Attention is directed to the uncertainties associated with the prediction of wave loads acting on the floating support structure of an offshore wind turbine operated in the North Sea. An assessment is made on the induced Morison wave loads acting on each member expected to occur during the operational life time of a semi-submersible floating offshore wind turbine. The determination of the ... [Show full abstract] existing uncertainty sources and their relative contribution to the total variability of the wave loads prediction is analyzed by using the first order second moment method. The study considers the influence of the wave height and peak period on the calculation of the water kinematics and on the total uncertainty as well as of the theory used to calculate the water kinematics itself. The results present an average value of uncertainty for wave load prediction in the North Sea which can be used in the assessment of the structural reliability of the floating foundation for offshore wind turbine.