Re: [PSUBS-MAILIST] Design depth

["Sean T. Stevenson" <cast55@telus.net>]

Jon - it seems pretty clear from the ABS rules what their intent is.  The 
usage factor eta as implemented in the ABS calculations runs from a minimum of 
0.5 (2x safety factor) for buckling modes, to a maximum of 0.8 (1.25 safety 
factor) for inter-stiffener strength.  This implies that ABS is ensuring that 
strength failure occurs before buckling by varying the value of eta, such that 
the limit pressure for any particular failure mode is the theoretical onset of 
failure (i.e. crush depth), and the limit pressure multiplied by the 
corresponding eta value for that mode of failure gives the maximum allowable 
working pressure.  The lowest of the maximum allowable working pressures for 
each mode gives the maximum allowable working pressure for the vessel, which 
corresponds to the maximum allowable rated depth.  Without any additional 
safety factor imposed by the operator, this maximum rated depth is also the 
maximum allowable operational depth, which would provide a minimum safety 
factor of 1.25 from crush, which would occur by inter-stiffener strength 
failure.



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