© Published under licence by IOP Publishing Ltd. In the design of supporting platforms of floating offshore wind turbines, global response analysis is essential to predict the response under various loads from wave, wind, moorings and the wind turbines. However, the literature of the global analysis of floating offshore wind turbines combining flexible modelling of the supporting platform and the potential flow theory for hydrodynamic evaluation is limited. In this study, the framework implementing the potential flow hydrodynamics to the time-domain analysis of the three-dimensional frame model was developed using modal decomposition for the hydrodynamic evaluations. The method utilized the assumption that the number of modes can be limited to those with larger contributions, which can lead to the reduction of the calculation cost. A spar-type floating offshore wind turbine was modelled to verify the implementation of the developed formulations. Results showed that the implemented method is effective in predicting the dynamic motion and deformation of floating offshore wind turbines under combined wind and wave loads. For the cases of the irregular waves, the results obtained with potential flow hydrodynamics showed smaller responses than the results from the Morison's equation in the natural frequencies of rigid body motion, which might be attributed to the steady and low frequency external forces introduced in the Morison's equation by considering the instantaneous position of the floater.