## 1 Introduction  
Taiwan is a populous and mountainous island. The location for wind turbine generator (WTG) installation is therefore limited. On the contrary, wind fields over the sea are much more attractive than those that belong to land, both in terms of quantity and quality of the wind energy [1]. As a result, the development of OWTGs (offshore wind turbine generators) has received great attention worldwide in the past two decades. However, the design and installation of OWTGs is much more complex than those of onshore WTGs due to the operating environments. One of the most important issues is the design and analysis of OWTG support structure. Since the water depth, soil type, and wave condition are much different according to the OWTG installed location, various types of substructure designs have been proposed such as fixed-bottom monopiles, gravity bases, tripods, jacket-type and floating structures. Due to the global demand for OWTGs, two special projects, Offshore Code Comparison Collaboration (OC3) and Offshore Code Comparison Collaboration Continuation (OC4), have been conducted by International Energy Agency (IEA) in 2005 and 2010, respectively. The goals of the two projects are to develop essential knowledge with respect to the OWTG design, for example, the mathematic model of OWTG system, the controller design, the dynamics of aerodynamic loading and support structure design, with the aid of transnational cooperation. Similar projects, namely INER-OC3 and INER-OC4, have been performed by the Institute of Nuclear Energy Research (INER) to develop the OWTG technologies of Taiwan. Since the jacket substructure will be applied to Taiwan OWTGs before 2020, the INER-OC4 project has been devoted to the design and analysis of jacket support structure. Assessment of structural reliability for OWTGs is a challenging task. Estimation of the loading effect is necessary due to the large variability in environmental conditions. A method to estimate the failure probability for OWTG support structures has been proposed [2]. Both monopile and jacket support structure were investigated. The uncertainties of soil stiffness and aerodynamic damping are evaluated by Monte Carlo integration. According to the simulation results, the system reliability of monopile is highly determined by the environmental uncertainties while the jacket support structure is more robust against the uncertainties and modeling errors. A similar research for the calibration of safety factors and fatigue design factors for OWTG substructures was reported. A reliability-based approach and a probabilistic model have been proposed to evaluate the acceptable reliability level for fatigue failure of OWTG substructures [3]. Based on the WTG component design data quoted from global manufacturers in USA, Asia and Europe, towers, blades, the jacket foundation support structure and generators contribute to 25%-30%, 15%-26%, 15% and 7% of an OWTG cost, respectively [4]. Being one of the main cost components in a typical offshore wind project, the substructure cost generally increases with the water depth, due to the complexity of the structural design and manufacturing process, as well as the additional material cost [5; 6]. A comparison study was conducted between a conventional four-legged and a newly-developed three-legged bottom fixed jacket substructure for OWTGs [7]. Fatigue and ultimate limit state analyses were performed to verify the efficacy of three-legged jacket substructure. Offshore wind turbines are subject to combined wind and wave loading, which have to take into account for the structural assessment. An integrated analysis method for complex OWTG support structure under wind and wave loadings was proposed by [PERSON] et al. [8]. The calculation process is implemented with the aid of software Excel, ASAS and Flex 5. The idea of this integrated and sequential approach is to completely substitute the Flex 5 foundation module by a more complex model in ASAS (chosen for the \"Offshore\" part) while maintaining the general approach in Flex 5.