Request for accuracy for a film...

[pbnj]

Hi guys
I'm researching a feature film script and I need some advice on a specific scenario...

I need to transfer two people - with no gear - from a pressurized vessel (like a mini submarine) to a wet diving bell in an emergency.

I want it to be as realistic as possible, but not necessarily as safe as possible. As with most movies based on tension, the odds of them transferring safely have to be 50/50.

1. At what depth below surface could somebody conceivably exit a pressurized vessel with no gear and swim a short distance to a wet diving bell?

2. Would a wet diving bell be believable at this same risky depth?

3. At 50/50 odds, what kind of physiological effects could one expect in attempting this?

4. Would those effects, say sustained for a minute or two swim between pressurized spaces, even be survivable with a wet bell, or would the pressure be too much and require a hyperbaric dry bell?

I'd prefer they can step out on to a ship deck upon surfacing without life-threatening complications, but they don't necessarily have to be walking in perfect health straight away.

Any help would be hugely appreciated and I'm open to hearing suggestions or complications I may not have considered.

Thanks, guys

Pete

 

[Storker]

Just a thought: have you looked into commercial saturation diving and what kind of depths that is done at?


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[Rhone Man]

Depends on a couple of things. Most submarines are not pressured to equilibrium with the pressure at depth in the same way a diving bell is. If you assume the people leaving the sub are breathing in a gas mix that is something approximating what we breathe at the surface, then they would be like a free diver and they would feel a massive constriction on their chests below about 100 feet. But theoretically? Free divers go bloody deep when they set records, so if you are in Hollywood, theoretically you could pull that maneuver off at several hundred feet. But it will be cold, and dark, and you would not be able to hold your breath for nearly as long as you might suppose. Provided the bell was close though, a good swimmer might make it.

Now if instead of a submarine you are in a pressurised environment (probably not a sub, but some form of underwater habitat), your chances get much better. It is still cold and dark, but you have effectively much more gas in your lungs at greater partial pressures, and you are in much better shape to swim to your bell, and you'd like your chances of swimming a similar sort of distance as you might be able to swim underwater at the surface. There was a scene similar to this in the movie Sphere. They overdid it a bit, but that's Hollywood for you.

 

[pbnj]

So, if a submersible is intended to maintain surface pressure for quick and easy entry/exit between explorations, then going from the submersible to a wet diving bell in an emergency would cause a dramatic increase in pressure?

If I'm understanding correctly, one of two things is ideal:

1. They would evacuate the submersible (which is at surface pressure), endure a minute or less of severe pressure and swim to a dry/hyperbaric bell set at close to the pressure their submersible was at?

In this instance, would that minute or less of increase in pressure during the swim create any severe problems?

2. They would need to pressurize their submersible to the equivalent pressure of the wet bell they were escaping to, making the swim easier and then a slower ascent to the surface in the bell?

In this instance, with a wet bell at, say, 300ft, how long would it take to safely increase the submersible from surface pressure to depth pressure required? Or even 50/50 movie stakes bare minimum?

---------- Post added May 5th, 2015 at 03:42 AM ----------

Just a thought: have you looked into commercial saturation diving and what kind of depths that is done at?


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I have a little.

The complication is that the submersible crew are shift workers. Enter on the surface, work for 8-12 hours at depth and then resurface. They never actually dive outside the vessel, so saturation is not required.

However, if relatively fast saturation is possible in an emergency, it's definitely an option for escaping to a wet bell at depth.

 

[Rhone Man]

Well, I think you get some kind of pressurisation when you go through the escape hatch (unless you get shot out like a torpedo). When they flood the chamber which allows you to exit the submarine, that will obviously build up the residual air pressure as it floods and vents. You would expect your last breath to be something close to ambient pressure before you leave the sub. And I don't think it would take very long to flood the chamber.

Generally speaking, sudden increases in pressure are not hugely problematic (except for the physical stress on the lungs). Sudden and dramatic decreases in pressure can be very bad indeed. By way of comparison, if you look at free divers who do "sled" diving (basically like attaching yourself to a giant anchor, plunging to some huge depth, releasing yourself and swimming up), they experience extremely rapid pressurisation. Whilst it is uncomfortable for the air spaces in your body, it is usually not especially harmful.

At a depth of 300 feet this would all be pretty dramatic, but certainly survivable (well, depending how far they had to swim to the bell). In a life or death situation I would figure I had a decent shot of getting even the members of my family who were not strong swimmers across to a bell which was 20 or so feet from a sub at that depth, assuming a relatively peaceful exit through a flooded access chamber. If you imagine your hero is young and athletic you could probably push that a bit and still not stretch the realism too far.

 

[Storker]

At those depths, work is either done from closed, surface pressurized vessels, or by sat divers. At 100m, you work up deco obligations so quickly that it's cost prohibitive to bring the workers to the surface between shifts.

For normal SCUBA diving, a descent speed of some 60 ft/min (20 m/min) is considered max. Your most obvious problem with faster descents will probably be equalizing your ears. So, at 300ft, that would take some five minutes.

Submarine crews have - at least in the past - been trained to exit the sub to do a free ascent. That requires a sluice chamber that the crew enters, pressure is increased until it balances the outside pressure, and the crew can the exit the sub. I don't know how fast the pressure is increased in those scenarios, but if you're opening the sub to exit, you need to equalize the inside pressure with the outside.

As soon as a person is pressurized and breathing gas at ambient pressure, nitrogen will start dissolving in the tissues, and at 100m depth, you'll have an almost instant deco obligation. Also, unless you're breathing a He mix, you'll probably be narked out of your skull, and the O2 partial pressure of air will be dangerously high at those depths.

 

[pbnj]

Interesting.

I see the freediving record is around 700ft. That's amazing.

Do you think flooding the submersible at, say, 300-400ft, grabbing a breath, exiting and making a straight shot up for the surface would be too unrealistic? This crew would be trained for diving, but not prepared for it in this emergency.

How long would it take a physically fit, trained diver with no gear to resurface from those depths?

It would certainly be more tense than a short swim to a diving bell, but I don't want to poop all over credibility.

I understand that freediving has less chance of the bends due to the time involved?

I'm not afraid for my heroes to vomit pathetically when they get to the surface, but their feat should be humanly heroic, not superheroic.

Thanks for your help so far, guys!

 

[doctormike]

I don't know much about sat diving, but I want to commend the OP for taking the time to ask diving experts about this topic. It's always frustrating for divers to see unbelievable, poorly written stuff in movies and on TV about diving... What's obvious to us might not be obvious to a non-diving writer!


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[pbnj]

Storker, that submariner emergency free ascent definitely meets my qualification of heroic. I would need to figure out a depth to launch from that had real 50/50 (or worse) stakes.

During the emergency, they lose contact with their surface ship. If I could get them to the diving bell, only to find it incapacitated because their surface ship was also devastated, it would be another twist of the knife and force them into a free ascent.

Again, it would come down to settling on a depth that was borderline crazy to attempt, but not impossible.

 

[Storker]

I don't know much about sat diving

Me neither

However, AFAIK Akimbo here at SB is or has been a commercial diver. He's probably one of the guys here who can answer some of the OP's questions best. Maybe the OP should drop him a PM and tell him about this thread?

http://www.scubaboard.com/forums/commercial-divers/467739-what-saturation-diving.html