A calculation method is presented of the wave-induced steady drift force and yaw moment on a very large floating structure (VLFS) comprising a multitude of floating columns. The theory is based on the momentum-conservation principle, and all necessary integrations are analytically implemented. Thus the resultant formulae include only the coefficients of the incident-wave and disturbance potentials at a large distance from the structure. A hierarchical interaction theory developed by the author is applied to determine the disturbance potential due to hydrodynamic interactions among a great number of floating columns and elastic motions of a thin upper deck. Experiments in head waves are also conducted using 64 truncated vertical cylinders arranged periodically in 4 rows and 16 columns. Good agreement is found between computed and measured results. Furthermore, through numerical computations in oblique waves, discussions are made on variation characteristics of the steady force and yaw moment particularly near trapped-mode frequencies.
Very large floating structures (VLFSs) are categorized with the configuration under the sea level into:
a pontoon type which looks like a simple plate with very shallow draft, and
a column-supported type in which a thin upper deck is supported by a large number of floating colurans. It is said that the pontoon type is advantageous in low costs for construction and maintenance, but the wave-induced motions may be relatively large. On the other hand, the column-supported type has reverse features; that is, the motions in waves may be small relative to the pontoon type, because incident waves will transmit through a gap between columns. However, the above recognition may not be the case. Recent studies on hydrodynamic interactions among many cylinders reveal that near-resonant modes occur at some critical frequencies and cause large wave forces on each element of the array.
