The design and analysis of offshore wind turbine foundations are traditionally based on deterministic time-domain simulations of a numerical model. The wind turbine, support structure and environmental conditions are represented by a large number of input parameters, whose uncertainties are accounted by applying partial safety factors. In this paper, the sensitivity of fatigue loads with respect to primary structural, geotechnical and metocean parameters are investigated for a 5 MW offshore wind turbine installed on a gravity based foundation. Linear regression of Monte Carlo simulations and Morris screening are performed for three design load cases. Results show that parameter significance rankings vary according to which design load case is considered. In general, uncertainties in the fatigue loads are highly influenced by turbulence intensity and wave load uncertainties, while uncertainties in soil property suggest significant nonlinear or interactive effects. This work provides insights to foundation designers and wind turbine manufacturers on which parameters must be assessed in more detail in order to reduce uncertainties in load prediction. (C) 2019 The Authors. Published by Elsevier Ltd.