Hierarchical remarshaling operations in block stacking storage systems considering duration of stay

• A new operation strategy, hierarchical remarshaling operation, is introduced.
• A statistical formulation for optimizing the new strategy is suggested.
• An algorithm to find the optimal solution is provided.
• A numerical experiment is performed to characterize the optimal solutions.

This study proposes a strategy for steel plate storage systems in port terminals where steel plates are stored after being discharged from a vessel. Minimizing the rehandling operation or the number of relocations of plates on top of the plate to be retrieved is an essential requirement in the retrieval of steel plates from the storage system. One popular method to minimize the number of rehandling steps is to segregate the storage yard into several zones in which only plates with a remaining duration of stay (DOS) in a pre-specified range are stacked. When the retrieval time of a plate approaches, it is moved to the next zone corresponding to the plate’s remaining DOS at that time. This process is called “remarshaling.” This paper discusses how to optimize the classification of the zones so that the total number of remarshaling operations is minimized. Mathematical expressions for analyzing the remarshaling operation are derived, and the results of numerical experiments that use data from a steel plate storage warehouse are provided to explore interesting characteristics of optimal solutions.