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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.
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.
"When a solid bulk cargo liquefies, it can shift or slosh inside a ship’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. At some point, the angle of list becomes so great that water enters the hull through the hatch covers, or the vessel is no longer stable enough to recover from the rolling motion caused by the waves. Water can also move from within the cargo to its surface as a result of liquefaction and subsequent sloshing of this free water can further impact the vessel’s stability. Unless the sloshing can be stopped, the ship is in danger of sinking."
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.
Likely it's the "free surface effect". Basically, if tanks or holds aren't full, when the ship rolls to one side, the cargo flows to that side too, moving the ship's center of gravity towards the low side. Get enough of a flow and the vessel can capsize, or the momentum of the flow can damage the ship:
The free surface effect is a mechanism which can cause a watercraft to become unstable and capsize.
It refers to the tendency of liquids — and of unbound aggregates of small solid objects, like seeds, gravel, or crushed ore, whose behavior approximates that of liquids — to move in response to changes in the attitude of a craft's cargo holds, decks, or liquid tanks in reaction to operator-induced motions (or sea states caused by waves and wind acting upon the craft). When referring to the free surface effect, the condition of a tank that is not full is described as a "slack tank", while a full tank is "pressed up".
Stability and equilibrium
In a normally loaded vessel any rolling from perpendicular is countered by a righting moment generated from the increased volume of water displaced by the hull on the lowered side. This assumes the center of gravity of the vessel is relatively constant. If a moving mass inside the vessel moves in the direction of the roll, this counters the righting effect by moving the center of gravity towards the lowered side. The free surface effect can become a problem in a craft with large partially full bulk cargo compartments, fuel tanks, or water tanks (especially if they span the full breadth of the ship), or from accidental flooding, such as has occurred in several accidents involving roll-on/roll-off ferries.
If a compartment or tank is either empty or full, there is no change in the craft's center of mass as it rolls from side to side (in strong winds, heavy seas, or on sharp motions or turns). However, if the compartment is only partially full, the liquid in the compartment will respond to the vessel's heave, pitch, roll, surge, sway or yaw. For example, as a vessel rolls to port, liquid will displace to the port side of a compartment, and this will move the vessel's center of mass to port. This has the effect of slowing the vessel's return to vertical.
The momentum of large volumes of moving liquids cause significant dynamic forces, which act against the righting effect. When the vessel returns to vertical the roll continues and the effect is repeated on the opposite side. In heavy sea states, this can become a positive feedback loop, causing each roll to become more and more extreme, eventually overcoming the righting effect leading to a capsize. While repeated oscillations of increasing magnitude are commonly associated with the free surface effect, they are not a necessary condition. For example, in the cases of both the SS Normandie and MS al-Salam Boccaccio 98, gradual buildup of water from fire-fighting caused capsizing in a single continuous roll.
Google "free surface effect" images
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.
the obvious solution to this is to have partitions inside the ship, to limit the amount of shift possible.
...
Except that might make loading and unloading cargoes such as bulk ores difficult or impossible, depending on the vessel and the port facilities.
The large machine in TFS sure looks a lot like a Hulett unloader, and those only work with wide-open, flat-bottomed holds.
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".