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Sökning: L773:0969 6016 OR L773:1475 3995

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  • Karlsson, Jenny, 1969-, et al. (författare)
  • Short-term harvest planning including scheduling of harvest crews
  • 2003
  • Ingår i: International Transactions in Operational Research. - 0969-6016 .- 1475-3995. ; 10:5, s. 413-431
  • Tidskriftsartikel (refereegranskat)abstract
    • The problem we consider is short-term harvesting planning for a total planning period of 4–6 weeks where we want to decide the harvest sequences or schedules for harvest crews. A schedule is an order or sequence of harvest areas assigned to each crew. The harvesting of areas is planned in order to meet industrial demand. The total cost includes harvesting, transportation, and storage. One considerable cost is due to the quality reduction of logs stored at harvest areas. There are a number of restrictions to be considered. Areas are of varying size and the composition of assortments in each area is different. Each harvest team has different skills, a different home base, and different production capacity. Another aspect is the road network. There is a cost related to road opening (restoring, snow removal). In this paper, we develop a mixed integer programming (MIP) model for the problem. The schedules are represented by 0/1 variables. With a limited number of schedules, the problem can be solved by a commercial MIP solver. We have also developed a heuristic solution approach that provides high-quality integer solutions within a distinct time limit to be used when more schedules are used. Computational results from a major Swedish forest company are presented.
  • Chattopadhyay, G., et al. (författare)
  • Decision on economical rail grinding interval for controlling rolling contact fatigue
  • 2005
  • Ingår i: International Transactions in Operational Research. - 0969-6016 .- 1475-3995. ; 12:6, s. 545-558
  • Tidskriftsartikel (refereegranskat)abstract
    • Rail players around the world have been increasing axle loads to improve the productivity of freight and heavy haul operations. This has increased the risk of surface cracks at curves because of rolling contact fatigue. Rail grinding has been considered an effective process for controlling these cracks and reducing risks of rail breaks. The complexity of deciding the optimal rail grinding intervals for improving the reliability and safety of rails is because of insufficient understanding of the various factors involved in the crack initiation and propagation process. This paper focuses on identifying the factors influencing rail degradation, developing models for rail failures and analyzing the costs of various grinding intervals for economic decision making. Various costs involved in rail maintenance, such as rail grinding, downtime, inspection, rail failures and derailment, and replacement of worn-out rails, are incorporated into the total cost model developed in this paper. Field data from the rail industry have been used for illustration.
  • Guajardo, Mario, et al. (författare)
  • Joint optimization of pricing and planning decisions in divergent supply chain
  • 2013
  • Ingår i: International Transactions in Operational Research. - : Wiley-Blackwell. - 0969-6016 .- 1475-3995. ; 20:6, s. 889-916
  • Tidskriftsartikel (refereegranskat)abstract
    • In divergent supply chains, such as in the oil industry, processing raw materials results in an outflow of multiple products. Final products are stored at international depots, from where they are ready to be shipped to the markets. Even if one company controls the entire chain, when production and sales organizations are decoupled, a relevant problem is to determine the internal prices of products at depots for achieving coordination. We propose an optimization model involving pricing and production decisions, and several constraints commonly used in divergent chains. In our approach, the producer incorporates the sellers behavior by expressing demand as a function of the internal price. As a result, our model serves as a coordination mechanism in trying to get an overall coordinated integrated solution in a decoupled reality. Numerical examples in single and multiple periods problems show the advantages of our approach over cost-based methods.
  • Lean, Hooi Hooi, et al. (författare)
  • On the role of commodity futures in portfolio diversification
  • 2021
  • Ingår i: International Transactions in Operational Research. - : WILEY. - 0969-6016 .- 1475-3995.
  • Tidskriftsartikel (refereegranskat)abstract
    • The last two decades have witnessed major financial crises that led investors to seek alternative assets and investment strategies to reduce their portfolio risk. In this article, we provide information on the role of commodity futures in designing portfolios and managing risk based on an appealing operational framework. Using more than 20 years of sample data, we first investigate the conditional mean and volatility dynamics of equity and commodity futures markets within a dynamic conditional correlation model setup. We then form alternative equity-commodity futures portfolios by changing the weights of commodity futures and examine if the diversified commodity-equity portfolios perform superior to the all-equity portfolios and four well-known investment strategies that suit most practitioners. Stochastic dominance approach shows that including commodity futures in diversified portfolios does not always improve the risk-return performance, except for gold in some particular portfolio setups. Accordingly, commodity assets have behaved like financial assets (stocks) and tend to be driven by the same pricing factors in general, which reduces the benefits of diversification.
  • Liu, Biyu, et al. (författare)
  • A min–max solution to optimise planned lead time in a remanufacturing system
  • 2019
  • Ingår i: International Transactions in Operational Research. - : John Wiley & Sons. - 0969-6016 .- 1475-3995. ; 26:2, s. 485-506
  • Tidskriftsartikel (refereegranskat)abstract
    • Determining a rational planned lead time (PLT) is a critical and difficult problem in production planning, especially for a remanufacturing system. This paper considers an optimisation problem of the PLT in a make-to-order remanufacturing system to coordinate disassembly–remanufacturing–reassembly and to improve performance. This optimisation model is designed to determine the PLT that minimises the inventory holding and shortage costs. Given the unknown distribution of remanufacturing time but with known first and second moments, this model is solved by a min–max approach, which can capture distributions with the same first and second moments. How the PLT and total cost are affected by yield rate with different first and second moments, unit holding cost, unit shortage cost and purchasing lead time are also investigated through numerical examples in this paper. The results of this study are shown to be consistent with practice and can be a support to decision-making in production planning and scheduling for remanufacturing.
  • Rönnqvist, Mikael, et al. (författare)
  • Collaboration and optimization in farmland exchanges
  • 2022
  • Ingår i: International Transactions in Operational Research. - : John Wiley and Sons Inc. - 0969-6016 .- 1475-3995.
  • Tidskriftsartikel (refereegranskat)abstract
    • To keep businesses competitive in world markets, the number of farms in most countries is decreasing while farm size is increasing, and parcels making up the farm area are extremely fragmented. It is common for farms to expand faster than available arable land near the farm center, leading to longer distances between the farm center and field areas. Consequently, logistic costs increase and affect the farm's profitability and the environment. Moreover, there is often substantial overlap between different farms, which implies that it is possible for farmers to collaborate with each other. We developed and proposed three optimization models to describe different levels of collaboration and sharing restrictions. Many practical considerations need to be weighed, such as the soil type and crops used. We collected detailed information on farms, parcels, and cost drivers in seven large instances for our case study in Sweden. One instance had 1431 farms and 32,091 parcels with 96 crop types. The resulting optimization models were very large; special pre-processing and optimization techniques were developed for their solutions. The theoretical potential for collaboration in logistic cost reduction was very large, ranging from approximately 10% to 50% depending on the form of the collaboration. Because a large part of costs comes from less use of diesel for transport resources, the reduction in cost is directly related to a reduction in CO2 emissions. © 2022 The Authors. International Transactions in Operational Research
  • Zhao, Yixin, et al. (författare)
  • An integer programming column generation principlefor heuristic search methods
  • 2020
  • Ingår i: International Transactions in Operational Research. - : Wiley-Blackwell. - 0969-6016 .- 1475-3995. ; 27:1, s. 665-695
  • Tidskriftsartikel (refereegranskat)abstract
    • There is an increasing interest in integrating column generation and heuristic approaches to efficiently solve large-scale discrete optimisation problems. We contribute in this direction. Based on the insights from Lagrangian duality theory, we present an auxiliary problem that can be used for finding near-optimal solutions to a discrete column-oriented model. The structure of this auxiliary problem makes it suitable for being addressed with a heuristic search method involving column generation. To this end, we suggest a large neighbourhood search strategy where the repair step is to solve a column generation type subproblem. The suggested search strategy and mathematical models involved need to be tailored to the problem structure. To illustrate important design options and computational behaviour, four applications are studied: bin packing, generalised assignment, a resource allocation problem and the fixed-charge transportation problem.
  • Fotedar, Sunney, 1989, et al. (författare)
  • Bi-objective optimization of the tactical allocation of job types to machines: mathematical modeling, theoretical analysis, and numerical tests
  • 2022
  • Ingår i: International Transactions in Operational Research. - 0969-6016. ; In Press
  • Tidskriftsartikel (refereegranskat)abstract
    • We introduce a tactical resource allocation model for a large aerospace engine system manufacturer aimed at long-term production planning. Our model identifies the routings a product takes through the factory, and which machines should be qualified for a balanced resource loading, to reduce product lead times. We prove some important mathematical properties of the model that are used to develop a heuristic providing a good initial feasible solution. We propose a tailored approach for our class of problems combining two well-known criterion space search algorithms, the bi-directional ε-constraint method and the augmented weighted Tchebycheff method. A computational investigation comparing solution times for several solution methods is presented for 60 numerical instances.
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  • Resultat 1-10 av 11
  • [1]2Nästa

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