Susceptibility of optimal train schedules to stochastic disturbances of process times

This work focuses on the stochastic evaluation of train schedules computed by a microscopic scheduler of railway operations based on deterministic information. The research question is to assess the degree of sensitivity of various rescheduling algorithms to variations in process times (running and dwell times). In fact, the objective of railway traffic management is to reduce delay propagation and to increase disturbance robustness of train schedules at a network scale. We present a quantitative study of traffic disturbances and their effects on the schedules computed by simple and advanced rescheduling algorithms. Computational results are based on a complex and densely occupied Dutch railway area; train delays are computed based on accepted statistical distributions, and dwell and running times of trains are subject to additional stochastic variations. From the results obtained on a real case study, an advanced branch and bound algorithm, on average, outperforms a First In First Out scheduling rule both in deterministic and stochastic traffic scenarios. However, the characteristic of the stochastic processes and the way a stochastic instance is handled turn out to have a serious impact on the scheduler performance

Microbial contamination and management scenarios in a Mediterranean coastal lagoon (Etang de Thau, France): application of a Decision Support System within the Integrated Coastal Zone Management context

1 - In the Thau lagoon (Southern Mediterranean Coast) the main anthropogenic pressure is represented by the urban development in the watershed, whilst oyster and mussel farming represents one of the main economical activities in the region. 2 - During the last decade, the increasing organic loads from watershed and urban settlements in the lagoon surroundings have caused a diffuse contamination by faecal bacteria. Also toxic algal blooms have been occurring, impairing water quality with major impacts on shellfish farming, fishery and bathing. 3 - In this study, indicators and scenarios identified for the lagoon have been integrated in a Decision Support System (DSS) to evaluate the best solutions for reducing pressures and improving both water quality and ecosystem status. 4 - The watershed has been analysed with reference to indicators of pollution sources and transfer rates to the lagoon. In parallel, socio-economic indicators and descriptors of urban growth and development have been assessed. Numerical models have been run in order to simulate the lagoon hydrodynamics in relation to both meteorological factors and watershed runoff. The impact of faecal bacteria contamination has been evaluated in terms of economical losses and social conflicts, arising from the restriction of shellfish farming and marketing during contamination events. Finally, the DSS prototype has been applied to the lagoon in support to management and future planning

Batch scheduling in a two-machine flow shop with limited buffer and sequence independent setup times and removal times

The problem of makespan minimization in a flow shop with two-machines when the input buffer of the second machine can only host a limited number c of parts is known to be NP-hard for any c>0 and c⩽n, where n is the number of jobs. In this paper we analyze this problem in the context of batch scheduling, i.e. when identical parts must be processed consecutively. In particular we study the case in which each batch requires sequence independent setup times and removal times. We then show that, if the size of the ith batch is larger than a value b*_i, then the makespan minimization problem can be formulated as a special case of TSP and solved in polynomial time. The cost structure of this TSP can be reduced to the one defined for the two-machine no-wait flow shop. Hence, we give a closed form expression for b*_i. Then, we prove that when the same algorithm is applied to batch sizes smaller than b*_i, the error goes to zero as the batch sizes approach the values b*_i

Love, justice, knowledge : a comparison of child custody and visitation disputes in the US and Sweden

EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Production Scheduling in a Steelmaking-Continuous Casting Plant

In this paper we describe an optimization procedure for planning the production of steel ingots in a steelmaking-continuous casting plant. The strict requirements of the production process defeated most of the earlier approaches to steelmaking-continuous casting production scheduling, mainly due to the lack of information in the optimization models. Our formulation of the problem is based on the alternative graph, which is a generalization of the disjunctive graph of Roy and Sussman. The alternative graph formulation allow us to describe in detail all the constraints that are relevant for the scheduling problem. We then solve the problem by using a beam search procedure, and compare our results with a lower bound of the optimal solutions and with the actual performance obtained in the plant. Computational experience shows the effectiveness of this approach

Expected shortfall for the makespan in activity networks under imperfect information

This paper deals with the evaluation of the expected shortfall or the conditional value-at-risk for the makespan in scheduling problems represented as temporal networks under incomplete and uncertain information. We consider temporal activity network representations of scheduling problems affected by uncertainties related to the activity durations and we assume that for these uncertainties only incomplete or imperfect information is available. More precisely, for each activity only the interval for its integer valued duration is known to the scheduler. We address the evaluation of the expected shortfall associated to a feasible schedule discussing its importance in scheduling applications. We propose lower and upper bounds, heuristics to determine a fast computational estimation of the expected shortfall, and an exact method for a class of activity networks. The experimental results show that the proposed method can enable to use the expected shortfall as optimization criterion for wide classes of scheduling approaches considering risk-aversion in different practical contexts