ssc-367 - Ship Structure Committee

4. ~arpkaya
spreading.
(Reference 6.8) provides further
discussion on
A significant effect of short crested seas is that they can cause
response in a direction orthogonalto the central direction, i.e. a
ship may develop considerableroll motion even though the vessel is
headed into the waves.
In the design and analysis of typical offshore platforms (i.e.,
conventional structures in shallow or moderate waterdepths)
spreading is generally neglected. However, for both typical and
nonconventional structures such as the tripod or an extended base
platform (see Figure 6-2) spreadingmay be significant. A platform
with very differentresponsecharacteristicsin two orthogonalaxes,
such as the extended-base platform, may be susceptible to larger
dynamic response in one axis. Even a typical platform, with a
natural period coinciding with the wave force cancellation
frequency, will be subjected to higher wave loading at the
cancellation frequency and neglecting of spreading may not be
conservative.
6.2
STRESS SPECTRUM
A stress spectrum is the stress energy distribution resulting from
loading the structurewith a particular sea spectrum.
6.2.1 Stress RAOs
In order to derive the stress statisticsa stress response spectrum
is developed. The stress response spectrum is the product of the
wave spectrum ordinates times the stress response amplitude
operators squared. The stress response amplitude operators (RAOS)
are the stresses representing a “unit amplitude” regular wave,
obtained by normalizing the input wave heights.
The stress responsesto a set of regularwaves coveringthe complete
frequency (or period) range and the complete direction range are
6-10
[,>—~q