Journal papers

A-L. Ladier, G. Alpan, Robust crossdock scheduling with time windows. Computers & Industrial Engineering 99, 16-28. Sept 2016.

Keywords:

Logistics, Robustness, Cross-docking, Crossdock truck scheduling, Project scheduling

Abstract Abstract
Cross-docking is a logistic technique that helps to fasten the good flows and to reduce inventory costs; but it requires a perfect coordination of the inbound and outbound trucks. The truck scheduling problem has been studied by many authors, but mainly in a deterministic case. And yet, many uncertainties can arise in the process: if a truck is delayed, or the process times change, does the truck schedule remain feasible and stable? This article proposes robust models for the truck scheduling model with time windows. The reformulations of the original model are based on classical techniques in robust optimization (minimax and minimization of the expected regret) but also on techniques from robust project scheduling (resource redundancy and time redundancy). The computation study carried out to compare the nine different models shows that the methods based on resource redundancy give the best results overall in the cross-docking case. Minimizing the average number of trucks docked at a given door is a good way to ensure robustness in the schedule, but it also increases storage.

A-L. Ladier, G. Alpan, Cross-docking operations: current research versus industry practice. Omega 62, 145–162. July 2016.

Keywords:

Materials handling, Cross-docking operations, Survey, Field-based research, Industry practice

Abstract Abstract
The technique of cross-docking, which consists in unloading trucks, sorting the items they contain and reloading them directly into outbound trucks in order to minimize temporary storage, has attracted researchers' attention in the past few years. The number of articles on the subject has been growing very fast, but largely detached from industry practice. In order to see whether the current state-of-the-art matches the industry practice, we propose a common framework that helps comparing the literature review regarding cross-docking operations with on-field observations and platform managers' interviews. Analyzing the gaps between the state-of-the-art and the industry practice helps drawing future research directions, in relation to industrial needs.

A-L. Ladier, G. Alpan, Crossdock truck scheduling with time windows - Earliness, tardiness and storage policies. Special issue (Scheduling in logistic and maintenance) of the Journal of Intelligent Manufacturing. doi:10.1007/s10845-014-1014-4

Keywords:

Cross-docking, Crossdock truck scheduling, Integer programming, Heuristics

Abstract Abstract
This article proposes to simultaneously plan inbound and outbound truck arrivals and departures in a cross-docking platform, as well as the internal pallet handling. The objective is to minimize both the total number of pallets put in storage and the dissatisfaction of the transportation providers, by creating a truck schedule as close as possible to the wished schedule they communicate in advance. The problem is modeled with an integer program tested on generated instances to assess its performance, especially regarding the computation time. It is proven to be NP-hard in the strong sense. Since the execution takes too long to be used on a daily basis by platform managers, three heuristics are also proposed and tested. Two are based on integer programs solved sequentially, the third one is a tabu search in which the storage part of the objective function is evaluated by a maximum ow model in a graph. Numerical experiments show in which conditions each heuristic performs best, which can help in choosing a solution method when confronted to a real-life problem.

A-L. Ladier, G. Alpan, B. Penz, Joint employee weekly timetabling and daily rostering: A decision-support tool for a logistics platform. European Journal of Operation Research 234, 1 (2014) 278-297. April 2014.

Keywords:

Manpower planning, Logistics, Mixed integer linear programming, Sequential solving

Abstract Abstract
To deal with their highly variable workload, logistics companies make their task force flexible using multi-skilled employees, flexible working hours or short-term contracts. Together with the legal constraints and the handling equipments’ capacities, these possibilities make personnel scheduling a complex task. This paper describes a model to support their chain of decisions from the weekly timetabling to the daily rostering (detailed task allocation).

We divide the problem into three sub-problems depending on the type of decision to be made: (1) workforce dimensioning, (2) task allocation for a week, and (3) detailed rostering for a day. The three decisions are made sequentially, the output of a step being the input of the next one. Each step is modeled as a mixed integer linear program which is described and commented.

The proposed models are tested with industrial data as well as generated instances. From the observations made in an industrial context, we show that our model is an actual management tool supporting the managers in their operational decisions. This tool is currently used by the company which provided us with the industrial data. Based on the results with the generated instances, we present the conditions under which the models can be solved within a reasonable amount of time, and we assess the robustness of the daily rostering when the input data changes.

G. Alpan, A.-L. Ladier, R. Larbi, B. Penz, Heuristic solutions for transshipment problems in a multiple door cross docking warehouse. Computers and Industrial Engineering 61, 2 (2011) 402-408. Octobre 2011.

Keywords:

Cross docking; Multiple docks; Scheduling; Heuristics; Transshipment

Abstract Abstract
Cross docking is a practice in logistics with the main operations of unloading products from an incoming truck, regrouping them with respect to their destinations and loading them directly into an outbound truck with minimum storage in between these operations. In this article, we study the transshipment scheduling problem in a multiple inbound and outbound dock configuration. The operations manager has several decisions to make: he can decide to transship products directly from inbound to an outbound truck, if an outbound truck is available; he can temporarily store certain products and have them loaded later on; or he can replace an outbound truck to facilitate direct loading. The objective is to find the best schedule of transshipment operations to minimize the sum of inventory holding and truck replacement costs. In this article, we present several heuristics to attain this objective. Numerical experiments are presented and the results are compared with the optimal solution to evaluate the performance of the heuristics.

International conferences

G. Dottavio, M. F. Andrade, F. Renard, V. Cheutet, A-L. Ladier, S. Vercraene, P. Hoang, S. Briet, R. Dachicourt, Logistical Optimization of Nuclear Waste Flows during Decommissioning. 8th International Conference on Energy Management (ICEM), Paris, France, oct. 2016.

Keywords:

Nuclear decommissioning, logistical optimization, decision-support framework, waste management.

D. Diekmann, U. Clausen, L. Eufinger, G. Alpan, A-L. Ladier, Combining mathematical optimization with discrete-event simulation to improve cross docking operations. International Conference on Operations Research of the German Operations Research Society (GOR), Hamburg, Germany, September 2016.

A.-L. Ladier, A. G. Greenwood, G. Alpan, Modeling issues when using simulation to test the performance of mathematical programming models under stochastic conditions. 29th European Simulation and Modelling Conference - ESM'2015, Leicester, United Kingdom, pp 117-121.

Keywords:

Discrete-event simulation, discrete mathematical programming optimization, validation, verification.

A-L. Ladier, G. Alpan, Integrating truck scheduling and employee rostering in a cross-docking platform. International Conference on Industrial Engineering and Systems Management 2015, pp 676-685.

Keywords:

Logistics, Cross-docking, Crossdock truck scheduling, Employee timetabling, Iterative approach

Abstract Abstract
In a crossdocking platforms, goods are unloaded, transferred and reloaded into trucks with little or no storage in between. The crossdock truck scheduling problem addresses the hard problem of coordinating the truck operations. However, crossdock operations are mostly done manually: it is therefore important to take staffing issues into account while building the truck schedule. This article shows how a truck scheduling model and an employee timetabling and rostering model can be combined to address both problems in an integrated manner. Three approaches are compared. The sequential approach consists in sequentially solving the two problems: from the truck schedule calculated first, a workload is deduced and used as input for the employee timetabling and rostering process. The iterative approach, inspired by Weide et al. (2010), consists in solving both problems one after another in an iterative manner until a stable point is reached. Two iterative procedures are proposed, employees-first and trucks-first.

A.-L. Ladier, G. Alpan, A. G. Greenwood, Robustness evaluation of an IP-based cross-docking schedule using discrete-event simulation. Industrial and Systems Engineering Research Conference, Montreal, Canada, June 2014.

Abstract
Integer programming (IP) models are powerful tools for operations planning, but may encounter problems when applied in a stochastic environment. We propose a methodology to evaluate the robustness of an IP model using discrete-event simulation. In our case, the IP model determines truck and pallet schedules for a cross-docking facility. A portion of the output from the IP model is used as input to a simulation model that represents the facility. The decisions regarding pallet transfers and truck departures are left to the simulation model to ensure flexibility. Random events are induced through the simulation in order to see how robust the IP-based schedules are against the uncertainties in: (i) the transfer time of pallets through the cross-dock, (ii) the unloading time of pallets from trucks, and (iii) the earliness or delay of the trucks arriving at the platform. The difference between the deterministic and the stochastic scenarios are evaluated regarding the number of pallets put into storage, truck docking and sojourn times. Robustness measures are proposed based on numerical experiments.

A-L. Ladier, G. Alpan, Scheduling truck arrivals and departures in a cross dock: earliness, tardiness and storage policies. International Conference on Industrial Engineering and Systems Management, Rabat, Morocco, October 2013.

Key words:

Cross docking, Logistics operations, Integer programming, Optimization, Scheduling heuristics
Abstract
This article proposes a model to simultaneously plan the truck arrivals, the trucks departures and the internal pallet handling in a cross docking platform. The objective is to minimize both the total number of pallets put in storage on the planning horizon, and the dissatisfaction of the transportation providers, by creating a truck schedule as close as possible to the wished schedule they communicate in advance. The problem is modeled with an integer program, which is tested on generated instances to assess its performances, especially regarding the computation time. Since the execution takes too long to be used by platform managers on a daily basis, two heuristics are also proposed and tested. We show in which conditions each heuristic performs best, which can help in choosing a solution method when confronted to a real-life problem.

A-L. Ladier, A. G. Greenwood, G. Alpan, H. Hales, Simulation-based assessment of the robustness of IP-based truck schedules for cross-docking operations. 26th European Conference on Operational Research, Rome, Italy, July 2013.

Abstract
A previously developed IP-based model for scheduling truck arrivals, truck departures, and pallet handling in a cross-docking platform is assessed using discrete-event simulation. Experimentation with a simulation model of basic cross-docking operations subjects the schedules to a more realistic environment (e.g., stochastic truck arrival times, pallets transfer times, truck content) than can be formulated in the IP model. Analysis of the simulation experiments, in terms of the schedules’ performance and robustness under pragmatic conditions, provides insights to help refine the IP model.

A. G. Greenwood, H. Hales, G. Alpan, A-L. Ladier, Performance evaluation of worker timetables in cross-docking facilities using simulation. 26th European Conference on Operational Research, Rome, Italy, July 2013.

Abstract
A previously developed general approach for generating worker timetables is applied to cross-docking facilities to evaluate its performance under varying operating conditions. The timetables, generated from MILP models, are tested under stochastic conditions - random process times, varying working availabilities, and early/late truck arrivals - using discrete-event simulation. Experimentation and analysis using the simulation models identify which MILP model assumptions need to be relaxed or which constraints need to be reformulated in order to provide more robust and useful worker schedules.

National conferences

A-L. Ladier, S. Vercraene, V. Cheutet, R. Dachicourt, Planification et gestion des flux de déchets issus du démantèlement d’installations nucléaires. Congrès annuel de la société française de recherche opérationnelle et d’aide à la décision (ROADEF), Compiègne, February 2016.

A-L. Ladier, G. Alpan, Planification intégrée des camions et des employés dans une platforme de cross-docking. Congrès annuel de la société française de recherche opérationnelle et d’aide à la décision (ROADEF), Marseille, February 2015.

Q. Christ, A-L. Ladier, B. Penz, Ordonnancement d'atelier avec lissage de consommation d'énergie dans une industrie chimique. Congrès annuel de la société française de recherche opérationnelle et d’aide à la décision (ROADEF), Marseille, February 2015.

A-L. Ladier, G. Alpan, Planification des opérations de cross-docking. Congrès annuel de la société française de recherche opérationnelle et d’aide à la décision (ROADEF), March 2014.

ROADEF Young Researcher Prize

A-L. Ladier, G. Alpan, A. G. Greenwood, Une approche optimisation-simulation pour la planification robuste des opérations sur une plateforme de cross-docking. 18èmes journées STP du GdR MACS, St Etienne, France, October 2013.


A-L. Ladier, A. G. Greenwood, G. Alpan, Cross docking operations: a simulation model for the validation and robustness assessment of IP-based truck schedules. Analytics Research Workshop of the Deep South Regional Chapter of INFORMS, Tuscaloosa, Alabama, United States, April 2013.

Key words:

cross-docking operations, optimization, truck scheduling, simulation, robustness

A-L. Ladier, G. Alpan, B. Penz, Optimisation séquentielle des emplois du temps dans une plateforme logistique. Actes du 12e congrès annuel de la société française de recherche opérationnelle et d’aide à la décision (ROADEF), vol II, pp. 627, St Etienne, France, March 2011.

Key words:

Planning, programmation linéaire, résolution séquentielle, logistique

Other communications

A-L. Ladier, Optimisation des opérations dans une plateforme logistique - Prise en compte de l’incertitude sur les flux d’arrivée et de la capacité des ressources internes. Séminaire d'équipe GCSP, November 2013.


A-L. Ladier, G. Alpan, Optimisation des opérations dans une plateforme logistique - Prise en compte des flux d’arrivée et de la capacité des ressources internes. Journées G-SCOP 2013, Saint Jorioz, France, June 2013.

Key words

Cross docking, opérations logistiques, planification robuste, simulation à évts discrets

A-L. Ladier, G. Alpan, Optimisation des opérations dans une plateforme logistique - Prise en compte des flux d’arrivée et de la capacité des ressources internes. Journées G-SCOP 2012, Saint Jorioz, France, June 2012.

Key words

Opérations logistiques, cross docking, planification robuste

Working papers

A.-L. Ladier, A. G. Greenwood, G. Alpan, H. Hales, Issues in the Complementary use of simulation and optimization modeling. Cahiers Leibniz n°211, January 2014.