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Guns, Explosions, Fires bleg:

Suppose that a ship's crew is fighting a gun battle with pirates. A stray bullet hits a storage tank. Could that bullet ignite a fire or cause an explosion by hitting:

the fuel tank?
crude oil?
liquid natural gas?
other commodities that might be shipped by sea, and if so, which ones?
Commenters who actually know an answer, and are not guessing, will be appreciated.

Gene Hoffman (mail) (www):
Bunker fuel and diesel are actually quite hard to ignite and are the primary fueling mechanisms of long distance vessels.

LNG is a whole other issue and would have a serious risks of explosion from gunfire. Crude oil is not going to have any real risk of fire or explosion from a conventional round.

There is some small fire risk with regards to RPGs starting fires that would spread to crude, but the flashpoints remain very high and therefor are very hard to ignite.

Dropping a match in bunker fuel or diesel will put the match out.

(Only a few thousand offshore NM)

-Gene
4.16.2009 1:39am
Jim at FSU (mail):
It would depend on the exact type of petroleum product being shipped, how full the container was and whether there was oxygen in the empty space. As I understand it, tanks for transporting flammable liquids (ie, fuels) are purged of oxygen before shipping to avoid the risk of sparks causing explosions. There is an atmosphere of inert gas over the petroleum products at all time. This would prevent an explosion, since petroleum product produce energy by being oxidized, something they cannot do alone, under an inert atmosphere.

It would also require that a projectile penetrate outer the hull of the ship and the side of the tank containing the petroleum product. If the pirates brought a 14.4mm machine gun with them, this might be sufficient, but I doubt anything less will penetrate the outer hull of the vessel. Although the outer hull might be penetrated by an RPG-7, the outer hull will shield the inner tank wall much like slat armor shields a vehicle- the separating space between the two barriers will allow the gaseous penetrator to dissipate. Even if the space were modest (say a foot or two), this would probably be enough to defeat any HEAT warheads small enough to be launched from a man-portable platform.

Additionally, even in an oxygen atmosphere, crude oil is not particularly volatile nor is the diesel/kerosene used to run most large ship motors. They might burn if the "fuel tank" of the vessel were pierced, but they wouldn't EXPLODE. Also, there will be fire suppression systems in the engine room, or at least their should be.
4.16.2009 1:39am
Jim at FSU (mail):
Cliff notes:
Without going on for pages, my hunch is that any substances hazardous enough to damage the ship are protected so that the ship can suffer collisions and other serious mishaps without being vaporized when the cargo cooks off.

I very much doubt that there is any Wages of Fear type transportation of explosives and fuel going on in the world today.
4.16.2009 1:44am
Malvolio:
The gods of such questions are Adam Savage and Jamie Hyneman and they say, basically, no.

At least about gasoline. Which makes sense, as gasoline is only inflammable when mixed with oxygen in a fairly narrow range of proportions.

And, yes, I'd be guessing here, but the same is probably true of LPG and especially crude oil. Don't forget, the bullet would have to be
1. fired
2. hit a tank with the fuel in it
3. penetrate the tough metal of the tank (which is designed to contain high-pressure fluid in heavy seas)
4. bring enough heat to ignite the fuel
5. somewhere find enough oxygen to support a fire

Basically, this is only going happen on an episode of 24.
4.16.2009 1:44am
cirby (mail):
On the other hand, think of what would happen if a pirate crew decided to go from simple hostage-taking to large-scale extortion.

Capture a liquid natural gas tanker, wire it with explosives, and sail it into a busy harbor. Ask for a few hundred million bucks.

They already captured one mid-sized LNG ship and ransomed it...
4.16.2009 1:57am
Jim at FSU (mail):
That wouldn't work, for reasons that should be obvious from my first post. LNG isn't an explosive, it's a relatively slow-burning petroleum fuel. If it were under a lot of pressure, shattering it (with a ton of explosives) would probably throw quite a bit of debris. It would also make quite an impressive fireball as it burned in the air. But it would not explode as if it were a tanker filled with nitroglycerin.

Tankers full of explosives have gone off in harbors before and it caused horrific damage. Here's one:
http://en.wikipedia.org/wiki/Halifax_Explosion

There have been others (and even a few from torts that I can't remember the specifics of) but I can't remember them off the top of my head.
4.16.2009 2:13am
Eric Wilner (mail) (www):
Malvolio: see also Episode 95, the James Bond special, in which they tried to explode a propane tank by shooting it. They did eventually manage to ignite the gas cloud from a punctured tank, using a stream of incendiary bullets.
Propane being a particularly easy thing to ignite (once it's out of the tank and mixed with air), it's quite surprising how hard they had to work at getting it to go off; I didn't really expect sparks from steel-core bullets to work, but even repeated tracer rounds didn't do the trick.
4.16.2009 2:43am
Frater Plotter:
Capture a liquid natural gas tanker, wire it with explosives, and sail it into a busy harbor. Ask for a few hundred million bucks.
For the same reasons that suicide hijacking doesn't work any more, this won't, either.

You know, we have these things called navies and coast guards that don't let them come in. I am reassured that the current C-in-C appears to have remembered why the hell we have a navy.
4.16.2009 2:55am
Jim at FSU (mail):
Here's the one I was thinking of, involving ammonium nitrate and munitions. It knocked people on their asses 10 miles away and was felt 250 miles away. Most of the town was completely obliterated:
http://en.wikipedia.org/wiki/Texas_City_disaster

Anyway, the exploding ship cases all involve the transport of vast quantities of explosives, not petroleum compounds.
4.16.2009 3:25am
Splunge:
Kopel, you should keep in mind that explosions and fires are very different things. Roughly speaking, an explosion only happens when the chemical reactants are well-mixed, so that the chemical reaction can happen over a large volume simultaneously. (It doesn't actually happen simultaneously, of course; it begins somewhere with an application of the necessary activation energy, and then a wave of reaction radiates out from there, its speed limited only by the speed at which mechanical energy can propagate in the material, roughly the speed of sound.)

A combustion, by contrast, exists when the reactants are not well-mixed, and one or more must be transported to some interface, typically by convection, i.e. actual mass transport, a considerably slower process than energy transport. A tank of liquid gasoline burns at the surface, and gas and oxygen must be transported to the interface to react. By contrast, if the gas is mixed as vapor with the oxygen, the reaction can take place throughout the volume immediately, and you can get an explosion.

Clearly a bullet can provide activation energy, but that will only get you an explosion if your reactants are pre-mixed. Of course, ships are carefully designed to avoid any mixing of chemically reactive species, since events as energetic as a mere bullet impact, e.g. a hundred-pound beam slipping and hitting a steel deck, are not uncommon. (For the same reason, if you leave your stove on but unlit, and your house fills halfway with methane, it's pretty sure to go off, even if you don't fire your gun.)

You're probably also talking about large explosions, and that means substantial amounts of mixing, which requires lots of energy (not mere bullets) and/or lots of time. So I'd suggest that the only way a bullet can set off an explosion is if some disastrous pre-existing condition has been set up -- and in that case, you probably won't need a bullet to set off the explosion anyway.

About starting a fire I have no idea. Starting fires isn't very hard, I would guess. You could just hit a fusebox, maybe.
4.16.2009 5:57am
tommears (mail):
It is possible to make a tank of fuel oil or LPG explode. Google the word BLEVE (boiling liquid expanding vapor explosion). Unfortunately you need a large, hot fire to get one of these started, not sudden ignition sources like bullets or RPGs. Of course you could start the fire with the RPG ;->
4.16.2009 6:53am
Porkchop:
I spent 2-1/2 years on a fleet oiler carrying 8 million gallons of DFM (Diesel Fuel, Marine) and JP5 (jet fuel -- basically kerosene). Neither is particularly volatile, although it's not a good idea to test that statement by dropping matches into the tanks.
4.16.2009 7:28am
Richard Aubrey (mail):
Key is the projectile has to penetrate the storage container. This is usually a quite sturdy steel tank. Some of its surfaces are rounded to fit within the hull with as little waste cubic as possible, and pressure vessels are rounded for strength.
So, first, penetrate the steel hull.
Then find the storage vessel.
Then not ricochet due to less than perpendicular impact.
Then have sufficient energy remaining to penetrate.
Then go through the ignition problems listed above.
It would take a heavy machine gun--50 cal or the Sov version, 14.5mm to get through the outer hull in the first place. Then it would have to find an ideal impact angle on the tank.
Problem is, there is no purpose to having one of these on a pirate boat since it's too heavy and awkward to use as an anti-personnel weapon from the boat, hauling it up the scaling ladder would require it be broken down....
You'd have small arms and machine guns in the area of 30cal, or 7.62, and then personal weapons such as the venerable AK.
The pirate is not fighting a ship, here. He's strictly anti-personnel, preferably by threat rather than actual death.
Smaller calibers, such as the M16 5.56mm are considered by old farts as not having enough penetrating power to go through a cardboard outhouse. So the M16 and its Sov counterpart, the AK 74 aren't a problem.
RPGs could be a problem, except as one commenter noted above, the offset between the outer hull which detonates the warhead and the inner hull--most tankers are now double-hulled--should diffuse the blast. Offset armor has been working since WW II when the Germans put sheets of steel bolted onto their tank turrets--some models--offset from the turret armor by about six inches.
4.16.2009 7:57am
GradyBarr (mail):
Distinguish LPG and LNG, which some refer to. LPG is a conventional petroleum-based fuel. LNG is Liqufied natural Gas, which is cooled to several hundred degrees below zero farenheit. There are apocalyptic scenarios with this, but it requires a large spill, the gradual warming and then vaporization of the fuel (which would come out as a supercooled liquid and then change to a gas that is heavier than air at first) and then an ignition source. In the scenarios, it is the most spectacular but also the most difficult.
4.16.2009 8:19am
rosetta's stones:
I think the experts have it covered above. There's a threat, but not an imminent catastrophe, sailing around.

To understand the reactions involved with fossil fuels, it might be good to check out how the military uses them in their "thermobaric weapons", which I believe the US used in Afghanistan. These are fuel-air weapons, which create an environment where the above reaction can occur, which small arms penetration of a storage tank can't readily do.
4.16.2009 8:52am
rosetta's stones:
I don't believe the thermobaric bombs use fossil fuels as combustible, but I believe the principles are similar.
4.16.2009 8:54am
Tom S (mail):
During the "Tanker War" (Iran-Iraq War) in the 1980s, VLCCs hit by Iraqi-launched Exocet missiles rarely caught fire, and their crude oil cargos were for the most part successfully salvaged.
4.16.2009 8:59am
John Burgess (mail) (www):
What Tom S said.

I was in the Persian Gulf during the 'Tanker War', working with the US Navy's Public Relations guys. They were concerned about RPG attacks on tankers--particularly LPG carriers--but not overly concerned. There was a certain amount of gallows humor about 'Giant Ronsons', but no actual experience of them.
4.16.2009 9:12am
pintler:
I will just point out that this experiment has been tried many, many times in e.g. WWII, with a relatively small chance of dramatic results. There were exceptions, e.g. American PT boats that ran on avgas.
4.16.2009 9:22am
Houston Lawyer:
We had a LNG spill outside of Brenham in 1992. Conditions were just right as it was a cool still morning and the gas escaped and stayed near the ground in a shallow valley. It hit a source of ignition and created a huge explosion. I heard it in Houston, about 80 miles away. It broke windows several miles out.

My history professor in college was with an Army unit that went to Texas City to help following that blast. He could tell captivating stories about it.

The Darwin Awards web site chronicled the story of a man who shot a butane tank with a .22 and then walked towards it with a lighter to see how close he could get before it ignited.
4.16.2009 9:58am
Gray Ghost:
It would take a heavy machine gun--50 cal or the Sov version, 14.5mm to get through the outer hull in the first place. Then it would have to find an ideal impact angle on the tank.
Problem is, there is no purpose to having one of these on a pirate boat since it's too heavy and awkward to use as an anti-personnel weapon from the boat, hauling it up the scaling ladder would require it be broken down....


I don't necessarily disagree with the conclusion but I think its very possible to have a .50 cal mg on a pirate sized craft. After all, .50 cals are sometimes mounted in the beds of pickup trucks. If you can do that you can mount one on a fishing boat or motor launch.

I would say in general it would be diffiuclt or impossible to explode or burn a merchant ship or tanker with a mere small arms bullet but very possible to do so once the ship has been taken over and the pirates have access to all spaces of the ship. Just blow a few holes in important tanks, let thinks leak and slosh around, add air then light it up. Depends in part on the volatility of the cargo or fuel.

But that I think is an argument for armed resistance to pirates not against resistance. Keep the pirates off the ship and you are relatively safe. Let them on and then they control the next steps. It's sort of analogous to the question of do you fight hijackers or let them take over the aircraft....we have learned it's better to fight them.

GG
4.16.2009 9:59am
FormerStudent:
The navy and Marines train people to board ships. So does the Coast Guard. In fact the Coast Guard does it all the time. We don't train them to board with water pistols.

Could an academic with too much time on his hands dream up a scenario leading to catastrophe? Yes. Is it likely that every ship with an armed crew is sailing to certain doom as their idiot crewmembers pour hundreds of armor piercing incendiary rounds into paper thin containers filled with highly unstable high explosives? No.
4.16.2009 10:09am
Richard Aubrey (mail):
GG. I was thinking of the mount. To shoot "up", the breech has to be "down". The shooter would have to be lying on the deck, trying to get his eye behind the thing. The closer, the more awkward the angle. If the boat were big enough, they could have a pintle mount and not raise the center of gravity too much. That would help. Problem then is, explaining the whole thing. You could pitch the 50 overboard, but there's still the mount....
You can announce your presence with a 30 just as well, or, for that matter, with a cowbell.
The point for the pirates so far is to get on deck with light weapons fast so as to keep the crew from either getting into secured spaces or fighting back. Pistols would do just fine under current circumstances, and you can move fast.
4.16.2009 11:15am
Monty:
What about a direct hit to a LNG tank from an RPG. You have the whole, essentially burned into tank...
4.16.2009 12:08pm
geokstr:
Unless I am not understanding the above comments, it appears that the concensus of experts is that it would be very very difficult to ignite the volatiles typically carried by or used to fuel merchant ships, particularly with the armaments normally carried by pirates. Additionally, it would seem to be against the pirates best interests to destroy the ship they wish to hold for ransom.

I made a suggestion on a previous piracy thread that perhaps we should try some passive defense techniques. Since the pirate boats sit low in the water and the merchant ships decks are usually several stories above the surface, they must be boarded with grappling hooks, unless the crew just gives up at the first threat and lets them come aboard.

So how about electrified or barbed wire fences around the decks, or some such defensive mechanism. If the pirates can't really do any significant damage while still on their own ship, wouldn't such devices allow the crew to tell them to piss off and keep on going?

That suggestion was immediately dismissed on the other thread because the pirates would supposedly be able to inflict heavy damage to the merchant. This thread seems to suggest otherwise.

Am I missing something here?

Of course, that may make the pirates willing to spend some of the hundreds of millions in booty the take each year and buy one of the old warships or submarines rusting in various Russian or former Soviet states' ports and retrofit it, but those are very slow and would be easily located and destroyed by modern military vessels, non?
4.16.2009 12:57pm
gerbilsbite:
Just got off the line with my father, who's a fire/explosion investigator. If a round has punctured a tank containing an accelerant, it is almost certain to be throwing off enough heat as a result of the friction to ignite the fuel. And if a sizable tank goes on any ship, the ship will be a total loss.

http://www.youtube.com/watch?v=wVZUVsHsWis

Remember: there's nothing that says the firefight would have to be inter-ship. If it takes place after the pirates board (as the scuffle did on the Maersk Alabama), then the requisite firepower to create an inferno is much lower.

Additionally, the most recent pirate attacks have involved, among other weapons, hand grenades. An incendiary device that finds a pool of accelerants will be sufficient to cause a fire, and a well-fueled shipboard fire areas can burn incredibly hot, which in turn increases the risk of combustion in nearby areas.
4.16.2009 1:38pm
Don Miller (mail) (www):
Volunteer Fireman, Hazmat Instructor, Former Machinist Mate, US Navy, Engine Room Supervisor

I have practical experience with this question.

Basic Fire Science:
1. No liquids will burn or explode. They must be converted to a vapor first.
2. The concentration of vapors must be in the flammable range to burn. Can't be too lean, can't be too rich
3. Everything is different

For a gunshot to cause an explosion, you must be dealing with something that is shock sensitive and self-oxiding, example Nitroglycerin. None of the chemicals you mentioned are shock sensitive, so just the gunshot won't do it. The bullet must hit a vapor pocket with the right vapor/oxygen mix to burn. Then the bullet needs to be hot enough to cause the vapors to ignite. While the average bullet has a high temperature, it doesn't have enough heat to raise the temperature of a large volume of vapor to the self-ignition temperature. Also, it is traveling fast, so it doesn't have time to transfer that heat.

So, to even start a fire the bullet has to hit in the right spot, it has to find the right conditions, it has to cause a spark for ignition. For the fire to build to an explosion, the fire must be contained long enough for pressure to build up.

A tracer bullet or an RPG is more likely to start a fire because they provide an ignition source.

Even an LNG tank is unlikely to explode initially. Because it is 100% Natural Gas in the tank, under pressure, there is no oxygen to burn in the tank. The bullet would cause a leak. The leak would turn into a vapor cloud, the vapor cloud could be ignited causing an explosion. Natural Gas is much lighter than air, so the vapor cloud will tend to rise away from ship board ignition sources, but a 2nd bullet, tracer, flare, or RPG could ignite it.

Propane is heavier than air, bulk containers of propane could rupture and the vapor cloud would sink into the ship, and could find a separate ignition source.

Long answer, boils down to this, Pressurized Gas containers are at the most risk from gunfire. The venting gas is the problem. Gunfire is low risk compared to RPGs.

Liquid Fuel containers are low risk items to gunfire. RPGs could, depending on the location of the hit, ignite the fuel in the tank. Most likely it would cause a leak, then the spilled fuel would be at risk for a secondary fire from a different ignition source.
4.16.2009 2:05pm
einhverfr (mail) (www):
This scenario doesn't make a lot of sense to me. You can have a lot of damage caused by fire, but explosion is a different matter.

There is a second issue that I can think of though. Imagine if pirates captured an LNG tanker in a convoy (per CHris99's suggestion, some armed crews but no military escort) and threatened to start ramming ships if there wasn't a surrender. Could the real or perceived threat from this sort of tactic allow pirates to end up holding a lot more ships hostage?

Another question involves possibilities of shaped charges in terrorist attacks on tankers. If Somali pirates develop ties to Al Qaeda would this be a possibility?
4.16.2009 3:21pm
Tom S (mail):
POLISARIO in the Western Sahara mounted 12.7mm machine guns on zodiac rafts, and used them to attack fishing boats. They also used RPGs.
4.16.2009 3:23pm
Richard Aubrey (mail):
TomS.
Polisario had a navy. Hadn't heard.
Question is what was the goal in attacking fishing boats. Destroying them or scaring them into staying ashore, or capturing them? If the latter, 14.5s and RPGs are not cool. A heavy MG round going through, say, wood or fiberglass and hitting the engine block means you have converted a motor-driven commercial vessel of some use to a raft.
The extra punch of a heavy MG is unnecessary for the piracy we see, and awkward to use, especially when close to the merchie. All downside.
I presume the Polisario didn't worry about having to explain the mount?
4.16.2009 3:39pm
Carolc (mail):
The short answer is NO.

The experiment has been performed. A direct Exocet missile hit on a LNG cargo tank during the Iran--Iraq war did not cause an explosion.

http://snipurl.com/g1i7p
4.16.2009 4:16pm
DennisN (mail):
@ einhverfr



There is a second issue that I can think of though. Imagine if pirates captured an LNG tanker in a convoy (per CHris99's suggestion, some armed crews but no military escort) and threatened to start ramming ships if there wasn't a surrender. Could the real or perceived threat from this sort of tactic allow pirates to end up holding a lot more ships hostage?


They could do the same with any freighter, albeit with less spectacle. Once a weapon like that is on the loose, it would be a trivial task to put a torpedo or a laser guided bomb into it.


Another question involves possibilities of shaped charges in terrorist attacks on tankers. If Somali pirates develop ties to Al Qaeda would this be a possibility?


An RPG uses a shaped charge warhead. A shaped charge, assuming it is fired such that it penetrates the fuel tank, makes rather a small hole. You'll have a nasty blowtorch fire. There isn't enough power in the warhead to rupture the tank.

During the Tanker Wars in the Persian/Arabian (depending on whose side you're on) Gulf, the Iranians found it difficult to damage large tankers with dedicated antiship missiles such as Exocets. Big ships are remarkably tough critters.

If a real navy were to attack large merchant vessels, the attack would preferably be made with torpedoes or large aerial bombs. Five inch guns would make a mess of a tanker, but it would take a lot of five inch fire to sink a freighter.
4.16.2009 4:17pm
Carolc (mail):
FYI, the Exocet missile uses a 165 kg shaped charge warhead.
http://www.fas.org/man/dod-101/sys/missile/row/exocet.htm

So that movie-plot threat doesn't work, either.

The LNG tanker not only did not explode, the cargo was salvaged. It did make an impressive fire, as the boiling LNG mixed with sufficient oxygen to begin combustion many meters from hole the missile made in one tank (actually dewar) of the attacked ship.
4.16.2009 4:25pm
Tom S (mail):
POLISARIO claimed that they were "protecting" the territorial waters of the "Saharwi Republic" against illegal fishing. The fishing boat crews were held hostage, often in return for their home country making political concessions to POLISARIO, such as allowing POLISARIO to open an interests office (Spain).

Until Morocco was able to keep POLISARIO out of the Western Sahara through the use a system of sand wall fortifications, POLISARIO had pretty much free run of the territory.

Iraq used Exocets, not Iran.
4.16.2009 4:51pm
Bernie (mail):
WWII, no LNG, but lots of petroleum, attacks by ships and submarines with much larger rounds than anything you could put on a pirate craft. A lot of fires, but very few explosions. Ships carrying munitions did explode.
4.16.2009 5:05pm
C/M:
Former deck officer (chief mate), maritime academy education, served on tankers (crude and product), bulk cargo vessels, container ships and self-propelled dredges—

1. Penetration of a fuel tank is unlikely (they're typically—though not always—below the waterline at normal drafts for most ships), the fuel is very heavy (closer to tar, must be heated to flow), and can be difficult to burn without substantial pre-heating. The engine room portion of the ship has other stuff that could, in theory, burn (lube oil, hydraulic fluid, diesel) but is unlikely in practice.

2. Crude oil is very flammable if exposed to oxygen; however, penetration of a cargo tank is unlikely. Virtually all ships are double-hulled (OPA '90) to reduce the possibility of spill resulting from collision or allision (thank Joe Hazelwood), so the round would need to penetrate the outer hull plating, travel through a ballast tank or other void space and retain enough energy to penetrate the cargo tank plating. I suppose a large enough round (.50 BMG) might manage to do so, but the spill would likely be confined to the void space. The cargo itself is maintained in an inert (non-flammable) environment within the cargo tank. If spilled into the void space, the atmosphere would quickly become too rich to support combustion.

3. I've never served on an LNG ship, but my reading (perhaps out of date by now) would again lead me to believe that penetration into the cargo tank is unlikely (again, absent a very energetic round such as a .50 BMG). The liquid is maintained in a cryogenic environment, a hence the cargo tank's walls are fairly thick and well insulated.

4. Virtually all types of cargo are carried by ship. Many of these cargoes could be hazardous, flammable or explosive, but as a general rule one may assume that exceptionally hazardous cargoes are carried with much greater care. Chemical product ships carrying ethers, alcohols or other cargoes with very low flashpoints, or exceptionally poisonous cargoes (e.g., cyanates, benzene) are double hulled, with greater subdivision of tanks and more exotic inerting systems, for example. I believe it unlikely that typical small arms fire could penetrate these cargo tanks.

Pirates seem to favor RPGs, at least for intimidation. I don't know anything about RPGs, have never spoken to anyone that has first-hand experience with them, and so I can't really comment about them. They seem pretty potent (at least the armor-piercing anti-tank variety) and it's possible that they could penetrate into a cargo tank with enough energy to create a sizeable spill that could be ignited from a random spark. I couldn't say.
4.16.2009 6:48pm
Richard Aubrey (mail):
Tom S. Wow. Thought I'd been keeping up. Thanks for the info on Polisario.
The info ref Spain's caving (shut up,Aubrey, ed.) and opening an interests office provides a bit of additional reason not to let this stuff get out of hand.
Sure, right now they're just swapping sailors for money. No big deal, and IT CAN'T, WON'T, EVER CHANGE SO DON'T GO TALKING ABOUT NO SLIPPERY SLOPE YOU WAR MONGER.
Beheading Kenyan civilians to get their buddies free?
Okay, take them elsewhere (note, not Spain).
4.16.2009 9:08pm
DennisN (mail):
I pretty much agree with C/M except that, in the context of a double hulled merchant vessel, a .50 BMG or 12.7mm is not a particularly energetic round. A four inch gun is considered a popgun at this scale.

A shaped charge round like an RPG depends on the warhead exploding at the optimal distance from the target. The copper lining of the conical charge is explosively formed into a rod of plasma that penetrates the armor. Change the distance, and the effectiveness drops off dramatically. That's why the glorified bedspring slat armor, retrofitted to armored vehicles, works. It detonates the warhead too far away from the real armor to allow it to penetrate.

So the effectiveness of a double hull will depend on the standoff distance and the thickness of the inner hull.

Once the shaped charge penetrates, it has punched a small hole in the armor. It destroys a tank by spraying the armor and the penetrator rod around the inside and setting things on fire or penetrating them with white hot fragments. You definitely don't want to be in there. In a fuel tank, that hot plasma is quickly quenched with cold oil. WW-II battleships used their fuel tanks, in part as armor.

So you now have a two inch hole leaking black oil. You send down a crew with a tapered wooden peg and a mallet. Seriously. If it's a LPG tanker, you have a blowtorch fire, but that's still controlable. There's no way an RPG is going to reupture a tank or do serious damage to a merchie unless some poor mope is standing in the wrong place.

RPGs are effective against tanks because the inside is tiny.
4.16.2009 11:16pm
Uno Hu (mail):
This is a question that has no practical answer. The absolute answer is "It depends completely on the size/energy of the incoming round." By way of brief explanation, a 7.62 NATO round, even if "AP" is unlikely to penetrate anything on an oceangoing vessel besides window or porthole glass; a .50 BMG AP round would penetrate the hull (probably not both layers of a double hull) but penetration of structures in the ship would be questionable. Move up to a 3.5 inch or 5 inch Naval Rifle round and it will penetrate anything on the average oceangoing merchant ship and explode inside to boot; or to a 16 inch battleship-sized Naval Rifle, and it'll shoot completely through the ship and come out the other side if AP (2700 lb projectile) , or blow one heckuva hole if an HE round (1700 lb projective with large bursting charge), cause massive internal damage, holing of watertight subdivision bulkheads, and flooding, and severely compromise the seaworthiness of the ship with one hit.

So, what size weapons are the pirate using now? What size can they get their hands on if they want to escalate?
4.17.2009 5:37pm
dougger (mail):
Normal copper clad or lead bullets will not generate a spark to cause ignition. A steel core bullet (as often seen in some military ball or armor piercing ammunition)could generate a spark but even then ignition depends on other factors such as fuel/air ratio etc.

Upshot: Very low risk of explosion or fire.
4.17.2009 9:05pm
Sictransit (mail):
White phosphorus burns fiercely upon coming into contact with oxygen; ignition is automatic. As a safety measure, it is shipped in containers that are filled with an inert liquid. A stray bullet that pierced such a container and drained the liquid would result in a fire. The destructiveness of the fire would largely depend upon whether the material of the container would stand up to the heat. If the container maintained its integrity except for one or two bullet holes, the fire would likely do little more than produce an enormous amount of smoke. If the container ruptured, the fire would be exceedingly difficult to contain.
4.18.2009 8:38am
markm (mail):
First, large ships have hulls that are several inches thick. Oceanic waves can be so large that even a super-tanker will sometimes be lifted by one wave in the middle with both ends hanging, alternating with being suspended between two waves with the middle hanging. It's not the framework that takes such loads, but the hull plating forming what is in essence a giant box beam. So .30 caliber weapons are no threat to the ship itself, although they might be quite useful to keep the crew back so they can't use firehoses to repel boarders. A .50 machine gun may or may not penetrate, but penetrating the hull and then the inner tank on a tanker seems pretty unlikely. RPG's with shaped charges should easily burn a hole through the hull, but I'd expect the directed blast to dissipate in the inter-hull space.

OTOH, if it's a single-hulled freighter loaded with grain, cotton, or other flammable cargo with airspaces (not to mention things like WP), I see a pretty good chance of an RPG igniting the cargo, and not much chance of the crew repelling pirates and controlling the fire at the same time. The crew would have to take to the liferafts and the insurers would take a big loss, but the only explosion hazard would be if the crew left the hatches dogged down so tightly that pressure built up.

Finally, if crude oil burned that easily, we wouldn't have so much trouble controlling oil spills. Just evacuate and flick a match. But actually, while a burning oil slick can be deadly to men in the water or small boats, and would thus drive off all rescue or salvage operations, it burns out long before the fuel is gone. And it's not explosive.

With petroleum distillates much lighter than crude or the bunker fuel used in steamships (a petroleum fraction just lighter than tar, often needing heated so it can be pumper), there is an explosion hazard if the inner hull/tanks are leaking into the space between the hulls and the spill is vaporizing faster than the ventilators can exhange the air. If the concentration reaches the right level, and then there's something to ignite it, *boom*. I think that a .50 BMG bullet coming through the outer hull would be hot enough from penetrating steal plate for ignition. But for this scenario, the pirates would have to attack once, with something heavy enough to penetrate both hulls, be repelled, and come back later for another try, after enough time for the vapors to build up, but not long enough for the crew to rig extra ventilation and pound plugs into the leaks. I'd put that hazard much lower than the chance that the pirates would win the first fight and be angry enough at the resistance to summarily execute captured crewmen.
4.18.2009 11:43am
Brooks Lyman (mail):
Because LNG (Liquified Natural Gas) is a cryogenic (read: damn cold) material, the tanks are not only steel or stainless steel to hold the cargo, but are insulated on all sides with thick layers of material such as balsa wood, plastic foam, etc. and in addition, have an outer steel casing.

So, it would take quite a bit to get through the ship's hull, then through the tank wall. As has been stated, it would probably take a lot more than the pirates are currently (or likely to be) carrying, with the possible exception of an RPG, and that would probably be defeated by the double-walled hull. Shooting at the top of the tank (on those ships with tanks with spherical ends sticking out above the deck) would probably fail because the angle of attack would cause a ricochet instead of penetration.

While I'm no expert, I would doubt that a .50BMG would penetrate a double-hulled ship, and it might not even make it through the outer skin. Now, something like a 20mm cannon might do the job, but that's pretty heavy ordnance for the current pirate craft.

And so long as the pirates' aim is to capture ships and crew for ransom, they will probably not resort to truly destructive weaponry. If piracy should mutate into Jihadist terrorism, all bets are off, of course....
4.18.2009 11:19pm

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