Rehandling Strategies for Container Retrieval

In this work, we consider the problem of optimizing the retrieval of containers from a bay in a fixed sequence. When there are other containers on top of the container to be retrieved, there may be alternative locations for those containers to be relocated. So it is essential to develop algorithms that find “good” locations in order to minimize the total time of the retrieval. In the literature, there are not many papers on this subject. The problem is discussed in some papers but only a few of them actually propose algorithms whose performance is demonstrated through extensive testing. In Chen (1999), factors causing unproductive moves during storage from operational to strategic levels are discussed in a top to bottom perspective. Kim et al. (2000) derive a methodology to locate export containers within a bay. An optimization model based on containers’ weight groups is formulated to find the location within the bay, minimizing the expected number of rehandles for an arriving export container. The container positioning problem is defined by Tranberg (2005). The problem is minimizing the total handling time of a block in an automated terminal. A linear mixed-integer model with a non-polynomial number of variables is formulated. In Lee & Hsu (2007), a multi-commodity network problem with side constraints is constructed for the pre-marshalling problem Kim & Hong (2006) deals with the problem of the retrieval of import containers from a single bay and is closet to our study. The problem is to find exact locations of relocated containers while retrieving all the containers from a bay according to a predetermined order. Another version of the problem is also considered. In this version, the retrieval sequence is not given as a permutation of all individual containers, but the sequence is among groups of containers. The model is formulated and first solved via a branch and bound search with the objective of minimizing the number of relocations.