Maintenance costs in offshore wind farms are one of the main reasons why it is still not competitive with its onshore counterpart. To lower the price ofenergy, the authors want to improve the modeling of offshore wind farms in order to help improving the applied maintenance strategies. In this paper, weinvestigate the repair of wind turbine components as an alternating renewal process. The time it takes to repair (renew) a component in a turbine, directlyinfluences the downtime of this turbine, or even larger parts of the wind farm. This downtime leads to production losses, which in turn raise the cost ofenergy. Previous investigations by two of the present authors showed that a variation in repair times has a significant influence of the production losses. Inthis paper, an alternate renewal process is investigated as tool to calculate the influence of the repair time duration on the production losses. We show that ifwe assume an exponential distribution of repair times, the distribution of the downtimes can be calculated analytically. The results are demonstrated with acase study, based on available failure rates and repair times. For other distributions, the calculation has to be done numerically, after fitting the parametersof the distribution to the available data. Further work with other distributions and actual repair time distributions is planned.
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