A reactive multi-component maintenance policy for offshore wind turbines

This paper treats the maintenance problem of offshore wind turbines. In this study we consider the critical components of the wind turbine. The turbine failure caused by the electrical generator, the gearbox and the rotor blade are called hard-type failure and these corresponding components are called hard-type components. The hard-type failure is self-declared and any hard-type failure leads to turbine failure. The fault of the control system is a soft-type failure and it can be revealed only through inspection. The soft-type failure does not cause the turbine failure but reduces production and causes a large loss. The hard-type failure is independent of the soft-type failure and the latter does not result in a hard-type/system failure. The three hard-type components are monitored by sensors which transfer two-state signals: normal operation and abnormal operation. In this paper three maintenance policies of different levels are considered: periodic routine, reactive and system-level maintenance. The periodic routine maintenance is the farm-level maintenance which is scheduled at fixed period. The reactive maintenance is carried out when an abnormal operation signal of one component is received. In this case only the latter component is maintained and the maintenance occurs usually between two successive routine maintenances. When during an inspection it is revealed that a component needs to be repaired or replaced, the turbine should be shutdown and hence it offers the opportunity to maintain the other components of the same turbine, in this case the maintenance is called a system-level maintenance. The time duration for replacements or repairs cannot be ignored. In this paper, the optimization of the maintenance policies is realized by minimizing the long run average maintenance cost.