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RE: [PSUBS-MAILIST] Hull Penetration and Reinforcement
Jon, Extract from ASME 8 div1 UG37.
(d) Design for External Pressure
(1) The reinforcement required for openings in
single-walled vessels subject to external pressure
need be only 50% of that required in (c) above,
where tr is the wall thickness required by the rules
for vessels under external pressure and the value of
F shall be 1.0 in all external pressure reinforcement
calculations.
Regards, Hugh.
-----Original Message-----
From: owner-personal_submersibles@psubs.org
[mailto:owner-personal_submersibles@psubs.org] On Behalf Of Jon Wallace
Sent: Tuesday, 6 April 2010 4:23 a.m.
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] Hull Penetration and Reinforcement
Hi Hugh,
Yes, I definitely agree we need to develop a guide for hull penetrations
including large viewports and conning tower. That's why I started this
discussion thread. Like you, I see a "hole" in the availability of this
design information and ABS rules are not much help in terms of guidance
for large hull penetrations because they require submittal of the design
for specific approval. I will contact Cliff off-list, however he is on
the mailing list and should be receiving this discussion thread. There
are a number of other people monitoring our mailing list who no doubt
are qualified to provide guidance here as well. In the absence of mail
delivery errors I generally assume that non-responders are acting
intentionally because of one of: a) too busy to respond; b) not
interested or have nothing to add to the discussion; c) do not want to
become involved for reasons of liability.
Until someone with an engineering background joins the discussion, we're
going to have to do the best we can. I did run numbers through your
spreadsheet and they look very similar to results I got with a program
(perl) I wrote using the A1, A2, A3 and A4 formulas from ABS 4-4-1A1 on
page 311, but I have some questions for you.
1) Am I correct that F=.5 for a penetration with an axis 90 degrees to
the longitudinal axis of the hull (ie conning tower)? I notice you are
using F=1 by default, which is worst-case (Line 27).
2) I don't see any reference in ABS that the calculated area for
external pressure is half of internal pressure (spreadsheet line 31).
Did you get that from ASME?
3) Lines 25 and 26, and the formula in line 30 appear to be based on
physical size of the thru-hull (or nozzle as ABS calls it), however the
"Reinforcement Limits" section 7.5.2 (4-4-1A1/7.5.2) appears to limit
the height that can be used to calculate reinforcement, to the smaller
of 2.5T or 2.5Tn. At least that's the way I interpreted it. Can you
comment on this?
4) I notice you are using a corrosion allowance, but all the ABS
definitions for T values (Tr, Tn, etc) state that the thickness should
be exclusive of the corrosion allowance.
I took a somewhat different approach to the problem and wrote a small
program (perl) using the A1, A2, A3, and A4 formulas from ABS and came
up with similar results to what you are showing on the spreadsheet.
Because of the way you organized the spreadsheet, it is easier to
visualize the problem and result that way, than with my program. I also
tested both programs using the dimensions of the large viewport
thru-hull from the K-350 and both programs confirm that those dimensions
are (minimally) properly designed according to 2010 ABS rules, which I
think resolves the issue of whether the K350 basic pressure hull design
of the 80s (not including issues that Alec raised) would pass current
ABS rules. If variable F in the ABS penetration reinforcement
calculations should be .5, then the viewport thru-hulls for the K350 are
overbuilt. If F is 1, then the K350 viewport thru-hulls meet current
2010 ABS requirements.
Of course, none of this directly applies to the conning tower
penetration since the ABS calculations are only applicable to holes 18
inches or less, on a 36 inch diameter hull. This is where the value of
F and "ring height" (as you call it in your spreadsheet) values become
important because I think we can make some reasonable assumptions based
upon the K350 design knowing that the viewports less than 18 inches in
diameter meet current ABS requirements. In my program, I can vary the
value of F, but I used a maximum "ring height" of 2.5T as I believe the
Figure 8 illustration and ABS text dictates. Using F=.5 and 2.5T, both
your program and my program show the K350 conning tower thru-hull (3/4
inch thick hunk of metal) being overbuilt, however we must remember it
appears this way because the calculations are based upon an 18 inch max
diameter thru-hull for a 36 inch diameter hull. If I use F=1, then both
programs show the CT thru-hull not meeting minimum requirements.
Knowing George wouldn't build something that way, I am left to conclude
F=.5. (I hope I'm right about this!)
Jon
Hugh Fulton wrote:
> Jon, the hull thickness has an effect on the height of the reinforcing
> allowed in the calc which is 6 times the hull thickness( i.e. 5 times plus
> the hull thickness) But according to ASME/ABS you are replacing the area
> cut out. I.e. thicker hull equals higher reinforcing allowed. Because it
is
> external pressure it is 1/2 what is required for internal pressure. How
> about running it past Cliff et co.for comment. I could be digging myself a
> hole here. I just think that it needs some simple guidance as it is a trap
> that a few have fallen into. The ASME code is a bit scary for some to
look
> at and I am offering a simplified approach for checking. It should always
> have a disclaimer and it should be checked out by a professional engineer
> but it would save people getting into a situation where they have assumed
> the wrong approach. I know that I almost fell into the trap myself before
I
> went through chapter and verse of the code. We have the hull thickness
and
> stiffener calc and we have the calc for the flat windows but we have no
> guide for the window rings or hatch rings. Don't you think we should have?
> Regards, Hugh.
>
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