Chapter 14.2. “Strategic production-distribution models: A critical review with emphasis on global supply chain models” Vidal, C.J. and Goetschalckx, M., “A critical review with emphasis on global supply chain models”, European Journal of Operation Research 98 (1997). 1-18. 1. Introduction 1.1. Motivation/ Objectives The analysis and design of production-distribution systems has been an active area of research for many years. Most research addresses a single component of the overall production-distribution system, such as purchasing, production and scheduling, inventory, warehousing, or transportation. To date there exists little research that addresses the integration of such single components into the overall supply chain. Another body of research considers the problem of coordinating two different functions of the system. Thomas and Griffin (1995) classify this body of research into three categories: buyer-vendor coordination, production-distribution coordination, and inventory-distribution coordination. Bhatnagar et al . (1993) present a similar classification, and distinguish between the integration of different functions, such as the production-distribution integration mentioned above, and the coordination of different levels of the company within the same function, such as the production planning of several assembly plants that are vertically integrated. 1.2. Definition Planning level에 따른 분류 및 정의 Three levels of planning can be distinguished depending on the time horizon, namely, strategic, tactical, and operational (see Ballou, 1992, pp. 32-35). The strategic level considers time horizons of more than one year, and requires approximate and aggregated data. The operational level involves short-term decisions, often less than an hour or a day, and requires transactional data. The tactical level falls in between those two extremes with respect to the time horizon, and the amount and accuracy of data required. As an illustration, consider the transportation function of the system. The selection of a transportation mode, the determination of the fleet size, and the dispatching of different vehicles, are examples of strategic, tactical, and operational decisions, respectively Definition of Strategic production-distribution model. The most comprehensive strategic problem is the optimization of the complete supply chain. The strategic design of a supply chain requires managers to determine: · the number, location, capacity, and type of manufacturing plants and warehouses to use; · the set of suppliers to select; · the transportation channels to use; · the amount of raw materials and products to produce and ship among suppliers, plants, ware-houses, and customers; and · the amount of raw materials, intermediate products, and finished goods to hold at various locations in inventory. Many models have been formulated for the strategic design of supply chains. We will call these models 'strategic production-distribution' models. We will distinguish between domestic and international strategic production-distribution models. The former models consider strategic production-distribution decisions within a single country or a unified group of countries, e.g., the continental United States. On the other hand, the latter models include global features and consider multiple countries for selecting vendors, and locating plants and warehouses. 1.3. Literature review Aikens (1985) presents an early review of the main facility location models, starting from the least complex, such as the simple uncapacitated model, and progressing to the more complex, such as the single-echelon multicommodity capacitated model. In addition, the author indicates that the dynamic multicommodity problem and the consideration of side constraints are the most promising topics for future research. Bhatnagar et al. (1993) present a review on models for multi-plant coordination, defining two levels of coordination. The first one, called 'general coordination', addresses the problem of integrating decisions of different activities, such as facility location, production, and distribution. The second level of coordination deals with the problem of linking decisions within the same activity for different levels in the firm; this level is called 'multi-plant coordination'. The 'general coordination' problem is further classified into three main categories: coordination between supply and production activities, coordination between production and distribution activities, and coordination between inventory and distribution activities. Geoffrion and Powers (1995) analyze the evolution of logistics as a corporate function, computercommunications technology, algorithms, data development and management tools, model features and software capabilities, and how companies use software for distribution system design. Additionally, the authors present some data-related theoretical topics, such as the aggregation of products and customers, the linearization of functions that describe inventory Issues, and the effect of errors in transportation rates, as topics for future research Thomas and Grifnn (1995) present a comprehensive review of the literature related to the coordination of two or more of the main stages of the supply chain, that is, procurement, production, and distribution. This is accomplished by describing the main references related to buyer-vendor coordination, productiondistribution coordination , inventory-distribution coordination, and strategic planning. They also present some topics for further research, which include the modeling of nonlinear transportation costs, life cycle constraints, general international supply issues, third-parties in international problems, and the determination of interface points within the supply chain. Slats et al. (1995) present a description of the characteristics of a logistic chain with emphasis on recent technological developments, such as Electronic Data Interchange (EDI) and Product Data Interchange (PDI), which improve processes and facilitate the flow of information along the supply chain. They claim that Operations Research (OR) and Management Science (MS) techniques such as mathematical programming, heuristics, and simulation, are essential to support the redesign of logistic processes. Verter and Dincer (1995) discuss the facility location decisions of multinational organizations. They highlight the need for coordination among all international entities of global companies in order to improve competitiveness. After discussing the planning process and coordination of a firm's global manufacturing strategy, Verter and Dincer present a literature review on the production-distribution system design problem. In the first survey, the authors conclude that most of the existing methods focus on the optimization of location and allocation decisions, ignoring important structural aspects such as capacity and technology content of manufacturing facilities. In the second survey, specific references related to international strategic production-distribution models are identified. 1.4. Complexities/ Difficulty to Solve Global supply chain models are more complex and difficult to solve than domestic models. In an international scenario, the flow of cash and the flow of information are more important and difficult to coordinate than they are In a single-country environment. The inclusion of different taxes and duties, differential exchange rates, trade barriers, transfer prices, and duty drawbacks is fundamental for a model to more accurately represent the real system. In addition, sources of uncertainty and qualitative factors, such as government stability and general infrastructure of a particular country, are critical issues for the strategic design of a global supply chain. These factors should be considered when de-signing any global supply chain, but they are very difficult to include in mathematical models. This article describes the existing models for global supply chain design, highlighting their contributions and limitations. 2. Optimization models 2.1. MIP Models Main characteristics of selected MIP strategic production-distribution models. Model characteristics [1] [2] [3] [4] [5] [6] [7] [8] Stochastic features Material requirements Material supply lead times Demand of products and parts X X** Transportation times Exchange rates Reliability of vendors Dynamic characteristics Static model(single-period) X X X X Multi-period model X X X X Dynamic demand only Status of facilities Fixed number, status, and location X X of facilities (flow optimization) Fixed location of facilities X X X X X X Xa X X X X X X X X (discrete location) Continuous facility location Capacities Production capacity at plants Supply capacity of vendors X X X X X X Capacity of transportation channels Distribution Center(DC) capacities X X Manufacturing line capacity Xb Other capacities Multicommodity model Single sourcing X X X X X X Customers from DC X Customers per commodity Xc X Xd DC per commodity Manufacturing plants from vendors Objective function Minimization of costs X X X Xf Maximization of profits X Xg Xe X X Multiobjective function Nonlinear costs considered Nonlinear DC costs X* X* X Concave production costs X Nonlinear transportation costs Nonlinear purchasing costs Number of echelons 1 1 0 1 0 0 N/C X X X X X X X 2h Fixed costs included Fixed production costs Fixed facility costs X X Vendor fixed costs X X X Fixed transportation channel costs Manufacturing line costs X Side constraints included Bill of materials Bounds on the No. of open facilities X X* X* X X X Xi Other side constraints X Xo Customer service features Customer demand satisfaction X Maximum time (distance) to X* X X X X X X X X Serve customers % of orders satisfied from the shelf X Inventory characteristics Pipeline inventory Cyclic inventory in facilities X* Xj X X X Safety stock: • Proportional to throughput • Stochastic (level of service) International features X Taxes and duties X Offset requirements X X X X Local content X X Methods of solution applied Benders decomposition X X Decomposition with goal constraint X A variant of Benders decomposition X Xk Factorization A heuristic method X Commercial MIP solver X LP solution by fixing X binary variables 0-1 variables and CPU time Maximum number of 0-1 variables 513 N/C N/C N/C N/C 60 l 887 Maximum number of constraints 8441 N/C 19841 3000 N/C N/C 6000 2297 191 m 58 n 593 Max. computational time (CPU sec) 64 N/C N/C Reference considered in this table: [1] Geoffrion and Graves (1974); [2] Geoffrion et al. (1978); [3] Brown et al. (1987); [4] Cohen and Lee (1989), (5) Cohen et at. (1989); (6) Cohen and Moon (1991); [71 ArntEen et at. (1995); [8] Cole (1995). * Optlonal (may be included in the model, according to the authors) ** Although this aspect is addressed by the authors, it is not explicitly included in the model. N/C Not clear from the paper (either the author(5) does (do) not present it, or the information Is very limited) or not applicable for the specific case. a b The model uses binary variables for vendor selection and plant opening. Multiproduct capacity constraints and upper bounds on products produced on each production facility (for example, ovens and packing lines) at each plant. c Single sourcing of customers per commodity and distribution channel. d Single sourcing of warehouses per commodity and distribution channel. e Savinfs from duty drawbacks are considered in the objective function. f Profitab31ity analysis may be included, but it does not influence the optimal solution. e Maximization of after-tax profit in all the countries. h At most two love]s of warehousing are allowed. i A production facility (ovens and packing lines) can be assigned only to one production plant. j Pipeline Inventories are implicitly considered in the costs included in the objective function. k Although factorization seems to be applied, the real method of solution is not clearly described. l According to the authors, the problems they solve have ‘a few hundred’ of binary variables. m The authors say that solutions require “about half an hour and the equivalent of several megabytes of main storage on an IBM 370/168”. n According to the authors, their methods of solution suffice to solve the problems “In a minute or so on a personal computer or workstation to within an integrality gap of 0.01 percent or better” o Configuration constraints are Included to represent piecewise-linear concave production costs. Geoffrion and Graves(1974) present an algorithm based on Benders De-composition to solve a multicommodity single-period production-distribution problem. The model represents a productiondistribution system with several plants with known capacities, distribution centers, and a number of customer zones; discrete candidate locations are considered for opening DC. Fixed and variable (linear) costs for DC, production costs, and linear transportation costs are included in the objective function. The constraints considered in the model are capacity at plants, customer demand satisfaction, single sourcing by customer zone, bounds on the throughput at DC, and linear configuration constraints on binary variables (logical constraints). Geoffrion et at. (1978) present a status report in strategic distribution system planning based on docomposition techniques. These model has single sourcing of customer zone by commodity, nonlinear facility throughput constraints, and tradeoffs between distribution and customer service. Cohen and Lee (1985) present a complete analysis related to the development of strategies to improve general manufacturing operations. The structure of the network consists of raw material vendors, intermediate and final product plants, DC, and customer zones. These model is a deterministic, periodic, mathematical program to minimize costs with a nonlinear objective function that extends the model by Geoffrion and Graves (1974) and it additionally considers opening and closing of plants, attendant production planning decisions, inbound raw material, and intermediate product flows. The method of solution is a heuristic embedded in a larger computerized manufacturing planning sup-port system (MPSS). Nodder and Dincer (1986) presented an international plant location model with financial capabilities. These formulation considers exchange rate fluctuations, market prices, international interest rates, and fixed costs in a stochastic environment. Production costs, transportation costs, import tariffs, and export taxes are implicitly included in the calculation of net revenue. The objective function is obtained as the difference between the expected value of after-tax profit and the product of a risk a version factor with the variance of profit. The constraints include the capacity at plants, an upper bound on market demands, financial constraints, and bounds on decision variables. But it does not consider multiple products and transportation channels, suppliers, multi-echelon distribution centers, inter-facility transshipments, and inventory costs. Brown et al. (1987) present a MIP multicommodity model that determines the opening/closing of plants, the commodities produced at each plant anddelivered to each customer, and the assignment of equipment (represented by facilities) to plants at NABISCO. Variable production and shipping costs, fixed costs of equipment assignment (facilities to plants), and fixed costs of plant operations are included in the objective function. The constraints considered in the model are customer demand satisfaction, balance constraints, multi-product capacity constraints, maximum number of facilities(equipment) assigned to each plant, single sourcing of facilities to plants, and upper bounds on products produced on each equipment at each plant. Cohen et al. (1989) present the main features that differentiate an international supply chain model from a single-country model. The most important characteristics are the necessity of treating multinational firms as global systems to obtain economies of scale in order to reduce raw material and production costs; the existence of duties, tariffs, and differential tax rates among countries; random fluctuation of currency exchange rates; and the existence of constraints not considered in single-country models, such as local content rules. To consider these characteristics, the authors formulate a normative model that is a dynamic, nonlinear MIP model. The main contributions of this model are the explicit inclusion of vendor supply contracts, which leads to the consideration of fixed vendor costs to opening contracts, and the inclusion of local content constraints. Cohen and Lee (1989) present a single-period multlcommodity model that analyzes resource deployment decisions for an international firm. it is a deterministic formulation that considers the maximization of after tax profit in all countries. while including variable production and purchasing costs, fixed setup and warehousing costs, transportation costs, and fixed vendor costs. Within the constraints, the model considers plant production capacity, material requirements at each plant (major components and subassemblies), balance constraints at plants and DC, demand limits, feasible flow constraints (system configuration constraints), capacity of suppliers, and offset requirements. Cohen and Moon (1991) present a mixed integer multicommodity model to find inbound raw material flows, assignment of product lines and specification of production volumes, and outbound finished product flows in a production-distribution network. Here, the location of facilities (vendors, plants, and DC) is given and fixed. The main contribution of this re-search is to provide an algorithm to solve some production-distribution models with piecewise linear concave costs of production. However, the model is very restricted because the plant loading problem under consideration assumes a fixed facility network configuration, with the DC as the final demand points. Cohen and Kleindorfer (1993) extend the work by Cohen et at. (1989) by describing a normative model framework for the operations of a global company Location, capacity, product mix, material flow, and cash flow features are decisions included in the model. The model framework consists of a Master Problem, which is a multi-period stochastic program; a single-period stochastic program subproblem; and a set of submodels that interact with both programs, such as a stochastic supply chain network model, a financial flow model, a stochastic exchange rate model, and a price/demand model. Brown and 01son (1994) present a new mathematical framework for dynamic row factorization with three algorithms for three different row structures: generalized upper bound rows, pure network rows, and generalized network rows. A common key aspect of these factorization methods is the identification of special structures in the bases of the LP. These structures are very common in supply chain models, and the authors apply their method in two of them. The first, ODS, is a multicommodity capacitated singleperiod model similar to the problem presented by Geoffrion and Graves (1974); here, the application of factorization to the Master Problems leads to very few Benders cuts. The second problem is DEC, presented in Arntzen et al. (1995), which is discussed below. The application of factorization to these supply chain models appears very promising for future developments in this area. Goetschalckx et al. (1994, 1995) present a generic model for the strategic design of productiondistribution systems, including visual capabilities. They present a generic production-distribution model of which certain features can be ignored depending on the application. Arntzen et at. (1995) present a multi-period, multicommodity mixed integer model to optimize a global supply chain. Its objective function includes variable production, inventory, and shipping costs; fixed production and production 'style' costs, and savings from credit earned for reexporting products. The constraints include customer demand satisfaction, balance of materials, global Bill of Materials(BOM), throughput capacity at each facility, production capacity for each facility per production style, system configuration constraints, and bounds on decision variables. Offset trade and local content, duty drawback, and duty relief are the international constraints included in the model. The main contribution of this model is the inclusion of offset trade, local content, and duty considerations in an international supply chain model that also includes BOM constraints. Cole (1995) develops a Strategic Inventory Location Allocation System (SILAS), in which he presents a multicommodity, multi-echelon, single-period mixed integer model for optimizing a strategic productiondistribution system. The main contribution of Cole's work is the consideration of normal demands and stochastic customer service by carrying safety stock. together with warehouse location, customer allocation, and channel selection. This model is applicable when safety stock costs are significant. The objective function includes plant fixed and closing costs, plant production and inventory costs, trunking transportation and inventory costs, fixed operating and closing costs of depots, variable ware-house costs, and total average warehousing inventory costs. The constraints include plant production capacity, logical constraints on channels and ware-houses, channels capacity, customer demand satisfaction, single sourcing of customers by distribution channel and product, limits on the distance and time from a warehouse to serve a customer, maximum on-hand inventory at warehouses, warehouse storage and handling capacities per warehouse type, balance constraints at warehouses, warehouse single sourcing by distribution channel and product, a set of constraints describing demand variances at warehouses, a set of linearized safety stock constraints, and bounds on decision variables. Vidal and Goetschalckx (1996) present the first attempt to include supplier reliability in the formulation of a strategic production-distribution model. Their formulation represents a zero-echelon system, and includes deterministic exchange rates, material flow linkage constraints, and a set of linearized suppliers' reliability constraints. These constraints assure that the probability of being on time of all suppliers shipping to each plant is at least a specified target value. 2.2. Other optimization approaches Burns et al. (1985) present an analytic approach to optimize freight distribution by truck from a supplier to many customers, considering transportation and inventory costs. The analytical formulas may be used to approximate distribution costs, requiring less data in comparison to traditional network modeling approaches Cohen and Lee (1988) develop a set of stochastic submodels including the optimization of material control operations, a serial production process, finished goods stockpile, and a distribution process. These submodels are linked and related to each other using a set of variables. Under some assumptions, the authors are able to optimize each submodel independently. 3. Additional issues for modeling In the design of a global supply chain model, it is strictly necessary to aggregate the suppliers, customers, and products into some kind of 'zones' for suppliers and customers, and into groups for products. Geoffrion (1977) presents a description of how to obtain an adequate method of aggregation of suppliers, when the analyst wants to gather a subset of items in a logistic planning model. Geoffrion and Powers (1980) present the main reasons for the need of optimization capability when solving comprehensive distribution planning problems. It allows the analyst to evaluate different' what-if' question that must be considered when studying the flexibility of the model. Among these what-if questions, we have changes in demands and costs, closing of facilities and transportation channels, plant capacity expansions, introduction of anew product or elimination of one, inventory policy comparisons, tradeoffs between customer service and costs, and changes in echelon structures. Lee and Billington (1992) present a qualitative discussion concerning the pitfalls and opportunities of managing supply chain inventory. The main pitfalls that can influence model formulation for a global supply chain are the inadequate performance measures for the complete supply chain, the inadequate definition of customer service, the exclusion of uncertainties. poor coordination, the consideration of incomplete methods of shipment, the incorrect treatment of inventory costs, and the separation of supply chain design from operational decisions. Ballou (1994) states the importance of the approach that analysts adopt when aggregating customers in location studies. He presents two major sources of error: the estimation of the total transportation cost to the cluster rather than to each customer individually, and the allocation of customers to facilities based on clustered demand instead of individual demands. The main goal of Ballou's research is to determine the influence of the number of clusters, their size, and the number of source points, on the total error obtained in location models for the continental United States. 4. Review on Cases Studies/ Applications Kogut (1985) discusses the importance of flexibility in global companies as a response to fluctuations in exchange rates, changes in government policies, and complexities in competitive moves. Lessard and Lightstone (1986), and Carter and Victory (1989)present a more detailed analysis on exchange rate fluctuations, describing the risks of the operating exposure of multinational companies, and the possible methods for hedging and managing this expo-sure. Ohmae (1989) discusses the basic characteristics of strategic alliances as fundamental tools for succeeding in a highly competitive global market. Fagan (1991), and Monczka and Trent (1991) pre-sent the benefits of global sourcing and discuss some of its disadvantages. In contrast, Davis (1992) discusses the inconveniences and possible negative consequences for companies that source globally. Fawcett (1992) highlights the importance of into-grated, strategic logistics to provide a coordinating mechanism for the efficient management of global manufacturing strategies. Min et at. (1994) present a generic stepwise approach based upon multiple criteria for selection of suppliers in an international purchasing scenario. Klassen and Whybark (1994) describe a study to determine and rank key barriers for the effective management of manufacturing operations under an international environment. Among the highest ranked barriers, the authors found the management's lack of global view, international manufacturing strategy, the management of global logistics, and cultural and linguistic differences among countries. As part of the integrative analyses, Goldsborough (1992) presents a complete analysis on global logistics management, establishing comparisons between domestic and international logistics, and describing basic factors of global information systems and Decision Support Systems (DSS). Bartmess and Corny(1993) illustrate the fundamental differences between location decisions based upon the traditional approach that overemphasizes direct labor costs, and location decisions based on the core competencies of the company. By presenting similar ideas, Maccormack et al. (1994) highlight the importance of making international location decisions based on key qualitative factors, such as adequate infrastructure and managerial issues, instead of doing it based only on cost factors, whose advantages are usually transitory. The authors present a four-phase decision making process for location decisions. 5. Further Research Issues There exist many research opportunities for developing more comprehensive global supply chain models that include BOM constraints, more stochastic factors, and qualitative aspects that are very important within a global environment. 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