## 1 Introduction  
In order to construct an offshore wind turbine (OWT), an engineer should select a proper support structure concept. The selection depends on various factors including water depth, environmental and seabed conditions, and installation equipment where water depth and seabed conditions are two basic factors. A few common types of offshore wind support structures include monopile, gravity, tripod, jacket and floating types [1]. Monopile (as shown in Figure 1a) and gravity structures are often found to be the most suitable for shallow water regions, where the water depth is lower than 30 m. Tripod and jacket structures, as respectively shown in Figure 1b,c, can be built in transitional water regions, which have a water depth between 30 and 50 m [1]. Floating types are expected to be the most cost-effective and reasonable solution for deeper waters [2]. While the monopile transfers the lateral and moment loads by bending to the soil, both the tripod and jacket dissolve the global moments to pairs of forces and transfer them as axial loads to the soil, and are advantageous especially in weak soils [3]. They are considered to be a relatively lightweight concept and have good stability and overall stiffness [4]. Compared to the tripod concept, the jacket foundation concept is more preferable in termsof scour, ship collision, complexity of joints, deflection at tower top and overall weight [3]. The study in this paper focuses on jacket-foundation supported OWTs. The jacket foundation concept consists of three parts: Piles, jackets and transition pieces. The jacket is characterized by three or four legs. It appears that the four-legged jacket supported by main piles is particularly suitable for the offshore wind industry [5]. The benefits of using the jacket foundation concept are that it is found to be endurable against environmental load, has a higher level of stiffness and lower soil dependency and is suitable for installations in deeper water or in waters with high waves and at sites with poor soil conditions. However, the structural analysis and design of jacket foundations are more complicated than other foundation concepts, which may push up the costs of fabrication and installation [6] and are therefore focused in this paper. In order to ensure structural safety of all of the components and serviceability of the OWT system, an accurate and detailed dynamic analysis of the complex OWT behavior is required. [PERSON] et al. (2014) presented numerical studies of the dynamic responses of a jacket OWT using both decoupled and coupled models [7]. In the decoupled model, the wind load is included through time-dependent forces and moments at a single node on the top of the tower. The coupled model is a hydro-servo-aero-elastic representation of the system. In the first decoupling method, internal loads obtained from the coupled model were applied directly as time-dependent point loads in the decoupled model, the point mass and the mass moments of inertia were not included [7] but the dynamic responses were significantly over-predicted. In their second decoupling method, the thrust and torque from an isolated rotor model were used as wind loads and a linear damper was introduced to account for the aerodynamic damping. However, there were considerable differences in the time histories and spectral plots of the tower base moment, tower top displacement and axial force under combined wind and waves. Excessive computation times on the complex system of jacket foundation-OWTs remains an issue. In order to cope with the excessive computation times on complex foundation types such as jackets and tripods, super element modeling was combined with the concept of modal truncation augmentation using the Craig-Bampton method, which consists in extending the reduction basis by adding'residual vectors' [8]. Their case study enabled efficient integrated simulation of OWTs. However, effects of soil and soil-pile interactions have not been accounted for in their method and case study.