## 1 Introduction
Several studies have examined the wave radiation boundary conditions in the time domain. The wave radiation problem was first solved by applying an artificial damping scheme [16]. In addition, a study was conducted by applying two damping terms (\(f\), \(h\)) to kinematic and dynamic free surface boundary conditions [17]. A dual damping system that combined the artificial damping zone with a piston-type wave absorber at the end boundary was proposed in [18]. An artificial damping scheme applying two damping terms (\(f\), \(h\)) only to the kinematic free surface boundary condition was developed [19]. In the two-dimensional fully nonlinear NWT development, two damping terms (\(f_{n}\), \(h_{n}\)) were used for kinematic and dynamic free surface boundary conditions, respectively [20]. An optimal ramp function and length of the artificial damping zone applicable to a three-dimensional NWT were proposed [9]. Four different shapes of ramp function applied to the damping coefficient were introduced to evaluate the damping performance of the artificial damping zone. In addition, it was confirmed that the total wave energy in the computational fluid domain was preserved despite the propagation of incident wave propagation. A three-dimensional, time-domain, fully nonlinear NWT was equipped with frontal, side, and end artificial damping zones [21; 22]. In addition, a three-dimensional NWT combining two radiation boundary conditions (Sommerfeld condition and artificial damping zone (ADZ)) was developed [23; 24]. The application of the combined conditions was very effective in removing waves from the sidewall and end boundaries. Studies using the ADZ technique based on the Rankine panel method are used mainly for time-domain analysis. On the other hand, a two-dimensional frequency domain program with an artificial damping zone was developed [25]. This was the numerical simulation for the wave radiation problem of a floating body in two-layer fluid. In this study, a three-dimensional frequency-domain numerical wave tank (FR-NWT) based on the Rankine panel method was developed to analyze the hydrodynamic performance of a heaving-buoy-type WEC integrated with a breakwater. Since this frequency domain analysis model can solve the radiation and diffraction problems in the rectangular fluid domain, various wave-ocean structural interactions can be calculated. The ADZ scheme, which is mainly used for the time-domain analysis, was first applied to the 3D frequency-domain, FR-NWT. In addition, to apply the ADZ of the optimal condition, the optimal length of ADZ, type of damping terms, and shape function of ramping function were estimated. The far-field radiation performance was compared by applying a combined radiation boundary condition (both ADZ and Sommerfeld radiation conditions). Based on the developed FR-NWT, the hydrodynamic performance of a hemispherical-buoy-type WEC combined with a seawall-type breakwater of infinite length was evaluated. The interaction between the floating WEC and the breakwater was used to amplify the global motion and generate power under specific conditions. The rest of this paper is organized as follows. Section 2 presents numerical schemes for developing the FR-NWT. Section 3 can be divided into two parts. The first part focuses on investigation of the radiation boundary condition suitable for application to the FR-NWT. The second part focuses on the motion amplification and power generation efficiency of the WEC integrated with the breakwater using the developed FR-NWT. Finally, Section 4 summarizes the results and future challenges.