The paper outlines challenges in improving wind turbine availability to reduce the cost of offshore wind energy. It reports the development of a design software tool for wind turbine gearboxes. It facilitates efficient conceptual designs of wind turbine gearboxes with combinations of epicyclic and parallel gear trains. Field loading conditions are obtained by analysing SCADA (Supervisory Control and Data Acquisition) data to support fatigue damage estimation based on Miner’s rule. Results show that considerable overloading conditions are present which would result in significant fatigue damages to the gearbox.