## 1 Introduction
This enables design calculations for both fatigue and extreme conditions without the need for a substitute monopile model. Design optimization of OWTG support structure is an expensive task, due to the highly-constrained, non-convex and nonlinear nature of the design problem. An analytical gradient-based method has been proposed by [PERSON] et al. [9] to solve the dynamic constrained optimization problem of OWTG support structures. The design sensitivities of the objective and constraint functions are evaluated analytically while the optimization of the structure is performed, subject to sizing, eigenfrequency, extreme load and fatigue load constraints. This framework was validated using the numerical OC4 OWTG jacket model. Another work regarding the ship impact and influence of gravity loads, wind force, and soil stiffness has been reported in 2015 [10]. Numerical analyses focused on the force transfer between legs and braces have been illustrated. These simulation results were then employed to fix the hypotheses for the development of a simplified tool based on analytical formulations. The current analysis methods for WTG system design can be divided into two types: Coupled and decoupled method. It can already be seen that WTG industry is moving forward to employ a more accurate partially or fully integrated approach for their detailed design and analysis of WTG systems. An integrated load analysis was proposed by [PERSON] and used successfully by REpower [11]. The detailed design process of support structure, the integrated WTG model and the integrated load simulation method are all introduced in this paper. Another work regarding the dynamic responses of a jacket-type offshore wind turbine has been reported by [PERSON] et al. [12]. The objective of this study is to evaluate the applicability of the computationally efficient linear decoupled model by comparing with the results obtained from the nonlinear coupled model. Numerical studies of dynamic response of the OC4 jacket-type offshore wind turbine using both linear decoupled and nonlinear coupled models have been carried out in that work. Comparisons between the linear decoupled models and nonlinear coupled model were evaluated through performing eigenfrequency analysis, a decay test, and dynamic simulations with only wave excitation and combined wind-wave conditions within the operational conditions of the OWTG. According to the results, the differences between the decoupled model and the coupled model are primarily due to the decoupling, not nonlinearity. The decoupled method can have significant impact on the accuracy of the structural analysis. However, this approach is still widely adopted in the WTG industry due to its simplicity to implement and efficiency. Since the development of offshore WTG system design in Taiwan is just conducted for few years, the decoupled method is helpful for the initial design phase. Generally speaking, most of the current OWTG support structure designs are based on the recommendations by IEC (International Electrotechnical commission) 61400-3 [13], though there are many other design guidelines proposed such as those by DNV (Det Norske Veritas) [14] and GL (Germanischer Lloyd) [15]. The development of OWTGs in Taiwan was conducted from 2011. The jacket substructure is going to be applied to Taiwan OWTGs before 2020. Due to the environmental considerations of Taiwan, the design and analysis of OWTG foundation has received much attention. For OWTG support structure analysis, the software Bladed can be employed. However, this commercial program is not adopted by every research teams due to its cost and the limitation of source code modifications. In OC4, different software and methods have been adopted and proposed by individual teams to accomplish the OWTG analysis. In this work, a simplified and low cost analysis procedure for OWTG jacket substructure has been proposed. The goal of INER-OC4 is to develop the technical capability for the design and analysis of jacket support structure against Taiwan environments.