Page 203 - Practical Design Ships and Floating Structures

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depending on the level of analysis. For example, to check the strength of deck and bottom plating in
the hull-girder strength level, calculations using vertical bending moment and sectional modulus can
provide satisfactory results.
The hydrodynamic force components consist of incident wave forces, diffraction wave forces and
motion-induced radiation forces (added mass and damping forces). The potential theory of fluid
mechanics with boundary element method using source distribution can be applied to numerically
calculate the hydrodynamic forces. Currently, hydrodynamic analysis software using a three-
dimensional modeling (preferred) or two-dimensional strip method is widely applied. A detailed
discussion of numerical techniques and other effects of loads (such as bow flare impacting, bottom
slamming, green water, ice loads, and accident loads) on the extreme response and fatigue are beyond
the scope of this paper, and will not be discussed further.

The wave heading a, is defined with respect to a FPSO (see Figure 4). Depending on the mooring type,
the wave probability at direction a, needs to be converted into FPSO local coordinates. For example, if
the turret-mooring system is adopted, the weathervaning should be considered, and some of the wave
headings can be removed.
The stress FRFs of a deck plating at twenty-four incident wave directions are calculated by using the
2D strip method and cross-sectional modulus for the purposes of illustration (3D hydrodynamic and
FE method can be used for general structural details). With the WSD and the FRF, the spectral density
function S!'(o) of the responses x (stresses or loads) to a wave spectrum can be determined.

4 EXTREME RESPONSE

Strength analysis generally involves assessing the yielding strength, buckling strength, ultimate
strength and fatigue strength. Details of structural strength and fatigue evaluation may be found in Bai
(2001) and ABS (2000). The first three aspects of structural strength are directly related to the extreme
values of stress responses, which will be discussed in this section. The fatigue strength assessment will
be covered later.