If a fire breaks out on a hydrogen-powered vessel, there is a serious risk that the hydrogen tanks could explode. The hydrogen must therefore be released in a controlled and safe manner. A new report from RISE, funded by the Swedish Transport Administration, points to a clear solution: a 9.5-metre-high vent mast that releases the hydrogen in a plume straight up into the air.

Compressed hydrogen is stored at very high pressure, usually in composite tanks. If exposed to fire, the material can weaken and, in the worst case, the tank can explode.

“On road vehicles, thermally activated pressure relief devices are used, while stationary hydrogen storage facilities use various types of vent masts to safely release the gas when necessary,” says Jonatan Gehandler at RISE, one of the authors of the report Hydrogen ship vent mast design – H2Vent.

So what works best on a ship? This is where things get complicated. Releasing hydrogen to prevent a more serious accident simultaneously creates new risks. The gas can spread across the vessel, ignite immediately and form a jet flame, or ignite later and cause a gas cloud explosion.

The project therefore evaluated six different vent mast concepts, using the Beluga24 high-speed catamaran as a reference case. The carbon-fibre ferry has capacity for 150 passengers and is designed for zero-emission commuter services.

“The gas must not expose people to any of these risks, but the biggest challenge arises if the hydrogen ignites immediately and forms a large jet flame. The thermal radiation from the jet flame places stricter requirements on the design of the vent mast than the risk of an explosion.”

The seemingly obvious solution, dividing the gas flow between several smaller outlets, actually made matters worse. This exposes the vessel to a larger part of the flame and therefore to more thermal radiation. The best solution turned out to be simply releasing the hydrogen through a pipe straight up into the air, producing a long, narrow jet flame.

“This keeps the radiation as low as possible. However, there is some uncertainty in our modelling. Radiation from a hydrogen flame mainly comes from hot water vapour, and some of this radiation is absorbed by water vapour in the surrounding air. I would like to investigate the magnitude of this effect further in the future.”

But even this simple solution is far from simple in practice. For the Beluga24, the modelling showed that a single vent mast would need to be around 9.5 metres high if people are to remain on the weather deck without exceeding the thermal radiation limit. According to the report, this presents a significant engineering challenge on a small passenger vessel.

“The risk, of course, is that the mast gets in the way. The Beluga24, for example, is intended for urban services and will need to pass beneath bridges. It could also affect the vessel’s stability,” says Jonatan Gehandler.

The report Hydrogen ship vent mast design – H2Vent was written by Jonatan Gehandler, Sixten Dahlbom, Chen Huang and Paul Adams (RISE), Marcus Runefors and Thushadh Wijesekere (Lund University’s Faculty of Engineering, LTH), and Niclas Kling (Poseidon Konsult AB).