[PSUBS-MAILIST] K3000 spherical shell calculations

[Sean T. Stevenson]

Per ABS, it is the maximum allowable working pressure, which corresponds to a critical limit pressure multiplied by a prescribed usage factor. Failure is not deterministic - if you performed destructive external pressure to failure tests on 1000 identical hulls, the resulting failure pressures would follow a bell curve distribution, and the "crush depth" could be said to be either the peak, or a prescribed number of standard deviations in advance of it, such that eg. 99% of such hulls are still intact at that pressure. I don't know specifically how the ABS rules equations were derived, so I can't speak to how ABS defines the failure pressure. In any case, the usage factor puts the maximum allowable working pressure well below the region where you need to worry about where exactly the line is. This is much more apparent when you look at the stiffened cylinder calculations, as there are a half-dozen or so ways such a hull can fail, so the usage factors are similar with the exception
of a strength failure, which has a lower factor. This is because strength failures are much more predictable than buckling failures, so the line is clearer (bell curve is narrower), and the lower usage factor ensures that such a failure will occur before any of the other modes.  ABS language does not include "crush" or "failure" - only "critical" pressures, which they do not elaborate on the definition of, and "maximum allowable working pressures".  Consensus seems to be that it is imprudent to design for typical working depth at the ABS maximum allowable, but what additional safety factor should be incorporated is unclear. There is some safety inherent in the ABS usage factor, and according to their own rules, a hull designed to operate at MAWP should pass classification. That said, additional safety over and above the MAWP is probably prudent, and would be achieved simply by designing for a MAWP deeper than your operating depth, but such safety is arbitrary and at the designer's
discretion.

Sean


On April 10, 2014 5:53:10 AM MDT, swaters <swaters at waters-ks.com> wrote:
>Sean,
>When you are talking about boyancy of the sphere, is the 1152m the
>design depth or crush depth. If it is the design depth, then that
>boyancy vs shell thickness in ASME 516 gr 70 is perfect! If not then
>might have to do some more thinking
>Thanks,
>Scott Waters
>
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>Sent from my U.S. Cellular© Smartphone"Sean T. Stevenson"
><cast55 at telus.net> wrote:Further to this discussion, additional safety
>factor or design crush depth beyond the ABS maximum allowable working
>pressure may be a moot point in this example, as the payload capacity
>(net buoyancy) of the shell in question (A516 grade 70, 6' inner
>diameter) becomes negatively buoyant at only 1152 m working depth,
>meaning that if you design for deeper than that, the shell becomes
>heavy enough that you need supplemental buoyancy to stay afloat.  With
>HY-80, the situation improves significantly, extending depth to 2347 m
>before going negative.
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-- 
Sent from Kaiten Mail. Please excuse my brevity.
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