## 1 Introduction  
In response to the growing demand for renewable energy, offshore wind turbines have recently seen their capacity increased from 1-3 MW to 7-12 MW. The increase of the turbine capacity means enlarged turbines and enlarged support structures undergoing extreme environmental conditions including wind, waves, and tides. This enlargement of the offshore support structure improves the strength and the stiffness but makes it subject to complicated wave forces with a wider exposed area and complex details. It is thus important to evaluate exactly the wave force and wave-induced overturning moment applied to the support structure to secure its structural safety [1; 2]. In the hydrodynamic and structural analyses conducted during the structural design, the wave forces acting on the support structure are calculated based upon the Morison equation or diffraction theory. As shown in Figures 1 and 2, the wave nonlinearity and wave diffraction of the wave force applied to the offshore support structure depends on its shape and size as well as on the sea levels and wave conditions including the wave height, period, and length [3; 4; 5; 6; 7; 8]. Shoaling is a phenomenon that occurs when a wave passes through a shallow water depth, in which the wave crest increases sharply and the wave trough flattens, as presented in Figure 1, and it is believed that nonlinear variation of wave height due to shoaling affects wave force [4]. Diffraction is a phenomenon in which fluid flow is diffused around a circle when the fluid passes through a large circle [7], as presented in Figure 2, which is known to generate diffracted wave force on the circular backside and to have lower wave pressure on the circular surface. Recalling that the monopile [9], GBS (gravity based structure) [10; 11], and hybrid support structures [12; 13] are currently preferred to support offshore wind turbines and that irregular waves are commonly applied to obtain a more accurate wave force [14; 15], the wave force acting on the offshore structure needs to be evaluated experimentally for specific shapes and specific design wave conditions in case of large structures because of the resulting higher nonlinearity and complexity of the wave force [3; 6; 7; 8; 16]. [PERSON] et al. [7] summarized the influences of the breaking waves on offshore wind turbine structures, in which considerable uncertainties in the estimation of hydrodynamic loads are caused by the breaking waves in shallow water. [PERSON] et al. [3] analytically investigated wave run-up and wave forces on a vertical circular cylinder and found that the run-up is higher when the wave is just about to break compared to the fully broken wave. [PERSON] et al. [6] studied the influence of wave condition and water depth on run-up and wave force including both regular and irregular waves. The results showed that the shape of the support structure substantially affects the maximum run-up level, increasing the expected run-up value. [PERSON] et al. [17] studied a fully nonlinear wave model to simulating breaking wave impact loads on offshore wind turbines. [PERSON] et al. [15] studied the wave forces on a monopile from a nonlinear irregular unidirectional wave model and reported that nonlinear irregular waves give rise to larger extreme wave forces than those predicted by linear theory. [PERSON] [18] and [PERSON] [19] studied the slamming forces from plunging breaking waves on truss support structures in shallow water. [PERSON] et al. [20] have studied of wave force on the array of a vertical cylinder in shallow water. [PERSON] et al. [5] investigated the shoaling evolution of the wave frequency-directional spectrum with field and laboratory observations and a numerical model. [PERSON] et al. [14] estimated the transformation of wave shoaling and breaking which are essential for nearshore hydrodynamics and the design of the coastal structure and evaluated some criteria for wave shoaling. [PERSON] et al.