Page 225 - Dynamic Loading and Design of Structures

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Figure 5.10 Contributions from linear, second and third order wave components to wave elevation of
a steep wave (Stokka, 1994).

with third order loads from the FNV theory. The method yields an improved representation of
second order forces. Although diffraction models yield estimates closer to model test results
than Morison-type formulations, currently available methods are generally amenable to
screening analysis of the ringing phenomenon. In this connection it is an advantage that
diffraction theories seem to yield conservative load estimates. For platforms with multiple
columns, the phenomenon is today best quantified by model tests.

5.4 CALCULATION OF WAVE LOAD EFFECTS

5.4.1 Dynamic models
Various dynamic models of marine structures, like those in Figure 5.1, are envisaged in this
section, ranging from simple ‘stick models’ as shown in Figure 5.11 to sophisticated finite
element models of the structure and foundation.
Excitation is due to wave loading and the structure, soil and water may contribute stiffness,
mass and damping, depending on the support conditions of the structure.
Global models of, for example, platforms and buoyant bridges are commonly based on
beam models. However, the caisson of gravity platforms is usually modelled as a rigid body.
The P-Δeffect for platforms with ‘large’ motion displacement could be taken into account by
linearized negative springs. Possible catenary