Syncrolift dry dock scheduling with a capacitated ship transfer system

Despite the rapidly increasing maintenance demand driven by an aging fleet and the growth of global fleet size, optimizing dry dock operations remains an underexplored area in academic research. This study addresses this gap by focusing on enhancing efficiency through improved dry dock scheduling, aiming to alleviate the growing disparity between maintenance needs and limited dry dock resources. We propose a practical syncrolift dry dock scheduling problem that incorporates a capacitated ship transfer system, which requires determining the optimal sequence of ship maintenance services to minimize total waiting time. A mixed-integer linear programming (MILP) model is developed to effectively tackle the challenges of this problem, particularly the ship transfer system—a critical bottleneck involving the syncrolift and railways—by modeling it as a series of segments with varying capacities and transfer restrictions. The model accommodates the complexities of the scheduling process, such as limited transfer capacity, diverse segment characteristics, bidirectional ship transfer flows, ship compatibility constraints, and sequence-dependent setup times. Recognizing the NP-hard nature of the problem, we introduce a novel column-generation-based heuristic method to solve the MILP model efficiently. The efficacy of the proposed solution method is validated through extensive numerical experiments using real operational data from the NOSCO shipyard. Results demonstrate significant improvements in operational efficiency, with ship waiting times reduced by 32.5% compared to first-come-first-served scheduling solutions.