Titanic sub trips were an oil & gas industry proving ground

[Mr Gus]

Cumulatively I am curious as to how much that "disregard & flippancy" has cost (presumably) taxpayers of various nations ..needlessly.
Likely a hell of a lot more than the sub.

Anyone know what the insurance would be likely to have been?

[Paul_F]

It was certainly unique in that it used a 1400mtr rated porthole for 3800mtr WD. This is why a 3rd party independent non partisan inspection regime is critical. We all need someone to reign us in now and then and play overseer even if we ourselves are paying for that to happen. It's gutwrenchingly .

Not entirely clear if that was the same porthole design - this detail is included in the 2018 lawsuit - but in any case the key point is that the porthole was **certified** for 1400m not that this was the depth at which it will fail. So far as I can tell it was probably designed for extreme depths but nobody had yet run the certification test campaign to prove it. That's seriously expensive, and they didn't want to pay for it so seem to have gone just with analysis.

But there expert opinion seems to be focussed on the carbon fibe tube section as the most likely to failue - and the principal reason it would have failed an independent certification. Anyone else going to that depth used a titanium sphere i believe.

I'm not a materials expert but my understanding is that carbon fibre is great under tension, but under compression it's the properties of the medium in which they're set that becomes the limiting factor. Although i guess it depends on how the fibres have been laid.

So there's a lot to unpack here:

1. Carbon Fibre is not inherently a bad or even unsuitable material, and I don't see a reason it couldn't be certified. The problem is simply that certification of a carbon fibre structure is very significantly more expensive than a steel or titanium one, and the key reason they wanted to use carbon fibre seems to have been to reduce cost (the reduced weight allowed them to use little or no syntactic foam).
2. Steel or Titanium spheres are common and either would work. Note the word spheres - the loading cases for a cylinder are different and that means the experience can't readily be read across.
3. Under compression in the direction of the fibre you should at least get some of the strength of the material - it's going to fail by buckling, essentially, and the force required from the matrix to keep it straight is far less than the total load on the fibre. Pack a load of them tightly together and you can handle compression pretty well.
4. In a sphere the loading us the same in all directions: cut it in half along any axis and the two halves will be compressed together by the net pressure of the water outside. The pressure vessel they used was essentially a sphere chopped in two with a cylinder inserted into it. For the cylinder, it's resisting both the loads of the half-spheres on the end but also the hoop stresses you see if you chop the thing lengthwise.
5. To handle this, you need both longitudinal and hoop-wound fibres to handle the load. They seem to have done this with a solution that works with good safety margins in simulation, but which is essentially reliant on a perfect build and no in-service damage. This is where all the comments about non-destructive testing come in - if there are any faults in the component, the structural strength is severely compromised. Repeated dive cycles or generally being thrown around and bumped at sea could both potentially cause such damage.

There was a report on the TV news earlier saying they refused to do destructive testing on the pressure vessel and instead relied on strain gauges with the comment that as soon as the strain gauges move its too late.

May have been garbled. They previously did what turned out to be a destructive test of a 1/3 scale model of the submarine in a pressure tank at a third-party test institute. The main finding was that the pressure hull deteriorated over multiple cycles, leading to a depth restriction of 3000m being applied. This was subsequently increased to 4000m without further testing, and the Operations Director was fired when he tried to insist on repeated non-destructive testing between pressure cycles. The hull appears to have failed at 3500m.