ssc-367 - Ship Structure Committee

second objective is to accurately evaluate the fatigue strength of
the structure components and to calculate the cumulative fatigue
damage basedon stress-historyand fatigue strength. While some of
the uncertaintiesoccur in nature,others are caused by shortcomings
in simulating the actual behavior.
Uncertainties in Predicting Stress Histor.v
It is necessaryto model the actual structure as closely as possible
to determine the applied loads and the response of the structure to
these applied loads. Since marine structures are typically
indeterminate structures, stresses are strongly dependent on the
structuralconfiguration,necessitatingcareful simulationof actual
member and joint behavior.
a) Hydrodynamic Loads Model
The ship structureloads model allows the use of strip methods
or 3-D flw solutions to determine the wave loads. The
accuracyof the wave loaddeterminationdepends on the ability
to accurately define the wave force coefficients, marine
growth, wave steepness, hydrostatic effects and hydrodynamic
effects.
The loads on a stationary semisubmersible or fixed platform
are typically determined from Morison’s equation. Fixed
platform loads are largely affected by the accuracy of wave
inertia and drag force coefficients, wave steepness, marine
growth and the shieldingeffectof componentmembers.’ The use
of a stick model is valid for a fixed platform, the use of a
stick model for a structure made up of large members will
result in inaccurate loads.
Becauselargememberswill disturb the flow, leading to highly
frequency dependent diffraction, a three-dimensional
diffraction theory is often used to determine the wave force
componentsto directly accountfor the effect of one member on
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