What happens if liquefied natural gas (LNG) hits the wall of the cargo tanks in a ship? New research shows that much higher pressure peaks can occur during impact than previously assumed. This insight is important for the design and safety of LNG ships and future liquid hydrogen transport systems.
The research from the team of physicist Devaraj van der Meer of the University of Twente was published in the scientific journal PNAS.
Normally, a thin layer of air prevents a liquid from hitting a surface directly. That gas acts like a cushion and dampens the blow. In LNG ships, that air has been replaced by vapour from the LNG itself. And that vapour can condense back into liquid during impact. As a result, the cushion disappears, and the load on the wall increases sharply.
From everyday sloshing to extreme impact
Everything we intuitively know about liquids impacting a surface comes from situations with air. Rain on the street. Waves against a quay. In all these cases, there is a layer of air between the liquid and the surface just before impact. That air is compressed, acting as a shock absorber. This effect is known as air cushioning.
But with LNG and liquid hydrogen, that air has been replaced by vapour from the same liquid. The liquid is in equilibrium with its own vapour. On impact, the vapour can liquefy again: condensing. ‘And that changes everything,’ says Van der Meer. ‘When that vapour layer disappears, the cushion disappears too. Then you no longer get a soft landing, but a much harder impact.’
Also read: LNG retrofits surge as quick fix for carbon reduction
Drops that hit harder than expected
To isolate that effect, Van der Meer and his team did experiments with a special material that boils at 34 degrees Celsius. This allowed them to work at room temperature, but use a liquid that behaves like much colder LNG (−162 degrees Celsius) or liquid hydrogen (−253 degrees Celsius).
In the PNAS study, the researchers dropped droplets on a surface. At low speeds, what you expect happens: a small vapour bubble remains trapped under the droplet. But as soon as the droplet impacts at a higher speed, something striking happens: the vapour disappears.
‘The vapour condenses faster than the droplet moves downwards,’ explains Van der Meer. ‘As a result, the pressure cannot be built up slowly. The dampening effect disappears.’ The result is a much harder collision than if the same droplet were to fall through air.
Not only theory, but also measured pressure
It didn’t stop at drops. In another study, the team had a metal disc smash into the surface of a bath of the same liquid. By only lowering the temperature slightly, the maximum pressure on impact increased up to fifteen times.
The effect became even greater with waves. In a large test set-up at the MARIN research institute in Wageningen, fifteen metres long, the researchers had breaking waves crash against a wall. With 99 pressure sensors, they measured what was happening.
With water in air, such a collision behaves exactly as predicted by existing models. But with water in its own vapour, the pressure sometimes rose up to a 100 times higher. The cause: a vapour bubble under the breaking wave that is not compressed, as in air, but collapses extremely rapidly because the vapour condenses.
Also read: Demand for LNG up 52% in Rotterdam last year
What does this mean for LNG and hydrogen?
LNG ships are not suddenly unsafe. Until now, a fundamental effect has so far not been adequately included in models and experiments. ‘Many safety studies work with an inert gas that cannot condense,’ says Van der Meer. ‘But in a real LNG tank, the vapour is anything but inert. It can simply become liquid again.’
According to him, designers and engineers must know this effect. ‘We show that physics is different from what was thought. That doesn’t mean that everything has to be done again, but it does mean that you can’t ignore this effect. We are in talks with parties from the industry that design LNG containers for ships. For us, it’s a starting point: we now know that the effect exists. The next step is to understand what this means on the scale of real transport tanks.’
Picture by ChatGPT.
