ssc-367 - Ship Structure Committee

The finite element models of increasing mesh refinement are often
used to obtain accurate stress range data locally in fatigue
sensitive areas. Thus, an overall coarse mesh model of the
structure used in the first stage of analyses is modified by
increasingmesh refinement in various fatigue sensitive areas. The
finite element models are typically built from membrane plate
elements,bendingplate elements,bar elementsand beam elementsand
further discussed in Section 5.
Because the individual joints and members define the global
structure, the boundary conditions should also reflect the true
response of the structure when subjected to the excitation
loads. For a bottom-supported structure, individual piles can be
simulated by individual springs. Whatever the support
characteristics,a foundationmatrix can be developed to represent
the foundation-structureinterface at the seafloor. It should be
noted that the foundation matrix developed for an extreme
environmentwould be too flexible for a milder fatigue environment.
Thus, the foundationmatrix developed should be compatiblewith the
applicable load range.
Stress Response Amplitude Operators (RAOS)
The stress RAOS or stress transfer functions are obtained by
unitizing the stress ranges. If the wave height specified is other
than the unit wave height (doubleamplitude of 2 feet or 2 meters),
stress ranges at each frequency are divided by the wave heights
input to generate the loads. Similarly, wind loads computed based
on cyclic wind velocities at each frequency are divided by the
respective velocities to obtain the unitized stress ranges.
Stress Concentration Factors [SCF] and Hot Spot Stresses
The stresses obtained from a stiffness analysis, and the RAOS
generated,representnominalor averagestresses. However,the load
path and the detailing of orthotropically stiffened plate or an
intersection of tubular members will exhibit hot-spot or peak
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