A Study Of Storage Policy And Maintenance Support Strategy In Warehousing Systems

Along with the development of Internet Technology and Electronic Commerce, logistics and supply chain becomes one of the most popular research topics since the end of last century. Wherein, warehousing systems, as important nodes in supply chains, receive great attentions from the scholars. In practice, the rapid development of B2C and C2C e-commerce brings the warehouses new challenges in order response.Under the business environment described above, this thesis studies the most important issues related with order response efficiency of warehousing systems:the storage policies of the system and the availability of the storage/retrieval (S/R) machine. According to these two research issues, this thesis can be partitioned into two parts.The first part of this thesis focuses on storage policies of the warehousing systems, analyzes how to improve the operational efficiency based on different storage policies. The discussions of this part lie in Chapters2and3. In Chapter2, by considering a finite number of items stored in the system, we relax the following basic assumption in the literature:required storage space of all the items equals their average inventory level, which is valid only if an infinite number of items are stored in each storage region. We first give the basic one way travel-time model based on a square-in-time (SIT) continuous storage rack. Then we relax the assumptions by considering non-SIT storage racks, a different type of ABC demand curve, different number of stored items, discrete storage racks and stochastic item demands, to validate the robustness of the basic model. After that, in Chapter3, we study the performance of different storage policies (including random storage, class-based storage and turnover-based storage) in a unit-load traditional warehouse with a finite number of items by considering the effect of realistic required storage space.The response efficiency depends on not only the storage policies, but also the availability of the S/R machine. Consequently, in the second part of this thesis, we focus on how to maximize the expected profit of storage enterprises through optimal maintenance strategies. In Chapter4, we focus on the optimization problem of the storage enterprise to determine the optimal spare inventory and cost reduction effort when it does the equipment maintenance itself. In the discussion, different from the literature, we focus on the maximal profit of the enterprise considering the operational requirement (i.e., profitability) of the S/R machine instead of a given availability requirement. Additionally, we extend the model to an alliance model by considering multiple enterprises in the same market. Based on the analysis of Chapter4, Chapter5studies the maintenance strategy in a decentralized scenario when the support service is provided by the maintenance suppliers. A Stackelberg game model is adopted to describe the relationship between the storage enterprises and the maintenance suppliers based on performance-based contract.Based on the analysis provided above, the main innovations and contributions of this thesis are summarized as follows:1) Through the analysis in Chapter2, we find that commonly a small number (3to5) of classes is optimal for the warehousing system. This is in consistent with that a small number of classes is very close to warehousing practice where often only three storage classes are used. In addition, we find that the shortest travel time is fairly insensitive to the number of storage classes in a wide range around the optimum. This suggests a manager can select a near optimal number of storage classes in an easy way and he or she should not be worried about the impact of storage-class reconfigurations.2) By considering the realistic required storage space of stored items in a warehousing system, we exam the effect of item demand structure (ABC demand curve) on storage policy performance in Chapter3. Different from the findings in the literature, we find that under the same storage policy, items with different ABC demand curves lead the system different average travel time/distance.3) Based on the theoretical findings presented in point1) and2), we give the optimal item classification and warehouse shape factor for AS/RSs and aisle-based traditional warehouses.4) A new maintenance strategy based on the profitability of the equipment is proposed in Chapter4. Through the discussion, we find that the optimal availability requirement of the equipment is not constant, but increasing with its profitability. Seeking high availability regardless with the profitability of the equipment will hurt the expect profit of the enterprise.5) When the maintenance service is provided by the suppliers, we model the after-sales service supply chain through game models considering the profitability of the equipment in Chapter5. The findings tell that (i) the shared inventory strategy is not always better than dedicate spare inventory for each enterprise from the suppliers’ perspective. When the unit cost of the spare part is relatively high, it is better for the supplier to set dedicate spare inventories.(ii) At the game equilibrium between the storage enterprises and the suppliers, all the enterprises will provide the same cost reimbursement to the suppliers even they hold different number of equipment with different profitability. This is a result of that with performance-based contract, each enterprise has two terms to influence the decisions of the supplier, especially the decision on spare inventory which influence the system availability directly. One is the cost reimbursement which provides a pull effect for the suppliers, and the other one is backorder punishment which provides a push effect. In the Nash game among the enterprises, it is a dominant strategy for the enterprises with large number of equipment or high profitability to select the same cost reimbursement as other ones and provide a high backorder punishment to push the spare inventory to the desired level.