Study On Distribution Optimization Method In The Vendor Managed Inventory System

Vendor managed inventory is an advanced inventory management mode of supply chain, which can effectively eliminate the uncertainty by demand fluctuation and enormously save the operational cost of supply chain. Although the mode of vendor managed inventory breaks free from conventions of inventory management and it embodies the thought of integrated supply chain management, it makes the vendor improving distribution service level as well. With the consumers'requirement of small batch and multi-item and distribution service time, the distribution cost gradually increases, but reasonable distribution plan can effectively reduce the cost and improve the service quality, so the distribution directly affects the implement of the mode of vendor managed inventory. The key problems of distribution are the routing plan and shipment consolidation policy, surrounding the two kinds of problems and based on the work of other scholars, this thesis develops more realistic model of vehicle routing problem. In order to understand of the tradeoffs of supply chain system operational cost, the integrated policy of shipment consolidation and stock replenishment is developed, and the solutions of the models are developed at the same time.The vehicle routing problem is the core of the study of routing plan, most of the models reported in the literature assume constant travel speed, but the travel time between two cities does not depend only on the distance traveled, but on many other factors including rush hours, accident, road and weather conditions, etc. In order to achieve more reasonable routing scheme, this thesis considers that the travel speed is dependent on the travel times of the day, at the same time in order to reduce the vehicle empty run, a more general vehicle routing problem (vehicle routing problem with backhauls) is considered.This thesis first develops the model of single object time-dependent vehicle routing problem with backhauls which object is minimizing total travel time, the characteristic of time-dependent travel speed is analyzed and the model of time-dependent travel speed is developed. Because the complexity of this problem, the most promising reactive tabu search algorithm is applied to solve the vehicle routing problem, which include generating the initial solution by modified saving heuristics method and then improving the result by the reactive tabu search. Computational study with the updated standard instances under different road condition shows that the resluts with the time-dependent speed are (almost) systematically better than those obtained with constant speeds, which means the more reasonable routing scheme can be attained with time-dependent speed. with the increasing degree of the road condition the best object values with time-dependent speed and constant speed are both increases, the number of the infeasible solutions with constant speed increases. The results by reactive tabu search algorithm are better than those by the parallel nearest neighbor search algorithm, which shows that the performance of the reactive tabu search algorithm is better than a general heuristic algorithm.Based on the single objective time-dependent vehicle routing problem with backhauls, the customers'service window is further considered, the objective are minimizing the total travel time and minimizing the total tardy time, which resembles two conflicting objectives of transportation total cost and service level, the bi-objective problem can transform into single objective problem by additive weighted method. The reactive tabu search algorithm is designed to solve this problem, at the first phase the initial solution is achieved by bi-objective saving heuristic method and insert heuristic method, at the second phase the bi-objective reactive tabu search algorithm is designed to improve the initial solution. Through tests on the standard instances the offset region of objective is achieved, the vendor can choose different weights to get the tradeoff of two objectives which satisfies both the vendor and the customer. The results by reactive tabu search algorithm are better than those by the nearest neighbor search algorithm, which shows that the performance of the reactive tabu search algorithm is better.Before making routing plan the vendor should consider the economic shipment number and shipment frequency and replenishment quantity, thus the vendor need consider shipment consolidation policy and stock replenishment policy. This thesis firstly analyze the characteristic of quantity-based shipment consolidation policies and the characteristic of vendor managed inventory system, then the system cost model based on the integrated policy of quantity-based shipment consolidation and stock replenishment is developed, in the literature the vendor's lead time is assumed zero, which means the vendor can receive the products as soon as he begins to make an order, this does not accord with the fact, under the demand of Poisson process the lead time influence the vendor's stock cost and transportation cost, at the same time the vendor should consider the customer'wait cost by the stock out in the lead time. Through the analysis of the integrated policy, the minimal total cost and corresponding integrated policy with different lead time is computed by the designed algorithm, then the influence of the integrated policy and total cost by the vendor lead time is analyzed, through computation study it shows that the shipment consolidation policy under the mode of the vendor managed inventory outperforms the immediate delivery policy, at last the system model parameter sensitivity is discussed.Based on the integrated policy of quantity-based shipment consolidation and stock replenishment this thesis further studies the integrated policy of time-based shipment consolidation and stock replenishment, firstly the characteristic of time-based shipment consolidation policies is discussed, then the system total cost model based on the integrated policy of time-based shipment consolidation and stock replenishment is developed.Through the analysis of the integrated policy of time-based shipment consolidation and stock replenishment, the algorithm is designed to solve the minimal total cost and corresponding integrated policy with different lead time. The advantage of two shipment consolidation policy is discussed by comparing the two shipment consolidation policy and then the cause is discussed, the same conclusion can be drawn with the lead time and without the lead time, at last the system parameter sensitivity of the integrated policy of time-based shipment consolidation and stock replenishment is discussed.