ssc-367 - Ship Structure Committee

waves in a short crested sea approach from a range of directions
centered about the central direction.
Directionality
For a fixed structure the direction of the sea will affect the
stressdistributionwithin the structure. Most fatigueanalysesare
performed for four or eight wave directions. When directional wave
scatter diagrams are available the sea direction can be matched to
the analysisdirection, and the fatiguedamage accumulated. If the
data availabledo not includewave directionality,directionscan be
estimated on the basis of wind roses or hindcasting.
Spreading
In order to model a short crested sea a “spreadingfunction” is used
to distribute the wave energy about the central direction. In
typical analyses the short crested sea is represented by a set of
long crested spectra coming from directions spread over -90 deg to
+90 deg from the central direction
to the specified short-crestedsea
and having a total energy equal
spectrum.
The directional spreadingfunction as defined by Kinra and Marshall
(Reference6.7) is often used in the following form.
D (8) = Cn COSn (1?)
where n is a positive integer and is measured from the central
direction. The coefficient C. should satisfy the following:
A typical n value for wind-driven seas would be 2, while an
appropriatevalue for a limited fetch (restrictedspreading)may be
6-9