RE: [PSUBS-MAILIST] FEA

FEA is Finite Element Analysis. Which means that you take small element cross sections of the model and mathematically analyse them for stresses. You apply theoretical loads or pressures on the parts. This is now done by computers. FEA packages are available with limited functionality with various solid modeling packages like Solid Works, and Solid Edge. Cosmos is one product and they have different levels for FEA. The consultants tend to use NASTRAN with FEMAP and Abaqus amongst others. But I am only a bunny in that field.

Yes the cheap one was a low cost report rather than incompetent but a competent person would not use a cheap package. The meshing of the model appears to be an important issue with the number of nodes used. However you need to talk to a consultant with years of experience and preferably one with experience on subs and ABS requirements. I used Predictive Engineering http://www.predictiveengineering.com but the cost starts at about $20,000 USD . Their site is very informativeand really nice people. They were recommended by an ABS listed consultant. You could always use an engineering student who has access to University equipment and programmes. However the ABS calculation programme will give you the correct Shell, ends and stiffener requirements but will not give you rotation of the viewport seats or impact loadings at depth. There is no easy way through it. Best option is to get Busby and go through all the depths, diameters, configurations of various submarines of note and tabulate them on a spreadsheet. You will very quickly get the feel of what works. Get an experienced engineer to review your design. Same rules as an Aeroplane. If it fails it is not a good look. Parachutes have more survivors than Steinke hoods. Hugh

From: owner-personal_submersibles@psubs.org [mailto:owner-personal_submersibles@psubs.org] On Behalf Of JimToddPsub@aol.com
Sent: Monday, 13 December 2010 2:26 a.m.
To: personal_submersibles@psubs.org
Subject: [PSUBS-MAILIST] FEA

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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