ssc-367 - Ship Structure Committee

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Reqular Waves in FrecjuencvDomain - Spectral
This method, based on the use of regular waves in the
frequency domain, requires linearization of wave loading.
Approximatingthe wave loadingby sinusoidallyvaryingforces,
and assuming a constant sea surface elevationdoes contribute
to some inaccuracies. However, these approximations also
allow equationsof motion to be solvedwithout having to carry
out direct time integration, thereby greatly facilitating
fatigue analysis work.
The approach chosen should depend on the structure type and
the environment. For most “rigid body” inertially driven
floating structures, frequency-domain spectral fatigue
analysis is recommended. However, for tethered structures
such as a TLP, and for structures in areas where large waves
contribute substantially to cumulative fatigue damage, the
effects of linearizationand inundation are substantial. In
these cases the preferredapproachmaybe time-domainspectral
fatigue analysis. Even time-domain solutions at several
frequencies may be sufficient to compare the RAOS obtained
from a frequency-domain solution and to calibrate them as
necessary.
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Wind Gust - Spectral
Most marine structures are designed to resist extreme wind
loadings,but they are rarely susceptibletocyclic wind gusts
that cause fatigue damage. Some structures, such as flare
towers or radio towers, support negligible equipment and
weights; as a result, they are often made up of light and
slender members, making them susceptible to wind-caused
fatigue damage.
As with analysisof the wave environment,structuressubjected
to wind turbulencecan be analyzed by quantifyingcyclic wind
forces and their associated frequencies. The total applied
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