Minimum usage factor specified by ABS is 1.25, specific to the inter-stiffener strength failure mode, which is far more predictable than the instability modes. Consequently, the usage factors on those modes are greater so that the design failure pressure is always deterministic. When you build from tough steel though, as is the case with ASTM A516 grade 70 N which many of us are using, the steel can actually yield and undergo plastic deformation to a significant extent (slow crush) and may not even reach its ultimate strength limit before hitting the safety factor on a buckling mode (implosion).<br><br>Thus, the maximum allowable working pressure specified by ABS is the pressure at the yield point at an interstiffener strength failure divided by 1.25, but this is still somewhat removed from the actual survivability limit, which has a probability distribution that effectively begins at the inter-stiffener strength failure with usage factor 1.0, and extends past the buckling modes (also at 1.0).<br><br>Sean<br><br><br>Sent from ProtonMail mobile<br><br><br><br>-------- Original Message --------<br>On Jun. 18, 2020, 06:49, Jon Wallace via Personal_Submersibles < personal_submersibles@psubs.org> wrote:<blockquote class="protonmail_quote"><br><html>
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<div class="moz-cite-prefix">I ran the numbers again in the
calculator and got the same numbers. If I change the usage factor
to 1.0 then I get 884 feet. I suppose when trying to ascertain a
theoretical crush depth a usage factor of 1.0 would be acceptable
in the calculator. It's been my understanding that ABS, Lloyds,
etc, look for a safety factor of about 1.5 which would put the 350
at 525 feet. That may explain the 600 foot test depth you
mentioned, but even so, my opinion is that's overkill.</div>
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<div class="moz-cite-prefix">Jon<br>
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<div class="moz-cite-prefix">On 6/17/2020 1:25 PM, Rick Patton via
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<div dir="ltr">Jon
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<div>OK sounds good. I was asking for the crush depth of a K-350
and the unmanned test depth for one hour is 600' so that
doesn't sound correct. Someone told me a while ago that
Ketteredge had put a 350 when first developed in a hyperbaric
chamber that was only rated for 1,200 and pushed it down to
that depth to see if it could take that pressure without
imploding and nothing happened so he knew that that design
would survive at least to that depth without failure. Can't
remember who told me that but does anyone know if that story
is correct?</div>
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<div dir="ltr" class="gmail_attr">On Wed, Jun 17, 2020 at 4:13
AM Jon Wallace via Personal_Submersibles <<a
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moz-do-not-send="true">personal_submersibles@psubs.org</a>>
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<div dir="ltr">Just point them to the website Rick, if
they ask. There's a link to facebook from there.</div>
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<div dir="ltr">I get 665 feet for the pressure cylinder
and 576 feet for the hull caps, but those are
theoretical best case limits. So 500-600 feet would be
a fair statement. Given all the fabrication variables
there is no way to predict a specific depth which is why
we use safety margins.</div>
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