From: Jon Wallace <jonw@psubs.org>
To: personal_submersibles@psubs.org
Sent: Mon, April 5, 2010 11:23:11 AM
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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