ssc-367 - Ship Structure Committee

analysis of a fixed platform. As discussed in Section 3.2.4
under Stress History Parameters, wave scatter diagrams are
used to develop wave height exceedance curves in each wave
direction and used to obtain the stress exceedance curves.
Consideringboth the effort needed and the questionablelevel
of accuracy of selecting wave heights to represent a wide
range of wave heights and periods, it may be better to
initiate a spectral fatigue analysis directly.
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Reqular Waves in Time Domain - Spectral
Because a spectralfatigueanalysis is carriedout to properly
account for the actual distribution of wave energy over the
entire frequency range, a sufficient number of time domain
solutions is required to define the stress ranges for
sufficientpairs of wave heightsand frequencies. A resultof
this procedure is development of another characteristic
element of spectral fatigue analysis, namely, the stress
transfer functions, or response amplitude operators (RAOS).
For each wave period in the transfer function, a sinusoidal
wave is propagated past the structure and a wave load time
history is generated. The equations of motion (structure
response) are solved to obtain a steady state response. A
point on the transfer functionat the wave period is the ratio
of the responseamplitudeto the wave ampl” tude. A sufficient
number of frequencies is required to incorporate the
characteristicpeaks and valleys.
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Random Waves in Time Domain - Spectral
The use of randomwaves avoidsthe necessityof selectingwave
heights and frequencies associated with the regular wave
analysis.
3-1o