The block relocation problem, which is also known as the container relocation problem, is an optimization problem to find an optimal sequence of operations for retrieving blocks (containers) from a container yard in a given order. The objective function to be minimized is the number of necessary relocations. In this study, we propose an efficient exact algorithm for two variants of the block relocation problem, those with distinct and duplicate retrieval priorities. In the former problem, the retrieval order of blocks is uniquely provided, whereas in the latter problem, it is given only among groups of blocks. The primary contribution of this study is a tighter lower bound of the number of relocations than existing ones. This enables us to construct a faster branch-and-bound algorithm. Its effectiveness is demonstrated by extensive numerical experiments. Note to Practitioners-A container terminal plays an important role in logistics chains by offering a temporary storage location for containers. Containers transported to a container terminal by vessels, trucks, or trains are stored in container yards. They are piled up vertically to form several stacks. A gantry crane is commonly used to move containers in a container yard, which can access only the topmost containers.When the container that we want to retrieve next from the yard is not on the top of a stack, those stacked above it should be relocated to other stacks. The reduction of such undesirable relocation operations is crucial in container operational decisions because the throughput of cranes is one of the key factors that affect the overall efficiency of container terminals. The algorithm proposed in this paper enables us to obtain an optimal relocation plan that minimizes the number of required relocations in a shorter computation time. When the number of stacks is not large, we can solve the problem to optimality without resorting to heuristic or metaheuristic approaches that only search for near-optimal solutions. Index Terms-Block relocation problem, branch-and-bound algorithm, container terminal, lower bound.Manuscript
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