The vast deepwater wind resource represents a potential to use offshore floating wind turbines to power much of the world with renewable energy. Many floating wind turbine concepts have been proposed, but dynamics models, which account for the wind inflow, aerodynamics, elasticity and controls of the wind turbine, along with the incident waves, sea current, hydrodynamics, and platform and mooring dynamics of the floater, were needed to determine their technical and economic feasibility. This work presents the development of a comprehensive simulation tool for modelling the coupled dynamic response of offshore floating wind turbines and the verification of the simulation tool through model-to-model comparisons. The fully coupled time-domain aero-hydro-servo-elastic simulation tool was developed with enough sophistication to address limitations of previous studies and has features required to perform loads analyses for a variety of rotor-nacelle assembly, tower, support platform and mooring system configurations. The developed hydrodynamics module accounts for linear hydrostatic restoring; non-linear viscous drag; the added-mass and damping contributions from linear wave radiation, including free-surface memory effects; and the incident-wave excitation from linear diffraction in regular or irregular seas. The developed mooring line module is quasi-static and accounts for the elastic stretching of an array of homogenous taut or slack catenary lines with seabed interaction. The hydrodynamics module, the moorings module, and the overall simulation tool were tested by comparing to results of other models, including frequency-domain models. The favourable results of all the verification exercises provided confidence to perform more thorough analyses. Copyright © 2009 John Wiley & Sons, Ltd.