ssc-367 - Ship Structure Committee

evaluated as explained in Section 5. For a vessel global effects
of port and starboard quartering seas are identical, allowing
reduction of applied loading cases. Similarly, for a platform with
two planes of symmetry several of the eight loading cases (45
deg. intervals)may be combined.
6.2.2
Res~onse Analysis
The response analysis squares the stress RAOS; multiplies them by
the spectrum ordinate; multiplies that product by the spreading
function;andsums/integratesoverdirections the resultsto produce
the stress spectrum.
The stress range spectra is integrated to allow determination of
various statistical parameters, including the zero-uncrossing
frequency, the mean squared value, etc., from which the short-term
probabilitystatistics preconstructed. The ’’Rayleigh’’ distribution
can be used to idealize the stress range associated with a
particular cell (Hs and T) in the scatter diagram. Then, the
fatigue damage associated with each block can be computed, the
cumulative damage thus incorporating the weighting effect of the
joint probability of wave scatter diagram. Since the damage for
each cell is computed numerically, this approach is generally
defined as the “short-termnumerical method.”
The typical loading response exhibits smaller stress cycles
interspersed among larger stress cycles, making it difficult to
identify the number of cycles contributing to fatigue damage.
Rainflow counting is the name of a large class of stress cycle
counting methods often applied to upgrade the short-term
statistics. The rainflow parameter, introduced by Wirsching
(Reference 6.9) is frequently used in upgrading stress spectra
statistics.
The stress range associated with a particular block of the wave
scatter diagram is random in nature and governed by a probability
density function. Such a density function, covering the fatigue
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