Moonpool-type floaters were initially proposed for applications such as artificial islands or as protecting barriers around a small area enabling work at the inner surface to be carried out in relatively calm water. In recent years, a growing interest on such structures has been noted, especially in relation to their use as heaving wave energy converters or as oscillating water column (OWC) devices for the extraction of energy from waves. Furthermore, in the offshore marine industry, several types of vessels are frequently constructed with moonpools. The present paper deals with the hydrodynamics of bottomless cylindrical bodies having vertical symmetry axis and floating in a water of finite depth. Two computation methods were implemented and compared: a theoretical approach solving analytically the corresponding diffraction problem around the moonpool floater and a computational fluid dynamics (CFD) solver, which considers the viscous effects near the sharp edges of the body (vortex shedding) as non-negligible. Two different moonpool-type configurations were examined, and some interesting phenomena were discussed concerning the viscous effects and irregularities caused by the resonance of the confined fluid.

The present contribution aims at presenting a semi-analytical solution methodology of the linear hydrodynamic diffraction and radiation problems induced by two coaxial moonpool-type floaters subjected to incident waves. The flow field around the floaters is subdivided into ring-shaped fluid regions, in each of which axisymmetric eigenfunction-expansions of the velocity potential are made. The applied theoretical formulation is complemented by numerical-panel methodologies, using pulsating singularities distributed over the floaters’ wetted surface. Extensive numerical results in terms of exciting forces and hydrodynamic coefficients are given by applying frequency-domain techniques. The effect of the water trapped between the solids’ vertical walls on the floaters’ hydrodynamics is investigated and discussed. The presented analysis led to some remarkable trapping phenomena in the enclosed water areas, broadening the resonance-frequency bandwidth of the floaters when compared to a single moonpool body.

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