Got it. Light bulb just went on. Thanks. On 12/11/2010 2:46 PM, Sean T. Stevenson wrote:
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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