ssc-367 - Ship Structure Committee

For each load component (in-phase and out-of-phase) at each
frequencyof agiven wave directionthe finite elementmodel is used
to generate local stress distributions. The stress range transfer
functions are then generated for each wave direction. Although
current computers are well suited to compute large problems, the
number of frequenciesnecessary to define the transfer function may
be small. Using the predominantload transfer function as guide a
limited number of frequencies (say4 to 6) maybe adequateto define
the other transfer functions. The use of a stress range
distribution parameter allows carrying out of a fatigue analysis
with relatively few structural analyses cases. The accuracy of
fatigue lives obtained largely depends on the validity of the
Weibull shape factor used.
The shape factors obtained by calibratingthe characteristicstress
range against spectral fatigue approach indicate that the single
most important variable affecting the shape factor is the
environment. While the shape factor may vary from 0.8 to 1.2,
depending on the route characteristicsand on structuregeometry, a
factor of 1.0 may be used when such information is not available.
Stationary Marine Structures
The accuracy of stress transfer function for a joint/detail of a
stationarymarine structuredependson many variables, includingthe
accuracy of applied loads, motion response characteristics and the
stress distribution. Hydrodynamic forces may be determined by
either Morison’s equation or by diffractiontheory. Since the wave
length-to-member sizes are small for most floating (i.e.
semisubmersibles,TLPs) structures, diffraction effects should be
accounted for.
A 2D or 3D diffraction analysis can be used to generate the
hydrodynamic coefficients. Then, utilizing these coefficients,
Morison’s equation can be used to generate theappliedl oads. As an
alternative,diffraction analysis can be used to generate the wave
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