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Analysis of Serial Inventory Systems with Batch Ordering and Nested Replenishment Schedule

Abstract: We consider an $N$-stage serial inventory system where material is produced/ordered in batches. Inventory replenishment between stages follows a nested schedule. Stochastic customer demand occurs at the most downstream stage. There are two types of fixed costs considered in this system: setup costs and order costs. The former is associated with each batch produced/ordered; the latter is incurred for each replenishment cycle. We study two models. The first (resp., second) assumes that batch sizes (resp., replenishment cycles) are fixed. The objective is to determine optimal reorder points and replenishment cycles (resp., batch sizes). These models represent two variants of the deterministic model (e.g., Maxwell and Muckstadt 1985 and Roundy 1985). For both models, we use the same approach to construct a lower bound on optimal cost and to develop an optimization algorithm. Since the algorithm is computationally infeasible for large systems, we further propose a heuristic that generates near-optimal solutions by solving at most $N$ single-stage systems with the original problem data. We find, among other insights, that managing these two systems is quite similar indeed.
Keyword: Multi-echelon inventory model