ssc-367 - Ship Structure Committee

A response scatter diagram could be generated by taking the wave
spectrum.for each sample used to.create the.wave scatter diagram and
performing a spectral analysis for the response. The computed
characteristics are then used to assign the percentage of occurrence
to the appropriate box in the response scatter diagram. This entails
considerablework and can be simplified by reducing the number of sea
spectra considered, as described below.
B.4.1
Special Family Method
All of the original sea spectra used to define the wave scatter
diagram must be available, in order to select a special family of sea
spectra to represent the whole population.
The sea spectra are first grouped by wave height bands, such as O to
2 ft significant wave height, 2 ft to 4 ft H~, etc. The average
properties of the spectra within a group are computed. Within each
group, which may contain thousands of sample sea spectra, a small set
of sea spectra are selected to represent all of the spectra in the
group. The small set will typically contain 4 to 10 spectra.
The spectra of a representative set are selected by a Monte Carlo
(Shotgun) process which randomly picks, say 8, spectra from the
group. The mean spectrum and the standard deviation of the spectral
ordinates about the mean spectrum are computed for the 8 spectra. A
weighted sum of differences in properties between the 8 spectra and
the total population of the group represent the “goodness of fit” of
that set of 8 spectra.
A second representative set of 8 spectra is then selected from the
group, and the “goodness of fit” of the second set is computed. The
better set (first or second) is retained and compared to a third
sample of 8, etc. The process is repeated many times, say 1000,
within each wave group.
R-18