[PSUBS-MAILIST] External Stiffeners

[Sean T. Stevenson via Personal_Submersibles]

I used to see doubler plates used all the time on mining machinery, despite the fact that these details are extremely sensitive to fatigue. Fatigue seems to be overlooked by a lot of junior designers. Failure after failure, and every time the same solution: cut it out, and replace with a casting which accomplishes the same thing but with only full penetration welding, and of course the casting allows for generous radii and mitigation of stress concentrations that you can't achieve with plate fabrication. Full penetration is required on hull welds. I would employ it everywhere I couldn't afford a failure.

Sean



On April 26, 2014 6:58:12 PM MDT, via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>Sean,
>
>
>Thanks for that. It just never occurred to me that you could actually
>get enough penetration (without excessive warpage) to qualify. And it
>seems not. My doubler plate analogy notwithstanding, of course. I was
>thinking back to the old days when Perry used doublers. ABS put the
>kibosh on that, and it was a big change to install doubled thicknesses
>(for lift pads, for instance). We ground a few old doublers down on a
>couple of bells and found (surprise, surprise) rust.
>
>
>Vance
>
>
>
>-----Original Message-----
>From: Sean T. Stevenson via Personal_Submersibles
><personal_submersibles at psubs.org>
>To: Personal Submersibles General Discussion
><personal_submersibles at psubs.org>
>Sent: Sat, Apr 26, 2014 8:42 pm
>Subject: Re: [PSUBS-MAILIST] External Stiffeners
>
>
>They are both permissible, provided that the stiffener section is
>symmetrical, and the web to hull weld is full penetration.  Vance
>refers
>to a "doubler plate" which is typically a reinforcement joined by
>perimeter welding only, which would be prohibited.  Any partial
>penetration weld joint creates an artificial crack at the interface
>which can precipitate a failure.  All of your pressure boundary welds
>must be full-penetration and 100% NDT inspected for defects.  With a
>thick section like the posted example against the hull, you can see how
>the acceptable technique would be difficult to accomplish, both to
>first
>create the fully fused full-penetration weld and then to do your
>subsequent 100% NDT inspection.  The amount of heat you would drive
>into
>the hull cylinder when welding the thick version is also apt to cause
>more warpage / shrinkage of the joint than welding the narrow version.
>Additionally, the purpose of the stiffener at all is to increase the
>area moment of inertia of the combined hull / stiffener section, which
>the version with the flange / outstand AWAY from the hull does more
>effectively (more efficiently for the same weight of material), in
>addition to being simpler to properly fabricate and inspect.
>
>Sean
>
>
>
>On 2014-04-26 15:02, Pete Niedermayr via Personal_Submersibles wrote:
>> I have been looking at external stiffeners and I see that the figures
>in the 
>ABS Hull Calculator shows them as (see Hull B pdf ) where as some subs
>(Curasub 
>and Aquarius) seem to have them as (see Hull B pdf). Are they both
>correct ?
>>
>> Thanks Pete
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