Dock Assignment and Truck Scheduling Problems at Cross-docking Terminals

In this paper, we consider the integration of dock assignment and truck scheduling problem at cross-docking terminals. The problem is first formulated as a 0-1 integer programming model. Since both dock assignment and truck scheduling problems are NP-hard, its integration is more difficult to solve. Thus we propose reduced variable neighborhood search (RVNS) algorithms to solve the problem. Computational experiments are carried out on four set of instances. The results show that RVNS is capable of finding good solutions in a much shorter computation time when it is compared with optimization solver Gurobi’s solutions

Application of Tabu Search to Scheduling Trucks in Multiple Doors Cross-Docking Systems

RÉSUMÉ : Cette recherche focus sur l’amélioration des cross-dockings en vue d’augmenter les niveaux de performance du service et de réduire les coûts. l’algorithme de la recherche avec tabous est étudiée pour trouver la séquence optimale d’entrée et sortie des remorques au cross-docking. L’objectif de cette recherche est de maximiser le nombre total de transferts directs entre le fournisseur et une destination finale commune de livraison. Dans les stratégies de distribution actuelles, l’objectif est de synchroniser les chaines du fabricant et du client. Le cross-docking implique de recevoir les produits d’un fournisseur pour plusieurs clients et d’occasionnellement consolider cela avec les produits d’autres fournisseurs pour des destinations finales de livraison communes. En résumé, l’approche examinée dans cette recherche donne une occasion significative pour l’amélioration des opérations au Cross-docking par la réduction du stockage des produits.----------ABSTRACT : Today’s supply chain management performance has been affected by continuously increasing pressure of market forces. The pressure of market includes demands on increased flow of products and throughput with less amount of storage, also customers demand for more products with lower operational costs and more value-added services provided to customers. Supply chain is responsible in cost reduction and service levels increase by providing transshipments across its members. However supply chain has to face fluctuations of demands with the short available lead times. Physical problem of warehouse limitations and also inventory costs and shipping affect the performance of supply chain. In today’s distribution strategies, the main goal is to provide a synchronization of customer chains and the suppliers. The objective is to reduce the inventory buffering between customers and suppliers. The idea of cross-docking is to receive different goods from a manufacturer for several end destinations and possibly consolidate the goods with other manufacturer’s items for common final customers; then ship them in the earliest possible time. The focus of this research effort is to improve cross-dock operations with the goal of increasing the service performance levels and reducing costs. Specifically, metaheuristics algorithm of Tabu search is investigated for finding optimal sequence of incoming and outgoing trailers at cross-docks. This thesis reviews available research literature on cross-dock operations. Tabu search for the truck scheduling problem is presented along with results. Tabu search algorithm is investigated for the truck scheduling problem in the multiple doors cross-docking with unknown incoming and outgoing sequences. The objective of this research is to maximize the total direct transfer of products from a supplier to common final delivery destinations. The algorithm is implemented in C++ and analyzed using different problem instances. The results gained from algorithm of Tabu search are compared with other iterative heuristic descent method. The results indicate that the Tabu search performs significantly better than the descent method for large problem instances. In general, the results present that a metaheuristic algorithm of Tabu search for multiple or single door cross-docking offers thelargest potential for improvement. In summary, the approach explored in this research offers significant opportunity to improve cross-dock operations through reducing storage of products

Truck scheduling problem in a cross-docking system with release time constraint

Abstract In a supply chain, cross-docking is one of the most innovative systems for ameliorating the operational performance at distribution centers. Cross-docking is a logistical strategy in which freight is unloaded from inbound trucks and (almost) directly loaded into outbound trucks, with little or no storage in between, thus no inventory remains at the distribution center. In this study, we consider the scheduling problem of inbound and outbound trucks with multiple dock doors, aiming at the minimization of the makespan. The considered scheduling problem determines where and when the trucks must be processed; also due to the interchangeability specification of products, product assignment is done simultaneously as well. Inbound trucks enter the system according to their release times', however, there is no mandatory time constraint for outbound truck presence at a designated stack door; they should just observe their relative docking sequences. Moreover, a loading sequence is determined for each of the outbound trucks. In this research, a mathematical model is derived to find the optimal solution. Since the problem under study is NP-hard, a simulated annealing algorithm is adapted to find the (near-) optimal solution, as the mathematical model will not be applicable to solve largescale real-world cases. Numerical examples have been done in order to specify the efficiency of the metaheuristic algorithm in comparison with the results obtained from solving the mathematical model

Scheduling Material Handling in Cross-Docking Terminals

RÉSUMÉ : La manutention au sein des plateformes de distribution est un problème d’ordonnancement. Le transport interne des produits doit en effet être synchronisé avec les arrivées et les départs des camions. Ce problème se retrouve dans toutes les plateformes de distribution où la manipulation des produits est effectuée manuellement par l’opérateur. Dans cette thèse, nous investiguons ce problème d’ordonnancement dans les plateformes de distribution. Nous mettons en relief les différentes facettes de ce problème et proposons une classification de ses différents sous problèmes. De manière générale, l'objectif est d'éviter les doubles manipulations (déplacer un produit d’un camion vers le stock, puis du stock vers un camion) qui doublent les coûts sans valeur ajoutée. Il faut minimiser ces doubles manipulations en orchestrant les transferts internes et la séquence de chargement/ déchargement des camions. Dans une première partie, nous analysons la structure du problème avec un modèle simplifié n’ayant qu’un quai de réception et un quai d’envois. Nous formalisons les décisions de manipulation interne et développons un algorithme optimal pour déterminer le meilleur plan de transfert de produits lorsque la séquence des camions est connue. Cet algorithme est utilisé comme fonction d’évaluation dans une recherche stochastique pour minimiser les doubles manipulations et optimisant les séquences de chargement/déchargement. Nous présentons ensuite un modèle de programmation linéaire en nombres entiers du problème général (ordonnancement des arrivées et départs de camions et transfert interne des produits). Nous proposons un algorithme de séparation et d’évaluation permettant une résolution efficace du problème. Nous proposons des structures de dominance et quelques inégalités valides permettant d’améliorer les performances de l’algorithme. Cette approche nous permet de résoudre à l’optimum en un temps raisonnable de très gros problèmes. Dans une seconde partie, nous étendons ces modèles au problème général avec plusieurs quais. Nous nous intéressons d’abord au terminal de type satellite où l’ordonnancement des camions d’entrée est connu. Ces plateformes opèrent en deux mouvements différents : l’ordonnancement et chargement pour le transport de nuit et celui pour les livraisons matinales. Nous donnons une représentation mathématique qui permet de résoudre les problèmes de petite taille. Pour ceux de plus grandes ampleurs, nous utilisons une heuristique. Les résultats numériques montrent la validité de cette approche. Finalement, nous généralisons le type de plateforme (les séquences d’arrivée et de départ sont à déterminer) et développons un nouveau modèle d’ordonnancement plus compact. Nous utilisons pour les grandes instances une recherche par voisinage. Nous mettons en place des voisinages originaux adaptés à ce type d’ordonnancement. Mots clés: Transfert de produits, ordonnancement, plateforme de transbordement, recherche stochastique, programmation à nombres entiers, heuristiques, recherche du plus proche voisin.----------ABSTRACT : Material handling in cross-dock is a relevant class of scheduling problems in distribution centers in which inner transhipment decisions need to be considered in addition to the processing order of trucks. The problem has applications in distribution centers where operators manually perform internal transhipment. In this dissertation, we investigate the problem of material handling inside cross-docking terminals. The main component of the problem is presented, followed by a classification scheme to express its diversity. Moreover, double handling identifies the main source of deficiencies in transferring operations. The objective is to synchronize the trucks’ loading and unloading sequences with internal transferring decisions to minimize excessive product displacement inside the terminal. First, the problem is studied for a conceptual model of the platform with single receiving and shipping doors. We formalize decisions on internal transhipment and develop an algorithm to determine the best transferring plan with restricted orders on processing trucks. This algorithm is employed as an evaluation function in a stochastic search framework to ameliorate the order of processing trucks and reduce the cost of double handling. Then, a mixed integer linear programming formulation of the general problem is introduced. The proposed model determines the joint schedule between processing order of trucks at inbound and outbound doors with an internal transhipment plan. A path branching algorithm is proposed. We present several structural properties and some valid inequalities to enhance the performance of the algorithm. This method could solve fairly large instances within a reasonable time. Second, we extend the developed models and approaches to schedule material handling process for a real platform with multiple doors. In the first installment, we focus on the satellite cross-docks that have limitations on the processing order of trucks at inbound door. These platforms operate in two separate shifts: consolidating pickup freight for overnight shipments and processing received products for early morning deliveries. A mathematical formulation of the problem is presented that can solve small instances with commercial software. In addition, a sequential priority-based heuristic is introduced to tackle the large problems. Numerical results depict the stability of this approach. Finally, in the second instalment, we study the general model with no restriction on the arrival and departure pattern of trucks and formulate a new mathematical model. This model has considerably fewer variables and constraints than the previous one. Moreover, a variable neighborhood search heuristic is developed to tackle real life problems. This method consists of several operators incorporated in a search subroutine to find local optima and a perturbation operator to alter it. The developed method is adopted for three scenarios concerning limitations imposed by the network schedule. The analyzes demonstrate economical savings in the cost of material handling. Keywords: Material handling; scheduling; cross-dock; stochastic search; Integer programming; heuristic; variable neighborhood search

Cross-Docking: A Proven LTL Technique to Help Suppliers Minimize Products\u27 Unit Costs Delivered to the Final Customers

This study aims at proposing a decision-support tool to reduce the total supply chain costs (TSCC) consisting of two separate and independent objective functions including total transportation costs (TTC) and total cross-docking operating cost (TCDC). The full-truckload (FT) transportation mode is assumed to handle supplier→customer product transportation; otherwise, a cross-docking terminal as an intermediate transshipment node is hired to handle the less-than-truckload (LTL) product transportation between the suppliers and customers. TTC model helps minimize the total transportation costs by maximization of the number of FT transportation and reduction of the total number of LTL. TCDC model tries to minimize total operating costs within a cross-docking terminal. Both sub-objective functions are formulated as binary mathematical programming models. The first objective function is a binary-linear programming model, and the second one is a binary-quadratic assignment problem (QAP) model. QAP is an NP-hard problem, and therefore, besides a complement enumeration method using ILOG CPLEX software, the Tabu search (TS) algorithm with four diversification methods is employed to solve larger size problems. The efficiency of the model is examined from two perspectives by comparing the output of two scenarios including; i.e., 1) when cross-docking is included in the supply chain and 2) when it is excluded. The first perspective is to compare the two scenarios’ outcomes from the total supply chain costs standpoint, and the second perspective is the comparison of the scenarios’ outcomes from the total supply chain costs standpoint. By addressing a numerical example, the results confirm that the present of cross-docking within a supply chain can significantly reduce total supply chain costs and total transportation costs

Horizontale en verticale samenwerking in distributieketens met cross-docks

Logistiek dienstverleners staan voor grote uitdagingen op het gebied van duurzaamheid, in het bijzonder vanwege de steeds kleiner wordende zendingen die just-in-time bij de klant moeten worden afgeleverd. Samenwerking tussen partners in de distributieketen en met concurrenten daarbuiten biedt kansen om deze uitdagingen het hoofd te bieden. Dit proefschrift richt zich op samenwerkingsvormen in distributieketens met cross-docks. Cross-docks zijn logistieke centra die bedrijven in staat stellen om kleine zendingen gegroepeerd te transporteren zonder dat daarvoor tussentijdse opslag nodig is. In een cross-dock worden goederen direct van inkomende naar uitgaande vrachtwagens verplaatst. Het succesvol toepassen van cross-docking vereist verticale samenwerking tussen partners in opeenvolgende stadia van de distributieketen. Horizontale samenwerking ontstaat tussen mogelijk concurrerende bedrijven die vergelijkbare activiteiten in verschillende distributieketens uitvoeren. Dit proefschrift presenteert theoretische modellen voor horizontale en verticale samenwerking in distributieketens met cross-docks en bestudeert oplossingsmethodieken waarmee de duurzaamheid van deze ketens kan worden verbeterd. Daarvoor worden concepten uit de vakgebieden informatiesystemen, Operations Research en Supply Chain Management gecombineerd. De in dit proefschrift beschreven classificatie van wiskundige cross-docking modellen onthult nieuwe onderzoeksvragen gericht op een betere afstemming tussen interne cross-dock processen en ketenlogistiek. Een simulatiestudie illustreert hoe geringe aanpassingen in de ketenlogistiek tot grote prestatieverbeteringen in het cross-dock leiden. Op het gebied van horizontale samenwerking is een methode ontwikkeld die de uitwisseling van ladingen tussen transporteurs systematiseerd. Een reeks casussen toont aan dat doorbraken in ICT ontwikkeling nodig zijn om samenwerkende transporteurs in staat te stellen gezamenlijk planningsbeslissingen te nemen

Cross-dock Scheduling with Known Shipment Unloading Order

Cross-docking is a logistics strategy which is widely used these days in different industries. Cross-docking takes place in a distribution docking centre and consists of trucks and dock doors on inbound and outbound sides.Products from suppliers get unloaded at inbound doors from incoming trucks, consolidated, transferred and loaded into outgoing trucks at outbound doors, with little or no storing them in between. We study two scenarios of cross-docking scheduling problem: scheduling inbound side with fixed outbound side scheduling and scheduling both inbound and outbound sides. In the former scenario, we introduce five mixed integer programming models with enhanced pre-processing and extensions to minimize the total number of tardy products. In the later scenario, we proposed new linear mixed integer programming models where transportation time between dock doors are considered. The objective in the second case is to minimize the maximum lateness of outgoing trucks. In both scenarios, we integrate the unloading order of shipments in incoming trucks into our models. Computational results show that taking advantage of that information helps improving the truck scheduling and assessing much more accurately the number of tardy products and lateness