The aim of this work is to investigate the dynamic behavior of a spar-type support for offshore wind turbines under the action of irregular waves in ultimate design conditions. The structure chosen is the U-Maine Hywind Spar Buoy, supporting the NREL 5MW turbine. A 1:30 scale field experiment is arranged, exploiting the unique characteristics of the Natural Ocean Engineering Laboratory (NOEL) of Reggio Calabria (Italy), which uses the occurring real sea states as scale models of severe ocean storms. The design of the model has been made in order to reproduce the prototype mass and geometrical properties and to assure its structural resistance to the local extreme conditions. Numerical analysis have been carried out, using the commercial code AQWA, to obtain RAOs, platform motion and tensions on the catenary mooring lines, in order to verify the design of the model and to be compared to the experimental results.