As wind turbines move into deeper water, floating structures might be preferred to fixed foundationsto reduce both their production cost and their installation cost. A floating foundation has the advantage that the turbine can be mounted on the floater in a sheltered area before the whole structure is towed to the wind farm. In this way the installation can be limited to the towing of the floating turbine and the mooring of the floater, which is quicker than for a fixed offshore wind turbine. This operation only requires tugs and anchor handling vessels which are more widely available and cost less than crane vessels needed for the installation of bottom fixed offshore windturbines. Thus a floating foundation cuts installation time and cost in comparison to a bottom fixed foundation. Large maintenance refit actions can also be carried out in the harbour for a smaller cost than at sea in the case of fixed wind turbines. However designing an offshore floating wind turbine (OFWT) brings new mechanical constraints to the nacelle and the rotor. The motions of the floater affect the performance of the wind turbine and vice-versa. In an economically viable concept, the mass of the floater would most likely not be much bigger than the mass of the wind turbine.