As the offshore wind turbines increase in size, the structural flexibility of the tower, the blades and the platforms are becoming more significant and may impact the dynamic response of the wind turbine. To assess properly the design life of a floating wind turbine, the modal analysis needs then to consider the interaction between all elements. In this study, a modal analysis of the IEA 15 MW wind turbine on a floating NereWind semi-submersible support is performed with finite element modelling and the hydro-structure calculation software HOMER. The natural modes of the tower are calculated in several configurations to study the impact of the local stiffness variations, the global platform flexibility, the added mass and the hydrostatic stiffness. This work will allow floating offshore wind farms developers and floating concept designers to understand better the impact of the structural and geometrical arrangement on the modal turbine behaviour.