Titan/Titanic tragedy

[verbotenwhisky]

The fire service has CF air cylinders for self contained breathing apparatus with operating pressure of 4500psi, and 5500psi units are starting to be offered. 5 year testing cycle, with a maximum service life of 15 years. Those are pressure cylinders (containing pressure) though not made to resist external pressures.

I've seen several people now (people into submersible design, not random internet people) who have commented on the unsuitability of CF for deep sea use. There was a CF deep sea vessel that went far deeper than Titanic but it was a more traditional sphere design and essentially disposable. It was designed as a one time use vessel from the start, as they believed the dive would result in damage rendering it unsafe for a second dive.

From these comments I get the feeling CF is like a rope, a rope is useful for pulling or lifting an item, but pretty useless for pushing. The CF strands are apparently very strong vs internal pressure (puling), but offer little strength against external pressures, the epoxy carrier providing most of the strength in that case.

On prior dives passengers reported hearing the resin matrix cracking. It seems each dive weakened the structure, and the last dive was the one dive to many in a submersible that was already made to just barely take the pressure.

Yea, as I noted the smaller diameters are far better for pressure, the 1.5 meter Spheroid didn't survive the pneumatic pressure test but that was after it was cycled once to 6750PSI. The cylinders you are talking about are likely not over 8" in diameter, the smaller diameters and around 24" long, are not as susceptible to the same stresses. The reason they are 5 year test is likely because they are DOT cylinders and that is the requirement regardless of the material.

Keep in mind shape has a great deal to do with it as well, for external pressure a sphere is much better, even presser on the outside. Cameron once said when he was diving on the Titanic that they dove with the light bars spread in case a light bulb imploded because the change in pressure at those depths on a single point along the hull could cause the sphere to lose integrity and implode. When a deep water drilling rig has its riser down to depth, 5 to 10,000 feet, they have to be careful to insure it is always full lest it collapses and it is healthy seamless alloy pipe.

 

[Inferno]

Pictures of the debris, though not great pictures, show the skin on the outside of the CF intact but no visible CF sections have been identified.
Also, the front view port is not in the nose section as expected.
This seems to indicate that the front port blew outward as the water crushed the inside with pressure. (It's just an educated guess).
The titanium reinforcement rings are coming up intact.

Near 100% conclusion the CF hull had a catastrophic failure.

 

[verbotenwhisky]

Its as simple as this: Internal pressure vessels are not the same as external pressure vessels.

One is exposed to tension forces, the other is compression.

Aerospace designs primarily deal with internal pressure vessels, not external.

As such, anyone who says they are an aerospace engineer designing external pressure vessels should be taken with much skepticism.

I can look up the specs and I can work out the formulae, but that doesn't mean I should be treading into the very specialized field of deep sea pressure vessels just because "I can do math".

You know, all of this for me is an interesting conversation and my speculations; but my speculation isn't a WAG, its a SWAG and there is a difference.

Yea, I can do the math too, I never said I was an aerospace engineer, I am a senior piping designer with 40 years of piping systems, pressure vessels and equipment under my belt. I have been involved in field and in office work with systems operating from a vacuum to in excess of 15000 PSI, from around -480 Degrees F to around 2000 degrees F. I have worked Industrial gas systems including cryogenic, Refining, Chemical and currently offshore Marine Oil and Gas systems. So, no I am not the smartest guy in the room, based upon conversation I have read in the forum I can tell you there are some extremely smart people here (that is why I am here to pay attention and learn from these guys); but, I do have a little experience in this area. Have I designed a sub? no, have I deigned and been involved with equipment used for sub-sea work? yep....

I attempt not to be insulting in the forums, I never know who I am talking to for one and I just attempt to be decent to others; have a great and safe day my friend....

 

[Aaron_W]

Just about everything you read indicates that CF is the wrong choice for a compression vessel in particular in a salt water environment, and concerns with both repeated compression cycling and salt water exposure. Add to the fact that different coefficients of compression between dissimilar materials as well as penetration attachment points to the CF, it was the wrong material in this application. It may have bought them some time if the CF cylinder was designed to the full intended 7", but then we ultimately do not know what the failure point was, as the viewing port was also not certified to 4000M, only 1300M. Compression and movement between the titanium end caps and the CF, the attachment of the titanium flanges, as well as the skimpy bolts, are all failure points. In many application parts are not reused or have a calculated life, because of corrosion or fatigue. There were so many points on the design.

‘Milliseconds from implosion’: Company behind missing Titan sub fired a director for raising safety concerns

The submersible went missing off the coast of Canada as it took passengers to see the Titanic shipwreck.

fortune.com

Carbon fiber is also EXTREMELY fragile and you should be very careful if using them in salt water. If any salt water gets into the resin matrix through a small scratch/crack it will seep into the carbon fibers themselves. Once the tank dries the water will leave but the salt crystals remain and will abrade through the fibers as the tank expands/contracts due to temperature/pressure changes and will radically reduce their life. They are not a smart choice for in-water applications, especially salt. Perfectly fine on land, but absolutely the wrong choice for water.

Despite all of the excellent properties of CFRCs, there are issues with using carbon fiber reinforced composites and metals together. The carbon fibers in CFRPs cause this material to become electrically conductive. The carbon fibers are electrically conductive and electrochemically very noble. Therefore, when a metal is electrically connected to a CFRP, it is more susceptible to galvanic corrosion.

There are well designed deep sea submersibles, the issue is it is very expensive to design and build. If it is just your life at stake, you take the risk and pay the penalty, it is another story if you expose others to your foolhardy inventions.

Same with SCBA cylinders. Composite aluminum wrapped CF cylinders now dominate, because of the weight savings. A composite 60 minute cylinder is still lighter than an aluminum or even heavier steel 30 minute cylinder. A composite 30 minute cylinder is much lighter and smaller. Aluminum and steel cylinders continue to be seen in service where their greater durability is an asset, such as powering air tools.

We had some steel cylinders that were 30+ years old and still passing their hydro-tests. We used them to run air powered rescue tools where the weight wasn't an issue, plus they were essentially free having long been replaced in use by CF for SCBA use. It didn't hurt that they were very tolerant to being knocked around on the ground as they were being used. The composite CF cylinders are far more prone to damage being dragged, dropped, or exposed to chemicals.

There are photos out there of a fire engine that had the side blown off when a composite SCBA cylinder failed hours after getting exposed to solvents. Old steel and aluminum cylinders often show signs of abuse, but it is just cosmetic, and they just keep chugging along decade after decade. I had to retire a lot of composite cylinders that looked almost new because they reached their 15th birthday. One reason a lot of small, not too busy fire departments and industrial air packs continue to order aluminum cylinders. Not a huge savings initially, but a significant cost savings over the long haul.

 

[Inferno]

After giving this a little thought, carbon fiber has a strong pull strength, and almost no push strength. The strength of push is dependent on the resin used.
One would think that the stress on the sub was in the resin and it may has well been fiberglass with the same resin.

 

[7milesup]

After giving this a little thought, carbon fiber has a strong pull strength, and almost no push strength. The strength of push is dependent on the resin used.
One would think that the stress on the sub was in the resin and it may has well been fiberglass with the same resin.

That is pretty much what @great white has been saying during this whole thread.

Compression and tension are NOT the same. An aerospace engineer does not translate into a deep-sea submersible engineer. I would believe that even having significant experience in high internal PSI pressure operating systems (as in post #133) would not necessarily transfer to high PSI external systems.
Even spacecraft operate at a (relatively) very low PSI differential. 14.7psi actually. The aircraft that is in my avatar was 8.4 psi if I remember correctly. Carbon fiber has been used extensively in the aviation and space industries for a number of decades now (Beechcraft Starship for starters), but when comparing deep sea ocean pressures to atmospheric pressures, we are now talking about apples and armadillos.

 

[great white]

You know, all of this for me is an interesting conversation and my speculations; but my speculation isn't a WAG, its a SWAG and there is a difference.

Yea, I can do the math too, I never said I was an aerospace engineer, I am a senior piping designer with 40 years of piping systems, pressure vessels and equipment under my belt. I have been involved in field and in office work with systems operating from a vacuum to in excess of 15000 PSI, from around -480 Degrees F to around 2000 degrees F. I have worked Industrial gas systems including cryogenic, Refining, Chemical and currently offshore Marine Oil and Gas systems. So, no I am not the smartest guy in the room, based upon conversation I have read in the forum I can tell you there are some extremely smart people here (that is why I am here to pay attention and learn from these guys); but, I do have a little experience in this area. Have I designed a sub? no, have I deigned and been involved with equipment used for sub-sea work? yep....

I attempt not to be insulting in the forums, I never know who I am talking to for one and I just attempt to be decent to others; have a great and safe day my friend....

I have no idea why you seemed to have isolated me out, but my comment was a general comment, not specifically directed at you…or anyone else for that matter.

Internal pressure cylinders are NOT exposed to the same forces as external pressure cylinders. just because an object or material works in tension doesn’t mean it performs the same in compression.

I intend no insult or slight. Its just simple fact. Accept it or not, it won’t change the physics of the matter.

I’ve become tired of having to say the same thing over and over. I’m done replying to this particular thread.

Have fun all, I’m out.

 

[verbotenwhisky]

One last comment. 7milesup said, "Compression and tension are NOT the same", that we agree on; but, you guys are completely missing my point.

Structures (pressure vessels, pipe, submarines, etc... ) regardless of material, regardless of compressive or tension loads all have one thing in-common, cycles. In any pressure based structure where there is a pressure delta (high one side and low on the other) the structure will always compress in or expand out toward the low pressure side and all of these structures suffer fatigue over their life span, This is the reason, they have a design life span or maximum number of cycles until they are retired. The design life span can be extended through NDT to determine the condition of the structure, but will never result in an indefinite period.

Metallic structures are relatively easy to perform NDT on, if they are suffering fatigue there will be signs such as cracks and deformation which are visible to inspectors because the materials are monolithic, Carbon Fiber filament wound structures are not a monolithic structure, they are a layered/laminated structure with resin forming the bond between each layer and these bond break down due to the pressure cycles and once you loose the bond each layer of carbon fiber filament isn't as strong on its own.

Scuba Tanks and Scott air packs. These smaller cylinders are actually more ridged due to their smaller diameters and short lengths, their shape, size and construction minimizes the cycle issue, but, they still have a maximum service life due to the cycle issue and the DOT acoustical test will determine what the actual limit is.

Great White, this comment "An aerospace engineer does not translate into a deep-sea submersible engineer." following my comment about a CF vessel failure when I was working at an aerospace (launch vehicle) company.

7milesup the de Havilland DH.106 Comet was the first commercial jet airliner and suffered a number of in flight breakups. At the time construction methods and materials, the shapes used for the external window etc... were all contributing factors which were exacerbated by the new pressurized cabin. the thing which caused the aircraft to break apart was found to be metal fatigue caused by the cycling.

 

[GT-6 Racer]

Pressure cycles, or more precisely stress cycles create material damage. The material properties determine how well it tolerates damage and how it may fail under accumulated stress damage. Carbon fiber, while strong and very stiff, has very poor damage tolerance. Watch an F1 car crash. Glass fiber composites are much less stiff, lower modulus, and not as strong for the same weight but are far more tolerant to damage. Cycles cause damage.

even Russian Alpha class nuclear subs had titanium pressure hulls and had severely limited number of deep dives due to work hardening damage to the pressure hull and the risk of catastrophic failure. Steel hull subs, don’t have such a serious problem.

this is likely failure of the engineering practice as much as anything else. However, without a detailed investigation of the design, materials, testing, etc, it’s all speculation.

As a practicing ME and ex-USN nuke sub sailor, There’s no way I would ever get in a carbon fiber submersible. no way to know what the remaining life left is…. High resolution radiography each dive ? I doubt it…

 

[great white]

FFS.

Talk about beating a dead horse.

I should have employed the ignore list after my last post. That oversight is now corrected.

I actually wish there was an “ignore thread” option. I’ll just have to make do with “unwatch”.