Page 216 - Practical Design Ships and Floating Structures

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Heave 3.47 0.22
Pitch 3.57 0.23
1 st Bending 4.52 0.28
2"d Bending 8.89 0.56
3rd Bending 17.95 1.12

4. RESULTS AND DISCUSSIONS

4.1 Motions and Deflections in Regular Waves

Surge motion of pontoon type VLFS is shown in Figure 3 and the vertical displacements at three points
(fore end, midship, aft end) of semi-submersible type VLFS are shown in Figure 4. The experimental
results were obtained from tests in linear transient water waves, although the experimental results
scatter at higher frequencies, the calculations for both rigid motion and deflections agree with
experiments well on the whole. The disagreements in Z-displacement at fore end can be considered as
the non-linear interaction effect of waves and structure, which are not included in calculations.
4.2 Steady Drgt Forces in Regular Waves

Steady wave drift forces (non-dimensioned) in surge direction are shown in Figure 5 for pontoon type
VLFS and in Figure 6 for semi-submersible type. On the whole, the correlations between calculation
and experiment are found to be well in agreement. The discrepancies in semi-submersible type are
associated with the accuracy in deflection prediction at fore end for the same reason as mentioned in
previous section. However, different tendency in two type models has been observed. Drift force of
pontoon type is negligibly small in relatively low frequencies (less than 5 radkec in model scale) and it
increases from 5 rad/sec rapidly. On the contrary, drift force of semi-submersible changes its value
considerably at relatively low frequencies when wave frequency changes. The frequency dependence
becomes more complicated than pontoon type.
In Figure 6, drift force component corresponded to the terms on right hand side of Eqn.6 are shown. It
can be confirmed that the contributions of relative wave elevation (term 1) and pressure drop due to
velocity potentials (term 2) are dominant components for steady drift forces.
Drift forces of 9 units rigid semi-submersible model (1/4 of VLFS model) are shown in Figure 7. It can
be stated that the agreement between the calculated and experimental results are comparatively well
except frequencies from 5 to 7 dsec. For rigid semi-submersible model, the drift force varies its
value with wave frequency remarkably and this could be explained as the effect of wave scattering
phenomenon among columns, which is the well-known fact for first order wave force.