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?

[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?

[Anonymous Coward]

This. Baffles for liquid transport (even fuel tanks in vehicles designed for non-cargo purposes) along with sectioning tanks is not even remotely new. So

a) treat bulk solid powders like bulk liquid
b) dehumidify the hold
c) have a spray down for offload if getting every last grain is desired (adding baffles and bulkheads increases surface area, which will increase losses due to product sticking to surfaces)

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?

[Entrope]

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.

Re: This makes it sink?

[Anonymous Coward]

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

This is why we have regulations. It levels the playing field so EVERYONE has this expense. And it also solves the "Someone elses problem" of ships sinking. The ship is insured, so the shipping company doesn't have any vested interest in safety. The people who run the company are safe on dry land, so they're just apt to say "shit happens". There's likely smaller companies where the captain/crew might also be the owners, but normally there's just no way for them to compete with MegaCorp, AND put in the baffles because of the downsides you mention.

Preventable

[BrendaEM]

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

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.