The offshore wind energy industry has been developing rapidly due to the increasing demand for renewable energy. As a result, offshore wind turbines are increasing in mass and height and are installed at larger water depths and further away from shore. A floating vessel is not limited by water depth and soil conditions, but is sensitive to wave-induced motions. Heerema Marine Contractors developed a new installation method using a floating vessel, where the rotor nacelle assembly (RNA) of a wind turbine is assembled on an assembly tower on deck and is installed in one lift. The research objective is to gain a better understanding of the dynamic behaviour of a rotor nacelle assembly, particularly due to the wind, focusing on the horizontal motions of the RNA, when it is hanging in the crane above the tower. To examine the dynamic behaviour of the RNA, separate models are made to assess the effect of the wind and waves individually in OrcaFlex and Liftdyn. This is done to identify which motions are due to the wind, and which motions are due to the waves. Finally, the wind and waves are coupled to gain insight in the coupled response. Furthermore, a case study is performed to prove its applicability for implementing and assessing the effect of using tugger lines on mitigating horizontal motions. In the models the blade pitch angle, crane position and responses for different wind and wave conditions are examined. From the different models is observed that the response due to wind cannot be neglected. Wave-induced motions are governing, due to large crane tip motions. The governing motion is a pendulum motion of the RNA, which is larger for longer wave peak periods. For both short and long wave peak periods, the horizontal motions have to be mitigated. In the case study, tugger lines are implemented to examine the effect on the RNA motions and to assess if the model can be used for developing concepts to reduce the motions. It is shown that providing stiffness in the horizontal plane and in roll help to reduce the sideward pendulum. For long peak periods, the RNA motion has to follow the opposing motion of the crane tip. More research is required to develop a concept that can follow the crane tip for short peak periods and oppose the motion of the crane tip for long wave peak periods.