ssc-367 - Ship Structure Committee

6.1.3 Scatter Diaqram
Wave scatter diagrams show the occurrences of combinations of
significantwave heightand averagezero-uncrossingperiod overmany
years.
$iqnificant Heiqht vs Zero-crossinq Period
Irregular waves do not have any consistent pattern of height or
period, but exhibitcompleterandomness. Irregularwave heightsand
periods are usually defined by the statistical properties of the
wave record or by the properties of the energy spectrum which
represents the random sea. The significantwave height is taken to
be four times the standard deviation of the recorded water surface
elevations,or if the sea is representedby a half-amplitudeenergy
spectrum, the significantwave height is four times the square-root
of the area under the spectrum. The average zero-uncrossingperiod
is the average of the time intervals between negative to positive
sign changes in the recorded water surface elevations, or is the
square-root of the area under the spectrum divided by the squareroot
of second moment of the spectrum (frequency in Hz).
The wave height and period distributionovertime can be obtained by
actual wave measurements. The heights and periods of all waves in
a given direction are observed for short periods of time at regular
intervals. A short time intervalof severalhours maybe considered
constant. For this sea state, defined as “stationary”, the mean
zero- uncrossing period, Tz, and the significant wave height, Hs,
are calculated. The Hs and Tz pairs are ordered, and their
probabilitiesof occurrencewritten in a matrix form, called a wave
scatter diagram. A typical wave scatter diagram, presenting
statistical data on the occurrence of significant wave height and
zero-uncrossingperiod for one direction is shown on Figure 6-1 and
further discussed in Appendix B.
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