Optimal preventive maintenance strategies for a wind turbine gearbox

Abstract

This paper investigates two maintenance strategies for wind turbine gearboxes. The first one is frequently adopted in practice. It consists in monitoring the state of the gearbox through its temperature. As soon as the latter reaches a predefined threshold level, production rate is drastically reduced by slowing down the wind turbine while cooling the gearbox for a certain period before recovering the desired output rate. As it becomes more frequent with time, the wind turbine operators will decide to renew the gearbox. The latter is replaced by a new identical one or submitted to an overhaul based only on the judgement of the maintenance agents. For this first strategy, an analytical model is developed to optimize the renewal period of the gearbox considering the balance between the cost of production loss and cooling each time the threshold temperature is reached, and the cost of renewal.The second strategy is a new one proposed in this paper. It suggests performing an imperfect preventive maintenance (PM) action each time the temperature threshold is reached, reducing hence the failure rate of the gearbox to a value between the current one and the one of a new gearbox. The imperfect preventive action is performed N times before the gearbox must be renewed. A mathematical model is also developed to simultaneously find the optimal number of PM actions to be performed before renewing the gearbox, and the optimal period for the maintenance crew to start the PM or renewal action after the instant at which the temperature threshold level is exceeded. This period being longer or shorter depending on the logistics in place to move the maintenance crew to the site and prepare for the intervention. Numerical examples are presented, a sensitivity analysis is performed, and the two strategies are compared. Optimal solutions are obtained for each strategy. Also, the result of the comparison shows that each strategy can be more economical depending on the reliability of the gearbox, and the different costs incurred, particularly the PM and the renewal related logistics costs.