The installation of floating wind farms in deeper water is encouraged by the stronger and steadier wind, the lower visibility and noise impact, the absence of road restrictions, but also the absence or shortage of shallow water. In the summer of 2009, the first large-scale floating wind turbine "Hywind" was installed. Hywind is a spar-buoy concept with three catenary mooring lines. Offshore floating wind turbines are a completely novel concept. The experience with modeling such turbines is still limited. A new basic model has been developed for a spar-type floating wind turbine. The requirement of the model is that it incorporates the most significant physical processes so as to be able to provide insight into the dominant physical behaviour of spar-type floating wind turbines. Various verification methods show that A.T.FLOW simulates load cases as expected and is a useful tool for assessing the physical behaviour of spar-type floating wind turbines. The coming two years the body forces and behaviour of the operating full-scale Hywind demo project is monitored. This data should be used to further test and validate A.T.FLOW and to guide further development of the model.