Caroline Rocha

Scheduling workover rigs for onshore oil production

Many oil wells in Brazilian onshore fields rely on artificial lift methods. Maintenance services such as cleaning, reinstatement, stimulation and others are essential to these wells. These services are performed by workover rigs, which are available on a limited number with respect to the number of wells demanding service. The decision of which workover rig should be sent to perform some maintenance service is based on factors such as the well production, the current location of the workover rig in relation to the demanding well, and the type of service to be performed. The problem of scheduling workover rigs consists in finding the best schedule for the available workover rigs, so as to minimize the production loss associated with the wells awaiting for service. We propose a variable neighborhood search (VNS) heuristic for this problem. Computational results on real-life problems are reported and their economic impacts are evaluated.

A hybrid heuristic for a multi-objective real-life car sequencing problem with painting and assembly line constraints

We address a multi-objective version of the car sequencing problem, which consists in sequencing a given set of cars to be produced in a single day, minimizing the number of violations of assembly constraints and the number of paint color changes in the production line. We propose a set of heuristics for approximately solving this problem, based on the paradigms of the VNS and ILS metaheuristics, to which further intensification and diversification strategies have been added. Computational results on real-life test instances are reported. The work presented in this paper obtained the second prize in the ROADEF challenge 2005 sponsored by Renault.

Revisiting p-Cycles / FIPP p-Cycles vs. Shared Link / Path Protection

While the advantages of p-cycles and FIPP p-cycles are well established, there has been no systematic analysis of how much bandwidth they consume in comparison with the shared link or path protection schemes. It was also recently observed that, even enumerating a huge number of cycles is not necessarily a guarantee for obtaining good quality solutions with the classical ILP models if tools for large scale programming such as, e.g., column generation techniques, are not used. For instance, a reduction of up to 37% of the solution cost for FIPP p-cycles can be obtained when using column generation instead of classical ILP modeling. We propose to investigate the bandwidth protection costs of p-cycles and FIPP p-cycles in comparison with those of shared link and path protection, using column generation models for the four protection schemes, and therefore obtaining the optimal values for all of them, out of any doubt. Accurate quantitative comparisons show that the average excess required bandwidth is about 6.6% for p-cycles and about 13.4% for FIPP p-cycles in exchange of a much faster restoration time.

A simple and effective genetic algorithm for the two-stage capacitated facility location problem ☆

This paper presents a simple and effective Genetic Algorithm (GA) for the two-stage capacitated facility location problem (TSCFLP). The TSCFLP is a typical location problem which arises in freight transportation. In this problem, a single product must be transported from a set of plants to meet customers demands, passing out by intermediate depots. The objective is to minimize the operation costs of the underlying two-stage transportation system thereby satisfying demand and capacity constraints of its agents. For this purpose, a GA is proposed and computational results are reported comparing the heuristic results with those obtained by two state-of-the-art Lagrangian heuristics proposed in the literature for the problem.

A unified framework for shared protection schemes in optical mesh network

While the advantages of p-cycles and FIPP p-cycles are well established, there has been no systematic analysis of how much bandwidth they consume in comparison with the classical shared link and path protection schemes. It was recently observed that, even enumerating a huge number of cycles, is not necessarily a guarantee for obtaining good quality solutions with the ILP models if tools for large scale programming are not used. We propose to investigate the bandwidth protection costs of p-cycles and FIPP p-cycles in comparison with those of shared link and path protection by applying the column generation technique to solve relaxed LP models for the four protection schemes, and then solving the resulting ILP models. Provably near-optimal solutions allow us to perform accurate quantitative comparisons on real-world networks.

Column generation bounds for numerical microaggregation

The biggest challenge when disclosing private data is to share information contained in databases while protecting people from being individually identified. Microaggregation is a family of methods for statistical disclosure control. The principle of microaggregation is that confidentiality rules permit the publication of individual records if they are partitioned into groups of size larger or equal to a fixed threshold value, where none is more representative than the others in the same group. The application of such rules leads to replacing individual values by those computed from small groups (microaggregates), before data publication. This work proposes a column generation algorithm for numerical microaggregation in which its pricing problem is solved by a specialized branch-and-bound. The algorithm is able to find, for the first time, lower bounds for instances of three real-world datasets commonly used in the literature. Furthermore, new best known solutions are obtained for these instances by means of a simple heuristic method with the columns generated.

Design of efficient node p-cycles in WDM mesh networks

p-cycles have been extensively studied under a single link failure scenario. Even though not as common, single node failures may occur as well, and the resulting consequences can be devastating. Failure-Independent Path-Protecting (FIPP) p-cycles provide end-to-end path protection, and thereby protection against node failures, assuming that precautions are taken.In this paper, we propose enhanced p-cycles, called node p-cycles, different from the FIPP p-cycle scheme, which can guarantee 100% protection against any single node failure. We design a scalable optimization model, which relies on a column generation formulation, in order to design the enhanced node p-cycles.Extensive comparative experiments have been conducted after designing a column generation framework for all compared protection schemes, in order to guarantee scalable models for each of them. Experiments include the (link) p-cycles, the node p-cycles of Onguetou and Grover (2009) 16, our enhanced node p-cycles and the FIPP p-cycles. We show that node p-cycles offering node and link protection only require slightly more spare capacity than link p-cycles, while requiring sometimes less, sometimes more spare capacity than FIPP p-cycles. For the comparison with the work of Grover and Onguetou (2009) 2, results show that our new node p-cycle scheme clearly outperforms their design in terms of capacity efficiency.

Shortcut Np-cycles with no loop backs

We propose a novel protection scheme, called shortcut N<i>p</i>-cycles (short for node <i>p</i>-cycles), which (<i>i</i>) offers 100% guaranteed protection against any single link or node failures in wavelength-division multiplexing (WDM) mesh networks, (<i>ii</i>) provides recovery paths, which are free of loop backs, and (<i>iii</i>) bestows some shortcuts on some recovery paths. For a given shortcut N<i>p</i>-cycle and a working path of which some links and nodes are protected by the shortcut N<i>p</i>-cycle, benefits (<i>i</i>), (<i>ii</i>) and (<i>iii</i>) are achieved through protection switching at the common nodes of the shortcut N<i>p</i>-cycle and the working path which are nearest to the end nodes of the path. As in <i>p</i>-cycle based schemes, shortcut N<i>p</i>-cycles correspond to a pre-cross connected shared protection scheme. To design shortcut N<i>p</i>-cycles, we develop a scalable column generation optimization model. Extensive numerical experiments have been carried out in order to evaluate the performance of shortcut N<i>p</i>-cycles. They show that shortcut N<i>p</i>-cycles are more capacity efficient that node <i>p</i>-cycles and FIPP (Failure Independent Path Protecting) <i>p</i>-cycles for full node protection, and that they have a smaller recovery time than FIPP <i>p</i>-cycles.