ssc-367 - Ship Structure Committee

Seasonal Variation
The annual wave scatter diagram is often separated out into monthly
or seasonal (spring, summer, fall and winter) scatter diagrams.
Because a fatigue environment covers many years, the seasonal or
monthly scatter diagram cell values may be added to produce the
annual diagram.
Directional Variation
Sometimes the wave scatter diagram is separated out by direction.
This may be important for fixed structures, because waves from one
directionmay cause a different stress distributionthan waves from
another direction.
Sea and Swell
Sometimes the wave scatter diagrams are separated into “sea” and
“swell”. The sea scatterdiagram shows the significantwave heights
and zero-uncrossingperiodsdefiningsea spectra. The swell scatter
diagram usually shows the heightsand periodsof long period regular
waves. This separated informationcan be helpful in analyzing the
structure, because the swell may be present a large percentage of
the time, and the swell is likely to be from a different direction
than the higher frequency waves producing unique stress
distributions.
6.1.4 Directionality and Spreadinq
The directions that have been referred to up to now have been the
“central”directionof the sea. Irregularwaves are often idealized
as two-dimensionalwith wave crests parallel in the third dimension
and all waves moving forward. Such an irregularsea is called long
crested. In reality, storms occur over a finite area and the wave
heightsdiminishdue to lateral spreading. If such waves meet other
waves from different directions, a more typical “confused” sea is
observed. A confused sea is referredto as a short-crestedsea. The
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