## 2 Modeling of Offshore Wind Turbine (OWT)  
### Modeling of the Seabed  
The testing seabed was prepared to simulate the ground conditions of the site where the OWT was installed. In the natural-frequency calculations, the ground condition was simplified as a rigid spring model. The dynamic behavior of the structure, based on the soil-foundation-structure interaction, depends on the stiffness of the soil-foundation system. Therefore, the aim was to ensure that the E of the soil model is close to the E at the site. Site ground modeling was conducted in three stages: (i) evaluation of soil elastic modulus of the site; (ii) assessment of the modulus value of the soil specimen; (iii) evaluation of the soil E of the soil model prepared for the centrifuge test. The E of the soil at the site was evaluated based on the results of the two underwater cone penetration tests (CPTs) performed at the same location near the site. The shortest distance between the CPT locations was approximately 85 m. The results of the field CPTs confirmed that the ground had a three-layered structure consisting of silty sand (SM), clay, and SM, as shown in Figure 1.  
The cone tip resistance (q\({}_{c}\)) values obtained from the CPT tests were converted to the shear wave velocities using empirical equations for sand and clay. The maximum shear modulus of elasticity G\({}_{\text{max}}\) and the E of the soil were calculated as follows:  
\[\text{G}_{\text{max}}=\rho\text{V}_{\text{s}}{}^{2}, \tag{1}\]  
\[\text{G}_{\text{max}}=\frac{\text{E}}{2(1+\upsilon)}, \tag{2}\]  
where \(\rho\), \(\text{V}_{\text{s}}\), and \(\upsilon\) are the density, shear wave velocity, and Poisson's ratio of soil, respectively. The soil properties of the site are summarized in Table 1. The maximum elastic modulus E\({}_{\text{max}}\) value was selected as the representative E value. The soil model was constructed to match the average target E\({}_{\text{max}}\) values. Owing to the limitations of the model experiments in simulating a large-scale soil structure within a model soil box of 900 mm in diameter and 700 mm in height, the model ground is highly simplified relative to the actual site. The upper SM layer and the clay layer were simulated as one layer because they had similar E values. As a result, the model ground was designed as a double layer, with an upper E value of 18.0 MPa and a lower E value of 48.5 MPa. Figure 2 shows the cross-section of the actual site and soil model. \begin{table}
\begin{tabular}{c c c c c} \hline  & **Depth (m)** & **Density (kN/m\({}^{3}\))** & **Poisson’s Ratio** & **E\({}_{\text{max}}\) (MPa)** \\ \hline Upper sand & 0.0–2.5 & 19.0 & 0.35 & 17.7 \\ Clay & 2.5–8.3 & 18.6 & 0.4 & 18.3 \\ Bottom sand & \textgreater{}8.3 & 19.0 & 0.35 & 48.5 \\ \hline \end{tabular}
\end{table} 
Table 1: Basic soil properties at the site. Figure 1: Cone tip resistance (q\({}_{c}\)) profiles. For the model test, we used Semanguem sand, which is a type of very fine natural sand collected from the reclaimed site of Semanguem on the west coast of Korea, and it was classified as SM based on the unified soil classification system (USCS). The soil properties of Semangeum sand are listed in Table 2. Oedometer tests were performed to obtain the relative density (D\({}_{\text{r}}\)) of the soil with the target stiffness based on the weight and initial volume of the soil in the cell. The Semangeum samples were compacted by altering the relative density values inside the cell, and a pair of bender elements were installed inside the cell. The upper confining pressure, which increased with depth, was simulated by weighting the upper cell, and in each case, the shear wave velocity (VS) of the soil specimen was measured through bender element tests. The tests were conducted at a D\({}_{\text{r}}\) of approximately 40-80%. Each cell was tested under dry and saturated conditions and simulated as columnar phases by measuring the vs. under different confining conditions.