Operations research models and methods for safety stock determination: A review

In supply chain inventory management it is generally accepted that safety stocks are a suitable strategy to deal with demand and supply uncertainty aiming to prevent inventory stock-outs. Safety stocks have been the subject of intensive research, typically covering the problems of dimensioning, positioning, managing and placement. Here, we narrow the scope of the discussion to the safety stock dimensioning problem, consisting in determining the proper safety stock level for each product. This paper reports the results of a recent in-depth systematic literature review (SLR) of operations research (OR) models and methods for dimensioning safety stocks. To the best of our knowledge, this is the first systematic review of the application of OR-based approaches to investigate this problem. A set of 95 papers published from 1977 to 2019 has been reviewed to identify the type of model being employed, as well as the modeling techniques and main performance criteria used. At the end, we highlight current literature gaps and discuss potential research directions and trends that may help to guide researchers and practitioners interested in the development of new OR-based approaches for safety stock determination.This work has been supported by FCT – Fundação para a Ciência e Tecnologia within the R&D Units Project Scope: UIDB/00319/2020, and by the European Structural and Investment Funds in the FEDER component, through the Operational Competitiveness and Internationalization Program (COMPETE 2020) [Project no. 39479, Funding reference: POCI-01-0247-FEDER-39479]

Inventory control and internal logistics operations management in the automotive industry : Application to Faurecia

Cette thèse traite de diverses problématiques liées à la gestion des opérations dans l'industrie automobile. Plus particulièrement, les modèles développés cherchent à améliorer la gestion des stocks et des opérations de logistique interne chez Faurecia, un équipementier automobile. Dans un premier temps, à l’aide d’un modèle de simulation à événements discrets, nous déterminons les niveaux de stocks nominaux optimaux et les dates de lancement de production optimales de produits finis dans les usines d’assemblage, de type longue distance. L'impact de l’utilisation de l'information avancée sur la demande en termes de réduction de coûts de stock et de pénalités de retard est évalué. Ensuite, nous proposons un modèle analytique approximatif pour optimiser les niveaux de stock de sécurité de composants qui minimisent les coûts de stock et des livraisons exceptionnelles.Le modèle proposé est appliqué dans une usine de type courte distance et présente des réductions de coût intéressantes par rapport au modèle de calcul utilisé en pratique. Avant de développer ce nouveau modèle, nous déterminons la loi de probabilité de la demande relative aux composants et nous proposons une méthode générale d'analyse de la demande de composants dans les systèmes d’assemblage à la commande. La dernière partie de la thèse traite de la gestion des opérations de cross-docking interne (dans l’usine) en comparant différentes politiques de cross-docking couramment utilisées en pratique. Ce travail permet de comparer les politiques (en termes de coût de stockage et de main d’œuvre) et d’identifier les contextes dans lesquels il est intéressant d’utiliser chacune d’elles. Plusieurs extensions et perspectives de recherche sont proposées à la fin de la thèse.This thesis addresses new research questions related to operations management in the automotive industry. More particularly, we aim at improving inventory control and internal logistics operations management in Faurecia, an auto parts maker. First, by using a discrete event simulation model, we calculate the optimal base stocks levels and optimal release lead times of finished goods in long-distance plants. The impact of using advance demand information on inventory holding and backorder penalty costs reduction is evaluated. Then, we propose an approximate analytical model to optimize components safety stock that minimizes inventory holding and rush ordering costs.The proposed model is applied in a short-distance plant and shows interesting cost reductions compared to the currently used calculation model. Before developing this new model, we determine the probability distribution of components demand and propose a general method of components demand analysis in Assemble-to-Order systems. We also study the management of internal (plant) cross-docking operations by comparing different cross-docking policies commonly used in practice. The cost related to each policy is assessed in terms of surface and man-hours. Several extensions and research perspectives are proposed at the end of the manuscript

Components Inventory Control in an Assemble-to-Order System with Rush Ordering

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The use of internal cross-docking in just-in-time plants

International audienceWe study the use of internal cross-docking in Just-in-Time plants. The considered plants consist of a cross-docking zone, a picking zone and assembly lines. The cross-docking zone is used to receive, store (temporarily) and ship pallets. Pallets are then broken into cases and stored in the picking zone which supplies the assembly lines. Cross-docking is used instead of mass storage in order to accelerate the physical flows of pallets. In this paper, we introduce different cross-docking policies (set of rules which specify how to store pallets) commonly used in practice. A cross-docking cost evaluation model is developed in order to compare the cross-docking policies considered in terms of required labour and surface costs

Evaluation of the Impact of Uncertain Advance Demand Information on Production/Inventory Systems

International audienceIn this paper, we conduct a simulation study to evaluate the impact of using imperfect advance demand information (ADI) in a single stage production/inventory system. We mean by ADI the fact that the production system receives customer orders in advance of their due-dates. We consider four types of ADI: 1) Perfect ADI, 2) ADI with imperfect due-dates, 3) ADI with imperfect demand quantities and 4) ADI with updates. The production system is controlled by a modified base-stock policy which has two parameters that are S, the base-stock level, and L, the production release lead time. We intend to shed light on the impact of imperfect ADI (ADI types 2, 3, 4) on the performance of production/inventory systems. The aim of the simulation study is twofold. Firstly, the objective is to find the optimal parameters of the modified base-stock policy and secondly, to evaluate the benefits of imperfect ADI in a production/inventory system

A comparison of different cross-docking organizations in a JIT manufacturing system; application in the automotive industry

International audienceCross-docking is a logistics strategy used to accelerate the flows and to reduce inventory levels. In this paper, we consider the case of cross-docking centers applied in JIT manufacturing systems. We make a comparison between different cross-docking positioning policies: per supplier, per time slot, per mixed small/big time slots and per mixed supplier/time slot. We intend to shed light on the impact of different characteristics of the system like the supplier arrival frequency and the capacity of cross-docks on the performance of each cross-docking policy. The case study of Faurecia, an automotive parts suppliers, is used to consolidate the insights of our paper. We find that it is better to use the mixed cross-docking because it permits to minimize the used surface and to facilitate the realized movements

The use of rush deliveries in periodic review assemble-to-order systems

International audienceWe calculate optimal safety stock in a periodic review (T,S) Assemble-to-Order system having multiple components and multiple finished goods. Customer orders for finished goods arrive according to independent Poisson processes, and cannot be neither backlogged nor lost. In case of potential component stock-out, the studied system uses rush deliveries from suppliers. For this setting, approximate expressions of the optimal safety stock that minimize the sum of inventory holding and rush ordering costs are developed. Exact optimal safety stocks are calculated using discrete event simulation, and compared numerically to the approximate expressions. The model is applied to a first-tier automotive supplier and yields to a significant reduction in terms of inventory holding and rush ordering costs. A sensitivity analysis on relevant system parameters such as components demand, assembly coefficients and unit rush ordering cost is conducted