Which items should be stored together? A basic partition problem to assign storage space in group-based storage systems

We consider a basic partition problem that subdivides Stock Keeping Units (SKUs) into disjoint subsets, such that the minimum number of groups has to be accessed when retrieving a given order set under a pick-by-order policy. We formalize this SKU partition problem and show its applicability in a wide range of storage systems that are based on separating their storage space into groups of SKUs stored in separate areas; examples are carousel racks and mobile shelves. We analyze the computational complexity and propose two mathematical models for the problem under consideration. Furthermore, we present an ejection chain heuristic and a branch and bound procedure. We analyze these algorithms and the mathematical models in computational tests.