Offshore wind farms are currently located predominantly in shallow water as it is possible to cost effectively install bottom fixed offshore turbines. Where shallow water sites are not available, floating offshore turbines could be a better solution than bottom fixed turbines. Currently three main concepts are promoted for the design of a floating wind turbine: a ballast stabilized floater (i.e. spar), a buoyancy stabilized floater (i.e. barge or semi-submersible) or a mooring stabilized floater (tension leg platform). In April 2011 the DeepCWind consortium visited MARIN to carry out model tests in the offshore wave basin with these three types of floating wind turbine platform. This paper reports a numerical study of a wind turbine supported by a semi-submersible floater. The response of the floating system to wind and wave conditions is compared to physical measurements at 1:50 model scale. The outcome of these comparisons is discussed in the conclusions of this paper.