Decarbonising shipping and transportation remains a significant challenge as storing electrical energy from renewables in significant capacity makes it of little use against the requirements of running a ship at sea where space and load requirements inhibit battery options. Liquid hydrogen (LH2), commonly used as fuel in the space industry, forms one potential solution where the required energy density/volume can feasibly be met for ships of all sizes1.
However, its use in the maritime sector poses significant technical and safety challenges around the scale of operations, number of usage points, and its proximity to personnel. Potentially critical scenarios for LH2 on ships can be described through a variety of phenomena including outflow, dispersion, accumulation, cryogenic exposure, ignition potential, explosion, and fire.
In 2018, the NPRA gave the go ahead for a hydrogen-electric ferry to serve one of the routes along the coast of Norway.2 One concept for zero-emission ferry transportation involves LH2 as a concentrated form of hydrogen storage (see below image).
To identify and quantify safety related issues which may need to be resolved, DNV, the independent energy expert and assurance provider, was commissioned by Norway’s Defence Research Establishment and the Public Roads Administration to conduct a series of experiments investigating the behaviour of large releases of LH2 into the atmosphere and in a closed but ventilated area.
The build and operation of the experimental facilities at DNV’s Research and Development Centre in Cumbria, UK, delivered a programme of research pertaining to the outdoor and confined leakage phenomena.
A total of 15 large releases (up to 50 kg.min-1) from a liquid hydrogen storage tanker were performed with variations in source tanker pressure, flow rate, orientation, ignition, and atmospheric conditions.
The outdoor releases were initially illustrated using predictions from DNV’s GasVLE, FROST and Phast software packages to analyse the expected behaviour for:
- Outflow – assessments of the mass flow versus pipeline and outlet conditions are made including assessments of the level of flashing (liquid mass fraction) within the pipe
- Pooling – measurements of ground surface and subsurface temperature around the releases were made and interpreted for the presence of liquid hydrogen or condensed products of air
- Dispersion – determination of the extents of the flammable limits near to ground level from each release
- Thermal properties – thermal radiation measurements recorded around each ignited release are interpreted against simple models and existing correlations and guidance for hydrocarbons.
Bulk LH2 delivery was from a bulk tanker located in a protective mound at the test site, which included a tall camera tower equipped with wind instrumentation. A 30x30m concrete pad was constructed to contain the release arrangement. This incorporated a 40m long vacuum insulated pipe with instruments to analyse temperature and pressure conditions, the release point was located in the centre of the 900m2 concrete pad. Some obstacles in the form of ISO containers and smaller infrastructure were located on the test pad, as shown in Figure 2 (below). The release of LH2 was directed both downwards and horizontally.
All experiments were conducted within a 250m exclusion zone and controlled remotely. This followed the method for purging and cooling the pipe that went from gaseous nitrogen (N2) to liquid nitrogen (LN2) to gaseous helium (He2) to cold gaseous hydrogen (H2) to LH2.
Measurements focussed on the temperature of the surface and subsurface of the concrete close to the release while dispersion measurements using oxygen sensors were taken further afield. The oxygen sensors measured the hydrogen gas concentration by examining the depletion of oxygen in the atmosphere. Two of the experiments were ignited after a delay to gather information on thermal radiation and overpressure.
The following videos show the varying views of the outdoor, downward and horizontal releases and an ignited confined release:
Using the legacy British Gas model FROST and the DNV PHAST models, prediction of the vapour quality by mass and outflow were considered, with knowledge of the geometry of the release point orifice and the saturations conditions near to it.
The liquid/vapour fractions along the pipe during releases were calculated based on the pressure decay and assuming isenthalpic expansion. Releases in all experiments produced high liquid mass fractions (i.e. the proportion of the mass flowing in the pipe being in the liquid phase) with experiments being driven above saturation pressure in the tanker producing higher liquid mass fractions (i.e. little or no flashing in the pipe).
One of the key considerations when assessing the risk of releases is how much of the two-phase flow ends up on the floor, forming a pool, perhaps, or raining out from the jet. Taking the measurements of the surface and subsurface temperature of the concrete and in the experiments allowed DNV to conclude there was no evidence for any LH2 beyond 0.5m from the releases in the downward releases conducted onto concrete in the programme and there was zero evidence of any rainout in the horizontal releases.
Having set or determined the source conditions by looking at the outflow, the dispersion and ultimately, the lower flammable limits (LFL) of the dispersing cloud was analysed. The trends in the experimental results were also represented in the model results:
- Reducing concentration with distance
- Reducing concentration with radial departure from the downwind direction of about 90o
- Peak gas concentration versus the peak dip in temperature, giving a nominally linear relationship, worsening as the concentration gets higher
- Higher concentrations at higher positions above ground level
- LFL was not exceeded in downward releases beyond 30m from the release point and 50m in the horizontal releases.
The relationship between the peak concentration and the peak temperature drop across several experiments was further compared to the predictions from GasVLE, a fluid properties calculation tool from DNV, the predictions are slightly below that of the actual observations. This may be because the GasVLE model does not include heat transfer from the ground, possibly evident in the measurements.
At the point of ignition there is an explosion effect creating an initial expanding fireball which can give rise to local dynamic overpressures with the potential to cause harm. At all measurement locations, in all ignited experiments, the peak pressure did not exceed 30mBar in the horizontal jet and around about 15mBar in the downwards jet.
The releases conducted as part of this programme provide a valuable, extensive and unique dataset in relation to the phenomena contributing to the severity of events following a loss of containment of liquid hydrogen. The outdoor release data has been compared to predictions from existing software models and found to be of a high quality and suitable for further model development / validation.
Hydrogen is one of the most suitable solutions to contribute to the replacement hydrocarbons in the future and its consumption is expected to grow significantly over the next three decades. Despite being a clean fuel, concern surrounds its chemical and physical properties and the gaps in engineering techniques, tools and implementation experience and the potential impact on safety. Further investigation and model analysis is recommended.
- Decarbonising Shipping: Setting Shell’s Course. https://www.shell.com/energy-and-innovation/the-energy-future/decarbonising-shipping.html
- NPRA Press Release on Ferry Contract Award: https://www.vegvesen.no/om+statens+vegvesen/presse/Pressemeldingsarkiv/Vegdirektoratet/hydrogenferje-avtale-gir-nullutslipp-i-ferjesektoren