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Hugh (and others),
Can you elaborate on the FEA starting with a definition, what
it consists of, how you located a vendor, etc. Are there levels of
FEA? That is, was the cheap one just represented to be a cursory,
low-cost report or was it incompetent? What about the good one?
Do you have any links for informational purposes or for vendors?
Thanks,
Jim
In a message dated 12/12/2010 3:10:17 A.M. Central Standard Time,
hc.fulton@gmail.com writes:
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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