  During the life of a wind farm, various types of costs arise. A large share
of the operational cost for a wind farm is due to maintenance of the wind
turbine equipment; these costs are especially pronounced for offshore wind
farms and provide business opportunities in the wind energy industry. An
effective scheduling of the maintenance activities may reduce the costs related
to maintenance.
  We combine mathematical modelling of preventive maintenance scheduling with
corrective maintenance strategies. We further consider different types of
contracts between the wind farm owner and a maintenance or insurance company,
and during different phases of the turbines' lives and the contract periods.
Our combined preventive and corrective maintenance models are then applied to
relevant combinations of the phases of the turbines' lives and the contract
types.
  Our case studies show that even with the same initial criteria, the optimal
maintenance schedules differ between different phases of time as well as
between contract types. One case study reveals a 40% cost reduction and a
significantly higher production availability -- 1.8% points -- achieved by our
optimization model as compared to a pure corrective maintenance strategy.
Another study shows that the number of planned preventive maintenance occasions
for a wind farm decreases with an increasing level of an insurance contract
regarding reimbursement of costs for broken components.
