xe2x80x9cOptimal conditionbased maintenance with general repair and two dependent failure modesxe2x80x9d Abstract   This paper proposes a conditionbased maintenance (CBM) policy for a system subject to dependent soft and hard failures. When the deterioration exceeds a predetermined level, the system experiences a soft failure that can be rectified by corrective general repair or corrective replacement. Three possible actions, namely no maintenance, preventive general repair, and preventive replacement, are considered at each inspection epoch. The hard failure rate depending on both age and deterioration is described by a proportional hazards model with the covariate characterized by a gamma process. The hard failure can be corrected by minimal repair, general repair, or by replacement. Different from previous CBM policies, no thresholds are used to define maintenance actions. The objective is to determine the optimal maintenance policy that minimizes the longrun expected average cost per unit time. The optimization problem is formulated in the semiMarkov decision process (SMDP) framework and solved by the policy iteration algorithm. Two practical numerical examples are provided to illustrate the proposed policy. The comparison with other CBM policies shows an outstanding performance of the proposed policy, as well as the significance of minimal repair and general repair in CBM decision making.

A Numerical Method for Wind Farm ConditionBased Maintenance Policy Assessment The cost of wind energy is greatly affected by wind farm reliability and maintenance management. In this paper, we focus on conditionbased maintenance (CBM) optimization for wind farms considering multiple turbines, while each turbine involves multiple components. In previous studies, economic dependency among multiple turbines and multiple components in a turbine were considered, and a simulationbased method was developed for wind farm CBM policy cost evaluation 1. The simulationbased method was flexible in modeling various scenarios and factors, but due to its samplingbased nature, there are variations in CBM cost evaluation, and the resulting CBM cost function surface is not quite smooth. This could lead to a challenge in the optimization process and cause local minima or convergence problems. Thus, an accurate numerical method is desired. In this paper, a numerical method is proposed to assess the overall maintenance cost of the CBM policy. Therefore, the optimal maintenance policy corresponding to the minimum maintenance cost is more accurately determined compared to the simulation method.