Tourist submersible visting the Titanic is missing 101

[waross]

Viewport collapse and water hammer.
I have a hard time accepting water hammer as a destructive force;
1: The shape of the Titan is remarkably similar to a snubber used to mitigate water hammer.
2: Water hammer is a result of inertia and in-compressability.
3: Based on a viewport diameter of 21 inches and a titanium end diameter of 66 inches, the area ratio is almost 10:1..
For every pound force of the water jet hitting the inner end cap, there will be 10 lbs of force opposing.
I am not ruling out a collapse of the viewport, the overall damage may have been done by impact with the sea floor.
In extreme ram pump applications, the discharge pressure peak may be limited by expansion of the steel impulse line under pressure.
The Titan had a lot of air helping to limit the pressure peak of any water hammer effect.
I may be wrong.
We are all waiting for conformation of the failure sequence.

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

 

[3DDave]

The problem with window as initial failure is one can see through the window for any flaws that are developing. It doesn't appear to have a progressive failure mode. With carbon fiber, if one fiber begins to give the load is transferred to others. The load transfer will continue until

The window had made several dives before - I would expect visible damage if the window was near failure.

I have seen such windows fail when used in high pressure gas applications - the high pressure gas diffuses into the material and when the pressure is relieved the plastic ruptures internally making a very visible mess. In the carbon fiber, such damage might be deep enough (where diffusion is slow to release) and not visible. More dives - more damage.

For example:

https://benkrasnow.blogspot.com/2011/10/effect-of-long-term-high-pressure-co2.html

 

[dik]

I suspect strongly that we will never get that... collapse likely occurred by failure of the viewing window (overloaded by a factor of 3) or by collapse of some highly stressed and possibly damaged carbon fibre material. There would likely be a 'split' split second between the two events and it's not likely possible to determine which was the chicken and which was the egg.

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

-Dik

 

[dik]

The difference between the two may be milliseconds...

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

-Dik

 

[Tomfh]

overloaded by a factor of 3

These windows have a safety factor of ~6 don't they? So even at 3x the rated depth it still has a "safety factor" of ~2, which is similar to the vessel.

Hopefully we get an answer as to which went first.

 

[dik]

I think I have more confidence in the strength of the window than I do in the strength of the CF hull.

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

-Dik

 

[Tomfh]

Agree. My money is on the hull.

 

[Tomfh]

Another Opinion: Design News Jun 23, 2023: Carbon Fiber Is Safe for Submersibles When Properly Applied
Lets not completely discount this technology until all of the facts are in. It may be that the investigation could find fault with Oceangate's implementation, rather than the entire class of "Carbon Fiber Compost" material.

Test Sample Composite Energy Technologies

That page has an interesting design tool for Carbon fibre submersible.

It says for 4000m depth rating with a factor of safety of 2.25 you need 11 inch thick walls.
It says 5 inch thick walls correspond to safety factor of 1.3
For a depth of 3600m with a safety factor of 1, it says 3 inch walls

 

[rodface]

>Edited to embed images

Something I've been particularly interested in is the damage to the "capture rim" that is visible on the end ring being hoisted off the ship in this photo:

The outer rim was almost entirely peeled away from the end ring. Very interesting failure, what does it suggest about the forces acting at the moment of the implosion?

I drew up the profile in that area based on what is shown in various videos/photos:

There are a number of things that I have questions about:

 

[Tomfh]

Very interesting failure, what does it suggest about the forces acting at the moment of the implosion?

When the internal shock wave hit the end cap, the cap started peeling the rim off the ring before the cap/ring assembly had a chance to separate from the collapsing hull. The internal shock pressure must have been much greater than the external water pressure on the domes.

 

[GregLocock]

Tomfh, your first design has 16876 lbf buoyancy in pure water, that's not a lot of margin for all the other bits and bobs.

Cheers

Greg Locock


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[Just Some Nerd]

The channel Real Engineering put out a video on the titan sub - seems primarily concerned with the failure hull (his PHD was on composites, which introduces a bit of bias in terms of what he focuses on here). Not up yet on Youtube, but it is out on Nebula for anyone who has a subscription, though much of it has already been brought up in this thread already (particularly in terms of Rush's attitude).

Main part I want to draw attention to is this paper he brought up: "A Review on Structural Failure of Composite Pressure Hulls in Deep Sea"

https://www.mdpi.com/2077-1312/10/10/1456

Some interesting tidbits in there regarding the different failure modes of composites under deep sea pressures. To my eyes as someone without any experience in composites, some of the additional failure modes for thick-shelled composite cylinders do not seem overly well understood or predictable.

----------------------------------------------------------------------

Why yes, I do in fact have no idea what I'm talking about

 

[dik]

Why would you notch the cylinder for the end ring? Just looking for problems... area of highest stress and you are adding a stress riser... might have been a tad less costly.

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

-Dik

 

[dik]

How does the weight of sea water compare to 62.4 pcf? Does the salinity change with depth? Just curious...

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

-Dik

 

[IRstuff]

I wonder if the carbon fiber cylinder would be able to support itself without the titanium end caps?

The video I've seen of one of the winding operations showed a metal spool upon which the carbon fiber was wound.

https://www.youtube.com/watch?v=Vi4J1LDS504

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[3DDave]

If a 5 inch thick wall of material isn't enough to provide stability from buckling, the quarter inch of mandrel isn't going to do much to improve that.

I am not sure about the reason for the notch; perhaps it was an attempt at elastic matching so the titanium rim would flex to match the carbon hull.

As to salinity - I think it does increase with depth - Google likely knows.

From Wikipedia:

The density of surface seawater ranges from about 1020 to 1029 kg/m3, depending on the temperature and salinity. At a temperature of 25 °C, the salinity of 35 g/kg and 1 atm pressure, the density of seawater is 1023.6 kg/m3. Deep in the ocean, under high pressure, seawater can reach a density of 1050 kg/m3 or higher.

So that puts the density increase from salinity at 2-3%.

Note that the water is compressed at depth and the amount of compression will be less than the amount of compression of the submersible. There was at one time a goofy theory that if water was more compressible than iron that at some depth the iron would float and so sinking ships might never reach the bottom. As Titanic and Oceangate proved that is not a correct theory.

 

[dik]

Thanks 3DD...

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

-Dik

 

[TugboatEng]

How does the weight of sea water compare to 62.4 pcf? Does the salinity change with depth? Just curious...

64 lb/cf³ for sea water.

 

[1503-44]

Don't discount the effect of water hammer.
12000 ft of water head can generate a 600mph jet.
Directed upward, 5200 psi jet, 600mph would have the same 12,000 ft height.
If a 600mph jet is halted in 1ms, a theoretical pressure spike of 1M psi could be reached.
At 5200psi, 12000 ft depth, water compresses about 2%.

Volume change of materials under pressure is according to their bulk modulus,

Carbon Fiber

Bulk Modulus values of CF from 200 to 700 GPA are reported.

Titanium

It would appear that any interface between the two materials would be complicated by their differential compressive characteristics, not to mention within each material themselves as interior pressure of 30 psia and outside pressure of 5000 psi means volume contraction as one moves from in to outside the tube. It seems that would tend to agrevate any tendencies to form surface cracks at any interface.

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

 

[K3VIN]

Is there anyway for the mods to lock this thread and start a new part 2? My browser is having quite the issue now rendering the page.