ssc-367 - Ship Structure Committee

approach, also called “consistent method” differs from the
conventionalanalysesmethod only in the generation of hydrodynamic
loads. The hydrodynamic loads for a conventional analysis are
typicallygenerated based on a method by Hooft (Reference5.5) with
a modified form of Morison’s equation. Although the conventional
method also yields reliable results in most cases, it should be
noted that the hydrodynamic interactionamong component members of
the structure is neglected. Figure5-2 shows that the appliedheave
and pitch loadingsbased on both consistentand conventionalmethods
are very similar for wave periods (4 to 8 sec.) that contribute
largely to fatigue damage. For larger wave periods (9 to 15 sec.)
representing less frequent larger waves, the consistent method
provides more reliable results.
Stiffness Model
Typicallythe hydrodynamicmodel,mass model and stiffnessmodel are
all developed from the same structuralmodel. The stiffness model
incorporates correct member cross-sectional areas and stiffness
properties,joint releases and boundary conditionsto allow correct
distribution of structuralmember loadings and stresses.
The stiffness analysis is performed for each wave period and
direction to obtain in-phase and out-of-phasemember stresses. It
is necessarythat nominal stressescomputed are realistic. Thus, if
stick members are used to represent large members with internal
chords and bulkheads,additionalfinite element study of such areas
may be necessary. By using the loads from stick model analyses as
the applied loads on a detailed finite element model of a joint,
accurate stress distribution can be obtained to define the nominal
stresses in each sub-componentof such complex joints.
5.2.3
Overview and Recommendations
Although allowable stressmethods may be used to size the component
members of marine structures and to develop better details, a
detailed fatigue analysis is recommended for each structure. Each
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