[PSUBS-MAILIST] G.L. summary Design Loads

[vbra676539 at aol.com]

At the risk of soap boxing:


Philosophically yes, but in real time, you would be building to your minimum, not your calculated thickness. If you specified 3 mm material and got something with a 2 mm spot from your supplier, I think you'd be shipping trash back to that supplier along with some strenuous WTF? commentary on the phone. You might be able to deal with a 4 mm plate, but never if it's the other way around. Same if it was 10mm. If I ordered 10 and got 9, the resultant discussions would border on downright impolite.


As we were shown in Vancouver, the DW2000s are built from pentagonal segments of plate pressed to shape primarily to maximize control over thickness and form, an expensive and time consuming process which is absolutely necessary for integrity to the design. I think they are half an inch thick, which means half inch NOMINAL, as in half inch at the very least.


Those are tough little hulls, capable of holding their shapes to probably twice (or more) of their operating and maybe even test depths. And I'll bet there was a lot of time spent verifying thickness with the new Aquarius type hull (Curasub) prior to rolling the first piece of steel to shape. And a lot of double checking later. In fact, I know there was, even though I wasn't there. I know because I've had a hand in building some of these things, and nobody cuts corners. Ever. It's just too critical.


When you go for certification, there are no 10% errors allowed in the basic transition from drawing to fabrication. And certainly not 33%. To maintain your depth parameters, you do it right the first time, or you do it again. Period. Not a bad philosophy with momma's favorite son's backside on the line every time he goes in the water. As Alan clearly illustrated, you don't design for 125% of operating depth. You design for 200%, or maybe 250%. The 125% thing is about verifying your safety margin.


George Kittredge told me once to just ignore some corrosion pits in my K-350, even though sonic mapping had shown nearly a hundred thousandths loss in three smallish areas (out of a 0.250 material thickness???). Maybe he was right for a 160 psi operating environment. That said, well...me?...I weld over-layed the hell out of them, then knocked the tops off the welds looking for porosity and to keep the bilges from looking like something Dr. Frankenstein stitched together. Wasn't happy until it was done and I had all my metal back.


In my view, less is never more down where the sun don't shine. The regs keep us honest, but the steel keeps us dry. As we say in the nuclear business, "We aren't here to avoid risk, we're here to manage it." Second guessing just doesn't come into it.


Vance



-----Original Message-----
From: Alan <alanlindsayjames at yahoo.com>
To: Personal Submersibles General Discussion <personal_submersibles at psubs.org>
Sent: Fri, Dec 6, 2013 11:38 am
Subject: Re: [PSUBS-MAILIST] G.L. summary Design Loads



Hi Jim,
Yes you are right.
I used feet as an illustration, where they use metric pressure
in their table. 
I like their scale system, because if you are building for a
shallow depth, with a thin sectioned hull, then any small margin of
error will have a bigger impact than on a thicker hull.
As a simple illustration, if you were building 3mm thick & ended up
2 mm that's a 33% error, whereas if you built 10mm thick & were out
by the same 1mm, that's only a 10% error.
Alan
Sent from my iPad

On 7/12/2013, at 5:11 AM, Jon Wallace <jonw at psubs.org> wrote:



              

      Jim,
      
      Yes, there's certainly a point in all brackets where it would      behoove the owner to bump up the nominal dive depth (operating      depth) to take advantage of lower ratios.  I believe ABS uses a      static 1.25 ratio for all vessels regardless of operating depth.       GL would appear to be more conservative than ABS in this category.
      
      Jon
      
      
      On 12/6/2013 6:03 AM, jimtoddpsub at aol.com wrote:
    
    
        
Hi Alan,
        
 
        
If I understand the requirements correctly,            then for designed maximum operating depths of 320 feet and            the 340 feet respectively the test depths would be as            follows:
        
320' x 1.4 = 448' test depth
        
340' x 1.25 = 425' test depth
        
 
        
And the            designed collapse depths would be:
        
320' x 2.4 = 768' collapse
        
340' x 2.0 = 680' collapse
        
 
        
Therefore if my designed operating depth is            near the lower limit of a bracket, increasing the stated operating              depth enough to push it into a deeper bracket would              actually lessen the requirements for            test depth and collapse.  Do I understand correctly?
        
 
        
Thanks,
        
Jim
      
    
    
  

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