ssc-367 - Ship Structure Committee

simulations of random waves. The basic DAF approach, allowing
combination of inertial loads compatible with static loads, is
further discussed by Digre et al (Reference 5.17). Typically,
simulation of the response is performed, the ratio of dynamic-tostatic
loads determined (i.e. DAF)
until the DAF stabilizes. Larrabee
and the process is repeated
(Reference5.18) also provides
further discussion on the logic beh” nd DAF approach.
5.3.4 Stiffness Model
The load and the stiffness models are essentially the same.
Typically, a three-dimensionalspace framemodel of the structureis
made up of individual joints and members, each defining the joint
and member incidence, coordinates, hydrodynamic coefficients,
etc. that are necessary for generationof environmentalloads. The
loads model, providedwith member cross-sectionareas and stiffness
properties,joint releases,and boundaryconditions,transformsinto
a stiffness model. The structure mass model incorporates the
correctmember sizes,joint coordinatesand boundaryconditions,and
can be considered a stiffness model. Static stiffness analysis
solution follows standard structural analysis technique. Dynamic
analysis is typically based on a modal (eigenvalue) analysis
solution; two modal analysis solution techniques may be used:
●
The subspace iteration technique is a Ritz-type iteration
model used on a lumped mass system that produces eigenvectors
and eigenvalues for a reduced set of equations. This is the
method of choice for most fixed offshorestructuressinceonly
a relativelysmall number of modes are required to adequately
model the total structure response.
● The Householder tridiagonalization technique first
tridiagonalized the dynamic matrix, then computes all
eigenvectors and eigenvalues by inverse iteration. This
technique is most appropriate for structures with a small
number of degrees of freedom, for structureswhere all modal
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