Semi-analytical solutions to wave diffraction of cylindrical structures with a moonpool with a restricted entrance

Wave diffraction of cylindrical structures with a moonpool with a restricted entrance is studied using a semianalytical method based on eigenfunction matching. The fluid domain around the complex structure hull is divided into five cylindrical subdomains. In each subdomain, an eigenfunction expansion of the velocity potential is obtained by satisfying the Laplace equation and the boundary conditions on the free surface, on the hull, and on the seabed. The unknown coefficients of eigenfunction expansions are obtained by the continuity of potential and its normal derivative across the matching surfaces between subdomains. The introduction of a parametric dissipation is realized by assuming an additional term in the boundary condition at the free surface of the moonpool. The dispersion equation is then modified so that the wavenumber becomes complex with a small imaginary part that ensures the decay of wave amplitudes. Free-surface elevation in the moonpool center, first-order wave loading, and second-order drift forces are evaluated. The good agreement of the free-surface elevation with measurements in model tests is obtained both for the peak period and for the amplitude at resonance by choosing an appropriate dissipation coefficient. It is shown and confirmed by model tests that the resonant period of water motion in the moonpool increases when the size of the moonpool entrance is reduced and that the motion amplitude decreases because of larger fluid dissipation through a smaller moonpool entrance. Furthermore, the first- and second-order wave forces are evaluated and compared with the numerical results of HydroStar based on the boundary element method.