State based models applied to offshore wind turbine maintenance and renewal

Offshore Wind Turbines (OWT) are seen as a means to increase renewable energy sources. Maintenance and renewal of OWT poses new challenges due to number of factors like marine environment, weather conditions, and uncertainties regarding new failure modes and mechanisms. In this paper we specially focus on degradation modeling of OWT components where the state information is used to determine when to perform maintenance and renewal taking into accounts the fact that the OWT will be unavailable in randomly distributed periods of times due to the weather conditions. The model includes maintenance and renewal costs, cost of loss production, and a simple weather window model. Two degradation model are evaluated, a phase type model and the gamma process. The proposed scheme is implemented on the gearbox which has the highest downtime in case of failure. It is expected that the efficient maintenance of gearbox will enhance the availability of OWTs in a better and cost effective way.