Reactive scheduling and control for shipbuilding

Abstract

Production process of shipbuilding is very complex in nature. To build an efficient shipbuilding scheduling system so as to reduce the inherent computational complexity, DAS (Daewoo Shipbuilding Scheduling System) has adopted a two-layered hierarchical architecture. In the hierarchical architecture, detailed scheduling for the lower-level assembly plants are delegated to the individual plant``s schedulers as far as the requirements from the higher-level dock scheduler are satisfied. However, if the lower-level scheduling is not satisfied, the higher-level scheduler should attempt to adjust the original requirements. So, the reactive adjustment at the higher-level scheduler is unavoidable. To accommodate the requested delays, we propose an algorithm that is composed of a series heuristics to minimize the impact of local adjustment. Four heuristics adopted are exchange sequences, slack time reduction, processing time reduction, and propagation of unabsorbable requests to succeeding blocks. The algorithm can be applied for not only reactive scheduling but also reactive control, because unanticipated events (e.g. machine breakdown, material unavailability, strike, due date delays, etc.) may quickly invalidate generative schedules. A prototype reactive scheduling system, DAS-REACT, is implemented as a subsystem of DAS using the expert system development tool UNiK on SUN SPARC station. A series of experiment using the real-world data are also carried out to guide the equilibrium of responsibility between the higher-level scheduler and the lower-level ones.