Page 219 - Practical Design Ships and Floating Structures

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(1) Near field theory can predict the steady drift forces on VLFS properly by accounting its elastic
deformation modes. In that, three-dimensional source method is effective in calculation of
hydrodynamic forces.
(2) The rigidity affects both deflection and drift forces considerably, however the flexibility brings
conflicted effects toward deflection and drift force, thus, it is necessary to apply an optimizing
technique from viewpoint of reduction of both hydro-elastic response and wave drift force.
(3) Semi-submersible type VLFS shows smaller drift forces at higher wave frequencies, but larger drift
force at lower wave frequencies than the pontoon type and this depends on the rigidity,
configurations of underwater body of the structures.
(4) The relative motion of structure affect the drift force dominantly for a beam like very large floating
structure in head sea wave condition, thus, it is vitally important to move away the natural
frequency of deflection from the frequency range of incident waves.

0 ET. (measunl)
n
33
s
L
9
v!
P
I3
-0.5
0 5 10 15
a, [dscc] a, [dsec]
Figure 6: Wave drift force components of rigid Figure 7: Wave drift force coefficients of
semi-submersible type floating structure semi-submersible type VLFS
3, I I , , , , , , , I
il

o [radsec]
Figure 8: Comparison of measured drift forces of pontoon type and semi-submersible VLFS

Acknowledgement
The authors are grateful to Mr. K. Miyakawa and Mr. T. Takayama from Yokohama national
University for their supports in the preparation of model and execution of experiments. Mr H. Abe and
Mr. T. Ishikawa and other students of seakeeping laboratory of YNU are also acknowledged for their
helps during experiments.