Tourist submersible visiting the Titanic is missing Part 2 68

[rodface]

@Brian Malone

The tube certainly seems sturdy; I've wondered how it was handled after manufacture. We've seen the video of them adhering an end cap; but how did they adhere the other end cap and then flip it over?

They were certainly pushing the envelope, but it seems that more funding and effort was needed to really take such a cutting-edge R&D effort to a safe conclusion.

 

[SnTMan]

I suppose that, given favorable COEs of the composite and mandrel that following an elevated temp curve, that the mandrel could shrink more than the cylinder, parting the two. Defer to the composites guys here

Wouldn't take much damage to the inner surfaces at mold removal to make the cylinder unusable.

Note my earlier post: CF Composite submersibles are possible. Just not by OG

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[rodface]

FEA study using Abaqus:

https://youtu.be/i-bvNHCCpFo

 

[SwinnyGG]

Mold releases are routine, but imagine trying to remove one cylinder in intimate contact with another with no draft

This part of the operation is solved. Mandrel is coated with mold release, after curing there is very little friction between the part being cast and the mandrel. All sorts of fiberglass and carbon components - think light poles, javelins, oar shafts, arrow shafts, baseball bats, carbon tubes for bicycle frames, pole vault poles, flag poles, etc etc etc are made by filament winding on a release-coated mandrel with no taper. Lots of those things have much thinner walls and are much more fragile than the hull shell would be, and they are easily removed without damage.

Given that the manufacturer of the shell is a reputable manufacturer of filament wound composites, I would not expect this part of the process to be a component of the root cause.

 

[waross]

I have a hard time seeing any failure mode where the end bells stay on

There were titanium rings glued to the ends of the carbon fibre hull.
These rings were bolted to the titanium end bells.
In the picture of one of the rings, there appears to be remnants of the glue and possibly fibre.
I suspect that the recovery crews removed the bolts holding the ring to the end bell.
The ring would have stayed with the end bell, unless the bolts were woefully undersized. I guess that that was possible.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[JStephen]

On the "FEA study using Abaqus"-
I have wondered out some of that would actually work.
You have tremendous forces applied on the hull, but also, a lot of mass has to move for that water to advance inward.
I assume that is a problem that may be theoretically solvable (ie, assume initial condition as spherical void in infinite compressible water at constant pressure, etc.)
In the case of the Abaqus model, he's probably assuming constant pressure on the outside, so the dynamic effects may be skewed considerably, as it is just the shell that has to be accelerated, rather than the shell plus tons of water.

 

[SwinnyGG]

a lot of mass has to move for that water to advance inward.

At the pressures you're dealing with at 4,000m of water depth, the masses of the the components involved mean very little.

At 4,000 m, the amount of force on one of the end bells, along the long axis of the sub, is roughly 8500 tons

 

[Tomfh]

In the case of the Abaqus model, he's probably assuming constant pressure on the outside, so the dynamic effects may be skewed considerably, as it is just the shell that has to be accelerated

Yes I agree that model is a bit questionable once things start moving. However either way the implosion would be a split second.

 

[1503-44]

It does look like 3ms is too fast to include water acceleration time.

If we try to include water acceleration (neglecting the mass of CF)
1 ft3 of water under a pressure of 5000 psi (720k/ft2) will accelerate at 358ft/s^2
Taking average velocity over a timestep, the CF shell transverses the roughly 5ft internal radius of the CF tube in still < 0.2s

But anyone inside would still have been totally destroyed in 0.1ms or less.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[SnTMan]

SwinnyGG, thanks for the clarification, I am not overly familiar with the processes, and I did not mean to imply damage from de-molding played any part in the failure.

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[SwinnyGG]

SwinnyGG, thanks for the clarification, I am not overly familiar with the processes, and I did not mean to imply damage from de-molding played any part in the failure.

No worries. It's a niche process, one of those things most people never get exposed to.

1 ft3 of water under a pressure of 5000 psi (720k/ft2) will accelerate at 358ft/s^2

Respectfully, you have a unit conversion error somewhere, or something. You're off by multiple orders of magnitude

1 m[sup]3[/sup] H[sub]2[/sub]O = 1025kg
Water pressure @ 3800m = 39,000 kPa = 39,000,000 N force over 1 m[sup]2[/sup]
f=ma
f/m=a
39,000,000N/1025kg = 38,050 m/s[sup]2[/sup]. 125,000 ft/s.

That's 38 meters/s of velocity added per millisecond. At the third millisecond of the event, the water/carbon/playstation controller/formerly human jelly is already traveling at 400+ f/s.

The total time to traverse the diameter of the hull is less than 10 milliseconds.

There's also another effect at play which hasn't been discussed really at all. At these pressures, the density of the water is increased 2.5-3% due to pressure effects. Any water instantaneously exposed to atmospheric pressure is also going to explosively expand, including gaseous cavitation of any dissolved gases present in the water. This effect increases the rate at which the 1 atm bubble inside the sub will close and equalize.

This is not something being bent under moderate pressure. The scenario we are discussing is literally an explosion in reverse. Huge amounts of pressure and energy are involved.

 

[1503-44]

You are right. I divided 720 by 1.98 slugs, not 720kips, lost 000 there so my number should be 720,000 / 1.98 slugs= 358,000 ft/s^2

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Tomfh]

This is not something being bend under moderate pressure. The scenario we are discussing is literally an explosion in reverse.

Yes it would be supremely violent. There is no
cushioning effect as the air gets compressed. No balancing of internal and external pressures as the sub fills up. This is an explosion running backwards.

Look at what happens with underwater explosives; the explosion grows a bubble, which expands until the energy is used up, then the bubble collapses again, only to rebound again.

In the case of the sub you start with a pre-made bubble, but it’s the same in terms of energetic collapse.

 

[MechinNC]

I think analyzing what happened in the last few milliseconds (or microseconds) of this crafts life is an interesting, albeit academic exercise. Kinda like analyzing what happens in an airliner when it hits the side of a mountain at 3-4-500mph. Well, it hit the mountain and bad things happened FAST. Got that.

Back to why this thing failed. A few predominate theories:

1. Acrylic view port blew inward. Could have happened. But in that case the water jet may have blown off the aft end bell, but would have expected most of the CF hull to remain at least partially intact, especially fwd.

2. Glue joint between TI end cap and CF hull. Could see that failing or extruding allowing a leak of high pressure SW into the hull. Again, in this instance would expect the hull to at least remain mostly intact.

3. Catastrophic crush failure of the CF hull. This is what I am putting money on. As it seems the end bells and retaining rings were recovered intact with little of the CF remaining attached (just going on photos, holding off solid judgement til solid analysis).

My thinking now is that high pressure exterior water got into the laminate and weakened it. 5-6000psi does some crazy things. My personal thought is osmosis, but I have nothing to base that on but gut feel. Water at that pressure moves through fibers, and once inside the laminate structure can do weird things.

And it all comes back to an apparent lack of a serious design review and test protocol.

Should have dove the thing 100 times to design depth, last dive to failure depth to confirm analysis. Seems that was not done, or anything like it.

 

[Tomfh]

Should have dove the thing 100 times to design depth, last dive to failure depth to confirm analysis.

100%.

There's been a lot of talk over the absence of "non destructive" testing. However you'd think a destructive test would be more compelling?. Composite Energy Technologies claims: "We’ve built vessels that we’ve cycled 200 times (to deep-sea pressures) and then brought to implosion and those fail at the same depth as new ones"

What about that James Cameron DeepSea Challenger, and the Steve Fossett sumbersible? Did they test those to failure?

 

[TugboatEng]

I imagine the Mariana Trench littered with CF submersibles taken to crush depth if deep sea tourism ever takes off.

 

[SnTMan]

CET PROVED their designs and would not put people in them.

OG BELIEVED in their designs, and did.

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[SwinnyGG]

1. Acrylic view port blew inward. Could have happened. But in that case the water jet may have blown off the aft end bell, but would have expected most of the CF hull to remain at least partially intact, especially fwd.

2. Glue joint between TI end cap and CF hull. Could see that failing or extruding allowing a leak of high pressure SW into the hull. Again, in this instance would expect the hull to at least remain mostly intact.

Either of these types of failures still result in catastrophic destruction of the hull. At pressures this great, with an effectively infinite supply of energy feeding whatever happens, typical intuitive thinking does not apply.

There is no scenario where a leak starts trickling water in somewhere and the sub takes 30 seconds to fill with water.

High pressure entry of water through the pressure vessel wall, at any point, results in a relatively large mass of water impacting something at very high speed and detaching it from everything else. What follows that is a series of extremely fast oscillations in pressure with huge spikes, as the air 'bubble' which was previously inside the hull collapses, in turn creating a series of very, very powerful shock waves in the water. It is literally exactly the same as what happens when a depth charge detonates underwater- the explosive generates a gas bubble, which then collapses at extremely high speeds. The shock waves from that collapse cause as much damage, and potentially more, than the original detonation did.

This is why the SOSUS system was able to record this event - it would've been nearly as loud as the detonation of an actual depth charge.

 

[dik]

Reminds me of the last thing that goes through a bug's mind when it hits your windscreen...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[IRstuff]

Huh, I only just noticed that there were actually two rings recovered, so were there only two ever, or were there four as implied in the FEA video?

TTFN (ta ta for now)
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