Joint condition-based maintenance and spare control under degradation-rate warning and defect validation

Critical components in offshore assets, such as wind turbine bearings, often undergo multi-stage degradation before failure, posing challenges for health-state identification and maintenance decision-making, especially when manual inspections are costly and imperfect. Consequently, this paper proposes a condition-based maintenance strategy integrating degradation-rate warning and defect validation. Specifically, 1) periodic condition monitoring obtains degradation levels, from which the degradation rate is estimated using two consecutive measurements. When the estimated rate exceeds a rate-warning threshold, a potential defect is indicated, triggering imperfect manual inspection for defect validation. 2) State-dependent spare ordering is incorporated, where spare procurement is triggered either when the monitored degradation level exceeds the degradation threshold in the normal state or when a defective state is reported by manual inspection, with replacement performed upon spare arrival. The proposed strategy establishes an interactive detection mechanism that coordinates condition monitoring and manual inspection, reducing unnecessary inspections and enhancing maintenance decision-making effectiveness. The monitoring interval, degradation threshold, and rate-warning threshold are jointly optimized to minimize the long-run average cost rate. A case study on offshore wind turbine bearings demonstrates the economic advantage of the proposed strategy over conventional approaches.