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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.
As far as i understand they have a mixture of solid particles surrounded by liquid, the particles are more compressible, so under pressure the whole structure loses coherence as effectively the solid fraction is reduced, since the liquid is less compressible.
I wonder if this could be helped by injecting some gas from the bottom after loading, so there are pockets where the granular solid is surrounded by compressible gas instead of liquid. If the density of the liquid is less than that of the solid surplus liquid would be driven to the top where it could be extracted.
It might help if the gas pockets are well enough dispersed.
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Window glass at room temperature has a nearly incalculable relaxation time, approaching the age of the universe itself. For all practical observations, this glass is a solid. But its solidity is in the eye of the beholder.
For a fun example of liquefaction, check out Mark Rober's video on YouTube.
Out of curiosity, do these bulk cargo vessels store their loads in one big hold, or are the holds compartmentalized? If the ships have multiple holds (say 3), each side by side and running lengthwise, then even if the loads in each hold liquefy and shift to port, the loads in the center and starboard holds may still have enough weight to counteract the shift.
A quick google search turns up this image, showing a stern to bow layout. So if the load in one hold shifts it is likely that all the others will shift too. So running the holds bow to stern and stacking them port to starboard would solve that issue. Given the size of these vessels I would assume that loads shifting forward or aft would be less of an issue or concern, but you could always make 6 holds by running 3 along the length of the ship and separating them midship.
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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."
Nope!
Ground water in the soil has nothing to do with earthquake liquefaction. Liquefaction can occur without any water present at all.
See, e.g., https://www.britannica.com/science/soil-liquefaction
TL:dnr: [vibration] causes otherwise solid soil to behave temporarily as a viscous liquid.
As for cargo ships filled with bulk wheat, rice, etc.; it's kinda stating the obvious that, while they are solids, in the sense that they are not liquids, they also aren't solid like a blocks of wood, rolls of steel, or pigs of aluminum. They behave much more like liquids.
Another recipe for disaster is fill a cargo hold full to the top with rice, seal the hatches, and then add some water (e.g. from a leak), and watch the ship literally burst apart in slow motion.
The ship is insured and the crew expendable.
One of the major bulk cargos cited was bauxite, aluminum ore. Though most ores are smelted near the mine, the economics of aluminum are weird because this element requires vast amounts of electricity to refine. It actually pays to mine bauxite in Australia but smelt it in places like New Zealand, where there is cheap hydro, or Iceland, where there is cheap geothermal electricity.
The ship is insured
If the shipping company redesigns their ships, they can reduce the likelihood of a ship being lost.
If they reduce the likelihood of a ship being lost, they reduce how much insurance pays out to them.
If they reduce how much insurance pays out to them, they can successfully argue for lower rates.
So I can only conclude that either, 1) it would cost more to redesign the ships than would be saved, or 2) the insurance companies don't know that the shipping companies are choosing to not do more to mitigate ships being lost at sea and so are not charging appropriately for that.
This is a common trope, but it's simply not true. The number of cargo ships lost at sea about equals the number of lives lost aboard those ships. That is, on average about 1 person dies for each ship that sinks.
The vast majority of people aboard a cargo ship which sinks are rescued. Life rafts are required by all shipping regulators. And satellite locator beacons have become so cheap that I suggest you get one if you do things like boating or hiking.. Their cost (a few hundred dollars, though a commercial model will run a few thousand) is much less than the liability and bad publicity of someone dying because your ship sank. When someone dies, it's usually because they were unable to reach the life raft in time (injured or blocked in due to the accident which sank the ship).
In fact, the fatality rate works out to (100 deaths) * (100,000) / (1.25 million) = 8 per 100,000. That makes it safer than a variety of jobs as mundane as taxi driver or landscaper. The fatality rate is right around the average for all jobs if you account for those people being aboard the ship 24/7, while people are at the other occupationss on average for less than 6 hours a day.