Floating breakwaters (FBs) are frequently used to protect marinas, fisheries, or other bodies of water subject to wave attacks of moderate intensity. New forms of FBs are frequently introduced and investigated in the literature as a consequence of technological advancements. In particular, a new possibility is offered by High-Density Polyethylene (HDPE) by extruding pipes of large diameters (e.g., 2.5 m in diameter) and with virtually no limit in length (hundreds of meters). By connecting two or three such pipes in a vertical layout, a novel low-cost floating breakwater with deep draft is devised. This note investigates numerically and experimentally the efficiency of this type of multi-cylindrical FBs in evaluating different geometries and aims at finding design guidelines. Due to the extraordinary length of the breakwater, the investigation is carried out in two dimensions. The 2D numerical model is based on the solution of the rigid body motion in the frequency domain, where the hydrodynamic forces are evaluated (thanks to a linear potential flow model), and the mooring forces do not include dynamic effects nor drag on the lines. The numerical predictions are compared to the results of a 1:10 scale experimental investigation. An atypical shape of the wave transmission ($k_t$) curve is found, with a very low minimum in correspondence with the heave resonance frequency. The results essentially point out the influence of the position of the gravity center, the stiffness, and the mutual distance among cylinders on $k_t$.

Accurate identification of hydrodynamic characteristics for wave energy converters is a prerequisite for designing effective control strategies and power-optimization models. The identification could, in principle, be based on physical model experiments. However, in practice, mechanical imperfections, particularly Coulomb friction at pile-hinge connections, introduce a non-sinusoidal force component that contaminates the measured signals and yields spurious values of added mass and radiation damping coefficients. Those coefficients are the cornerstone for a power optimization study for the next stages. This paper presents a method to identify and quantify the dissipation contribution due to mechanical limitations by performing forced-oscillation tests on a 1:20-scale rectangular-shaped floating breakwater wave energy converter with a moonpool, tested in the wave flume lab of the University of Padova at different frequencies and amplitudes. Dry condition tests, carried out with and without the friction due to the pile constraints, characterize the mechanical dissipation and spurious dry stiffness independently of any hydrodynamic effects. Tests in water (wet tests) for both heave and pitch degrees of freedom are used to identify hydrodynamic characteristics contaminated by the square-like component due to friction. Overall, the procedure enables the acquisition of corrected force signals to better identify hydrodynamic characteristics (added mass, radiation damping, and frequency response function).