Coupled rotor-floater response analysis is essentially important for the design of Rotor Nacelle Assembly (RNA) and floating support structure of Floating Offshore Wind Turbine (FOWT). The authors have developed an analysis code UTWind for analysis of the coupled structural response. Blades and floater are modeled as frame structure with beam elements. Lumped mass model is use for mooring. Aerodynamic load on blade is calculated by Blade Element Momentum Theory (BEM), and hydrodynamic load is calculated by Hooft’s method, and Morison equation was modified to be applicable to cylindrical element with cross section with two axes of lines symmetry. The equations of motion of rotor, floater and mooring are solved in time domain by weak coupling algorithm. The numerical results by the code were compared with responses measured by experiment in wave and wind-and-wave coexistence field with/without blade pitch control and showed good agreement. Response by negative damping was reproduced by the code and showed good agreement with experiments.

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