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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.
Why does the ship sink, though? Is the material stable in its granular form but without the water binding it is it corrosive or something? TFS wasn't very helpful in explaining why this effect is dangerous or what is being done about it at all. It, however, explained the effect itself fairly well.
-SaNo
I guess the economics of letting the occasional ship sink with lives lost, is cheaper than securing the load.
https://www.youtube.com/c/BrendaEM
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.
"By the way if anyone here is in advertising or marketing... kill yourself." -- Bill Hicks
For a fun example of liquefaction, check out Mark Rober's video on YouTube.
Mineral Ores. Basically they have a hold full of dirt. If it's loaded during rainy season it's a hold full of mud and can act just like a landslide does in an earthquake.
There is absolutely no "mystery" here.
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.
The only thing necessary for evil to triumph is for it to be pitted against a slightly greater evil
Grain too contains water. Drying is necessary to prevent spoilage but it cannot drive out all water and then there is humidity to consider.
This is a remarkably good summary for Slashdot.
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.
In this case, bauxite.
The PDF linked from the article has a FAR better explaination:
http://www.imo.org/en/MediaCen...
Baffles will increase mass, weight and make loading and unloading of cargo much slower.
And that is somehow worse than the loss of the ship and possibly the crew?
As for the second point, what will all the specialised ships do whilst not being employed for a single task?
Sit idle. Yes this will make the cargo cost more to carry. That's just how the cookie crumbles sometimes. Ships for liquefied natural gas don't get converted to haul coal when not carrying cargo. If safety demands a specialized ship then so be it.
The best and simplest solution is to find out why cargoes are liquefying, its not something that happens that often.
And then what? They evidently already know why they are liquefying. The question is what to actually do about it which will almost certainly involved some amount of change to ship design and cargo procedures.
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.