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Thanks Jon for all your trouble,
I realize that in this field details are
important, but sometimes the semantics appear to complicate something
rather simple and instinctively to allow for. however I will follow your
suggestions and familiarize myself with the links you have supplied.
Thank you
Cheers
Les
----- Original Message -----
Sent: Friday, December 17, 2010 1:55
PM
Subject: Re: [PSUBS-MAILIST] Design
depth
Hi Les,
I'm afraid I cannot adequately explain failure
lobes and so I won't even try. In layman terms they refer to the number
of "dents" created when a cylinder fails by general instability (I
think). However I do not understand why a particular cylinder of
specific dimensions would fail with two lobes instead of three, four, five, or
six lobes.
The following video link shows a good series of
illustrations of failure lobes. At 3:01 they show what appears to be a
one lobe failure. At 3:06 it shows what they describe as "A near
perfect 6 lobe failure". So these will at least show you what a
failure lobe looks like. http://www.dailymotion.com/video/x86dsc_collapse-of-model-submarine-pressur_tech
The
following link also does a good job of illustrating local buckling and general
instability. Scroll down about two-thirds and look for Figures 11.1,
11.2, 11.3, etc. In Figure 11.2 you can see that the cylinder appears to
have failed with two lobes. Be careful with this link...you might need
to piece it together carefully if it gets split into separate lines by the
mailing list mailer.
http://books.google.com/books?id=rv0QXKI0HvMC&pg=PA289&lpg=PA289&dq=buckling+vs+general+instability&source=bl&ots=WYPwdqGZU6&sig=mtT5QVjTCj8_sLX4o9CZakPsUL8&hl=en&ei=zc8KTc3TKYGBlAea5J2tDA&sa=X&oi=book_result&ct=result&resnum=2&ved=0CCEQ6AEwAQ#v=onepage&q&f=false
The
multiple references to "maximum allowable working pressure" are specific to
the kind of failure being calculated. Cylinder failure can be by general
instability, buckling, inter-stiffener strength, etc. From previous
discussion about this, the goal is to adjust your cylinder specifications so
that all the "maximum allowable working pressure" results are as close as
possible to each other. You won't ever get any of them exactly the same,
but extreme differences are not preferable.
The usage factor is
essentially a safety margin. Use the defaults and you will end up with
the ABS recommended "Design Depth" or maximum operating depth. Ok, Alan,
Jim, and Sean, did I get that right this time???
:)
Jon
On 12/16/2010 9:27 PM, Les & Anna wrote:
Hi Jon
You referred to the psubs spread sheet for
ribbed calculations,
I have been playing with this spread sheet
and perhaps you or someone else can, excuse my ignorance, tell me to what
the following changeable colums refer to'
1. n - "no of circumferential lobes for failure
calculation" ?
2. n - "maximum allowable working
pressure?".........(several times under several headings with different
values applied.) also what units?
3. n - "usage factor?"
Also not sure if it is me or the site, but the
activation to the acrobat document site for the "mettalic pressure
boundary components" does not appear to
work.
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