ssc-367 - Ship Structure Committee

loads directly. Since the diffraction effects are strongly
dependent on frequency, a wide range of frequenciesmust be used.
The response of the floating structure to applied wave loadings
depends on its own geometry, stiffness and mass properties. Water
plane area and its distribution (i.e., hydrostatic-stiffness)and
mass properties directly affect the natural periods and the heave,
pitch and roll response amplitudes. For a tethered structure,such
as a TLP, tether stiffnesswill predominate hydrostatic stiffness.
Tether pretensionswill control surge and sway natural periods and
response amplitudes. The primary damping is due to wave radiation
and viscous effects.
It is recommended that the “consistent approach” discussed in
Section 5.2.2 is used to accuratelygenerate hydrodynamicloads. A
finite element model of the structure can be used to obtain the
solution to the motions analysis and determine the stress
distributions. As an alternate,a stickmodel maybe used to obtain
solutions to the equations of motion and to define global
deformations and forces. Then, additionalfinite element models of
various interfaces will be necessary to determine local stress
distributions accurately. The boundary conditions for the finite
element models will be the stick model deformations.
5.3
BOTTOM-SUPPORTEDMARINE STRUCTURES
This section discusses bottom-supportedmarine structures that are
represented by three-dimensional space frames and composed of
cylindrical shells. The dynamics of a large gravity-type bottom
supported structure dynamics are somewhat similar to those of a
fixed platform. However, the characteristicsof excitationalloads
on gravity structureshave more in common with floating structures.
5.3.1 Load or HydrodynamicsModel
A wave force acting on a single stationary element is due to both
the accelerationof water particles (inertialforce) and the kinetic
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