The demand for developing sustainable and renewable energy is increasing due to the greenhouse gas effect and the reducing availability of fossil fuels. Wind energy is one of the cleanest and most sustainable renewable resources. People mainly use two types of wind turbines - onshore and offshore, to harvest the wind energy. Compared with onshore wind, offshore wind resources allow more reliable generation of electricity. In shallow water, bottom-fixed turbines may be used. However, as water depths increase it becomes increasingly difficult to build suitable fixed foundations, and floating offshore wind turbines become attractive. The OC3-Hywind is an offshore floating wind turbine (OFWT) supported by a spar platform and held in place with three mooring lines. It is important to study the dynamic responses of the floating platform and the mooring system behaviour so that the OFWT can be designed and constructed safely and economically and generate electricity with reliability. However, there are not many studies which have validated the numerical simulation results for floating offshore wind turbines with physical experiments while published studies rarely show the details of the mooring line motion behaviour for OFWTs. This thesis investigates the dynamic behaviour of a spar-type OFWT including its mooring systems, analysed under various environmental loads both by using state-of-the-art numerical software and conducting an experiment campaign at the Kelvin Hydrodynamics Laboratory. The free decay test has been carried out first to get the spar platform's motions natural frequencies and damping characteristics. A range of regular and irregular waves has been applied to the platform (both with and without realistic mooring lines) to obtain the platform motion RAO (response amplitude operator) - which can be used to predict the platform dynamic responses under other wave conditions - and to examine the reliability of the numerical predictions under realistic sea conditions. For the mooring line tensions and motion, the software shows its limitations in the calculation. A non-linear snatching phenomenon has been observed at the tank for some wave frequencies, which has rarely been discussed in published research. It is very important to study this non-linear behaviour as it is shown that snatching leads to the high instantaneous mooring line loads and platform accelerations, which could cause the failure of the mooring lines in the real structure.