Pilar Lino

Project Scheduling with Stochastic Activity Interruptions

In this chapter we address the problem of scheduling the activities of a resource-constrained project, some of which may be interrupted by an uncertain amount of time. The resources may be, for example, machines in a jobshop, computers with specialized software packages (as those needed for engineering designs), or highly specialized technicians.

A branch and bound algorithm for minimizing the number of crossing arcs in bipartite graphs

Abstract Acyclic directed graphs are widely used in many fields of economic and social sciences. This has generated considerable interest in algorithms for drawing “good” maps of acyclic diagraphs. The most important criterion to obtain a readable map of an acyclic graph is that of minimizing the number of crossing arcs. In this paper, we present a branch and bound algorithm for solving the problem of minimizing the number of crossing arcs in a bipartite graph. Computational results are reported on a set of randomly generated test problems.

Criticality Analysis in Activity-on-Node Networks with Minimal Time Lags

This paper is a systematic study of the critical nature of activities and relationships in AON networks with minimal time lags in the unlimited resource case, when activity splitting is not allowed. An analysis of all possible cases produces an extended and now complete classification of critical activities in six classes: normal, reverse, neutral, bicritical, increasing normal and decreasing reverse. We also present a procedure for classifying any critical task depending on the early start/finish times and on position within the critical paths.

A tabu thresholding algorithm for arc crossing minimization in bipartite graphs

Acyclic directed graphs are commonly used to model complex systems. The most important criterion to obtain a readable map of an acyclic graph is that of minimizing the number of arc crossings. In this paper, we present a heuristic for solving the problem of minimizing the number of arc crossings in a bipartite graph. It consists of a novel and easier implementation of fundamental tabu search ideas without explicit use of memory structures (a tabu thresholding approach). Computational results are reported on a set of 250 randomly generated test problems. Our algorithm has been compared with the two best heuristics published in the literature and with the optimal solutions for the test problems, size permitting.

Static and dynamic policies with RFID for the scheduling of retrieval and storage warehouse operations

Warehouses are essential components of logistics and supply chains. The performance of warehouse operations significantly affects the efficiency of the whole chain it belongs to. Radio frequency identification (RFID) is an emerging technology capable of providing real-time information about the location and properties of tagged object(s), such as people, equipment or products. The objective of this article is threefold, to propose and compare different offline and online policies for the scheduling of warehouse operations, to design a tool that allows the decision maker to compare policies and environments without putting them into practice, and to study the benefits that can be obtained if RFID is used in a particular type of warehouse. To this end, we have developed a stylised model that captures and generalises the main characteristics of the structure, routing and sequencing operations of a given real warehouse. The model incorporates several realistic features never or rarely discussed in the literature in the presence of RFID, for example, due dates in the orders that have to be performed and congestion in the warehouse due to the presence of multiple vehicles performing the orders. We have also developed a set of heuristic routing and sequencing procedures that take and, alternatively, do not take into account real time information, and compare their performance via simulation on a set of randomly generated, although realistic, warehouse scenarios. Computational results show the effect in terms of due data fulfilment and tardiness minimisation if the RFID technology is installed and offline and online management policies are considered.

Time and work generalised precedence relationships in project scheduling with pre-emption: An application to the management of Service Centres

In this paper we present an application of project scheduling concepts and solution procedures for the solution of a complex problem that comes up in the daily management of many company Service Centres. The real problem has been modelled as a multi-mode resource-constrained project scheduling problem with pre-emption, time and work generalised precedence relationships with minimal and maximal time lags between the tasks and due dates. We present a complete study of work GPRs which includes proper definitions, a new notation and all possible conversions amongst them. Computational results that show the efficiency of the proposed hybrid genetic algorithm and the advantages of allowing pre-emption are also presented.

Minimizing weighted tardiness of jobs with stochastic interruptions in parallel machines

Abstract In this paper, we address the problem of minimizing expected total weighted tardiness of jobs that have stochastic interruptions and that are processed on a set of parallel machines. Our research generalizes the problem of scheduling parallel machines to minimize total weighted tardiness. The proposed solution method is based on the scatter search methodology and implements an innovative structured combination procedure. Extensive computational testing with more than 400 problem instances shows the merit of the proposed solution method.

Heuristic algorithms for a storage location assignment problem in a chaotic warehouse

The extensive application of emerging technologies is revolutionizing warehouse management. These technologies facilitate working with complex and powerful warehouse management models in which products do not have assigned fixed locations (random storage). Random storage allows the utilization of the available space to be optimized. In this context, and motivated by a real problem, this article presents a model that looks for the optimal allocation of goods in order to maximize the storage space availability within the restrictions of the warehouse. For the proposed model a construction method, a local search algorithm and different metaheuristics have been developed. The introduced algorithms can also be used for other purposes such as to assess when and how it is convenient to perform relocation of stored items to improve the current level of storage space availability. Computational tests performed on a set of randomly generated and real warehouse instances show the effectiveness of the proposed methods.

A multi-objective approach for a project scheduling problem with due dates and temporal constraints infeasibilities

In this paper, we study a multi-mode resource-constrained project scheduling problem (RCPSP) which considers time and work generalised precedence relationships with minimal and maximal time lags and due dates where each activity requires only one unit of resource (e.g. a worker, a machine, etc.). To find a feasible solution for this problem is NP-hard and therefore for instances where a feasible solution has not been found, an appropriate real-life approach would consist of providing the decision-maker with a collection of quality solutions with a trade-off between due dates and temporal constraints violations. We propose a multi-objective evolutionary algorithm for the generation of an approximation to the optimal Pareto front with the objectives of minimising the project tardiness, and the time and work precedence relationships infeasibilities. The algorithm is basically a genetic algorithm with a multi-objective management of the evolution and is complemented by several local searches. Local Searches are based on justification of strong components. In the literature, this methodology has been previously used successfully with activities, but never with strong components. Computational experiments have been carried out to test the relative efficiency of different versions of the algorithm.

A heuristic algorithm for project scheduling with splitting allowed

In this article, we analyze the precedence diagramming method, the only published algorithm for time-only project scheduling with activity splitting allowed. The criteria used in this method (forward and backward pass computations) for deciding when an activity has to be interrupted are shown to be invalid in some situations. We look into the causes of these failures and propose new formulae that always provide feasible solutions. The new algorithm has been tested on 240 randomly generated problems ranging up to 600 activities and 7,200 precedence relationships, resulting in an average deviation from optima of less than 1 percent.