ssc-367 - Ship Structure Committee

load analysisof adetailed three-dimensionalmodel of the platform.
An eigenvalue (modal) analysis is also performed on the same model
to determine platformnaturalperiods and mode shapes. The platform
global dynamic responses are determined by separating each applied
static wave loading into its Fourier series components and solving
directly for the dynamic response (This method of solution is
detailed in Appendix E of Ref. 5.15). Spectral analyses for both
the static and dynamic responsesare then performedand the spectral
inertial load calculated. Inertial load sets are then developed
from the modal results of the previous eigenvalue analysis which
produce the calculated global spectral inertial response (This
method of inertial load development is detailed in Appendix F of
Ref. 5.15).
Such analyses can be repeated for various wave spectra, structural
damping, platform peri?d, etc. at a relatively nominal increase in
analysis time and computer cost. Therefore,this method facilitate
parametric studies to assess fatigue sensitivity of the platform.
Of the three spectral analysisoptions availableto define the wave
loading, the frequency-domainsolution, providing member and joint
in-and out-of-phase wave loads is most frequently used due to its
simplicity. For an iterativedesign process, an analysis approach
utilizing random waves or regular waves in time domain is
appropriate but not frequently used due to both time and cost
constraints. Thus, a hybrid time and frequency domain method is
well suited for spectral fatigue analysis of a bottom-supported
structure. Figure 5-5 illustratesthe scatter of fatigue lives as
a function of the analysis method chosen.
Another appropriate procedureto define hydrodynamicsandwave-force
model, proposed by Kint and Morrison (Reference5.16), is based on
a short extract from a random simulation substituted for a design
wave. The proposedprocedureoffers a valid and a relativelysimple
alternative to the conventional regular wave analysis. Inertial
loads due to structure response can be obtained and dynamic
amplification factors (DAFs) determined by performing a number of
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