## 1 Introduction  
As the importance of renewable energy has increased due to the Kyoto Protocol and the Paris agreement in United Nations Framework Convention on Climate Change (UNFCCC), interest in the development and utilization of marine renewable energy has also increased [1]. Wave energy is one of the most abundant alternative energy sources [2]. Oscillating water column (OWC)-type wave energy converters (WECs), point-absorber-type WECs, and overtopping-type WECs are common forms of WECs. Among them, the point-absorber type can have the highest efficiency because it instantly converts the motion of a floating body into electrical energy. Many studies have focused on the power efficiency of WECs [3, 4, 5]. One way to increase the power efficiency is that WECs could be integrated with other marine structures, such as breakwaters or seawalls [6]. By integrating WECs with the breakwater, the reflected or re-reflected waves from the breakwater can improve the performance of WECs. Additionally, more frequent extreme weather conditions in the future could increase flooding of seawall-type breakwaters, leading to catastrophic coastal flooding. Thus, various studies have been conducted to strengthen the resilience of coastal protection by placing structures such as pillars in front of the breakwater or changing the shape of the breakwater [7, 8, 9, 10, 11]. In general, various numerical analyses have been used to evaluate the hydrodynamic performance of WECs, including frequency-domain analysis, time-domain analysis, and spectral-domain analysis [6]. In particular, frequency-domain analysis based on linear potential theory has been used for the initial design of the floating body because the calculation is fast and efficient. The wave-body interaction problem is important for analyzing the dynamic response of floating structures, such as WECs. Many studies on wave-body interaction problems have been conducted since the 1980s. The wave-body problem can usually be solved by numerical analysis using the free-surface wave Green function or Rankine source panel method, assuming potential flow [12]. The wave-body problem can be divided into a diffraction problem and a radiation problem. Solving the radiation problem requires a proper radiation condition to make an open sea condition. One of the well-known radiation boundary conditions is the Sommerfeld condition [13]. This boundary condition is easy to apply and does not require additional free surface meshes. Although this boundary condition has the advantage of not requiring an additional free surface mesh, it is ineffective under nonlinear wave conditions [14]. The reason is that the Sommerfeld radiation condition is a method of forcibly radiating from the end boundary using the information of propagated incident waves. Therefore, there is a limit to radiating nonlinear waves or scattered waves by objects at the end boundary. In the Rankine panel approach used for time-domain numerical wave tank (NWT) development, it is important to establish far-field radiation conditions. In addition, because the time domain NWT has the same conditions as the physical experimental wave tank, it is essential to solve the reflection problem of the incident wave to satisfy the open sea condition [14]. Therefore, an artificial damping zone scheme was applied to satisfy the open sea conditions in the time-domain NMT. In this scheme, by adding artificial damping terms to the free surface boundary condition, all reflected waves except the incident wave were forcibly attenuated in the incident wave boundary. All waves are forcibly attenuated at the end-wall boundary. On the other hand, the artificial damping scheme requires additional free-surface panels because the length of the artificial damping zone requires at least one wavelength to properly attenuate reflected waves [14; 15]. Many studies on the artificial damping system focusing on minimizing the damping length and maximizing damping performance have been reported.