Decision Support and Optimization in Shutdown and Turnaround Scheduling

Large-scale maintenance in industrial plants requires the entire shutdown of production units for disassembly, comprehensive inspection and renewal. It is an important process but causes high out-of-service cost. Therefore a good schedule for a shutdown and and an analysis of possible associated risks are crucial for the manufacturer. We derive models and algorithms for shutdown scheduling that include different features such as time-cost tradeoff, precedence constraints, hiring external resources, resource leveling, different working shifts, and risk analysis. Our experimental results show that our methods solve large real-world instances very fast and yield an excellent resource utilization. A comparison with solutions of a mixed integer program on smaller instances proves the high quality of the schedules that our algorithms produce within a few minutes. Our algorithms work in two phases. The first phase supports the manager in finding a good makespan for the shutdown. It computes an approximate project time cost tradeoff curve together with a stochastic evaluation of the risk for meeting a particular makespan t. Our risk measures are the expected tardiness at time t and the probability of completing the shutdown within time t. In the second, detailed planning phase, we solve the actual scheduling optimization problem for the makespan chosen in the first phase heuristically and compute a detailed schedule that respects all side constraints. Again, we complement this by computing upper bounds for the same two risk measures, but now for the detailed schedule. The shutdown problem has many relationships with well established areas of scheduling, and we also give an overview on the large variety of scheduling problems involved.