ssc-367 - Ship Structure Committee

Jr 2*$ *dti= Jr 2*S*du
eeee.
Therefore, the encounter frequency need only be used to select the
response amplitude operator and the integration is still over the
stationary frequency, u.
i.e.,
re = r(we) = r(w +
2
VU/g)
B.2.3.3 Short-Crested Seas
The usual mathematical representation of a sea spectrum is onedimensional
with the random waves traveling in a single direction
with the crests and troughs of the waves extending to infinity on
either side of the direction of wave travel. A one-dimensional
irregular sea is also referred to as a long-crested irregular sea.
In the real ocean the waves tend to be short-crested due to the
interactionof waves from different directions.
A two-dimensional spectrum (short-crested sea) is created from a
standard one-dimensional mathematical spectrum by multiplying the
spectrum by a “spreading function.” The most commonly used spreading
function is the “cosine-squared”function.
f(lp)= (2/Tr)*cos2$
where * is the angle away from the general wave heading,
(-lT/2q%/2)
The cosine-squared spreading function spreads the sea spectrum over
an angle +/- 90 degrees from the general wave heading.
To incorporatemulti-directional or short-crested irregular seas into
a spectral analysis, the RAOS for a range of wave headings must be
obtained. A spectral analysis is performed for each heading using
the one-dimensional sea spectrum. The results of the one-dimensional
analyses are then multiplied by integration factors and summed.
B-n