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Source: TÜV SÜD.

Green hydrogen produced with electricity from renewable sources could slash carbon emissions by several million tonnes per year in Germany alone. Beyond its application in the steel and chemical industries, the energy carrier can also be used for energy storage and in fuel-cell drive systems in the transport sector. Given this, the German Federal Ministries for Economic Affairs and for Digital and Transport have invested a total of over 8 billion euros since mid-2021, funding around 60 large-scale hydrogen projects from hydrogen production to transport and industrial use.¹

Using available infrastructure

With a service life of up to 100 years, pipelines and storage caverns are particularly ecological and economical solutions for gas transport and storage. In addition to roughly 500 000 km of pipelines transporting gas throughout Germany, there are 40 000 km of pipelines for cross-regional and cross-border transport. With diameters of up to 1.4 m and service pressures of up to 100 bar, the pipes are generally also suitable for transporting hydrogen. This is supported by historical fact; up to the mid-20th century, “city gas” contained up to 50 % of hydrogen. Using the existing infrastructure would further increase the sustainability of the transition to hydrogen as an energy carrier.

However, for this approach to be successful, various types of steel must be tested for their resistance to hydrogen, taking into account the current state of the art in this field and appropriately adjusted safety and maintenance strategies.

Hydrogen embrittlement

High-strength steels involve the risk of hydrogen-induced cracking. Minimal flaws in the structure of the material, inclusions, impurities, or cyclic mechanical stresses may cause the protective oxide layers of the metal to corrode, enabling hydrogen atoms to diffuse into the material and accumulate at flaws in the steel’s crystalline lattice structure. Because pipelines in particular, are exposed to pressure fluctuations, permanent avoidance or exclusion of damage in the passive oxide layers is impossible. Fluctuations in the internal operating pressure of a pipeline are due to various factors including injection and withdrawal processes.

Hydrogen deposition reduces the material’s plastic deformation capability and thereby its ductility, resulting in embrittlement and causing microscopic cracks. Continued accumulation of hydrogen atoms at the crack tips and cyclic loading cause these cracks to propagate. The extent of hydrogen embrittlement depends on the grade and structure of the steel and its type of production. Higher strength values and rougher surfaces increase the risk of hydrogen deposition.

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In principle, hydrogen reduces fracture (crack) resistance by up to 50 per cent and accelerates crack propagation even at relatively low partial pressures. It also lowers contraction at break, but not tensile strength. These influences of hydrogen on steel must be taken into account in pipeline assessment.

Using fracture-mechanics analysis

Fracture-mechanics analysis is applied in examining pipelines and their materials for their suitability for transporting hydrogen, and in calculating the expected service life. In the case of known flaws, the experts will assess the integrity of the component.

Fracture mechanics are also applied to new pipelines, e.g. to identify the detection limits in non-destructive testing of the material and weld seams and to calculate the inspection intervals for future operations.

The propagation behaviour of existing cracks in particular can be mathematically quantified. Fracture-mechanics analysis looks not only at the material-specific parameters, but also at stresses and distortions in the presence of the respective fluid.

Generally, it can be said that stresses at the crack tip are theoretically unlimited and interactions between crack geometry and loading are highly complex. Fracture mechanics use the factors of stress intensity and rate of energy release to describe local stress conditions at the tip of the crack and crack-propagation behaviour.

Since diffusion of hydrogen atoms into the lattice structure of the metal is a function of time, the frequency at which the workpiece is loaded also plays a critical role. This applies all the more given that cyclic loading causes varying operating pressures and may therefore further accelerate crack growth, which is slower when the operating pressure is high and the pressure amplitude low than vice versa. 

Visualisation in a diagram

A failure assessment diagram (FAD) is used to examine and evaluate flaws that may result in component failure from an integrated perspective with the help of fracture mechanics. The factors of loading intensity (L) and stress intensity (K) describe component stress with regard to the plastic collapse of the residual cross-section, and material strain at the tip of the crack with regard to brittle fracture. LR stands for the ratio of existing stress in the residual cross-section to load at plastic collapse, whereas KR stands for the ratio of existing load at the tip of the crack (stress intensity) to the material’s fracture toughness.

Together, LR and KR define the position of an evaluation point in the FAD (Figure 1). The green FAD curve indicates the limit values. Parameters below this curve are still acceptable, while parameters above the curve are unacceptable. The blue point indicates a specific case of evaluation. The analysis of past loading cycles can be used to make predictions about future loading cycles. The expected growth of an initial crack and the length of time until the crack turns into an unacceptable flaw can be mathematically calculated, so that experts can calculate the service life of a pipeline.

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Figure 1: Example of analysis of static load, FAD curve

Clarifying how H2 influences the material

In the USA, most steel types listed in accordance with the ASME Code have been analysed; in other words, their parameters (material characteristics in a hydrogen atmosphere) are known. However, where some steel types are concerned, fracture (crack) resistance and fatigue crack growth in a hydrogen atmosphere have yet to be determined or may be subject to changes caused by certain alloy elements or heat treatment processes.

For European materials in particular, experts must first determine how hydrogen will impact the relevant parameters before they can complete fracture-mechanics analysis. DVGW, a German recognised standardisation body for the gas and water industry, has launched a research project on this topic. TÜV SÜD is represented on the relevant committees and engages proactively in discussion and development of the pertinent safety concepts, which will be published shortly: DVGW Technical Rule – Code of

Practice G4643 (M) “Fracture-Mechanical Assessment Concept for Steel Pipelines with a Design Pressure of more than 16 bar for the Transport of Hydrogen” is scheduled for publication in March 2023.

Normative basis

All gas pipelines – irrespective of whether they transport natural gas, pure hydrogen or a mixture of the two – fall under the German Energy Management Act (EnGW). Under the German Regulation on High-Pressure Gas Lines (GasHDrLtgV), conversion of existing natural gas lines to hydrogen transport represents a major change and must be reported. The pipeline operator must prove that the conversion was completed expertly, professionally and in accordance with the state of the art. The technical requirements are described in DVGW Technical Rule – Standard G463 and/or DVGW Technical Rule – Code of Practice G409.

Benefiting from third-party expertise Acting on behalf of pipeline operators, TÜV SÜD is currently reviewing the conversion of existing natural gas pipelines to hydrogen. In their review, the experts consider all factors influencing service life as well as all documents on planning, construction and operation. The experts also point out measures that are suitable for determining, evaluating or upgrading the condition of pipeline infrastructure. By providing support in the form of safety strategies and fracture-mechanics analyses, TÜV SÜD is helping to achieve safe, secure and carbon-neutral energy management.

Dr. Johanna Steinbock, Expert Fracture Mechanics Analysis, TÜV SÜD Industrie Service
Jan Sachse, Head of Department Plant Safety, TÜV SÜD Industrie Service
Dr. Albert Großmann, Expert High-Pressure Pipelines, TÜV SÜD Industrie Service