Page 258 - Practical Design Ships and Floating Structures

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Sm = J2sF (om
where C,,,, : wave amplitude
om : wave circular frequency
s,,, : random phase angle
Ao : interval between wave circular frequencies
The wave exciting forces on the irregular waves are derived from Eqn. (14).

The deck wetness is defined when a bottom point of VLFS is below free surface and slamming when
free surface is below a bottom point of VLFS in Eqn. (24) and (25).

2, (t) = z(t) - sft) > fieebord
2, (t) = z(t) - g(t) < -draft
where, Z, : the local relative vertical motion

In the frequency domain, the response amplitude operator(R40) of relative vertical responses in
regular waves can be expressed as follows.

The relative responses for incident waves is described by the Rayleigh density function, the following
relations in the frequency and time domain are satisfied for the probability of deck wetness and
slamming.

P~ =exp[-f2 /2rn,l
P,! = exp[-dz l2m, ]
where, f: freebord of VLFS
d : drafi of VLFS

4 NUMERICAL RESULTS AND DISCUSSIONS
Fig.1 shows the VLFS model of length 1,20Om, breadth 24Om, depth 4Sm, draft l.Om, and rigidity
4.559x1O9kgfrn. The water depth is 20m. The location of slamming and deck wetness is expressed by
circle in Fig. 1. VLFS is subdivided into 60x8 flat shell elements.
Fig.3 shows a wave spectrum used in the prediction. Its significant wave height is 4.81m, wave
period is 1 Ssec, and incident wave angle is 180 degree.
Fig. 4 shows the comparison of hydroelastic vertical displacements in the frequency domain analysis
with those in the time domain analysis. The wave period is 8 seconds and its height is 2m. The
responses predicted by two methods show good agreements and the time domain analysis method is
validated.