Slashdot Mirror
Mystery of the Cargo Ships That Sink When Their Cargo Suddenly Liquefies (theconversation.com)
An anonymous reader writes (condensed for space): On average, ten "solid bulk cargo" carriers have been lost at sea each year for the last decade. Solid bulk cargoes -- defined as granular materials loaded directly into a ship's hold -- can suddenly turn from a solid state into a liquid state, a process known as liquefaction. And this can be disastrous for any ship carrying them -- and their crew. A lot is known about the physics of the liquefaction of granular materials from geotechnical and earthquake engineering. The vigorous shaking of the earth causes pressure in the ground water to increase to such a level that the soil "liquefies." Yet despite our understanding of this phenomenon, and the guidelines in place to prevent it occurring, it is still causing ships to sink and taking their crew with them.
Solid bulk cargoes are typically "two-phase" materials as they contain water between the solid particles. When the particles can touch, the friction between them makes the material act like a solid (even though there is liquid present). But when the water pressure rises, these inter-particle forces reduce and the strength of the material decreases. When the friction is reduced to zero, the material acts like a liquid (even though the solid particles are still present). A solid bulk cargo that is apparently stable on the quayside can liquefy because pressures in the water between the particles build up as it is loaded onto the ship. This is especially likely if, as is common practice, the cargo is loaded with a conveyor belt from the quayside into the hold, which can involve a fall of significant height. The vibration and motion of the ship from the engine and the sea during the voyage can also increase the water pressure and lead to liquefaction of the cargo. You can read more on this here.
Solid bulk cargoes are typically "two-phase" materials as they contain water between the solid particles. When the particles can touch, the friction between them makes the material act like a solid (even though there is liquid present). But when the water pressure rises, these inter-particle forces reduce and the strength of the material decreases. When the friction is reduced to zero, the material acts like a liquid (even though the solid particles are still present). A solid bulk cargo that is apparently stable on the quayside can liquefy because pressures in the water between the particles build up as it is loaded onto the ship. This is especially likely if, as is common practice, the cargo is loaded with a conveyor belt from the quayside into the hold, which can involve a fall of significant height. The vibration and motion of the ship from the engine and the sea during the voyage can also increase the water pressure and lead to liquefaction of the cargo. You can read more on this here.
From the linked article:
When a solid bulk cargo liquefies, it can shift or slosh inside a shipâ(TM)s hold, making the vessel less stable. A liquefied cargo can shift completely to one side of the hold. If it regains its strength and reverts to a solid state, the cargo will remain in the shifted position, causing the ship to permanently tilt or list in the water. The cargo can then liquefy again and shift further, increasing the angle of list.
the obvious solution to this is to have partitions inside the ship, to limit the amount of shift possible.
Also, picking "the right ship for the job" such that your cargo comes as close as possible to completely filling the hold to the top, to limit the amount of possible shifting.
I'm just surprised that the pressures added by "drop-filling" the cargo at port have any effect on the possibility of liquifying long after the ship has sailed. I would have expected that only the vibrations during the voyage would have affected it.
I wonder how much of a role uneven loading at port plays? Like if the hold is filled from only a relatively small number of hold covers, leading to cargo that's in roughy pyramid-shaped piles in the hold. If they have just barely enough cohesion to maintain that pyramid shape, I could definitely see how that could shift suddenly and significantly on a rolling sea. Once the shift starts, it's like the article describes, with the entire mass moving as a liquid, a lot like an avalanche, until the pressure drops below critical. And then the cargo "freezes" in place in its new position, quite likely creating a dangerous imbalance in the load.
I've always found watching avalanche videos to be fascinating, how snow, seemingly solid, can flow like a river, and then suddenly stop as if hit by a freeze ray, cementing everything in place. Trees, cars, people, buildings, everything is moved like it's being carried away by a tsunami, and then suddenly it all just stops. Landslides are the same eerie way. It's like god is playing "red-light-green-light" with giant hunks of material.
I work for the Department of Redundancy Department.
Not arriving at port also makes unloading much slower...
In this case, bauxite.
The PDF linked from the article has a FAR better explaination:
http://www.imo.org/en/MediaCen...
If you're right, that merely elevates the article from "crackpot claptrap" to "shoddily written clickbait". The mystery is less what happens than why it happens only to some ships, and why ship owners don't take the safety measures that are described in other comments here.
The article and accompanying discussion addresses these questions and was technically interesting and high quality. "A lot is known about the physics of the liquefaction [...] Yet despite our understanding of this phenomenon (and the guidelines in place to prevent it occurring), it is still causing ships to sink and take their crew with them."
The technical answer is that the existing guidance on stowing and shipping solid bulk cargoes is too simplistic. Liquefaction potential depends not just on how much moisture is in a bulk cargo but also other material characteristics, such as the particle size distribution, the ratio of the volume of solid particles to water and the relative density of the cargo, as well as the method of loading and the motions of the vessel during the voyage.
The economics make it clear that it's not worth spending huge amounts of money (that also make ships more awkward to load) for an event that's comparatively rare. It's doubly not worth spending money on refits when we don't even have a good physics model of it, one that's backed by data+observations. And if we kitted out some experimental ships but they proved to be in the 99.9% of ships that aren't affected by the phenomenon, then we won't have gathered any data.
The main idea in this thread was lengthwise bulkheads. I've never seen a ship designed that way. Not sure why. Maybe because it's a bulkhead that would need to be seriously strong (to stop the millions of tonnes) and would contribute nothing to the structure of the ship; only weight. Another idea was pumps to remove the water. Those would have to be exceptionally robust to handle being hammered by lumps of bauxite, and not get clogged by the finer bauxite. Maybe you could put a pump behind a mesh, if you could invent a mesh that would stand up to the millions of tons. The article suggests that another cause might actually just be the *speed* of loading. If that's true, how would we measure+test that? Could we invent a different loading technique that's mostly as fast? It'd be massively cheaper than lengthwise bulkheads.
There's a good video too: https://www.youtube.com/watch?...
In the video, the Australian Maritime Safety Organizations suggests a different preventative measure which is - captains should be aware that this is a phenomenon, aware of what are the warning signs, should pay attention to those first signs, and should consider seeking a Port of Refuge.
In short - excellent article, interesting phenomenon that even though we know about liquefaction and know about ocean shipping most of us still wouldn't have thought of, is deeper than "just add bulkheads".