Wind turbine reliability depends on the reliability of some key components. Maintenance of wind turbines is conducted twice a year at 6-month intervals. Typical maintenance would involve a thorough inspection of the entire system replacement of fluids lubrication and servicing of mechanical parts. There has been an increasing focus on reducing the cost of wind power particularly operating and maintenance (O&M) costs which include day-to-day operations preventive (scheduled) maintenance and unscheduled maintenance due to unexpected failures of wind turbine components.