Special Focus: Environment and Safety
T. HAJILOU and M. A. EIDEM, DNV, Bergen, Norway; and E. ØSTBY, DNV, Høvik, Norway
Hydrogen (H2) as an energy carrier is key in the inevitable transition from fossil fuels to renewable energy. Pipelines are considered a viable option for H2 transportation, either pure gas or mixed with natural gas. In this context, using new pipelines and utilizing existing infrastructure through re-qualification are possible options.1
The safe and efficient use of existing natural gas pipelines (initially not designed for H2 transportation) to transport pressurized H2 gas can play a prominent role in the global decarbonization of energy.2 For example, repurposing Norway’s vast subsea pipelines network, which spans 8,800 km, can act as a driving force to achieve Europe’s goal of being the first climate-neutral continent by 2050.1,3
The available H2 pipelines, usually with diameters of less than 20 in., were designed for onshore applications and typically operate at the low H2 pressure of 40 bar. These pipelines can operate at less than 50% of their specified minimum yield strength. However, the need for offshore pipelines to transport gaseous H2 at higher pressures and longer distances necessitates using pipelines with larger diameters and higher-grade steels.
Additionally, it has been found that blending 5%–20% H2 gas in natural gas pipelines may not significantly increase the risk of leakage or pipeline fracture.2,4 However, new offshore pipelines must meet the requirements for transporting 100% H2 gas with 100 bar–200 bar pressure, where the risks become more prominent.
Tackling H2 embrittlement. Certain challenges arise when transporting H2 through steel pipelines. The absorption of a hydrogen atom into metallic materials can cause H2 embrittlement (HE), which is the degradation of the mechanical properties of the pipeline.5
HE was first identified by Johnsen in 1875.6 He published experimental results showing that the mechanical properties of a piece of iron had altered when subjected to an acid. According to Johnsen, after a few minutes of immersion (half a minute will sometimes suffice) in strong hydrochloric or dilute sulfuric acid, a piece of iron broke after being bent once on itself; however, before immersion, it would bear bending on itself and back again two or three times before breaking.
The extraordinary decrease in toughness and breaking strain of the treated iron is more remarkable because it is not permanent. Eventually, the metal slowly regains its original toughness and strength.6
Testing for H2 fatigue. An HE-susceptible metal experiences cracking when exposed to enough H2 underloading. H2 decreases the material’s ductility, especially in the areas under stress concentration, and makes the steel pipelines susceptible to cracking under static loading. Additionally, gas pressure fluctuation inside the pipe and load level alternation can cause H2-enhanced fatigue crack growth (HEFCG).
HEFCG in gaseous H2 shows two distinguishable regimes. The first is an unaccelerated regime at a relatively low-stress intensity factor range (however, in the presence of high static stresses, it can still lead to an increased growth rate), and the crack is propagating by ductile blunting and re-sharpening. The second is the accelerate regime, which is observed at a relatively high-stress intensity factor range with the fracture surface covered by quasi-cleavage features.7 The accelerated HEFCG of steel in H2-rich circumstances raises concerns about transporting H2 via pipelines (FIG. 1).
Understanding challenges for H2 transportation. Research has shown that adding small amounts of impurity, such as oxygen (O2), carbon monoxide (CO) or carbon dioxide (CO2), to the H2 or H2 and methane mixtures can increase the time taken to achieve the equilibrium level of H2 solubility in the gas transmission pipeline steels.8
Observations on crack tip shape, metallography, fracture toughness and crack growth rate measurements support the conclusion that O2, as an impurity in the level of 100 volume parts per million (vppm), can prevent or slow atomic hydrogen’s entry into steel by preferential adsorption on the steel surface at the crack tip.8 These inhibiting gases in the H2 gas change the physisorption or the catalytic dissociation of the H2 molecules on the surface and decrease the rate of the H2 uptake.2 However, uncertainties surrounding the beneficial effects observed can be dependent on the operation.
The affected mechanical properties of steel pipelines increase the need for re-qualification procedures and new guidelines to be introduced to estimate the service life of pipelines carrying high-pressure gaseous H2. Available standards (e.g., ASME B31.12) cannot cover the necessary parameters for the design and operation of offshore H2 pipelines.1
However, an alternative standard for submarine pipeline systemsa includes gaseous H2 as a listed transport product—additional considerations are required to meet the target safety level for its increased use as an energy carrier. Consequently, it is necessary to update the standards to reach design and material requirements that do not compromise pipeline integrity and safety.9
In response, the author’s company launched a joint industry project, H2Pipe, in 2021 to develop guidelines for offshore H2 pipelines. The objective of H2Pipe Phase 1 was to revise the guidelines for designing and operating H2 pipelines.1 Phase 2 was launched in Q1 2023 and will last for 2 yr: it comprises a comprehensive experimental test program to enhance the understanding of the governing HE mechanisms and how gaseous H2 affects the integrity of pipeline materials.9
Setting material standards. New re-qualification procedures and guidelines have been implemented to enhance the requirements for better-equipped laboratories, where the effect of high-pressure pure gaseous H2—or its mixture with natural gas—on the fatigue crack growth rate and fracture toughness of steel pipelines can be evaluated.
A technology centerb established a H2 lab focusing on material performance in high-pressure gaseous H2. The H2 lab is equipped with two 10-t servo-electro machines. The autoclaves—with a capacity of 5 liters each—can be attached to the servo-electro machine, and fracture mechanic tests can be completed onsite in the presence of high-pressure H2 gas in static and dynamic conditions. Autoclave pressure can be elevated up to 270 bar. The lowest possible actuator rate is 0.00001 in./min, which enables precise strain measurements.10 The testing setup is equipped with direct current potential drop crack propagation measurement with high accuracy.11H2T
b DNV Technology Centre in Bergen
1 DNV, “Design and operation of hydrogen pipelines-Phase 1 of the H2Pipe JIP,” 2021
2 The Foundation for Scientific and Industrial Research (SINTEF), The Norwegian University of Science and Technology (NTNU), “HyLINE - Safe pipelines for hydrogen transport,” November 2019, online: https://www.sintef.no/en/projects/2019/hyline-safe-pipelines-for-hydrogen-transport/
3 Laureys, A., R. Depraetere, M. Cauwels, T. Depover, S. Hertelé, K. Verbeken, et al., “Use of existing steel pipeline infrastructure for gaseous hydrogen storage and transport: A review of factors affecting hydrogen induced degradation,” ScienceDirect, May 2022, online: https://www.sciencedirect.com/science/article/abs/pii/S187551002200124X
4 DNV, “DNVGL-RP-F112 Duplex stainless steel - design against hydrogen induced stress cracking,” September 2021, online: https://www.dnv.com/oilgas/download/dnv-rp-f112-duplex-stainless-steel-design-against-hydrogen-induced-stress-cracking.html
5 Robertson, I. M., P. Sofronis, A. Nagao, M. L. Martin, S. Wang, D. W. Gross, K. E. Nygren, et al., “Hydrogen embrittlement understood,” Metallurgical and Materials Transactions A, June 2015, online: https://link.springer.com/article/10.1007/s11661-015-2836-1
6 Johnson, W. H., et al., “On some remarkable changes produced in iron and steel by the action of hydrogen and acids,” Proceedings of the Royal Society of London, December 1875, online: https://royalsocietypublishing.org/doi/10.1098/rspl.1874.0024
7 Birenis, D. et al., “Interpretation of hydrogen-assisted fatigue crack propagation in BCC iron based on dislocation structure evolution around the crack wake,” Acta Materialia, September 2018, online: https://www.sciencedirect.com/journal/acta-materialia/vol/156/suppl/C
8 Atrens, A., E. Gray, J. Venezuela, J. Hoschke, M. Roethig, et al., “Feasibility of the use of gas phase inhibition of hydrogen embrittlement in gas transmission pipelines carrying hydrogen: A review,” JOM, October 2022, online: https://link.springer.com/article/10.1007/s11837-022-05559-8
9 DNV, “Design and operation of hydrogen pipelines—Phase 2 of the H2Pipe JIP,” 2023.
10 Interactive Instrument, “Load frame and creep controller instruction user manual,” June 2023.
11 American Society for Testing and Materials (ASTM), “ASTM E1820- 20b, Standard test method for measurement of fracture toughness.”
About the authors
TARLAN HAJILOU is the Senior Material and Hydrogen Engineer at DNV in Bergen. She earned an MS degree in materials selection and characterization. She completed her graduate studies at the Norwegian University of Science and Technology, focusing on H2 embrittlement. Following her studies, Dr. Hajilou served as a post-doctoral researcher at the Norwegian University of Science and Technology for 3 yr, where she specialized in studying the interaction between H2 and metals, particularly in carbon steels and nickel superalloys.
MADS ARILD EIDEM is the Head of Section for the DNV Technology Centre in Bergen, Norway, with expertise in materials technology, corrosion, H2, coating, failure investigation and large-scale testing. Eidem has further experience in line and portfolio management, technology development and project sponsor roles. He is responsible for the section resources and operational activities. Eidem earned an MS degree in project and program management and business development from SKEMA Business School, Paris, a BBA degree from Handelshøyskolen BI, and served as a submarine officer educated at the Royal Norwegian Naval Academy.
ERLING ØSTBY is the Principal Specialist at the DNV Technology Centre in Høvik, Norway. His main fields are structural integrity, fracture mechanics, small- and large-scale experimental testing, and numerical modeling. Dr. Østby has in-depth knowledge of the fracture properties of steels in brittle and ductile regimes. Regarding modeling of fracture in steels, he has been involved in activities from atomistic simulations to the continuum scale, including molecular dynamics, quasicontinuum method, crystal plasticity, cohesive zone modeling and general continuum and damage mechanics approaches. Dr. Østby earned an MS degree and PhD from the Norwegian University of Science and Technology.