Plant shutdown maintenance workforce team assignment and job scheduling

An important and challenging real-life problem is considered, involving workforce assignment and job scheduling for shutdown maintenance in a large oil refinery. A limited number of maintenance employees must be divided into several teams that work in parallel on different maintenance tasks. The objective is to minimize the shutdown cost by minimizing the total shutdown period, i.e., the time to complete all the maintenance tasks. Different team sizes are possible, and the size of the given team determines the speed of finishing the assigned maintenance tasks. Constraints include job availability (arrival) times, and precedence relations between different jobs. This problem can be considered as a resource-constrained parallel-machine scheduling problem, in which the objective is to minimize the makespan, and both the number and the speeds of the machines are decision variables. An integer programming model of this problem is formulated, but optimum solution is difficult because the problem is NP-hard. Therefore, a two-stage heuristic solution algorithm is developed and shown to be quite effective for solving this problem.