Tore Grünert

Planning models for long-haul operations of postal and express shipment companies

Abstract Postal and express shipment companies face the task of providing service between a number of fixed locations. The increasing competition and service orientation of this market sets new standards for planning. One must meet the required service level without letting costs get out of control. This paper identifies important planning tasks which have to be addressed at these companies and defines corresponding optimization models. The relationship between these models and their use within a decision support system are highlighted as well as future research challenges stemming from this application area.

Local Search for Vehicle Routing and Scheduling Problems: Review and Conceptual Integration

Local search and local search-based metaheuristics are currently the only available methods for obtaining good solutions to large vehicle routing and scheduling problems. In this paper we provide a review of both classical and modern local search neighborhoods for this class of problems. The intention of this paper is not only to give an overview but to classify and analyze the structure of different neighborhoods. The analysis is based on a formal representation of VRSP solutions given by a unifying giant-tour model. We describe neighborhoods implicitly by a set of transformations called moves and show how moves can be decomposed further into partial moves. The search method has to compose these partial moves into a complete move in an efficient way. The goal is to find a local best neighbor and to reach a local optimum as quickly as possible. This can be achieved by search methods, which do not scan all neighbor solutions explicitly. Our analysis shows how the properties of the partial moves and the constraints of the VRSP influences the choice of an appropriate search technique.

Sequential search and its application to vehicle-routing problems

Local search is the most frequently used heuristic technique for solving combinatorial optimization problems. It is also the basis for modern metaheuristics, like, e.g., Tabu Search, and Variable Neighborhood Search. The paper introduces sequential search as a generic technique for the efficient exploration of local-search neighborhoods. One of its key concepts is the systematic decomposition of moves, which allows pruning within local search based on associated partial gains. The application of theoretical concepts to several well-known neighborhoods of the vehicle-routing problem (VRP) is demonstrated. Computational tests show substantial speedup factors, e.g., up to 10000 for the 3-opt* neighborhood. This underlines the superiority of sequential search over straightforward techniques in the VRP context.

A Hybrid Tabu Search/Branch-and-Bound Algorithm for the Direct Flight Network Design Problem

This paper introduces a network design problem with a structure that is encountered in many transportation processes. The general organization of these systems is as follows. Freight has to be transported between a large number of origins and destinations. To consolidate the freight, it is first shipped to a terminal. Next, it is transported directly to a terminal where it is re-loaded and shipped to its destination. The task is to decide which terminals have to be used and how the freight is transported among the terminals. We describe an application where the terminals are airports and the freight is letter mail. Here, the transportation between terminals is by air, which is required due to temporal constraints. The economical impact of these decisions is huge because air transportation is costly and the process is repeated every night. We show how the problem can be modeled as a capacitated warehouse location problem with side constraints and propose a hybrid Tabu Search/Branch-and-Bound algorithm, which solves large real-world instances with acceptable computation times.

The design of a letter-mail transportation network by intelligent techniques

Many transportation providers such as package delivery companies and postal service organizations face the problem of designing a transportation network in order to service their customers. This network must balance the requirement of customers delivery under tight time window constraints with the goal of low-cost operations of the fleet. Until recently this task was usually performed by planners without sufficient software aid. This paper describes a decision support system (DSS) which has been designed in order to assist planners of the German postal service, the Deutsche Post AG. It helps the planners in designing improved plans which can be generated either manually or with the help of sophisticated intelligent optimization techniques. Both optimization and system design factors, which influenced the design of the DSS are classified.

A Note on Single Alternating Cycle Neighborhoods for the TSP

This paper investigates two different local search approaches for the TSP. Both approaches are based on the general concept of single-alternating cycle neighborhoods. The first approach, stems from the famous heuristic suggested by Lin and Kernighan and the second is based on the notion of stem-and-cycles developed by Glover in the early nineties. We show that the corresponding neighborhoods are not identical and that only a subset of moves can be found when Lin & Kernighan’s gain criterion is applied.

Finding all k-cliques in k-partite graphs, an application in textile engineering

Abstract In many practical cases one has to choose an arrangement of different objects so that they are compatible. Whenever the compatibility of the objects can be checked by a pair-wise comparison the problem can be modelled using the graph–theoretic notion of cliques. We consider a special case of the problem where the objects can be grouped so that exactly one object in every group has to be chosen. This object has to be compatible to every other object selected from the other groups. The problem was motivated by a braiding application from textile technology. The task is to route a set of thread-spools (bobbins) on a machine from their origins to their destinations so that collisions are avoided. We present a new model and algorithm in order to solve this important practical problem. Scope and purpose An important contribution of Operations Research is to develop computer-based tools for assistance in decision making. In this paper we show how techniques from combinatorial optimization can assist textile engineers in finding a control of a newly developed braiding machine. We have developed new models and have employed shortest path algorithms (Ahuja RK, Magnanti TL, Orlin JB. Network flows, theory, algorithms, and applications, Englewood Cliffs, NJ: Prentice-Hall, 1993. p. 93–165), and a branch and bound algorithm for the maximum clique problem (Johnson DS, Trick MA, (editors). Cliques, coloring, and satisfiability, DIMACS Series in discrete mathematics and theoretical computer science, Providence, RI: American Mathematical Society, 1996.) to compute controls. The determination of controls is a highly complex task, which can take days or even weeks if performed manually. Hence, computer-assistance is a necessity in practice. Indeed, the tools we have developed are employed in practice and have increased productivity enormously.

Modelle zur taktischen Planung von Frachttransporten

Die technologischen und wirtschaftlichen Rahmenbedingungen der Transportplanung im Frachtbereich haben sich in den vergangenen Jahren stark verandert. Die Durchdringung der Unternehmen mit Computern und der Einsatz von Datenbanken ermoglichen eine genauere und umfangreichere Planung der Transportprozesse. Ein wichtiger Teil ist die taktische Planung, wo es darum geht, vorhandene Standorte und Verkehrstrager mittelfristig so einzusetzen, das einerseits die Kundenwunsche an Service und Zuverlassigkeit erfullt werden und andererseits die Kosten dafur nicht zu hoch werden. In diesem Beitrag werden die wichtigsten Klassifikationskriterien der taktischen Transportplanung sowie die quantitativen Modelle als Basis einer rechnerunterstutzten Planung vorgestellt.