ssc-367 - Ship Structure Committee

the sea. By assuming thatthe response is linear, the response of
the offshore structure to a regular wave is equal to the RAO times
the regular wave amplitude. By assuming that the response to one
wave does not affect the response to another wave, the response of
the offshore structure to a random sea is the sum of its responses to
each of the constituent regular waves in the random sea. The
response is therefore a collection of responses each with a different
amplitude, frequency, and phase.
The energy of each constituent wave is proportional to the wave
amplitude squared. The energy of the response to a constituentwave
of the random sea is proportional to the response squared, or is
proportional to the RAO squared times the wave amplitude squared.
The response energy may also be represented by a spectrum from which
characteristics of the response may be derived. From the response
spectrum characteristics and the wave spectrum characteristics, a
“transfer function” can be obtained which relates the response and
wave characteristics.
B.2.1
Spectrum Analysis Procedure
The spectral analysis procedure involves four steps: 1) obtaining
the response amplitude operators, 2) multiplying the wave spectrum
ordinates by the RAOS squared to get the response spectrum, 3)
calculating the response spectrum characteristics, and 4) using the
response spectrum characteristics to compute the random sea response
transfer function.
The RAOS are usually calculated for a discrete set of wave
frequencies, and the discrete RAOS are then fit with a curve to
produce a continuous function. The singular term “RAt)” is used both
to signify a single response amplitude to wave amplitude ratio and to
signify the continuous function through all of the RAOS. Any
response that is linearly related (proportional) to wave amplitude
may be reduced to an RAO function. Typical responses are motions,
accelerations, bending moments, shears, stresses, etc.
B-3