http://repub.eur.nl/res/aut/2453/ List of Publications en http://repub.eur.nl/static-eur/img/logo.png http://repub.eur.nl http://repub.eur.nl/res/pub/37684/ 2012-02-01T00:00:00Z Sequencing unit-load retrieval requests has been extensively reported on in the literature for conventional single-deep automated warehousing systems. A proper sequence can greatly reduce the makespan when carrying out a group of such requests. Although the sequencing problem is NP-hard, some very good heuristics exist. Surprisingly, the problem has not yet been investigated for compact (multi-deep) storage systems, which have greatly increased in popularity the last decade. This article studies how to sequence a group (or block) of storage and retrieval requests in a multi-deep automated storage system with the objective to minimize the makespan. Currently utilized sequencing heuristics for the multi-deep system are adapted in this article and in addition a new heuristic, Percentage Priority to Retrievals with Shortest Leg (PPR-SL), is proposed and evaluated. It is shown that the PPR-SL heuristic consistently outperforms all of the other heuristics. Generally, it can outperform the benchmark First-Come First-Served (FCFS) heuristic by between 20 and 70%. The nearest neighbor heuristic that performs very well in conventional single-deep storage systems appears to perform poorly in the multi-deep system, even worse than FCFS. In addition, based on FCFS and PPR-SL, robust rack dimensions that yield a short makespan, regardless of the number of storage and retrieval requests, are found. http://repub.eur.nl/res/pub/37690/ 2012-02-01T00:00:00Z In this study we consider a pick-and-sort order picking system, in which batches of orders are picked simultaneously from different (work) zones by a group of order pickers. After picking, the orders are transported by a conveyor to a next station for consolidation and packing. Packing can only occur when an order has been picked completely. For a given number of workers, each assigned to a single zone, a larger number of zones reduces pick time (since travel time reduces), but increases waiting time for completion at the packing stations, because more partial batches needing assembly arrive at the packing stations. Our aim is to determine the optimal number of zones such that the total (picking and packing) time to complete a batch is minimised. We solve this problem by optimally assigning items to pick routes in each zone. We illustrate the method with data taken from a distribution centre of one of the largest online retailers in The Netherlands. http://repub.eur.nl/res/pub/37198/ 2011-10-01T00:00:00Z This paper provides an overview of stochastic research in warehouse operations. We identify uncertainty sources of warehousing systems and systematically present typical warehouse operations from a stochastic system viewpoint. Stochastic modeling methods and analysis techniques in existing literature are summarized, along with current research limitations. Through a comparison between potential and existing stochastic warehouse applications, we identify potential new research applications. Furthermore, by comparing potential and existing solution methods, methodological directions relevant to practice and largely unexplored in warehouse literature are identified. http://repub.eur.nl/res/pub/25835/ 2011-07-22T00:00:00Z The present research investigates antecedents of safety performance in warehouses. Specifically, we study what factors influence the number of accidents that have occurred in the past three and a half years in 78 Dutch warehouses. Based on prior research in (behavioral) operations management, safety management, and organizational behavior, we identify hazard reducing systems (HRS), safety-specific transformational leadership (SSTL), and safety consciousness (SC) as potential predictors of safety performance. Path analysis on data from a survey among 78 warehouse managers and 1033 warehouse employees shows, in line with prior research, that HRS is a strong predictor of safety performance. Importantly, our results also suggest that SSTL may be an even more important predictor of safety performance than HRS. SSTL affects safety performance directly (contrary to our expectations SC does not mediate this relationship) and strongly predicts safety performance even after controlling for the effects of HRS. SSTL also mediates some of the effects of HRS on safety performance. Subsequently, we propose that leaders are critical in fostering safety on the work floor. http://repub.eur.nl/res/pub/21769/ 2011-05-16T00:00:00Z The problem considered is that of forecasting demand for single-period products before the period starts. We study this problem for the case of a mail order apparel company that needs to order its products pre-season. The lack of historical demand data implies that other sources of data are needed. Advance order data can be obtained by allowing a selected group of customers to pre-order at a discount from a preview catalogue. Judgments can be obtained from purchase managers or other company experts. In this paper, we compare several existing and new forecasting methods for both sources of data. The methods are generic and can be used in any single-period problem in the apparel or fashion industries. Among the pre-order based methods, a novel 'top-flop' approach provides promising results. For a small group of products from the case company, expert judgment methods perform better than the methods based on advance demand information. The comparative results are obviously restricted to the specific case study, and additional testing is required to determine whether they are valid in general. http://repub.eur.nl/res/pub/22722/ 2011-02-17T00:00:00Z Sequencing unit load retrieval requests has been studied extensively in literature for conventional single-deep automated warehousing systems. A proper sequence can greatly reduce the makespan when carrying out a group of such requests. Although the sequencing problem is NP-hard some very good heuristics exist. Surprisingly the problem has not yet been investigated for compact (multi-deep) storage systems, which have greatly increased in popularity the last decade. This paper studies how to sequence a group (or block) of storage and retrieval requests in a multi-deep automated storage system with the objective to minimize the makespan. We adapt well-known sequencing heuristics for the multi-deep system, and propose and evaluate a new heuristic: percentage priority to retrievals with shortest leg (PPR-SL). Our results show the PPR-SL heuristic consistently outperforms all the other heuristics. Generally, it can outperform the benchmark first-come first-served (FCFS) heuristic by 20-70%. The nearest neighbor (NN) heuristic that performs very well in conventional single-deep storage systems, appears to perform poorly in the multi-deep system; even worse than FCFS. In addition, based on FCFS and PPR-SL, we find robust rack dimensions yielding a short makespan, regardless of the number of storage and retrieval requests. http://repub.eur.nl/res/pub/39944/ 2011-01-01T00:00:00Z In the summer of 2009, soon after the winners of the annual Safest Warehouse of the Year Awards were being lauded at an industry conference, journalist Marcel te Lindert wondered out loud in his regular column for the Dutch magazine Logistiek, why it was that there were more questions raised about safety issues than there were answers. http://repub.eur.nl/res/pub/20308/ 2010-02-01T00:00:00Z is paper studies the performance of static and dynamic scheduling approaches in vehicle-based internal transport (VBIT) systems and is one of the first to systematically investigate under which circumstances, which scheduling method helps in improving performance. In practice, usually myopic dispatching heuristics are used, often using look-ahead information. We argue more advanced scheduling methods can help, depending on circumstances. We introduce three basic scheduling approaches (insertion, combined and column generation) for the static problem. We then extend these to a dynamic, real-time setting with rolling horizons. We propose two further real-time scheduling approaches: dynamic assignment with and without look-ahead. The performances of the above five scheduling approaches are compared with two of the best performing look-ahead dispatching rules known from the literature. The performance of the various approaches depends on the facility layout and work distribution. However, column generation, the combined heuristic, and the assignment approach with look-ahead consistently outperform dispatching rules. Column generation can require substantial calculation time but delivers very good performance if sufficient look-ahead information is available. For large scale systems, the combined heuristic and the dynamic assignment approach with look ahead are recommended and have acceptable calculation times. http://repub.eur.nl/res/pub/19574/ 2010-01-01T00:00:00Z This paper develops a novel framework to evaluate the integral performance of order picking systems with different combinations of storage and order picking policies. The warehousing literature on order picking mostly considers minimizing either elapsed time or distance as the sole objective, whereas warehouse managers in a supply chain have to look beyond single-dimensional performance and consider trade-offs among different criteria. Thus managers still need a unified and efficient framework to select a portfolio of appropriate order picking policies from a multi-criteria and contextual perspective. Our framework-combining data envelopment analysis, ranking and selection, and multiple comparisons-provides an efficient methodology to simultaneously analyze several interrelated problems in order picking systems with multiple performance attributes, such as service levels and operational costs. We demonstrate our approach through comprehensive evaluations of order picking policies in three low-level, picker-to-parts rectangular warehouses facing demand variations. http://repub.eur.nl/res/pub/16898/ 2009-10-07T00:00:00Z In the past thirty years the full turnover-based storage policy as described by Hausman et al. (1976, Management Science 22(6)) has been widely claimed to outperform the commonly used ABC class-based storage policy, in terms of the resulting average storage and retrieval machine travel time. In practice however, ABC storage is the dominant policy. Hausman et al. (1976) model the turnover-based policy under the unrealistic assumption of shared storage, i.e. the storage space allocated to one product can only accommodate its average inventory level; no specific space is reserved to store the maximum inventory of a product. It appears that many authors citing Hausman et al.’s results overlook this assumption and use the resulting storage and retrieval machine travel times as if it were valid for full turnover-based storage. Full turnover-based storage is a dedicated storage policy where the storage space allocated to one product must accommodate its maximum inventory level. This paper adapts the travel time model of Hausman et al. to accommodate full turnover-based dedicated storage. Surprisingly, the result of the adapted travel time model is opposite to that of Hausman et al. (1976) but, in line with practice, it supports that ABC (2- or 3-) class-based storage normally outperforms full turnover-based storage. http://repub.eur.nl/res/pub/13935/ 2009-10-01T00:00:00Z This paper proposes an approximation model based on queuing network theory to analyze the impact of order batching and picking area zoning on the mean order throughput time in a pick-and-pass order picking system. The model includes the sorting process needed to sort the batch again by order. Service times at pick zones are assumed to follow general distributions. The first and second moments of service times at zones and the visiting probability of a batch of orders to a pick zone are derived. Based on this information, the mean throughput time of an arbitrary order in the order picking system is obtained. Results from a real application and simulation show that this approximation model provides acceptable accuracy for practical purposes. Furthermore, the proposed method is simple and fast and can be easily applied in the design and selection process of order picking systems. http://repub.eur.nl/res/pub/17231/ 2009-10-01T00:00:00Z Given the increased pressure on short delivery lead times, minimizing customer order throughput times is an important objective in warehousing operations. Warehouse managers can influence the responsiveness of their system through a number of controls, such as the order batching policy, the capacity of the picking and sorting operations, and the picking policy used (pick-and-sort versus sort-while-pick). This paper studies the impact of these controls on the average customer order throughput time in a numerical way, for warehouses with time window batching and separate picking and sorting functions. http://repub.eur.nl/res/pub/20974/ 2009-05-01T00:00:00Z Time-access regulations and vehicle restrictions are increasingly used, especially in western Europe, to improve social sustainability in urban areas. These regulations considerably affect the distribution process of retail chain organizations as well as the environmental burden. This paper studies the impact of governmental time windows, vehicle restrictions, and different retailers' logistical concepts on the financial and environmental performance of retailers. We use a case study with two cases that differ in their drop sizes as input for an experiment. The retailers provided all organizational, flow, and cost data of the distribution process between their distribution centers and their stores. We use these data to calculate the impacts of different scenarios on the retailers' financial and environmental performances based on a fractional factorial design in which urban policies and the retailers' logistical concepts are varied, using vehicle routing software. We test the propositions with a third case. We show that the cost impact of time windows is the largest for retailers who combine many deliveries in one vehicle round-trip. The cost increase due to vehicle restrictions is the largest for retailers whose round-trip lengths are restricted by vehicle capacity. Vehicle restrictions and time windows together do not increase a retailer's cost more than individually. Variations in delivery volume and store dispersion hardly influence the impact of urban policy and the retailer's logistical concept decisions. http://repub.eur.nl/res/pub/14615/ 2009-01-30T00:00:00Z A warehouse needs to have sufficient open locations to be able to store incoming shipments of various sizes. In combination with ongoing load retrievals open locations gradually spread over the storage area. Unfavorable positions of open locations negatively impact the average load retrieval times. This paper presents a new method to manage these open locations such that the average system travel time for processing a block of storage and retrieval jobs in an automated warehousing system is minimized. We introduce the effective storage area (ESA), a well-defined part of the locations closest to the depot; where only a part of the open locations –the effective open locations-, together with all the products, are stored. We determine the optimal number of effective open locations and the ESA boundary minimizing the average travel time. Using the ESA policy, the travel time of a pair of storage and retrieval jobs can be reduced by more than 10% on average. Its performance depends hardly on the number or the sequence of retrievals. In fact, in case of only one retrieval, applying the policy leads already to beneficial results. Application is also easy; the ESA size can be changed dynamically during storage and retrieval operations. http://repub.eur.nl/res/pub/15031/ 2009-01-20T00:00:00Z Compact, multi-deep three-dimensional (3D), Automated Storage and Retrieval Systems (AS/RS) are becoming more common, due to new technologies, lower investment costs, time efficiency and compact size. Decision-making research on these systems is still in its infancy. This paper studies a particular compact system with rotating conveyors for the depth movement and a Storage/Retrieval (S/R) machine for the horizontal and vertical movement of unit loads. The optimal storage zone boundaries are determined for this system with two product classes: high- and low-turnover, by minimizing the expected S/R machine travel time. We formulate a mixed-integer non-linear programming model to determine the zone boundaries. A decomposition algorithm and a one-dimensional search scheme are developed to solve the model. The algorithm is complex, but the results are appealing since most of them are in closed-form and easy to apply to optimally layout the 3D AS/RS rack. The results show that the S/R machine travel time is significantly influenced by the zone dimensions, zone sizes and ABC curve skewness (presenting turnover patterns of different products). The presented results are compared with those under random storage and it is shown that significant reductions of the machine travel time are obtainable by using class-based storage. http://repub.eur.nl/res/pub/20982/ 2009-01-01T00:00:00Z Compact, multi-deep (3D) automated storage and retrieval systems (AS/RS) are becoming increasingly popular for storing products. We study such a system where a storage and retrieval (S/R) machine takes care of movements in the horizontal and vertical directions of the rack, and an orthogonal conveying mechanism takes care of the depth movement. An important question is how to layout such systems under different storage policies to minimize the expected cycle time. We derive the expected single-command cycle time under the full-turnover-based storage policy and propose a model to determine the optimal rack dimensions by minimizing this cycle time. We simplify the model, and analytically determine optimal rack dimensions for any given rack capacity and ABC curve skewness. A significant cycle time reduction can be obtained compared with the random storage policy. We illustrate the findings of the study by applying them in a practical example http://repub.eur.nl/res/pub/15149/ 2008-09-11T00:00:00Z One of the challenging questions that online retailers are currently facing is how to organize the logistic fulfillment processes both during and after a transaction has taken place. As new information technologies become available that allow picking information to be conveyed in real time and with the ongoing need to create greater responsiveness to customers, there is increasing interest in applying dynamic picking in the warehouses of online retailers. In a Dynamic Picking System (DPS), a worker picks orders that arrive in real time during the picking operations and the picking information can dynamically change in a picking cycle. Models to describe and analyze such systems via stochastic polling theory are presented and closed-form expressions for the order line waiting times in a DPS are derived. These analytical results are verified by simulation. It is shown that the application of polling-based picking can generally lead to shorter order throughput times and higher on-time service completion ratios than traditional batch picking systems using optimal batch sizes. It is demonstrated that the proposed analysis method can be applied to minimize warehouse cost and improve service. http://repub.eur.nl/res/pub/15150/ 2008-09-11T00:00:00Z An approximation method based on G/G/m queuing network modeling using Whitt's queuing network analyzer to analyze pick-and-pass order picking systems is presented. The objective of this approximation method is to provide an instrument for obtaining rapid performance estimates (such as order lead time and station utilization) of an order picking system. The pick-and-pass system is decomposed into conveyor segments and pick stations. Conveyor segments have a constant processing time, whereas the service times at a pick station depend on the number of order lines in the order to be picked at the station, the storage policy at the station and the work practices. The proposed approximation method appears to be sufficiently accurate for practical purposes. It can be used to rapidly evaluate the effects of the storage methods in pick stations, the number of order pickers at stations, the size of pick stations, the arrival process of customer orders and the impact of batching and splitting orders on system performance. http://repub.eur.nl/res/pub/15968/ 2008-09-01T00:00:00Z At Bloemenveiling Aalsmeer (VBA), about 19 million flowers have to be distributed daily to customers in a building of 1 million m 2 within a few hours. With the increasing daily number of customer orders, the congestion in the main distribution area increases. As a consequence, the makespan exceeds the available time, and flowers arrive too late at the customers. This paper investigates the concept of zoning in the distribution process, where distributors work in teams for a fixed group of customers in a specific zone of the distribution area. Customers are assigned to zones to balance workload. We show by simulation that this way of organizing the distribution process reduces congestion and leads to considerable improvements in both makespan and transaction lead time. http://repub.eur.nl/res/pub/11878/ 2008-03-01T00:00:00Z In this paper, we consider a newly designed compact three-dimensional automated storage and retrieval system (AS/RS). The system consists of an automated crane taking care of the pallets' movements in the horizontal and vertical direction. A gravity or powered conveying mechanism takes care of the pallets' depth movement in the rack. Our research objective is to analyze the system performance and optimally dimension the system. For single-command cycles, the crane's expected retrieval travel time is the same for gravity and powered conveyors; we give a closed-form expression. From the expected travel time, we calculate the optimal ratio between three dimensions that minimizes the travel time for a random storage strategy. In addition, we derive an approximate travel time expression for dual command cycles for the system with powered and gravity conveyors, respectively, and use it to optimize the system dimensions. Finally, we illustrate the findings of the study by a practical example. http://repub.eur.nl/res/pub/10733/ 2007-12-03T00:00:00Z In this paper, we discuss an approximation method based on G/G/m queuing network modeling using Whitt’s (1983) queuing network analyzer to analyze pick-and-pass order picking systems. The objective of this approximation method is to provide an instrument for obtaining rapid performance estimates (such as order lead time and station utilization) of the order picking system. The pick-and-pass system is decomposed into conveyor pieces and pick stations. Conveyor pieces have a constant processing time, whereas the service times at a pick station depend on the number of order lines in the order to be picked at the station, the storage policy at the station, and the working methods. Our approximation method appears to be sufficiently accurate for practical purposes. It can be used to rapidly evaluate the effects of the storage methods in pick stations, the number of order pickers at stations, the size of pick stations, the arrival process of customer orders, and the impact of batching and splitting orders on system performance. http://repub.eur.nl/res/pub/11907/ 2007-11-01T00:00:00Z Local governments in Western Europe increasingly use city time-access regulations to improve social sustainability. These regulations significantly influence the distribution process of retail chain organizations. This paper studies the impact of governmental time-window pressure on retailers’ logistical concepts and the consequential financial and environmental distribution performance. We determine which dimensions in the retailer's logistical concept determine its cost and emission sensitivity to increasing time-window pressure. Our research is based on a multiple case study of fourteen Dutch retail cases in different sectors and with different store formulas. The retailers provided all organizational, flow and cost data of their secondary distribution (between distribution center and stores). We use these data to calculate the impacts of different time-window pressure scenarios, including the current situation, using vehicle routing software. It appears that cost and emissions increases are moderate, when few cities are affected. However, as more cities are affected, costs and emissions increase considerably, particularly if time-window lengths become shorter. Time-windows harmonized between cities lead to fewer negative effects. We find various dimensions that contribute to reducing a retailer's sensitivity to time-window pressure. We formulate conclusions hypothesizing the links between time-window pressure, its effects, and the dimensions that determine these effects. http://repub.eur.nl/res/pub/11877/ 2007-10-16T00:00:00Z Order picking has long been identified as the most labour-intensive and costly activity for almost every warehouse; the cost of order picking is estimated to be as much as 55% of the total warehouse operating expense. Any underperformance in order picking can lead to unsatisfactory service and high operational cost for the warehouse, and consequently for the whole supply chain. In order to operate efficiently, the order-picking process needs to be robustly designed and optimally controlled. This paper gives a literature overview on typical decision problems in design and control of manual order-picking processes. We focus on optimal (internal) layout design, storage assignment methods, routing methods, order batching and zoning. The research in this area has grown rapidly recently. Still, combinations of the above areas have hardly been explored. Order-picking system developments in practice lead to promising new research directions. http://repub.eur.nl/res/pub/10180/ 2007-05-10T00:00:00Z Compact, multi-deep (3D), Automated Storage and Retrieval Systems (AS/RS) are becoming more common, due to new technologies, lower investment costs, time efficiency and compact size. Decision-making research on these systems is still in its infancy. We study a particular compact system with rotating conveyors for the depth movement and a Storage/Retrieval (S/R) machine for the horizontal and vertical movement of unit loads. We determine the optimal storage zone boundaries for such systems with two product classes: high and low turnover, by minimizing the expected Storage/Retrieval (S/R) machine travel time. We propose a mixed-integer nonlinear programming model to determine the zone boundaries. A decomposition algorithm and a one dimensional search scheme are developed to solve the model. The algorithm is complex, but the results are appealing since most of them are in closed-form and easy to apply to optimally layout the 3D AS/RS rack. The results are compared with those under random storage, and show that a significant reduction of the machine travel time can be obtained. Finally, a practical example is studied to demonstrate the use and validate our findings. http://repub.eur.nl/res/pub/11876/ 2007-01-01T00:00:00Z Order batching problem (OBP) is the problem of determining the number of orders to be picked together in one picking tour. Although various objectives may arise in practice, minimizing the average throughput time of a random order is a common concern. In this paper, we consider the OBP for a 2-block rectangular warehouse with the assumptions that orders arrive according to a Poisson process and the method used for routing the order-pickers is the well-known S-shape heuristic. We first elaborate on the first and second moment of the order-picker’s travel time. Then we use these moments to estimate the average throughput time of a random order. This enables us to estimate the optimal picking batch size. Results from simulation show that the method provides a high accuracy level. Furthermore, the method is rather simple and can be easily applied in practice. http://repub.eur.nl/res/pub/8129/ 2006-11-28T00:00:00Z This paper studies the performance of static and real-time scheduling approaches in vehicle-based internal transport (VBIT) systems, which can be found in manufacturing and warehouse facilities. We propose three heuristic approaches for static VBIT problems (insertion, combined and column generation), extend them to a dynamic, real-time setting and compare their performance over a rolling time horizon. This time horizon can be seen either as a fixed-time interval in which advance information about loads’ arrivals is available, or as a fixed number of loads which are known to become available in the near future. We also propose two dynamic assignment approaches: with and without look-ahead, respectively. Performance (primarily average load waiting time) of the above five dynamic scheduling approaches is compared with two nearest-vehicle-first rules (with and without look-ahead), which are the best vehicle dispatching rules known from literature and which are commonly used in practice. Experimental results show that, if sufficient prior information is available, our dynamic scheduling approaches consistently outperform vehicle dispatching rules. Results also reveal that guide-path layout, load arrival rate and variance, and amount of load pre-arrival information have strong impacts on the performance of vehicle control approaches. Column generation or the combined heuristics are recommended in small or medium-scale VBIT systems, whereas for large scale VBIT systems, both the combined heuristic and the dynamic assignment approach with look ahead perform best. http://repub.eur.nl/res/pub/11887/ 2006-11-01T00:00:00Z Factory gate pricing (FGP) is a relatively new phenomenon in retail distribution. Under FGP, products are no longer delivered at the retailer distribution center, but collected by the retailer at the factory gates of the suppliers. Owing to both the asymmetry in the distribution networks (the supplier sites greatly outnumber the retailer distribution centers) and the better inventory and transport coordination mechanisms, this is likely to result in high cost savings. A mathematical model was used to analyze the benefits of FGP. The main contribution of this paper is its practical approach to transport consolidation in this recently emerging supply chain concept in retail distribution. Extensive numerical results for a large real-life case study of the Dutch retail distribution are presented to show the effect of the orchestration shift from supplier to retailer, the improved coordination mechanisms, and sector-wide cooperation. http://repub.eur.nl/res/pub/8020/ 2006-10-09T00:00:00Z Local authorities increasingly use time-access regulations to improve social sustainability issues, such as the attractiveness of a city centre, the shopping climate, or to reduce the nuisance caused by urban freight transport. However, these time-windows increase delivery costs and the environmental burden. This paper evaluates five different time-window schemes on their social, environmental, and economic impacts. The first scheme examines the current time-window policy scheme. In the second scheme time-windows are harmonized between different cities. The third scheme moves all deliveries to the night. The fourth and fifth schemes evaluate the consequences of the proposal by the Dutch committee for urban distribution (committee Sakkers). The fourth scheme includes noise-legislation for delivering during the night, the fifth does not. This research includes interviews with several Dutch policy-making officials and is further based on a multiple-case study of fourteen large retail chains in different sectors and with different formulas. The results show that the current time-window scheme performs worst. The best time-window scheme would be a combination of the proposal of the committee Sakkers and the harmonization scenario. http://repub.eur.nl/res/pub/7839/ 2006-06-23T00:00:00Z In this paper, we consider a newly-designed compact three-dimensional automated storage and retrieval system (AS/RS). The system consists of an automated crane taking care of movements in the horizontal and vertical direction. A gravity conveying mechanism takes care of the depth movement. Our research objective is to analyze the system performance and optimally dimension of the system. We estimate the crane’s expected travel time for single-command cycles. From the expected travel time, we calculate the optimal ratio between three dimensions that minimizes the travel time for a random storage strategy. In addition, we derive an approximate closed-form travel time expression for dual command cycles. Finally, we illustrate the findings of the study by a practical example. http://repub.eur.nl/res/pub/7831/ 2006-06-19T00:00:00Z Compact, multi-deep (3D) automated storage and retrieval systems (AS/RS) are becoming increasingly popular for storing products with relatively low turnover on a compact area. An automated storage/retrieval crane takes care of movements in the horizontal and vertical direction in the rack, and a gravity conveying mechanism takes care of the depth movement. An important question is how to layout such systems to minimize the product storage and retrieval times. Although much attention has been paid to 2D AS/RS, multi-deep systems have hardly been studied. This paper studies the impact of system layout on crane travel time. We calculate the rack dimensions that minimize single-command cycle time under the full-turnover-based storage policy. We prove the expected travel time is minimized when the rack is square-in-time in horizontal and vertical directions and the conveyor’s dimension is the longest. We compare the model’s results with the performance of the random storage policy and show a significant crane travel time reduction can be obtained. We illustrate the findings of the study by applying them in a practical example. http://repub.eur.nl/res/pub/11875/ 2006-05-16T00:00:00Z This paper presents a review on design and control of automated guided vehicle systems. We address most key related issues including guide-path design, determining vehicle requirements, vehicle scheduling, idle-vehicle positioning, battery management, vehicle routing and deadlock resolution. We discuss and classify important models and results from key publications in literature, including often-neglected areas, such as idle-vehicle positioning and battery management. In addition, we propose a new dispatching-rule classification, a framework for design and control of automated guided vehicle systems, and suggest some fruitful research directions. http://repub.eur.nl/res/pub/7330/ 2006-02-06T00:00:00Z Japanese companies value service and quality highly and they put much effort in realising this. However, survey research carried out in 2001 among senior managers of Japanese logistics companies in the Netherlands, indicated that these efforts do not result in significant performance differences compared to western companies. In this exploratory paper, we report of company visits and interviews with managers of Japanese logistics companies in Western Europe. They described a clash of cultures underlying their operations, prohibiting them from achieving performance excellence. The causes focus around two key factors: the unique concept of Japanese service, based on future rewards which are absent in Western Europe; different employment circumstances in Western Europe, which make Japanese human resource management ineffective, and the Japanese career development system which makes that Japanese managers do not always have the right focus in their job abroad. We conclude that Japanese subsidiaries in Western Europe should keep on nourishing their unequalled service standards, while simultaneously adapting to efficiency standards of Western business practices in order to become more successful in the future. http://repub.eur.nl/res/pub/7322/ 2006-02-01T00:00:00Z Order picking has long been identified as the most labour-intensive and costly activity for almost every warehouse; the cost of order picking is estimated to be as much as 55% of the total warehouse operating expense. Any underperformance in order picking can lead to unsatisfactory service and high operational cost for its warehouse, and consequently for the whole supply chain. In order to operate efficiently, the orderpicking process needs to be robustly designed and optimally controlled. This paper gives a literature overview on typical decision problems in design and control of manual order-picking processes. We focus on optimal (internal) layout design, storage assignment methods, routing methods, order batching and zoning. The research in this area has grown rapidly recently. Still, combinations of the above areas have hardly been explored. Order-picking system developments in practice lead to promising new research directions. http://repub.eur.nl/res/pub/11906/ 2006-01-01T00:00:00Z Urban freight transport has many sustainable aspects. It contributes to economic vitality and the competitiveness of a region. However, the less socially and environmentally friendly effects such as noise, pollutant emissions, and nuisance, are usually the central theme in designing urban freight transport policies. Restricting policies as time access restrictions and vehicle restrictions gain popularity among Dutch local authorities. More than half of the municipalities uses time access restrictions. In spite of the popularity of these policy measures, the effects on the distribution of retailers, the environment and the transport costs are not known yet. In this paper we present three case studies of food retailers, in order to examine the effects of time access restrictions and vehicle weight restrictions. We discuss the impacts on the transport costs and the distribution processes (on the retailer’s side) and the environmental impacts. To find these effects we design five scenarios, in which we vary the length of time access restrictions, the allowed weight of the weight restriction and the number of cities in which these regulations are used. In these scenarios we adapt, based on the likely reaction of the involved food retailers, the distribution activities to fit the regulations. The results show that the vehicle weight restriction as well as the time access restrictions have a negative impact on the retailers’ transport costs. In most cases we see that the policy measures also cause an increase in the pollutant CO2 emissions that are emitted during the distribution of the goods. The time access restrictions cause a cost-increase that develops more or less convexly, as more stores are affected by this policy measure. The turning point is around 45% of the stores affected. The weight restriction causes linear cost-increase as more stores are affected. Both policy measures affect especially the roundtrips that combine multiple less than truckload orders for different stores. The policy measures have a different effect on different retailers. http://repub.eur.nl/res/pub/7090/ 2005-11-16T00:00:00Z Local governments in Western Europe increasingly use city time-access regulations to improve social sustainibility. These regulations significantly influence the distribution process of retail chain organizations. This paper studies the impact of governmental timewindow pressure on retailers’ logistical concept and consequential financial and environmental distribution performance. We determine which dimensions in the retailer’s logistical concept determine its cost and emission sensitivity to increasing time-window pressure. Our research is based on a multiple case study of fourteen Dutch retail cases in different sectors and with different store formulas. The retailers provided all organizational, flow and cost data of their secondary distribution (between distribution center and stores). We use these data to calculate the impacts of different time-window pressure scenarios, including the current situation, using vehicle routing software. It appears that cost increases are moderate, when few cities are affected. However, as more cities are affected, costs increase considerably, particularly if time-window lengths become shorter. Time-windows harmonized between cities, lead to less negative effects. We find various dimensions that contribute to reducing the retailer’s sensitivity to timewindow pressure. We formulate conclusions hypothesizing the links between timewindow pressure, its effects, and the dimensions that determine these effects. http://repub.eur.nl/res/pub/11888/ 2005-09-18T00:00:00Z We study the case of a catalogue/internet mail order retailer selling style goods and receiving large numbers of commercial returns. Returned products arriving before the end of the selling season can be resold if there is sufficient demand. A single order is placed before the season starts. Excess inventory at the end of the season is salvaged and all demands not met directly are lost. Since little historical information is available, it is impossible to determine the shape of the distribution of demand. Therefore, we analyze the distribution-free newsboy problem with returns, in which only the mean and variance of demand are assumed to be known. We derive a simple closed-form expression for the distribution-free order quantity, which we compare to the optimal order quantities when gross demand is assumed to be normal, lognormal or uniform. We find that the distribution-free order rule performs well when the coefficient of variation (CV) is at most 0.5, but is far from optimal when the CV is large. http://repub.eur.nl/res/pub/11874/ 2005-09-01T00:00:00Z Order picking has been considered as the most critical operation in warehousing. Recent trends in logistics demand faster but more reliable order picking systems. The efficiency of an order picking process greatly depends on the storage policy used, i.e. where products are located within the warehouse. In this paper, we deal with the most popular storage policy that is class-based (or ABC) storage strategy. Particularly, we investigate the problem of determining the optimal storage boundaries (zones) of classes in each aisle for manually operated warehouses. We first propose a probabilistic model that enables us to estimate the average travel distance of a picking tour. We found that the differences between results obtained from simulation and the model were slight. Using the average travel distance as the objective function, we present a mathematical formulation for the storage zone optimization problem. However, the exact approach can handle only small size warehouse instances. To circumvent this obstacle, we propose a heuristic for the problem. Numerical examples we conducted show that the heuristic performs very well in all the cases http://repub.eur.nl/res/pub/11886/ 2005-08-29T00:00:00Z Purpose - This paper aims to report a survey carried out among European distribution centres (EDCs) in The Netherlands. EDCs are forerunners in implementing advanced logistics systems, since they are responsible for the distribution of a manufacturer's products to customers in a larger part of Europe, Middle East and Africa, often with strict service level agreements. Design/methodology/approach - Warehouses with different outsourcing relations (own-account, dedicated outsourced and public outsourced) are compared on operational aspects, performance, and future developments. Moreover, within the group of own-account warehouses, especially Asian and American operations are tested for differences. Findings - Outsourced warehouse operations appear to have a higher percentage of error-free deliveries and to be more flexible than own-account operations. Based on the literature it was expected that a difference in productivity and quality between American and Asian EDCs would be found. Some differences in operations are found. For example, Asian EDCs put more effort in quality methods and involve more native managers. However, significant differences in productivity and quality levels were not found. Apparently, the differences in operations do not have a substantial effect on the warehouse performance. Originality/value - This paper is a first attempt to compare advanced warehouse operations from different origins, either operated by service providers or not. http://repub.eur.nl/res/pub/6730/ 2005-06-28T00:00:00Z In this paper, we consider a newly-designed automated storage and retrieval system (AS/RS). The system consists of an automated crane taking care of movements in the horizontal and vertical direction. A gravity conveying mechanism takes care of the depth movement. The aim of the research was to facilitate the problem of optimal design and performance evaluation of the system. We estimate the crane’s expected travel time for single command cycles. From the expected travel time, we calculate the optimal ratio between three dimensions that minimize the travel time for a random storage strategy. Finally, we illustrate the findings of the study by a practical example. http://repub.eur.nl/res/pub/6555/ 2005-05-18T00:00:00Z In this study we consider a pick-to-pack orderpicking system, in which batches of orders are picked simultaneously from different(work) zones by a group of order pickers. After picking, the orders are transported by a conveyor to the next station for packing. Our aim is to determine the optimal number of zones such that the overall (picking and packing) time to finish a batch is minimized. We solve this problem by optimally assigning items to pick routes in each zone. We illustrate the method with data taken from a distribution center of one of the largest online retailers in the Netherlands. http://repub.eur.nl/res/pub/11873/ 2005-05-01T00:00:00Z Products can be transported in containers from one port to another. At a container terminal these containers are transshipped from one mode of transportation to another. Cranes remove containers from a ship and put them at a certain time (i.e., release time) into a buffer area with limited capacity. A vehicle lifts a container from the buffer area before the buffer area is full (i.e., in due time) and transports the container from the buffer area to the storage area. At the storage area the container is placed in another buffer area. The advantage of using these buffer areas is the resultant decoupling of the unloading and transportation processes. We study the case in which each container has a time window [release time, due time] in which the transportation should start. The objective is to minimize the vehicle fleet size such that the transportation of each container starts within its time window. No literature has been found studying this relevant problem. We have developed an integer linear programming model to solve the problem of determining vehicle requirements under time-window constraints. We use simulation to validate the estimates of the vehicle fleet size by the analytical model. We test the ability of the model under various conditions. From these numerical experiments we conclude that the results of the analytical model are close to the results of the simulation model. Furthermore, we conclude that the analytical model performs well in the context of a container terminal. http://repub.eur.nl/res/pub/11872/ 2005-04-15T00:00:00Z On-line vehicle dispatching rules are widely used in many facilities such as warehouses and manufacturing facilities to control vehicles' movements. Single-attribute dispatching rules, which dispatch vehicles based on only one parameter, are usually used. However, multi-attribute dispatching rules prove to be better in general. In this paper, we study the impact of reassigning moving vehicles on some good dispatching rules, both single- and multi-attribute, in the literature. Results suggest that reassigning moving-to-park vehicles has a significant positive effect on reducing the average load waiting time. We evaluate the dispatching rules' performance using the experimental design of a real-life case study. The performance criteria are: minimizing the average load waiting time, keeping the maximum load waiting time as short as possible and utilizing better vehicles. The results show that the combined dispatching rules which integrates multi-attribute dispatching and vehicle reassignment yields the best performance overall. http://repub.eur.nl/res/pub/1809/ 2004-11-19T00:00:00Z Internal transport systems using automated guided vehicles (AGVs) are widely used in many facilities such as warehouses, distribution centers and transshipment terminals. Most AGV systems use online dispatching rules to control vehicle movements. In literature, there are many types of dispatching rules such as single- and multi-attribute dispatching rules. However, a dispatching rule that is good for all cases does not exist. In this research, we study a specific type of AGV environments which has not received much attention from researchers - AGV systems with many vehicles as can be seen in airport baggage handling systems. We propose two new multi-attribute dispatching rules for this type of environment and compare their performance with that of two of the best dispatching rules in literature. Using simulation we show that the new multi-attribute dispatching rules are robust and perform very well. http://repub.eur.nl/res/pub/1810/ 2004-11-19T00:00:00Z The order batching problem (OBP) is the problem of determining the number of orders to be picked together in one picking tour. Although various objectives may arise in practice, minimizing the average throughput time of a random order is a common concern. In this paper, we consider the OBP for a 2-block rectangular warehouse with the assumptions that orders arrive according to a Poisson process and the method used for routing the order-pickers is the well-known S-shape heuristic. We first elaborate on the first and second moment of the order-picker's travel time. Then we use these moments to estimate the average throughput time of a random order. This enables us to estimate the optimal picking batch size. Results from simulation show that the method provides a high accuracy level. Furthermore, the method is rather simple and can be easily applied in practice. http://repub.eur.nl/res/pub/1443/ 2004-08-06T00:00:00Z Factory Gate Pricing (FGP) is a relatively new phenomenon in retail distribution. Under FGP, products are no longer delivered at the retailer distribution center, but collected by the retailer at the factory gates of the suppliers. Owing to both the asymmetry in the distribution networks (the supplier sites greatly outnumber the retailer distribution centers) and the better inventory and transport coordination mechanisms, this is likely to result in high savings. A mathematical model was used to analyze the benefits of FGP for a case study in the Dutch retail sector. Extensive numerical results are presented to show the effect of the orchestration shift from supplier to retailer, the improved coordination mechanisms, and sector-wide cooperation. http://repub.eur.nl/res/pub/1452/ 2004-08-06T00:00:00Z In this paper, we study the problem of scheduling and dispatching vehicles in vehicle-based internal transport systems within warehouses and production facilities. We develop and use two rolling horizon policies to solve real-time vehicle scheduling problems. To solve static instances of scheduling problems, we propose two new heuristics: combined and column-generation heuristics. We solve a real-time scheduling problem by applying a heuristic to dynamically solve a series of static instances under a rolling horizon policy. A rolling horizon can be seen either as a fixed-time interval in which advance information about loads’ arrivals is available, or as a fixed number of loads which are known to become available in the near future. We also propose a new look-ahead dynamic assignment algorithm, a different dynamic vehicle-scheduling approach. We evaluate these dynamic scheduling strategies by comparing their performance with that of two of the best online vehicle dispatching rules mentioned in the literature. Experimental results show that the new look-ahead dynamic assignment algorithm and dynamic scheduling approaches consistently outperform vehicle dispatching rules. http://repub.eur.nl/res/pub/11871/ 2004-08-01T00:00:00Z Guided vehicles (GVs) are commonly used for the internal transportation of loads in warehouses, production plants and terminals. These guided vehicles can be routed with a variety of vehicle dispatching rules in an attempt to meet performance criteria such as minimizing the average load waiting times. In this research, we use simulation models of three companies to evaluate the performance of several real-time vehicle dispatching rules, in part described in the literature. It appears that there is a clear difference in average load waiting time between the different dispatching rules in the different environments. Simple rules, based on load and vehicle proximity (distance-based) perform best for all cases. The penalty for this is a relatively high maximum load waiting time. A distance-based rule with time truncation, giving more priority to loads that have to wait longer than a time threshold, appears to yield the best possible overall performance. A rule that particularly considers load-waiting time performs poor overall. We also show that using little pre-arrival information of loads leads to a significant improvement in the performance of the dispatching rules without changing their performance ranking. http://repub.eur.nl/res/pub/1323/ 2004-06-21T00:00:00Z This paper presents a review on design and control of automated guided vehicle systems. We address most key related issues including guide-path design, estimating the number of vehicles, vehicle scheduling, idle-vehicle positioning, battery management, vehicle routing, and conflict resolution. We discuss and classify important models and results from key publications in literature on automated guided vehicle systems, including often-neglected areas, such as idle-vehicle positioning and battery management. In addition, we propose a decision framework for design and implementation of automated guided vehicle systems, and suggest some fruitful research directions. http://repub.eur.nl/res/pub/1324/ 2004-06-21T00:00:00Z On-line vehicles dispatching rules are widely used in many facilities such as warehouses to control vehicles' movements. Single-attribute dispatching rules, which dispatch vehicles based on only one parameter, are used commonly. However, multi-attribute dispatching rules prove to be better in general. In this study, we introduce new dispatching rules and evaluate their performance compared to several good dispatching rules in literature, using the experimental design of a real case study. The performance criteria are minimizing the average load waiting time while keeping the maximum load waiting time as small as possible and better utilize vehicles. The experiments show that newly introduced hybrid dispatching rule yields the best performance overall. http://repub.eur.nl/res/pub/11870/ 2004-01-01T00:00:00Z Ankor is a wholesaler of tools and garden equipment, carrying such well-known brands as Skandia. Its warehouse is under continuous pressure to improve its efficiency while it is confronted with several specific requirements in its order picking, like the requirement to retrieve heavy products first to prevent damage to other, breakable products. Our research goal was to determine a good combination of policies for storage assignment (assigning products to storage locations) and routing (determining the sequence in which to retrieve products from storage to meet customer demand) for Ankor's situation. We adapted existing solution techniques for this problem, overcoming the special characteristics of Ankor's operations. With these adapted techniques, we cut the average route length in the order-picking operation by 31 percent. As a result of our study, Ankor implemented a new storage and routing strategy. The study also showed further improvement potentials in the picking process, which Ankor adopted as well. All improvements led to a reduction in the number of order pickers of more than 25 percent. http://repub.eur.nl/res/pub/975/ 2003-10-17T00:00:00Z We study the case of a catalogue/internet mail order retailer selling seasonal products and receiving large numbers of commercial returns. Returned products arriving before the end of the selling season can be resold if there is sufficient demand. A single order is placed before the season starts. Excess inventory at the end of the season is salvaged and all demands not met directly are lost. Since little historical information is available, it is impossible to determine the shape of the distribution of demand. Therefore, we analyze the distribution-free newsboy problem with returns, in which only the mean and variance of demand are assumed to be known. We derive a simple closed-form expression for the distribution-free order quantity, which we compare to the optimal order quantities when gross demand is assumed to be normal, lognormal or uniform. We find that the distribution-free order rule performs well in most realistic cases. http://repub.eur.nl/res/pub/11868/ 2003-05-16T00:00:00Z At container terminals, containers are transshipped from one mode of transportation to another. Within a terminal different types of material handling equipment are used to transship containers from ships to barges, trucks and trains and vice versa. Over the past decades, ships have strongly increased in size, up to 8000 TEU (Twenty feet equivalent unit container). In order to use these big ships efficiently, the docking time at the port must be as small as possible. This means that large amounts of containers have to be loaded, unloaded and transshipped in a short time span, with a minimum use of expensive equipment. This paper gives a classification of the decision problems that arise at container terminals. For various decision problems, an overview of relevant literature is presented. Quantitative models from this literature, which try to solve the problems are discussed. Finally, some general conclusions and subjects for further research are given. http://repub.eur.nl/res/pub/278/ 2003-03-04T00:00:00Z In this article we focus on product return handling and warehousing issues. In some businesses return rates can be well over 20% and returns can be especially costly when not handled properly. In spite of this, many managers have handled returns extemporarily. The fact that quantitative methods barely exist to support return handling decisions adds to this. In this article we bridge those issues by 1) going over the key decisions related with return handling; 2) identifying quantitative models to support those decisions. Furthermore, we provide insights on directions for future research. http://repub.eur.nl/res/pub/215/ 2002-07-12T00:00:00Z This work aims at investigating the influence of picking batch size to average time in system of orders in a one-aisle warehouse under the assumption that order arrivals follow a Poisson process and items are uniformly distributed over the aisle's length. We model this problem as an M/G[k]/1 queue in which orders are served in batches of exactly orders. The average time in system of the M/G[k]/1 queue is difficult to obtain for general service times. To circumvent this obstacle, we perform an extensive numerical experiment on the average time in system of the model when the service time is deterministic (M/D[k]/1) or exponentially distributed (M/M[k]/1). These results are then compared with the corresponding times in system of the actual model taken from simulation runs. A variance analysis is carried out and its result elicits that the M/D/[k]/1 queue is a very good approximation for the average time in system of orders. Correspondingly, the optimal picking batch size of the real system can be approximated by the optimal batch size when service time is deterministic. http://repub.eur.nl/res/pub/576/ 2002-05-01T00:00:00Z Already for a long time retailers take back products. In this paper we explore the factors contributing to the decision of combining vs. separating inbound and outbound flows during the return handling process. We do so through a comparative analysis of the operations in nine retailer warehouses, which can be divided in three groups: food retailers, department stores and mail order companies. We identify both aggravating factors and facilitating actions for return handling. Furthermore, we bring about recommendations for practice. At the end we put forward propositions that are useful in feeding studies on return handling efficiency. In particular, we conjecture over the impact that return volume and product diversity have on the decision for combining vs. separating the reverse and forward flows. http://repub.eur.nl/res/pub/168/ 2002-02-22T00:00:00Z In the warehouse of Ankor, a wholesaler of tools and garden equipment, various problems concerning the storage and retrieval of products arise. For example, heavy products have to be retrieved prior to light products to prevent damage. Furthermore, the layout of the warehouse differs from the layout generally assumed in literature. The goal of this research was to determine storage locations for the products and a routing method to obtain sequences in which products are to be retrieved from their locations. It is shown that despite deviations from the "normal" case, similar savings in route length can be obtained by adapting existing solution techniques. Total labor savings are far less than expected on basis of assumptions made in literature. With a minimum of adaptations to the current situation the average route length can be decreased by 30 %. There is no need for complex techniques. http://repub.eur.nl/res/pub/11866/ 2002-01-01T00:00:00Z Warehousing is becoming more and more a critical activity in the supply chain to outperform competitors on customer service, lead times, and costs. However, if warehousing is to be a source of competitive advantage, then the implementation of a warehouse management information system (WMS) is a necessary condition to achieve efficiently the high performance of warehousing operations required in today's marketplace. A major practical question is then whether a given warehouse should implement a standard or a tailor-made WMS. A standard WMS offers many advantages. On the other hand, a standard WMS remains largely making compromises between the way a warehouse wants to work and the way the system allows the warehouse to work. In certain environments, such compromises might seriously degrade warehouse performance. An exploratory field study of warehouses with recently implemented WMSs was conducted to first understand the empirical reality and then build up a theory linking the constructs warehouse complexity and warehouse planning and control structure. http://repub.eur.nl/res/pub/11880/ 2002-01-01T00:00:00Z Presents a study which explored the factors contributing to the decision of combining versus separating inbound and outbound flows during the return handling process. Comparative analysis of the operations in nine retailer warehouses; Factors which facilitate actions for return handling; Recommendations for practice; Impact of return volume and product diversity on the decision for combining versus separating the reverse and forward flows. http://repub.eur.nl/res/pub/11882/ 2002-01-01T00:00:00Z In this paper, the relation between the constructs operational complexity, Web-based orientation of the company and the company's distribution structure used for the fulfillment of Internet customer orders is investigated in the food home shopping branch. A model is proposed with relations between these constructs, which is researched through a survey among food e-tailers. A positive association between operational complexity and the distribution structure used could be established, meaning that more complex operations (with a full-line assortment and a large number of orders) tend to have special (Internet orders only) distribution centers for the fulfillment of Internet customer orders. Companies with a store infrastructure tend to keep using this existing infrastructure (unless the number of orders becomes large) and new Internet-only companies tend to use special Internet-orders only warehouses, unless the number of orders is small, in which case cooperation with existing stores is preferred. http://repub.eur.nl/res/pub/116/ 2001-09-28T00:00:00Z Already for a long time retailers take back products. In this paper we explore the factors contributing to the decision of combining vs. separating inbound and outbound flows during the return handling process. We do so through a comparative analysis of the operations in nine retailer warehouses, which can be divided in three groups: food retailers, department stores and mail order companies. We identify both aggravating factors and facilitating actions for return handling. Furthermore, we bring about recommendations for practice. At the end we put forward propositions that are useful in feeding studies on return handling efficiency. In particular, we conjecture over the impact that return volume and product diversity have on the decision for combining vs. separating the reverse and forward flows. http://repub.eur.nl/res/pub/11854/ 2001-08-16T00:00:00Z This paper considers a parallel aisle warehouse, where order pickers can change aisles at the ends of every aisle and also at a cross aisle halfway along the aisles. An algorithm is presented that can find shortest order picking tours in this type of warehouses. The algorithm is applicable in warehouse situations with up to three aisle changing possibilities. Average tour length is compared for warehouses with and without a middle aisle. It appears that in many cases the average order picking time can be decreased significantly by adding a middle aisle to the layout. http://repub.eur.nl/res/pub/11855/ 2001-06-15T00:00:00Z This paper considers routing and layout issues for parallel aisle warehouses. In such warehouses order pickers walk or drive along the aisles to pick products from storage. They can change aisles at a number of cross aisles. These cross aisles are usually located at the front and the back of the warehouse, but there can also be one or more cross aisles at positions in between. We describe a number of heuristics to determine order picking routes in a warehouse with two or more cross aisles. To analyse the performance of the heuristics, a branch-and-bound algorithm that generates shortest order picking routes is used. Performance comparisons between heuristics and the branch-and-bound algorithm are given for various warehouse layouts and order sizes. For the majority of the instances with more than two cross aisles, a newly developed heuristic appears to perform better than the existing heuristics. Furthermore, some consequences for layout are discussed. From the results it appears that the addition of cross aisles to the warehouse layout can decrease the handling time of the orders by lowering average travel times. However, adding a large number of cross aisles may increase average travel times because the space occupied by the cross aisles has to be traversed as well. http://repub.eur.nl/res/pub/1662/ 2001-02-19T00:00:00Z Reverse logistics, that is, all operations related to the reuse of used products, excess inventory and packaging materials, gain increasing attention globally both for their promising financial potentials, the sustainable growth alternative they offer and the environmental positive impact they have. In this paper, we introduce reverse logistics and we explain how the adoption of e-commerce provides new possibilities to existing business models and what are the new e-business models in reverse logistics that have emerged. We compare these three new e-business models, namely, returns aggregators, specialty locators and integrated solution providers on a number of aspects and identify keys for their competitive advantage. Finally, we discuss conceptual and actual opportunities for these e-business models to thrive and advance and present some e-commerce tools that are being developed with the aim to address the distributed, dynamic and knowledge-intensive aspects of applications that contribute to the advancement of e-businesses in the field of reverse logistics. http://repub.eur.nl/res/pub/11856/ 2001-01-01T00:00:00Z This paper describes the development of a minimum flow algorithm to determine the number of automated guided vehicles (AGVs) required at a semi-automated container terminal. At such a terminal the containers are transported by AGVs from the quay cranes to the automated stacking cranes and vice versa. A model and a strongly poly~lomial time algorithm are developed to solve the case in which containers are available for transport at known time instants. http://repub.eur.nl/res/pub/11852/ 1999-01-01T00:00:00Z In this paper, the orderbatching problem in warehouses is investigated. Batching, or clustering orders together in the picking process to form a single picking route, reduces travel time and, as such, this phenomenon can be encountered in many warehouses. The reason for its importance is that orderpicking is a labour intensive process and, by using good batching methods, substantial savings can be obtained. The batching and routing problems are complex to solve. In practice, simple methods are used for the batching problem, such as first-come first-served (FCFS), i.e. combine orders in the route in the arriving sequence until the pick-device is full. Once clusters of orders have been formed, the calculation of the travel time for the routes requires the solution of a number of travelling salesman problems (one for each route). Two groups of heuristic algorithms are evaluated: the Seed algorithms and the somewhat more complex (and CPU time consuming) Time Savings algorithms. The performance of the algorithms is evaluated using two different routing strategies: the so-called S-shape and Largest gap strategies, which are well known in theory and practice. The heuristics are compared for travel time, number of batches formed and also for robustness. An algorithm is robust if it can be applied to any warehousing situation and still yield good results. Since the problem has to be implemented in existing Warehouse Management Systems and has to be solved online many times a day, the simplicity, transparancy and CPU time are also important in most warehousing situations. It is demonstrated that even simple order batching methods lead to significant improvement compared to FCFS. Seed algorithms are best in conjunction with S-shape and a large capacity of the pick device. Time savings algorithms perform best in conjunction with Largest gap and small pick device capacity. If CPU time becomes important, then using simple Seed algorithms should be considered. http://repub.eur.nl/res/pub/2272/ 1999-01-01T00:00:00Z http://repub.eur.nl/res/pub/11845/ 1998-01-01T00:00:00Z The problem of finding efficient orderpicking routes is studied for both conventional warehouses, where pickers have a central depot for picking up and depositing carts and pick lists, and modern warehouses, where orderpicking trucks can pick up and deposit pallets at the head of every aisle without returning to the depot. Such environments can be found in many warehouses where paperless picking is performed from pallet locations with pickers having mobile terminals receiving instructions one by one. In order to find orderpicking routes with a minimal length in both the situations of a central depot or decentralized depositing, the well-known polynomial algorithm of Ratliff and Rosenthal (1983) that considered warehouses with a central depot is extended. In practice, the problem is mainly solved by using the so-called S-shape heuristic in which orderpickers move in a S-shape curve along the pick locations. The performance of the new algorithm and the S-shape heuristic are compared in three realistic orderpicking systems. http://repub.eur.nl/res/pub/11836/ 1994-01-01T00:00:00Z In this paper, an approximation method is discussed for the analysis of pick-to-belt orderpicking systems. The aim of the approximation method is to provide an instrument for obtaining rapid insight in the performance of designs of pick-to-belt orderpicking systems. It can be used to evaluate the effects of changing the layout of the system, the number of picking stations, the number of pickers, the conveyor speed, the number of bins to be processed per day, the number of orderlines per bin, etc. Especially in the design phase, modeling and analysis speed is more important than accuracy. The method presented in this paper is based on Jackson network modeling and analysis. The method is fast and sufficiently accurate. The method is used by Ingenieursbureau Groenewout B.V., for early-stage evaluation of design alternatives of pick-to-belt orderpicking systems and general transportation systems. http://repub.eur.nl/res/pub/11832/ 1989-01-01T00:00:00Z In most of the literature on performance analysis of production systems, buffer are assumed to be controlled locally. In automated production systems buffers are not always the result of local physical space restrictions, but may also be software- controlled. Software-controlled buffers allow a more efficient use of the production units and a better control of the work in progress. Some of these possible control principles are known from production control and goods flow theory. We present a model for production systems in which the goods flow is continuous. The effect of different control mechanisms on the performance of the system (that is throughput and throughput time) is studied. Examples of control systems are BSC (Base Stock Control), Workload Control and control by local buffers only. It appears that for our model many of these control systems yield equivalent performance measures. http://repub.eur.nl/res/pub/11829/ 1988-01-01T00:00:00Z In this paper complex production systems are studied where a single product is manufactured and where each production unit stores its output in at most one buffer and receives its input from at most one buffer. The production units and the buffers may be connected nearly arbitrarily. The buffers are supposed to be of finite capacity and the goods flow is continuous. For such netwroks it is possible to estimate the throughput by applying repeated aggregation over the production units. The approximation appears to be best when the network shows some resemblance with a flow line. http://repub.eur.nl/res/pub/11830/ 1988-01-01T00:00:00Z In this paper, an approximation method is given for the analysis of flow lines with a continuous product flow. The algorithm is an improvement of an earlier developed method. It is applied here to lines in which the machines are unreliable with exponentially distributed life and repair times. All intermediate buffers have finite capacity. The algorithm consists of repeated decomposition and aggregation steps, in which two-stage lines are approximated by a single machine. The method appears to be fast and performs well for lines which are not too long. http://repub.eur.nl/res/pub/11826/ 1987-01-01T00:00:00Z Presents a multi-stage flow lines with intermediate buffers approximated by two-stage lines using repeated aggregation. Characteristics of the aggregation method; Problems associated with the analysis and design of production lines. http://repub.eur.nl/res/pub/11828/ 1987-01-01T00:00:00Z We consider two-stage production lines with an intermediate buffer. A buffer is needed when fluctuations occur. For single-product production lines fluctuations in capacity availability may be caused by random processing times, failures and random repair times. For multi-product production lines fluctuations are also caused by different processing time ratios for different products and by set up times. We examine whether it is possible to use the results developed for single-product flow lines, where the production units have exponentially distributed life- and repair times, for the multi-product case. As an example the case of a consumer electronics factory is studied. http://repub.eur.nl/res/pub/11817/ 1983-01-01T00:00:00Z Bargaining solutions are considered which have the following four properties: individual rationality, Pareto optimality, independence of equivalent utility representations, and independence of irrelevant alternatives. A main result of this paper is a simple proof of the fact that all such bargaining solutions are risk sensitive. Further a description is given of all bargaining solutions satisfying the four mentioned properties. Finally, a continuous bargaining solution, satisfying the first three properties, is given which is not risk sensitive.