## 1 Introduction  
Offshore structures suffer from various types of impulsive pressure loading, such as slamming, sloshing, and green water. Most studies have focused on estimating the slamming pressure and maximum pressure accordingly [1]. Based on the peak pressure, the slamming load is calculated as the equivalent design pressure. According to the results of several studies, not only the peak pressure but also the duration of pressure play an important role in predicting the range of damage to the structure [2]. Figure 1 shows the structural damage caused by slamming loads when the marine structure operates in extreme conditions. The slamming impact load may be described by the magnitude of the  
load, rise time, and duration. The phenomenon of slamming is strongly nonlinear and affected by the behavior and shape of the structure. Severe pressure due to slamming can be exerted on floating offshore wind turbines and could exacerbate their structural damage and deteriorate their strength. The authors of [3] conducted a repeated drop experiment using a bottom model of a ship with a deadrise angle of 10\({}^{\circ}\). The central deformation was measured after each test, and, in most classifications, the slamming impact pressure was assumed to be static and the dynamic nature was ignored. The basis of this assumption was that the duration of the impact pressure was greater than the natural frequency of the impacted structure, as reported by the authors of [3]. A free-drop experiment using a steel model was performed to determine the effects of elasticity on the slamming pressure. The slamming pressure was measured within a very short time as the test model was dropped freely onto the water surface [4]. The first pressure measured is due to slamming and the second pressure is due to the interaction with the natural frequency of the structure. The second pressure was large enough to not be ignored compared to the first pressure. In general, when the duration of non-dimensional pressure is less than 1, the behavioral characteristics of the structure are determined by the impulse values, regardless of peak pressure and duration. Conversely, when the duration of non-dimensional slamming pressure is greater than 1, the behavioral characteristics of the structure are determined by the peak pressure. However, if the duration of the pressure is similar to the natural frequency of the structure, the behavioral characteristics of the structure are affected by both peak pressure and duration [5; 6]. Even a single wave acting several times leaves permanent deflections over the impacted region. Meanwhile, [7; 8; 9] and [10] included structural damage due to slamming. In this paper, the inverse of the duration of pressure was compared with the natural frequency of the structure. Therefore, units for both parameters are the same as Hz. Recently, as the importance and interest in renewable energy has increased, further research has been conducted into installing wind turbines at sea. Korea's offshore wind turbines have excellent geographical conditions. The Yellow Sea, with low water depth, is favorable for the construction of fixed wind turbines. The East Sea, with a relatively high-water depth, is favorable for the installation of floating wind turbines and facilitates the creation of large-scale complexes. In the case of offshore wind turbines, many types of technical aspect should be considered before design. Among these, damage to the structure caused by slamming loads should be investigated. According to recently published papers and case studies, design life in the context of structural damage due to slamming loads is being actively investigated. Figure 2 shows the presence of slamming loads on a 1:40-scale floating offshore wind turbine model during experiments under irregular wave conditions. As shown in Figure 2, the floating wind turbines experience bottom slamming, wave run-up slamming, and horizontal slamming. Under these conditions, certain combinations of slamming loads can be harmful.