Re: [PSUBS-MAILIST] Crush Depth Calcs

[Cliff Redus <cliffordredus@sbcglobal.net>]

Frank,

 

There are a lot of variables as you rightly mention that make it difficult to accurately predict the crush depth.  As such most designers opt not for an accurate crush depth but a ?conservative? crush depth. This leaves room for many unknowns.  If you follow the ABS guidelines both for calculating the maximum allowable working depth and their fabrication process, you will have a conservative number with a big honker factor of safety build into the design.  The ABS calculation will also identify the weakest part of the hull design, which will set the maximum acceptable operating depth.  Finally, ABS guidelines call for an unmanned pressure test at 1.25* the maximum allowable operating depth.  Does this process guarantee success, no!  But there has never been an ABS A1 classed sub fail structurally while operating within it maximum operating depth to my knowledge.

 

ABS guidelines require any design feature not specifically called out in the rules to undergo extensive testing and be supported with an FE analysis. 

 

As to a simple way to calculate a reinforced cylindrical shell with elliptical or spherical heads, I would recommend you download the ABS pressure hull stress analysis spreadsheet at the Psub.org site.  You can then use the sheet to analyze each of your stiffened cylindrical sections as well as each head (if they are different).  You would then try different shell thickness, stiffener designs, stiffener spacing with the goal of coming up with a design that meets the max depth you want to operate and exhibits the most accurately predicted failure mode (failure by yielding of the shell between the stiffeners in an accordion fashion).  There are several good reference books that cover hull design at the PSUB.org site. 

 

Typically, designers do not recalculate the stress field after local yielding.  They rely on staying well below the elastic limit of the steel by relying on these large factors of safety and rigidly adhering to operating procedures that do not allow operation below the maximum design depth.

 

I hope this helps.

 

Cliff

ShellyDalg@aol.com wrote:

Hello psubbers.

I have a question on the whole "crush depth" thing.( actually, a couple of questions.)

First, how can you factor in all the variables  of hatch design, window design, thru-hull pens, weld attachments, stresses built in from weldments, additional reinforcing around penetrations, and god knows what else I might put on this thing, and still come up with an accurate guess ( and guess it must surely be ) of when my negative pressure envelope is gonna come crashing in on me?

Doesn't any one of these ( or other ) changes to a tank type shape create a need for a separate calculation on each area's configuration and ability to withstand pressure until the time of failure?

And doesn't the plastic deformation of an area of the hull, due to reaching that moment of failure, change the whole calculation for the adjacent areas, and this must be factored into  the failure threshold of each area affected by the point of failure.

I'm a welder, not a mathematician. Does anybody have a simple calculation of the " plain" tank for the Kittredge  design?

If a guy has a tank 48" round,  quarter inch thick, hemisphere ends, no stiffeners, basically your average propane tank, how deep can it go before it crushes.

My thoughts on this are, Find the weakest point in the envelope, and that's your crush depth.

Pumping a pressure tank down with a vacuum pump won't work. The dynamics of the steel, method of fabrication, weld quality, inherent stresses, and flaws in the material or welds, all change the way steel reacts to external or internal pressure variations.

Destructive testing is the best way to determine "crush depth" but it's just too expensive to do.

And even if you do it, and get the golden guarantee, knowing just where that hull is gonna implode, now we have to calculate what the new stress of the dive cycle has had on the individual hull components, to find the new crush depth.

What percentage of safety factor is acceptable, ( how much risk are you willing to accept )

If you're cruising along, and crash into a rock, cause the surge/current pushed you, and get a dent in your hull, do we have to recalculate everything?

Too much math.

I'm looking for a depth, that a simple tank will go to, without getting really flat.

Anybody got any ideas?

Frank D.

Great works are performed not by strength but by perseverance. 
Samuel Johnson