The Maritime Technologies Forum (MTF) has released a new report offering guidelines and key safety considerations for developing liquefied hydrogen bunkering systems and procedures.
The report titled “Guidelines for the development of liquefied hydrogen bunkering systems and procedures” discusses the potential use of hydrogen as a zero-emission fuel to meet the IMO Strategy on Reduction of GHG Emissions from Ships by 2050. It acknowledges the lack of experience in the maritime sector with hydrogen as cargo and fuel, and the increased risks associated with its use as a fuel compared to LNG.
As of the writing of this report, there are no international standards covering the bunkering of liquefied hydrogen. However, information collected from ongoing developments in ISO for related areas, as well as the experience gained from the Norwegian ferry “Hydra”, serves as the basis for the guidelines and recommendations of this report.
Based on the findings of the publication, MTF has outlined the following key observations:
- The properties of hydrogen, and in particular the very low temperature of liquified hydrogen, mean that experience gained from bunkering arrangements for liquid natural gas, LNG, cannot be re-used directly.
- The bunkering process will be more complex than it is for LNG, since no nitrogen can be present inside the piping systems when liquified hydrogen is introduced, as this will freeze and clog the systems.
- The material choices and need for more insulated components and piping will also be slightly different.
- The development of vessel-specific procedures for bunkering operations, such as more automated bunkering procedures, will be necessary.
- The added complexities will mean that the need for crew training and certification is of even higher importance than for other bunkering processes.
- The Safety Management Systems should be updated to cater for the additional safety aspects with liquified hydrogen bunkering, as outlined in another work carried out by MTF, ‘Guidelines to develop and implement Safety Management System for alternative fuels onboard ships’.
Commenting on the guidelines and the planned submission to the International Maritime Organization (IMO), Alf Tore Sørheim, Acting Director General of Shipping and Navigation at the Norwegian Maritime Authority said:
With the current lack of international standards covering bunkering of liquefied hydrogen, these guidelines are especially important to help industry develop and advance safer bunkering operations. This is why the Flag state members of MTF are jointly submitting this report to the IMO to provide recommendations and offer a framework to consider when developing liquefied hydrogen bunkering requirements.
Further commenting on the report, Knut Ørbeck-Nilssen, CEO, DNV Maritime, said:“Hydrogen is going to continue to play an important role in the energy transition, both as a marine fuel and as a cargo, which is why it is critical to develop standards to support its safe bunkering. The introduction of these guidelines and their submission to IMO are key steps in addressing the challenges around liquefied hydrogen bunkering.”
Properties of Hydrogen
- Flammability and Explosivity
Hydrogen, as a highly flammable fuel, mixes with oxygen whenever air is allowed to enter a hydrogen vessel, or when hydrogen leaks from any vessel into the air. Hydrogen is flammable over a very wide range of concentrations in air (4 – 75% vol in air). As a result, even small leaks of hydrogen have the potential to ignite. The wide flammability range of hydrogen also leads to larger flammable clouds compared to LNG, which has a flammability range of 5-17%, and hence will quickly reach a concentration that is too rich for ignition.
The flame speed of hydrogen is very high (265-325 cm/s), which also incurs the risk of detonation (DDT), quicker pressure peaks and higher explosion pressures. At the same time the required minimum ignition energy for hydrogen is very low, meaning that hydrogen can ignite with a weaker ignition source (as little
as static electricity is sufficient for certain concentrations). Hydrogen is colourless and odourless therefore leaks are difficult to detect and same goes for the flame that is almost invisible to human eyes. Hydrogen gas is lighter than air and provides a fire and explosion hazard in confined spaces. While the risk of
explosion in open air is lower, it should not be neglected.
- Hydrogen Permeation and Embrittlement
As the molecules of hydrogen gas are smaller than all other gases, it will leak through many materials considered airtight or impermeable to other gases, by diffusion through the material itself. This property makes hydrogen more challenging to contain than most other gases. The properties of the hydrogen
molecule will also incur the risk of hydrogen embrittlement to metallic materials. Higher risk of leakage through small crevices is also a direct consequence of the molecule size.
- Cryogenic
The boiling point of liquid hydrogen is extremely low, which creates several specific additional challenges in the design of ships and associated bunkering systems. First, the materials must be suitable for the fuel temperature, but also, at -253°C, both oxygen and nitrogen will become solid. This means that nitrogen
cannot be used as an inert gas in the same way as for LNG systems. If there is any N2 left in the systems when liquid hydrogen is introduced clogging of filters and fouling of valves and other sensitive equipment may occur. It is also necessary to consider the risk of air condensation on the outside surfaces, to mitigate
the risks arising from presence of concentrated oxygen such as cold effects on equipment and increased fire risk. The low temperature also means that it can cause severe frostbite and burns when in contact with skin. In case of liquid hydrogen leakage, surrounding structure may suffer brittle fracture if not constructed using low temperature material.
Hydrogen Dispersion
The dispersion properties of hydrogen when released from containment can be considered both positive for safety due to its buoyancy, and a safety risk due to the volumes and flammability range. In the case of a leakage of liquid hydrogen, the expansion ratio between the liquid and gas phase of 1:848 creates a
very large volume of flammable gas. The expansion rate also means that liquid hydrogen leakages cause a considerably larger flammable cloud size compared with leakage of hydrogen in the gas phase, and while a higher storage pressure increases the expansion ratio of gaseous hydrogen, the expansion ratio
of compressed hydrogen still remains well below liquefied hydrogen (Hydrogen gas stored at 250 barg typically has an expansion ratio of 1:240).
Additionally, compared with LNG, the flammable cloud sizes are also exacerbated by the larger flammability range of hydrogen (4-75%). However, as hydrogen also very readily disperses in air and is extremely buoyant, this property can be used to mitigate the explosive nature of hydrogen, by arranging the installations in open air to allow the rapid dispersion of hydrogen. This is a typical safety barrier for land-based installations with single walled piping and components.
- Toxicity and Asphyxiation
Hydrogen is not toxic, but it can displace oxygen in the air, which can lead to asphyxiation.
Additionally, it’s important to note that in April, the Maritime Technologies Forum (MTF) released guidelines that highlight recommendations for developing and implementing Safety Management Systems (SMS) under the International Safety Management (ISM) Code.
