the cloud, rather than deflagration (progressive combustion). He illustrated this with reference to full-scale testing including impressive videos of exploding buildings.

He then turned to hydrogen embrittlement. In susceptible steels under stress, hydrogen trapped at dislocations and sub-grain boundaries can facilitate crack initiation. It can also migrate to the tips of cracks, reducing the energy for crack propagation.

In tensile testing, hydrogen greatly reduces elongation to fracture. In cyclic loading the effect of hydrogen on crack growth per load cycle under fluctuating stress can be very large (28 times greater in one example). However, a factor that complicates design is that, since diffusion of hydrogen to a crack tip is time-dependent, low frequency stress fluctuation is generally worse in accelerating crack growth than rapid cycling over the same stress range.

This leads a key point in design of hydrogen pipelines: the first consideration must be how the pipeline will be operated. In this regard, large but slow variations in pressure are likely to have a disproportionate impact on crack growth and hence, pipeline life. Olivier likened this to the wear of car tyres. One ‘burn-out’ can reduce the remaining life by more than a year’s normal driving.

In the past, the design of the existing shorter-range hydrogen pipelines has typically been based on pressure vessel design codes for operation within the elastic range. This is not economic for long pipelines. Instead, these must be designed on limit state principles, particularly resistance to accidental events. This is where fracture control becomes a critical factor.

Olivier then turned to the topic of running fracture in pipelines, considering whether an accidental penetration of the pipeline can grow catastrophically. The two types of running fracture are ductile (crack growth rate less than 300m/s) and brittle (500 to 1000 m/s). Generally, because hydrogen embrittlement depends on diffusion, hydrogen will not change running fracture from ductile to brittle.

In principle, the same applies when hydrogen is blended as significant component of natural gas mixtures for transport using existing long-distance gas pipelines. However, it is essential to understand the properties of the steel from which the pipeline was made. Steels manufactured today are much cleaner than steels commonly used in pipelines 30 or 40 years ago. These older steels typically have more and less uniformly distributed carbides, around which hydrogen can accumulate. Oliver referred briefly to testing methods, noting the preference for rising displacement methods for testing for crack growth.

Before answering questions from the audience, Oliver finished the presentation by highlighting the critical importance of applying Safety in Design principles. This involves determining safety class and safety zones and requires full-scale testing to calibrate modelling. He stressed that the starting point for safe design must be the intended operation, and a specific safe operation procedure must be an integral output from the design process.

In conclusion, Olivier noted that with pipe wall thickness design for hydrogen pipelines being governed by crack growth, integrity management might need to rely more on prediction than inspection. This points to a key area for future research.