You guys have fallen head over heels in love with a logic trap, heavily basted with semantics. Give it up. Design depth is your operating depth PLUS the percentage required for testing. It means the depth to which the pressure vessel may be taken repeatedly without incurring irrecoverable damage.
125% doesn't make some people happy. I'll just stick in my 2 cents and tell you that I have done a whole bunch of dives in vehicles built and tested to that standard. Crush depth is an arbitrary derivative of the overall calculations, suggesting that all things being perfect, if you go this deep (whatever THIS happens to be) then the hull MAY suffer irreparable damage, and if it does, then THIS is the site of most probable damage. The THIS is typically a measure of the weakest link in the design, which is specific to the design, not general discussion. It is NOT a weak link, but simply the first failure point.
A C-class Perry for instance would be rated for 1200 feet maximum operating depth. It's design depth (and unmanned test depth) would be 1500 feet and it's crush test.....hmmm, I forget. About 1800 and change, I think. Maybe 19 and change. That kind of thing is in the initial ABS calculation package, if you happen to have it. However, it is not normally in the general ABS certs. I do not have a crush depth number for Gamma, for instance, but do have her certifications.
For general interest, Nekton Alpha, Beta and Gamma are all rated at 1000 feet and were all tank tested to 1500. Crush depth was ascertained by model testing, which verified the calculations handily (just over 2000 feet). Perry was doing repetitive construction on two proven primary designs (tubular hull, internal or external ring stiffeners). Once proven, in other words, all they had to do was replicate and prove that they had done so.
As to accepting one safety factor over another, I'd say 125% should be the accepted norm, based on experience and history. Anything over that is a personal choice. I feel one way, somebody else feels another. Potato, potahto.
Shallow boats are easy to double up on. The old Submaray, for instance, was 3/8" plate and they held it to 300 feet for safety. The PC-8, on the other hand, was also 3/8" plate and I can't even tell you how many times I've been to her operating maximum of 800 feet. Different construction techniques. Different design. Different results.
Vance
-----Original Message-----
From: Jon Wallace <jonw@psubs.org>
To: personal_submersibles <personal_submersibles@psubs.org>
Sent: Fri, Dec 10, 2010 4:53 am
Subject: Re: [PSUBS-MAILIST] Design depth
Jim,
Design depth is derived from the parameters (material type, hull
thickness, number of ribs, spacing of ribs, etc) used in ABS/ASME
formulas to determine the maximum external pressure that the
submarine can operate in. I don't think ABS or anyone else can
reliably calculate "fail depth" or "crush depth" because those
depths are highly dependent upon quality of material, fabrication,
vessel use/maintenance, etc. The crush depth could be some
percentage above or below design depth, however you could only find
out definitively by testing the vessel to destruction. For that
reason, you don't want to perform a manned dive to 375 feet. Make
sure that is an unmanned test. You'll note that ABS Underwater
Vessels Section 3.5 limits the manned test dive to the design depth.
Jon
On 12/10/2010 9:04 AM,
JimToddPsub@aol.com wrote:
Jon,
I need to read and study the whole ABS
publication thoroughly, but from what I've read is seems
that the Design Depth I would need to claim would be 300
feet. During the survey I would need to dive in increments
to 375 feet (300 x 1.25). The vessel would then be rated to
300 feet. Per ABS, Design Depth is the depth to which the
vessel is "designed and approved to operate." It's going to
be approved to 300. If I want to have it approved for
anything more than that, I'll have to have it surveyed to
125% of whatever I want its operating depth to be.
What is surprising is that [so far] I see
nothing in ABS that specifies how the Design Depth must be
derived. It seems rather arbitrary rather than stating
something such as "60% of the calculated fail depth."
Jim
In a message dated 12/10/2010 1:24:29 A.M. Central
Standard Time,
jonw@psubs.org writes:
A
different approach is to say, I know I don't want to dive
deeper than 300 feet (my max operating depth) and I want a
2x safety margin. That means using material capable of
withstanding 600 feet depth. Now with this approach you
certainly may not pass a 1.25 overpressure test for a 600
foot design depth, however you've already self imposed a
2x safety margin. If you wanted ABS certification, then
you simply claim the design depth as 480 feet. Now you
meet the ABS requirement (480 x 1.25 = 600) and also are
still well within your 300 foot self-imposed depth limit.