## 3 Test Results
### Wave Forces According to Wave Periods of Irregular Waves  
Figures 16-18 plot the results of the wave force tests measured for each of the considered test models under irregular wave conditions. Table 6 summarizes the maximum amplitudes of the wave forces to irregular wave conditions for the real-scale models, and Figure 19 plots the maximum amplitude of the wave forces for the real-scale models subjected to irregular waves. The measured wave force to wave period under irregular wave exhibits a tendency different from wave force under regular wave which presents a tendency that the wave force gradually decreases the overall wave period. In view of Table 6 and Figure 19, most of the support structures undergo gradual decrease of the wave force when the wave period increases from wave IR-#1 (7.5 s) to wave IR-#2 (9.5 s) at the three sea levels, which indicate similar tendency with regular wave condition. However, a slight increase of the wave force occurs when the wave period increases from wave IR-#2 (9.5 s) to wave IR-#3 (13.7 s). The wave force shows large variation according to the wave period with ranges of about 0.74-1.00 at HSWL and 0.62-0.80 at MSL. However, smaller variation is observed at LSWL with a range of 0.96-1.14. Furthermore, at the LSWL, the wave force to the long-period wave (13.7 s) tends to be slightly higher than that of the short period wave (7.5 s), contrary to the test result in the regular wave condition in which the wave force to the long-period wave (13.7 s) tends to be lower than that of the short period wave (7.5 s). This is because shoaling is likely to occur in the LSWL and the irregular wave contains a high wave height component so that the wave force change due to shoaling in an irregular wave can be more sensitive than that in the regular wave condition. In addition, the change in the wave force of the irregular wave caused by shoaling becomes more apparent in the long period than in the short period wave. Figure 14: Relationship between wave conditions of this study and wave breaking criteria. Figure 15: _Cont._Figure 16: Measured wave forces for monopile under irregular waves: (**a**) IR-#1: HSWL; (**b**) IR-#1: LSWL; (**c**) IR-#3: HSWL; (**d**) IR-#3: LSWL. Figure 17: Measured wave forces for GBS under irregular waves: (**a**) IR-#1: HSWL; (**b**) IR-#1: LSWL; (**c**) IR-#3: HSWL; (**d**) IR-#3: LSWL. Figure 18: _Cont._  
Figure 15: Measured wave pressure: (**a**) monopile; (**b**) GBS; (**c**) hybrid. Judging from the test results with respect to the support structure type, the monopile shows a wider variation of the wave force than the GBS and Hybrid. The decreasing ratios of the wave force when the sea level lowers from HSWL to MSL and LSWL are about 0.74, 0.62 and 1.14 for the monopile, 0.79, 0.68 and 0.96 for the GBS, and 0.88, 0.78 and 1.01 for the hybrid, respectively. This is the similar tendency with the test result in the regular wave, where the larger the diameter of the support structure, the larger the diffraction effect and the effect of increasing the wave force by shoaling is reduced. In GBS and hybrid, the wave forces at LSWL are somewhat higher than that the HSWL and MSL overall wave periods. As described in Sections 3.2 and 3.3, it is considered that, as the water depth is lowered, the larger projected area of these two types affects the increase of the wave force, and the high wave height component included in the irregular wave has an influence on the increase of the wave force.