# New Tendencies in Wind Energy Operation and Maintenance
## 3 Overview of Wind Farm O&M Research
### Maintenance and Life Cycle Cost (M_cost)  
The investment and maintenance costs are relatively insignificant [57, 102] with respect to the variables of excessive cost, fixed cost and capital cost of the conventional generation system. However, according to [103], cost reduction remains necessary for wind energy systems so that renewable energy systems can be competitive. According to the Condition Monitoring of Offshore Wind Turbines report, the investment costs [11] for future offshore wind farms built from larger units, costs are expected to be EUR 1.4 to 2.0 k/kW. The analysis of the maintenance costs of wind farms shows that up to 40% of the investment is related [49] to unexpected component failures leading to unscheduled costly modifications. The operation, maintenance and unscheduled interruption costs make great targets for wind energy production [78] but are very risky. The operation and maintenance have [36] an impact on the failure rates of wind turbines. Many research works focus on finding the best maintenance strategy that minimizes the total cost and maximizes annual energy produced by a turbine, and [80] takes into account the frequency of maintenance as a variable decision, keeping in mind that downtime of a turbine is very important in an eventual failure, and results in a loss [30] of production and income. Since the maintenance is limited to appropriate time windows, there is a need for decision support tools to ensure that maintenance is carried out in a timely manner, with minimal cost and minimal downtime. [PERSON] et al. [104] have, to this end, a methodology for the estimation of the remaining useful life of the main bearing of a wind turbine. Offshore wind energy is identified as an emerging future energy growth technology, so the operation and maintenance require major access methods. The economic viability of offshore wind farms are subject to favorable wind conditions [105] compared to onshore farms, offsetting the additional installation and maintenance costs to a greater extent. The most effective way to reduce costs would be to continuously control [106] the state of the systems. This allows early detection of the health degeneration of the wind turbine. Due to the large amount of information that this requires, the use of neural networks [52]to deal with all the information and try to detect failures in some equipment at an early stage is a promising technique for cost reduction. Wind turbine condition monitoring can significantly reduce [33] the wind farm maintenance cost, especially offshore. However, the high cost and various capacity constraints [12] of available developments have delayed its extensive use. The stochastic simulation model constructed in [58] quantifies the added economic value of implementing a system for condition monitoring of a gearbox. There are proposals for programming models that extend classical maintenance [39] to include the expected market prices and weather conditions. Other research studies show [107] the benefits of opportunistic maintenance to significantly reduce maintenance costs. The method of calculating the life cycle cost is used to study various technologies and to select cost-effective concepts, such as in [50], which evaluates and compares two types of vertical axis wind turbines. [PERSON] et al. [108] estimated the total maintenance cost of control device pitch, which is responsible for controlling the turbine blades. [PERSON] and [PERSON] [109] carried out a generic case study, where the wind turbine costs included in the model were assessed costs due to inspections, repairs and production loss, and the costs were compared with two maintenance strategies, with and without the inclusion of periodic inspections. [PERSON] and [PERSON] [73] analyzed the inspection interval and maintenance cost under periodic and no periodic inspections and they described optimal maintenance policies. The risk-based methodology [55] presents a cost-effective manner to minimize life cycle costs in asset management.