KEYWORD: offshore wind turbine; FAST; SACS; semi-integrated analysis method; fully coupled analysis method ABSTRACT: The semi-integrated analysis method was used extensively in the design of offshore wind turbine in China. Recent researches have proved that the wave loads and sub-structure responses have significant effects on the aerodynamic loads of the blades especially in the operation conditions. In order to take these effects for OWT design into account, the fully coupled analysis method was presented. Using Fortran language, a FAST-SACS joint analysis module was developed to carry out time history verification of internal forces of sub-structure member. The comparisons between fully couple and semi-integrated analysis method are performed. RESEARCH BACKGROUND IntroductionThe offshore wind energy plays an important role in renewable energy sources research. There are some word class offshore wind farm are being under construction in China. The fixed bottom substructure were widely used in offshore wind turbine design in China. In recent years significant efforts have been made to improve fixed bottom sub-structure design method. Böker C. (2010) developed a finite element code and coupled with Flex5 to research the fixed bottom OWT analysis method and further investigate the excitation of local dynamics in fully coupled model. Yan F., et al. (2012) based on NREL 5MW wind turbine and a spar researched the differences of coupled and limited coupled methods, they found that the heave result of limited coupled model is much greater that the couple model. Gutierrez J., et al (2013) presented a new engineering software based on FAST and another commercial software in order to perform optimization design of offshore wind turbine, an important advantage is the parametric design capability. Burn (2011) research the influence of different foundation modeling methodologies on the dynamic response of offshore wind turbine sub-structure. Cordle A., et al (2011) researched the relative importance of the various external conditions in OWT design based on a jacket OWT subjected to wind and wave loading. The semi-integrated analysis methodStep 1, derive the equivalent stiffness and mass matrixes at the flange point (transfer point-TP) on the basis of FE model of OWT sub-structure, such as monopile, tripod, pentapod or jacket.Step 2, calculate aerodynamic loads and wind turbine loads of transfer point, establish FE model of bladehub-nacelle-tower partial structure in the aero-elastic analysis code, model the sub-structure and the foundation using the equivalent stiffness and mass matrixes in this step.Step 3, apply the transfer point wind turbine loads to the FE model in step 1, in order to update the equivalent stiffness matrix. Figure 1 illustrates the analysis procedure of the semi-integrated analysis method.