As Carsten says calcs like the calculator are old type calcs, good for simple vessels but once you start putting holes in funny places and multiple holes then you need FEA or a good safety margin. Preferably both.
When you see the FEA results the weak spots look obvious. Cheap FEA can make you change design unnecessarily or give you false sense of security. If you don’t have the tools and not prepared to pay for proper design stick to a proven design and proven fabrication methods and materials.
Pioneering costs money. Regards, Hugh
From: owner-personal_submersibles@psubs.org [mailto:owner-personal_submersibles@psubs.org] On Behalf Of Alan James
Sent: Sunday, 12 December 2010 7:13 p.m.
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] Design depth
Hugh,
How did the Psubs calculator compare with your latest FEA?
I know you said you did a cheaper FEA & it was off the mark by a large percentage.
& congratulations on passing the test.
Alan
From: Hugh Fulton <hc.fulton@gmail.com>
To: personal_submersibles@psubs.org
Sent: Sun, 12 December, 2010 1:42:49 PM
Subject: RE: [PSUBS-MAILIST] Design depth
Having just been through it all I did design depth for 400 ft. Tested at 515 ft (1.25 minimum) for 2 cycles. Having seen Phil's comments I wish I had done 1.33 or 1.5 times design for the
test. ABS requires minimum crush depth of 1.5 times design depth but the surveyors want to see more than that for safety. Mine on FEA was 2.25 times design. I was advised to go to at least 2 times as ABS inspectors likely to question all other aspects much harder. ABS also say not less than 0.375" thickness but the inspectors do not strictly go by that as per Captain K. However all the roundness is a mission with thin (5/16" (8mm)) shell. Much easier to make with thicker shell material due to shrinkage around viewports, hatches etc. Carsten is absolutely correct with doing FEA design as you can see where the problems are and correct them. By the way I would love to see the new designs of the 3-man and 3 + 1. Now that is going to be a fun sub. Hugh
-----Original Message-----
From: owner-personal_submersibles@psubs.org [mailto:owner-personal_submersibles@psubs.org] On Behalf Of Sean T. Stevenson
Sent: Sunday, 12 December 2010 8:47 a.m.
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] Design depth
Jon - it seems pretty clear from the ABS rules what their intent is. The
usage factor eta as implemented in the ABS calculations runs from a minimum of
0.5 (2x safety factor) for buckling modes, to a maximum of 0.8 (1.25 safety
factor) for inter-stiffener strength. This implies that ABS is ensuring that
strength failure occurs before buckling
by varying the value of eta, such that
the limit pressure for any particular failure mode is the theoretical onset of
failure (i.e. crush depth), and the limit pressure multiplied by the
corresponding eta value for that mode of failure gives the maximum allowable
working pressure. The lowest of the maximum allowable working pressures for
each mode gives the maximum allowable working pressure for the vessel, which
corresponds to the maximum allowable rated depth. Without any additional
safety factor imposed by the operator, this maximum rated depth is also the
maximum allowable operational depth, which would provide a minimum safety
factor of 1.25 from crush, which would occur by inter-stiffener strength
failure.
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