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.

Comments

This makes it sink?

[sanosuke001]

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.

Re: This makes it sink?

[Anonymous Coward]

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.

Re:This makes it sink?

[gshegosh]

"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."

Re: This makes it sink?

[v1]

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.

Re:This makes it sink?

[dwywit]

I guess fitting baffles or compartments to bulk carriers costs more than the insurance when a ship goes down.

Re: This makes it sink?

[mjwx]

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.

Baffles will increase mass, weight and make loading and unloading of cargo much slower.

As for the second point, what will all the specialised ships do whilst not being employed for a single task? Freighters cost millions to make, every day they sit empty or idle is a day they're costing money.

The best and simplest solution is to find out why cargoes are liquefying, its not something that happens that often. 10 ships a year. There are an estimated 11,000 bulk carriers in service.

Re: This makes it sink?

[Anonymous Coward]

Not arriving at port also makes unloading much slower...

Re:This makes it sink?

[ljw1004]

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".

Preventable

[BrendaEM]

I guess the economics of letting the occasional ship sink with lives lost, is cheaper than securing the load.

Could injecting gas from the bottom help?

[gotan]

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.

Liquefaction

[MagicM]

For a fun example of liquefaction, check out Mark Rober's video on YouTube.

Re:Cargo?

[tazan]

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.

Compartmentalization

[Nidi62]

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.

Summary

[bluegutang]

This is a remarkably good summary for Slashdot.

Re:Cargo?

[AndroSyn]

In this case, bauxite.

The PDF linked from the article has a FAR better explaination:

http://www.imo.org/en/MediaCen...

Yes it will cost more

[sjbe]

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.

Re:Yes it will cost more

[thegarbz]

And that is somehow worse than the loss of the ship and possibly the crew?

In-humanness of the response aside, the risk is quite low, ships are replaceable, and crews are typically from poor countries where life is cheap. These aren't your western well paid sailors who are mourned and whose companies get sued into oblivion for providing unsafe work locations.

Unfortunately the answer to your question is yes.

Sit idle. Yes this will make the cargo cost more to carry.

Only if the costs are spread across the industry. If the costs are only carried by the one company prioritizing safety over cost then cargo won't cost more to carry, it will simply put one competitor with a conscience out of business.

If safety demands a specialized ship then so be it.

Are you talking about the customer who doesn't want to pay or the hauler who doesn't want to bear an additional cost when their competitors don't?

The question is what to actually do about it which will almost certainly involved some amount of change to ship design and cargo procedures.

Indeed. But the answer is not to jump to expensive and impractical solutions that won't see implementation without a concerted effort across the entire industry. Shit man for hazardous cargo the industry in the past 30 years hasn't even agreed to a standardised way to emergency shutdown their unloading pump. Good luck getting them to implement something that actually cost them money.

Re:Yes it will cost more

[Solandri]

In-humanness of the response aside, the risk is quite low, ships are replaceable, and crews are typically from poor countries where life is cheap. These aren't your western well paid sailors who are mourned and whose companies get sued into oblivion for providing unsafe work locations.

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.

Re:Cargo?

[Applehu Akbar]

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.