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Bizarre phenomenon is causing bulk carriers to sink – how a solid load becomes a liquid?

Posted September 13, 2018

Every year around 10 bulk carriers mysteriously sink. Some of these accidents cost lives and even if the crew manages to evacuate, the ship and its contents are very expensive. Bulk carriers are ships that carry their typically granular load directly in their hulls (ass opposed to containers). They cargo is heavy and hard, but the reason for so many accidents is actually it becoming ‘liquid’.

This is what a bulk carrier looks like – its hull is filled with granular material. Image credit: Nsandel via Wikimedia

Bulk carriers carry all sorts of things – from grain to aluminium ore. In fact, the latter one is usually the culprit in these kind of accidents. In 2015 a 190 meter long bulk carrier Bulk Jupiter sank near Vietnam coast. 18 people died and only one of the crew members managed to survive. The reason of the accident was suddenly liquefied aluminium ore load. Aluminium ore, by the way, is rocks. It is dug out in mines and prepared for transportation and processing. It is literally rocks, but they can suddenly become ‘liquefied’ which is dangerous for these big ships.

International Maritime Organization has warned bulk carrier operators to be careful with these kind of cargos, because they may go liquid all of a sudden. And no, aluminium ore does not dissolve in water, even though water does play a part in this process. Aluminium ore is usually transported with some water, but it behaves just like a solid load. However, everything changes once the ship starts vibrating.

Bulk carriers are loaded from the top, oftentimes with rocks. Image credit: Jvillegas via Wikimedia (CC BY-SA 4.0)

Waves and the engines introduce some vibration to the ship. Just from researching earthquakes scientists know that shaking changes properties of bulk materials. Vibration breaks the bonds between the particles and they start moving rigorously in relation to one another. Because they are moving, such material suddenly starts acting like a liquid, despite actually being solid.

There is some liquid in aluminium ore when it is being loaded into the ships – it allows for easier loading and unloading. When there is more and more of these rocks in the hull of the ship, the pressure starts to climb. Water squeezes its way through between different particles of the material and the catastrophe is just one step away – only vibration is needed to cause problems.

This is how liquefaction of solid granular materials looks like

The cargo starts sloshing around from one side to another. Eventually, it stick closer to one side, causing listing. Once that happens it can suddenly become a solid again, which pushes the ship further on its side. Eventually the tipping point is reached and the accident is inevitable. Of course, there are ways to minimize this risk. International Maritime Organization has strict rules about the water content in this kind of cargo. However, it doesn’t help that much, because not everyone complies with these regulations and aluminium ore can liquify even with a smaller concentration of water.

A better approach is to monitor water pressure in the cargo compartments – there are tools for that. Engineers are also trying to create surface monitoring technologies that would allow noticing liquefying pretty early.

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