[PSUBS-MAILIST] ABS Hull calulation Spreadsheet - How to use.

Fri May 1 16:02:34 EDT 2015

Scott.  First read thesection in the ABS rules in chapter 6 on MetallicPressure Boundary Components.  If you don’t,you will be lost.  The spreadsheetis broken into four tabs (Instructions, Cylindrical shells, Spherical and Ellipticalhead and Figures.)  Decide what you wantanalyze, i.e., shell plus stiffeners or the heads.Make a firstpass estimate of the shell thickness and the stiffener design dimensions andspacing.  For an FEA fora given loading you normally look for where in the structure you reach themaximum stress levels.  You stay way awayfrom the material yield point and beef up the weak area in an iterativeprocess.  The ABS hullanalysis goes about this differently. What is does is calculate a bunch of different Pa (Maximum allowableWorking Pressure) each associated with a different fail mode.  For example if your light stiffeners are two wimpy,then the failure mode would be predicted to be overall buckling in whichmultiple light stiffeners cave in.  Inthis case, the ABS sheet would give a low number for the Pa associated withthis event.  Let say your lightstiffeners are strong enough so that you don’t have overall buckling.  Then ABS rules are going to start pointing toother failure modes as indicated by a low Pa. For example, you may reach the yield point on the material at mid baybetween the light stiffeners.   If thiswas the case, and it was the lowest PA, then you would look and bumping up theshell thickness.  There is anotherfailure mode known as localized buckling. In this case, the shell buckles in between the light stiffeners in theform of dents.  The ABS calls theselobes.  For localized failure, there canbe 1, 2, 3 , 4 …of these lobes or dents. So the ABS sheets ask you to try 1, then 2 then 3 … as the number ofdents and for each, number, it recalculates the Pa.  The calculation you are interested in is thenumber of dents or lobes that associated with the lowest Pa.  IF this failure mode occurred, this would bethe number of lobes.So after youput your input data in and sweep through different numbers of lobes or dents, youleave n set to this valve that gives the minimum Pa for localized buckling.The sheet doesone other test, in which it looks at light stiffener tripping.  It looks at the design for the light stiffenerand calculates does it trip or not.  Ifit fails this test, you would beef up the light stiffener. Now you are readyto step back and look at what overall failure mode is predicted.  You do this by looking at each of Pa.  The Pa that has the lowest value is thepredicted failure mode.  This could be 1)Overall Buckling, 2) localized buckling or 3) Reaching the yield stress mid bay4) reaching the yield near the light stiffer in the shell. or 5) stiffenertripping.As a generalrule, prediction of reaching yield is more are accurate than the bucklingcalcs.Most designerswould purposefully design their boat with most of the PA fairly close but withmid bay yielding as the predicted failure mode.To build in afactor of safety to the calculation, the ABS uses safety factors for eachcalc.  This is the Greek letter eta. Thedefault values in the sheet are given by ABS. No factor of safety would be an eta = 1.Finally at theend of the day you have a magic Pa.  The spreadsheetthen calculates the hydrostatic pressure associated with Pa to determine themaximum safe operating depth.  To seewhat a crush depth would be, you can rerun the spreadsheet and relay all thesafety factors (eta=1) to one.  Hope thishelps.Cliff
      From: via Personal_Submersibles <personal_submersibles at psubs.org>
 To: Personal Submersibles General Discussion <personal_submersibles at psubs.org> 
 Sent: Friday, May 1, 2015 2:03 PM
 Subject: Re: [PSUBS-MAILIST] exotic steel

Cool. Would someone be able to spend a few minutes explaining how to use the sheet? Also how do you use different materials? I see 516gr 70 is plugged into it.  Thanks,
Scott Waters  
-------- Original Message --------
Subject: Re: [PSUBS-MAILIST] exotic steel
From: Jon Wallace via Personal_Submersibles
<personal_submersibles at psubs.org>
Date: Fri, May 01, 2015 8:49 am
To: Personal Submersibles General Discussion
<personal_submersibles at psubs.org>

Scott,

I use the excel spreadsheet calculator that Cliff Redus created a few years ago.  You can download it from the web site.
PSUBS.ORG -> Resources & Reference -> Design Tools -> Calculators & Data Sheets -> Stiffened Cylinder Calculator

Make sure you "Enable" macros.  There are multiple tabs at the bottom of the sheet, one of them is for spheres and elliptical heads.  It takes a little time to learn how to use it, but once you've "got it" in your head the spreadsheet is really easy to use.  I've gone through it line by line and Cliff did a fantastic job converting the ABS specs into a useable tool.

Jon

On 5/1/2015 10:40 AM, via Personal_Submersibles wrote:

Jon, Where / how do you do the calcs? Is it difficult? I have never had anyone show me, so I have had to rely on others to do the clacs for me. As I get closer to solidifying my next submarine, being able to do more of the design work myself would be really helpful Thanks,Scott Waters 

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