| 1. |
- Almgren, Torgny, 1962, et al.
(författare)
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Optimization models for improving periodic maintenance schedules by utilizing opportunities
- 2012
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Ingår i: Proceedings of 4th Production and Operations Management World Conference, July 2012.
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Konferensbidrag (refereegranskat)abstract
- We present mathematical models for finding optimal opportunistic maintenance schedules for systems, in which components are assigned maximum replacement intervals. Our mod- els are applied to safety-critical components in an aircraft engine, for which maintenance opportunities naturally arise since entire modules are sent to the workshop when mainte- nance is required on one or more components. Case study results illustrate the advantage of the mathematical models over simpler policies, the benefit of coordinating the maintenance in economically dependent systems, and that our models can be utilized also for strategic investment decision support.
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| 2. |
- Almgren, Torgny, 1962, et al.
(författare)
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The opportunistic replacement problem: analysis and case studies
- 2011
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We consider an optimization model for determining optimal opportunistic maintenance (that is, component replacement) schedules when data is deterministic. This problem generalizes that of Dickman, Epstein, and Wilamowsky [21] and is a natural starting point for the modelling of replacement schedules when component lives are non-deterministic. We show that this basic opportunistic replacement problem is NP-hard. We show that the convex hull of the set of feasible replacement schedules is full-dimensional, and that all the necessary inequalities also are facet-inducing. We show that when maintenance occasions are fixed, the remaining problem can be stated as a linear program; when maintenance costs are monotone with time, the latter is solvable through a greedy procedure. Results from a series of case studies performed in the areas of aircraft engine and wind turbine maintenance are also reported. These illustrate the advantages of utilizing opportunistic maintenance activities based on a complete optimization model, as compared to simpler policies.
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| 3. |
- Almgren, Torgny, 1962, et al.
(författare)
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The opportunistic replacement problem: theoretical analyses and numerical tests
- 2012
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Ingår i: Mathematical Methods of Operations Research. - : Springer Science and Business Media LLC. - 1432-2994 .- 1432-5217. ; 76:3, s. 289-319
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Tidskriftsartikel (refereegranskat)abstract
- We consider a model for determining optimal opportunistic maintenance schedules with respect to a maximum replacement interval. This problem generalizes that of Dickman et al. (J Oper Res Soc India 28:165–175, 1991) and is a natural starting point for modelling replacement schedules of more complex systems. We show that this basic opportunistic replacement problem is NP-hard, that the convex hull of the set of feasible replacement schedules is full-dimensional, that all the inequalities of the model are facet-inducing, and present a new class of facets obtained through a {0,1/2}-Chvátal–Gomory rounding. For costs monotone with time, a class of elimination constraints is introduced to reduce the computation time; it allows maintenance only when the replacement of at least one component is necessary. For costs decreasing with time, these constraints eliminate non-optimal solutions. When maintenance occasions are fixed, the remaining problem is stated as a linear program and solved by a greedy procedure. Results from a case study on aircraft engine maintenance illustrate the advantage of the optimization model over simpler policies. We include the new class of facets in a branch-and-cut framework and note a decrease in the number of branch-and-bound nodes and simplex iterations for most instance classes with time dependent costs. For instance classes with time independent costs and few components the elimination constraints are used favorably. For fixed maintenance occasions the greedy procedure reduces the computation time as compared with linear programming techniques for all instances tested.
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| 4. |
- Austeng, Dordi, et al.
(författare)
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Incidence of and risk factors for neonatal morbidity after active perinatal care : extremely preterm infants study in Sweden (EXPRESS)
- 2010
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Ingår i: Acta Paediatrica. - : Wiley. - 0803-5253 .- 1651-2227. ; 99:7, s. 978-992
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Tidskriftsartikel (refereegranskat)abstract
- Aims: The aim of this study was to determine the incidence of neonatal morbidity in extremely preterm infants and to identify associated risk factors. Methods: Population based study of infants born before 27 gestational weeks and admitted for neonatal intensive care in Sweden during 2004-2007. Results: Of 638 admitted infants, 141 died. Among these, life support was withdrawn in 55 infants because of anticipation of poor long-term outcome. Of 497 surviving infants, 10% developed severe intraventricular haemorrhage (IVH), 5.7% cystic periventricular leucomalacia (cPVL), 41% septicaemia and 5.8% necrotizing enterocolitis (NEC); 61% had patent ductus arteriosus (PDA) and 34% developed retinopathy of prematurity (ROP) stage >= 3. Eighty-five per cent needed mechanical ventilation and 25% developed severe bronchopulmonary dysplasia (BPD). Forty-seven per cent survived to one year of age without any severe IVH, cPVL, severe ROP, severe BPD or NEC. Tocolysis increased and prolonged mechanical ventilation decreased the chances of survival without these morbidities. Maternal smoking and higher gestational duration were associated with lower risk of severe ROP, whereas PDA and poor growth increased this risk. Conclusion: Half of the infants surviving extremely preterm birth suffered from severe neonatal morbidities. Studies on how to reduce these morbidities and on the long-term health of survivors are warranted.
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| 5. |
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| 6. |
- Besnard, Francois, 1983, et al.
(författare)
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A stochastic model for opportunistic maintenance planning of offshore wind farms
- 2011
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Ingår i: 2011 IEEE PES Trondheim PowerTech: The Power of Technology for a Sustainable Society, POWERTECH 2011; Trondheim; 19 June 2011 through 23 June 2011. - 9781424484195
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Konferensbidrag (refereegranskat)abstract
- A sound maintenance planning is of crucial importance for wind power farms, and especially for offshore locations. This paper presents a stochastic optimization model for opportunistic service maintenance of offshore wind farms. The model takes advantage of 7 days wind production ensemble forecast and opportunities at corrective maintenance activities in order to perform the service maintenance tasks at the lowest cost. The model is based on a rolling horizon, i.e. the optimization is performed on a daily basis to update the maintenance planning based on the updated production and weather forecasts. An example based on real wind data is used to demonstrate the value of the proposed approach. In this example, it is shown that 32% of the cost for production losses and transportation could be saved. © 2011 IEEE.
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| 7. |
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| 8. |
- Ehrstedt, Christoffer, et al.
(författare)
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Weekly vinblastine is a therapeutic option in recurrent/refractory pediatric low-grade gliomas
- 2012
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Ingår i: Neuro-Oncology. - : Oxford University Press (OUP). - 1522-8517 .- 1523-5866. ; 14:suppl 1, s. i70-i70
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- BACKGROUND: In a majority of cases efficient treatment of low-grade gliomas in the pediatric population is achieved by surgery, sometimes accompanied by chemotherapy according to the LGG SIOP 2003 protocol. However, some cases of LGG is refractory, treatment options in these cases often consists of LGG SIOP 2003 relapse protocol or radiotherapy. Vinblastine can be used as a secondline chemotherapy.METHODS: Four patients with refractory low grade gliomas were given vinblastine intravenously. These patients had previously failed chemotherapy and/or radiation for unresectable low-grade glioma. Tumor location has differed, 1 brainstem, 1 optic pathway, 1 thalamus, 1 cerebellar. Three of the patients were given vinblastine at a dose of 6mg/m2 weekly, the fourth patient received a 50% dose reduction because of intolerable side-effects. The treatment was given for at least 12 months in three of the cases.RESULTS: There have been significant reduction of tumor size in the 3 patients who have received vinblastine for at least 12 months. Response to treatment has been followed at three months interval with MRI. None of the patients have been forced to discontinue the treatment because of intolerable side-effects. The fourth patient has been treated for three months and follow-up with MRI indicates a slight reduction of tumor size.CONCLUSION: Vinblastine should be considered as a secondline chemotherapy in refractory low grade gliomas. Extended administration (>12 months) seems to be tolerated well. If untolerable side effects dose reduction should be tried.
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| 9. |
- Gad, Helge, et al.
(författare)
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MTH1 inhibition eradicates cancer by preventing sanitation of the dNTP pool
- 2014
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Ingår i: Nature. - : Nature Publishing Group. - 0028-0836 .- 1476-4687. ; 508:7495, s. 215-221
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Tidskriftsartikel (refereegranskat)abstract
- Cancers have dysfunctional redox regulation resulting in reactive oxygen species production, damaging both DNA and free dNTPs. The MTH1 protein sanitizes oxidized dNTP pools to prevent incorporation of damaged bases during DNA replication. Although MTH1 is non-essential in normal cells, we show that cancer cells require MTH1 activity to avoid incorporation of oxidized dNTPs, resulting in DNA damage and cell death. We validate MTH1 as an anticancer target in vivo and describe small molecules TH287 and TH588 as first-in-class nudix hydrolase family inhibitors that potently and selectively engage and inhibit the MTH1 protein in cells. Protein co-crystal structures demonstrate that the inhibitors bindin the active site of MTH1. The inhibitors cause incorporation of oxidized dNTPs in cancer cells, leading to DNA damage, cytotoxicity and therapeutic responses in patient-derived mouse xenografts. This study exemplifies the non-oncogene addiction concept for anticancer treatment and validates MTH1 as being cancer phenotypic lethal.
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| 10. |
- Gustavsson, Emil, 1987, et al.
(författare)
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Preventive maintenance scheduling of multi-component systems with deterioration costs
- 2012
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We introduce the preventive maintenance scheduling problem with interval costs (PMSPIC), which is to schedule preventive maintenance of components of a system over a finite discretized time horizon, given a common set-up cost and component costs dependent on the lengths of the maintenance intervals. We present a 0-1 integer linear programming (0-1 ILP) model for the PMSPIC which was originally presented by Joneja (1990) to model the joint replenishment problem. We show that most of the integrality constraints can be relaxed and that the linear inequality constraints define facets of the convex hull of the feasible set. We present three applications demonstrating that the PMSPIC can be used to model several types of maintenance problems with deterioration costs. The first considers rail grinding. If the interval between the grinding occasions increases, then the sizes of the rail cracks increase, which implies that more grinding passes must be performed, generating a higher maintenance cost. We presume a deterministic model for crack growth and optimize the scheduling of the rail grinding on a set of track sections. Our second application concerns two approaches for the scheduling of component replacements in aircraft engines. In the first approach a bi-objective problem, simultaneously minimizing the cost for the scheduled preventive maintenance and the probability of unexpected stops, is formulated. The second approach considers the minimization of the sum of costs of preventive and expected corrective maintenance, without rescheduling. We also demonstrate that if rescheduling is allowed, then the 0-1 ILP model can be used as a policy by re-optimizing the schedule at a component failure, thus utilizing the opportunity for preventive maintenance. We evaluate the use of such a strategy in a simulation of the engine. The third approach considers components’ replacement in wind mills in a wind farm, extending the PMSPIC to consider several systems with a joint set-up cost. As for the aircraft engine application, we use the 0-1 ILP model as a policy for deciding upon replacement decisions allowing for rescheduling, and evaluate it by simulating the joint system. In each of the three applications, the use of the 0-1 ILP model is compared with age or constantinterval policies, resulting in a reduction of maintenance costs by up to 15% compared with the respective best simple policy.
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| 11. |
- Gustavsson, Emil, 1987, et al.
(författare)
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Preventive maintenance scheduling of multi-component systems with interval costs
- 2014
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Ingår i: Computers & industrial engineering. - : Elsevier BV. - 0360-8352. ; 76, s. 390-400
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Tidskriftsartikel (refereegranskat)abstract
- We introduce the preventive maintenance scheduling problem with interval costs (PMSPIC), which is to schedule preventive maintenance (PM) of the components of a system over a finite and discretized time horizon, given a common set-up cost and component costs dependent on the lengths of the maintenance intervals. We present a 0–1 integer linear programming (0–1 ILP) model for the PMSPIC; the model is identical to that presented by Joneja (1990) for the joint replenishment problem within inventory management. We study this model from a polyhedral and exact solutions’ point of view, as opposed to previously studied heuristics (e.g. Boctor, Laporte, & Renaud, 2004; Federgruen & Tzur, 1994; Levi, Roundy, & Shmoys, 2006; Joneja, 1990).We show that most of the integrality constraints can be relaxed and that the linear inequality constraints define facets of the convex hull of the feasible set. We further relate the PMSPIC to the opportunistic replacement problem, for which detailed polyhedral studies were performed by Almgren et al. (2012a). The PMSPIC can be used as a building block to model several types of maintenance planning problems possessing deterioration costs. By a careful modeling of these costs, a polyhedrally sound 0–1 ILP model is used to find optimal solutions to realistic-sized multi-component maintenance planning problems. The PMSPIC is thus easily extended by side constraints or to multiple tiers, which is demonstrated through three applications; these are chosen to span several levels of unmodeled randomness requiring fundamentally different maintenance policies, which are all handled by variations of our basic model. Our first application considers rail grinding. Rail cracks increase with increasing intervals between grinding occasions, implying that more grinding passes must be performed—thus generating higher costs. We optimize the grinding schedule for a set of track sections presuming a deterministic model for crack growth; hence, no corrective maintenance (CM) will occur between the grinding occasions scheduled. The second application concerns two approaches for scheduling component replacements in aircraft engines. The first approach is bi-objective, simultaneously minimizing the cost for the scheduled PM and the probability of unexpected stops. In the second approach the sum of costs for PM and expected CM—without rescheduling—is minimized. When rescheduling is allowed, the 0–1 ILP model is used as a policy by re-optimizing the schedule at a component failure, which then constitutes an opportunity for PM. The policy manages the trade-off between costs for PM and unplanned CM and is evaluated in a simulation of the engine. The third application considers components’ replacement in wind mills in a wind farm, extending the PMSPIC to comprise multiple tiers with joint set-up costs. Due to the large number of components unexpected stops occur frequently, thus calling for a dynamic rescheduling, which is evaluated through a simulation of the system. In each of the three applications, the use of the 0-1 ILP model is compared with age or constant-interval policies; the maintenance costs are reduced by up to 16% as compared with the respective best simple policy. The results are strongest for the first two applications, possessing low levels of unmodeled randomness.
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| 12. |
- Gustavsson, Emil, 1987, et al.
(författare)
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Primal convergence from dual subgradient methods for convex optimization
- 2012
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- When solving a convex optimization problem through a Lagrangian dual reformulation subgradient optimization methods are favourably utilized, since they often find near-optimal dual solutions quickly. However, an optimal primal solution is generally not obtained directly through such a subgradient approach. We construct a sequence of convex combinations of primal subproblem solutions, a so called ergodic sequence, which is shown to converge to an optimal primal solution when the convexity weights are appropriately chosen. We generalize previous convergence results from linear to convex optimization and present a new set of rules for constructing the convexity weights that define the ergodic sequence of primal solutions. In contrast to previously proposed rules, they exploit more information from later subproblem solutions than from earlier ones. We evaluate the proposed rules on a set of nonlinear multicommodity flow problems and demonstrate that they clearly outperform the ones previously proposed.
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| 13. |
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| 14. |
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| 15. |
- Larsson, Ann I., 1965, et al.
(författare)
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Embryogenesis and larval biology of the cold-water coral Lophelia pertusa : Lophelia Larvae
- 2014
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Ingår i: PLoS ONE. - : Public Library of Science (PLoS). - 1932-6203. ; 9:7
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Tidskriftsartikel (refereegranskat)abstract
- Cold-water coral reefs form spectacular and highly diverse ecosystems in the deep sea but little is known about reproduction, and virtually nothing about the larval biology in these corals. This study is based on data from two locations of the North East Atlantic and documents the first observations of embryogenesis and larval development in Lophelia pertusa, the most common framework-building cold-water scleractinian. Embryos developed in a more or less organized radial cleavage pattern from ∼160 μm large neutral or negatively buoyant eggs, to 120-270 μm long ciliated planulae. Embryogenesis was slow with cleavage occurring at intervals of 6-8 hours up to the 64-cell stage. Genetically characterized larvae were sexually derived, with maternal and paternal alleles present. Larvae were active swimmers (0.5 mm s-1) initially residing in the upper part of the water column, with bottom probing behavior starting 3-5 weeks after fertilization. Nematocysts had developed by day 30, coinciding with peak bottom-probing behavior, and possibly an indication that larvae are fully competent to settle at this time. Planulae survived for eight weeks under laboratory conditions, and preliminary results indicate that these planulae are planktotrophic. The late onset of competency and larval longevity suggests a high dispersal potential. Understanding larval biology and behavior is of paramount importance for biophysical modeling of larval dispersal, which forms the basis for predictions of connectivity among populations. © 2014 Larsson et al.
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| 16. |
- Larsson, Torbjörn, et al.
(författare)
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Ergodic convergence in subgradient optimization - with application to simplicial decomposition of convex programs
- 2012
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Ingår i: Contemporary Mathematics. - Providence, Rhode Island : American Mathematical Society. - 0271-4132 .- 1098-3627. ; 568, s. 159-186
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Tidskriftsartikel (refereegranskat)abstract
- When non-smooth, convex minimization problems are solved by subgradient optimization methods, the subgradients used will in general not accumulate to subgradients that verify the optimality of a solution obtained in the limit. It is therefore not a straightforward task to monitor the progress of subgradient methods in terms of the approximate fulfilment of optimality conditions. Further, certain supplementary information, such as convergent estimates of Lagrange multipliers and convergent lower bounds on the optimal objective value, is not directly available in subgradient schemes. As a means of overcoming these weaknesses in subgradient methods, we introduced in LPS96b, LPS96c, and LPS98 the computation of an ergodic (averaged) sequence of subgradients. Specifically, we considered a non-smooth, convex program solved by a conditional subgradient optimization scheme with divergent series step lengths, and showed that the elements of the ergodic sequence of subgradients in the limit fulfil the optimality conditions at the optimal solution, to which the sequence of iterates converges. This result has three important implications. The first is the finite identification of active constraints at the solution obtained in the limit. The second is the establishment of the convergence of ergodic sequences of Lagrange multipliers; this result enables sensitivity analyses for solutions obtained by subgradient methods. The third is the convergence of a lower bounding procedure based on an ergodic sequence of affine underestimates of the objective function; this procedure also provides a proper termination criterion for subgradient optimization methods. This article contributes first an overview of results and applications found in LPS96b, LPS96c, and LPS98 pertaining to the generation of ergodic sequences of subgradients generated within a subgradient scheme. It then presents an application of these results to that of the first instance of a simplicial decomposition algorithm for convex and non-smooth optimization problems.
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| 17. |
- Lindroth, Peter, 1979, et al.
(författare)
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Approximating the Pareto Optimal Set using a Reduced Set of Objective Functions
- 2010
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Ingår i: European Journal of Operational Research. - : Elsevier BV. - 0377-2217. ; 207:3, s. 1519-1534
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Tidskriftsartikel (refereegranskat)abstract
- Real-world applications of multi-objective optimization often involve numerous objective functions. But while such problems are in general computationally intractable, it is seldom necessary to determine the Pareto optimal set exactly. A significantly smaller computational burden thus motivates the loss of precision if the size of the loss can be estimated. We describe a method for finding an optimal reduction of the set of objectives yielding a smaller problem whose Pareto optimal set w.r.t. a discrete subset of the decision space is as close as possible to that of the original set of objectives. Utilizing a new characterization of Pareto optimality and presuming a finite decision space, we derive a program whose solution represents an optimal reduction. We also propose an approximate, computationally less demanding formulation which utilizes correlations between the objectives and separates into two parts. Numerical results from an industrial instance concerning the configuration of heavy-duty trucks are also reported, demonstrating the usefulness of the method developed. The results show that multi-objective optimization problems can be significantly simplified with an induced error which can be measured.
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| 18. |
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| 19. |
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| 20. |
- Lindroth, Peter, 1979, et al.
(författare)
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Optimizing truck tyres
- 2014
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Ingår i: Orbit medlemsblad for Dansk Selskab for Operationsanalyse og Svenska OperationsAnalysFöreningen. - 1601-8893. ; :23, s. 12-14
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Tidskriftsartikel (refereegranskat)
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| 21. |
- Nilsson, Julia, et al.
(författare)
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An evaluation approach for opportunistic maintenance optimization models for nuclear power plants
- 2010
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Ingår i: Power and Energy Society General Meeting, 2010 IEEE. - : IEEE. - 9781424483570
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Konferensbidrag (refereegranskat)abstract
- Nuclear power constitutes one of the main electricity production sources in several countries, e.g. in Sweden, with about half of today's electricity production. Driving forces for cost-efficiency have pushed the development of new methods for maintenance planning and optimization forward. Reliability Centered Asset Management (RCAM) is one of these new approaches, and maintenance optimization is one way to perform quantitative analysis which is a feature of RCAM. This paper proposes a stochastic framework for evaluating opportunistic maintenance optimization models for the generation of replacement schedules for shaft seals in feed-water pump systems in nuclear power plants. Results show that the extended discounted model has a good overall performance, when compared to other strategies. Moreover, it is shown that it is the best strategy for high values of the constant cost for maintenance and the second best for lower values of the constant cost.
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| 22. |
- Patriksson, Michael, 1964, et al.
(författare)
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The opportunistic replacement problem with individual component lives
- 2011
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We consider an extension of the opportunistic replacement problem, which has been studied by Dickman, Epstein and Wilamowsky [3], Andréasson [2], and Andréasson et al. [1], that allows the individuals of the same component to have nonidentical lives. Formulating and solving this problem defines a first step towards solving the opportunistic replacement problems with uncertain component lives. We show that the problem is NP-hard even with time independent costs, and present two mixed integer linear programming models for the problem. We show that in model I the binary requirement on the majority of the variables can be relaxed; this is in contrast to model II and Andréasson’s [2] model. We remove all superfluous variables and constraints in model I and show that the remaining constraints are facet inducing. We also utilize a linear transformation of model I to obtain a stronger version of model II, model II+, that inherits the polyhedral properties of model I. Numerical experiments show that the solution time of model I is significantly lower than the solution times of both model II and Andréasson’s model. It is also somewhat lower than the solution time of model II+.
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| 23. |
- Patriksson, Michael, 1964, et al.
(författare)
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The stochastic opportunistic replacement problem: A two-stage solution approach
- 2011
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In Almgren et al. 2009 we studied the opportunistic replacement problem, which is a multicomponent maintenance scheduling problem with deterministic component lives. The assumption of deterministic lives is a strong simplification, but valid in applications where critical components are assigned a technical life after which replacement is enforced. Here, we study the stochastic opportunistic replacement problem, which is a more general setting in which component lives are allowed to be stochastic. We consider a stochastic programming approach for the minimization of the expected cost over the remaining planning horizon. Further, we present a means to compute lower bounds on the recourse function. The lower bounds are used in the construction of a decomposition method which extends the integer L-shaped method to incorporate stronger optimality cuts. In order to obtain a computationally tractable model, a two-stage sample average approximation scheme is utilized. Numerical experiments on problem instances from the wind power and aviation industry as well as on two test instances are performed. The results show that the decomposition method is faster than solving the deterministic equivalent on the three more complex instances out of the four instances considered. Furthermore, the numerical experiments show that decisions based on the stochastic programming approach yield a lower average total maintenance cost compared to that of decisions based on simpler maintenance.
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| 24. |
- Rydholm Hedman, Ann-Marie, et al.
(författare)
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Hip fracture patients’ cognitive state affects family members’ experiences : a diary study of the hip fracture recovery
- 2011
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Ingår i: Scandinavian Journal of Caring Sciences. - : Wiley-Blackwell. - 0283-9318 .- 1471-6712. ; 25:3, s. 451-458
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Tidskriftsartikel (refereegranskat)abstract
- Background: Many patients with hip fractures suffer from dementia disease, which has shown to affect the outcome of recovery strongly, as well as care and treatment. As most hip fracture patients are discharged home early after surgery, caregiving often falls on family members – spouses, daughters, sons, or even neighbours become informal carers.Aim: To explore how hip fracture patients’ cognitive state affect family members’ experiences during the recovery period.Methods: Eleven diaries written by family members’ of hip fracture patients were analysed by means of qualitative content analysis.Findings: The analysis generated two main categories with four categories. The first main category was; ‘Being a family member of a cognitively impaired patient’ with the categories ‘Dissatisfaction with lack of support’ and ‘Emotional distress due to the patient’s suffering’. The second main category was ‘Being a family member of a cognitively intact patient’ with the categories ‘Satisfaction with a relative’s successful recovery’ and ‘Strain due to their caring responsibilities’. Being a family member of a patient with cognitive impairment and a hip fracture meant being solely responsible for protecting the interests of the patient; in regard to care, rehabilitation and resources. The family members were also burdened with feelings of powerlessness and sadness due to the patients’ suffering. On the contrary, family members of cognitively intact hip fracture patients had positive experiences. The family members expressed pleasure from seeing their close ones make progress. However, when the healing process was delayed this led to strain on the family members.Conclusions: The findings suggest the hip fracture patient’s cognitive state is more decisive than the hip fracture itself for the family members’ experiences.
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| 25. |
- Sabartova, Zuzana, 1987, et al.
(författare)
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An optimization model for truck tyres selection
- 2014
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Ingår i: Proceedings of the 4th International Conference on Engineering Optimization, ENGOPT 2014; Lisbon; Portugal; 8 September 2014 through 11 September 2014. - : CRC Press. - 9781138027251 ; , s. 561-566
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Konferensbidrag (refereegranskat)abstract
- To improve the truck tyre selection process at Volvo Group Trucks Technology which is currently based on convention rather than on scientific methodology, an optimization model has been developed with the aim of determining an optimal set of tyres for each vehicle and operating environment specification. The overall purpose is to reduce the cost of operation, which is in this case measured by fuel consumption and tyre wear, while preserving the levels of other tyre dependent features such as startability, handling, and ride comfort. We have developed a joint model of the vehicle, the tyres, and the operating environment. The model is based on vehicle dynamics equations describing the vehicle and is implemented in Matlab and Simulink. To be able to distinguish between different tyres, the part of the model describing tyres must be able to describe more complex properties than the commonly used Pacejka's model. Hence, a surrogate model of the function describing the rolling resistance coefficient of a truck tyre and regression models of vertical and lateral stiffness have been developed and inserted into the part of the model describing tyres; the surrogate model is based on sample points evaluated through a finite element analysis. The road is then generated based on the operating environment classification of the actual truck. Since the resulting optimization model has a simulation-based objective function and simulation-based constraints, a global derivative-free optimization algorithm has to be used to solve the problem. Characteristics of available solvers for the resulting optimization problem, i.e., rbfSolve, EGO, ConstrLMSRBF, and NOMAD, are discussed.
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| 26. |
- Shafiee, Mahmood, 1982, et al.
(författare)
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A Redundancy Optimization Model Applied to Offshore Wind Turbine Power Converters
- 2013
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Ingår i: PowerTech (POWERTECH), 2013 IEEE Grenoble. - 9781467356695
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Konferensbidrag (refereegranskat)abstract
- Unexpected failures, high operation and maintenance (O&M) cost, and low accessibility are critical issues for offshore wind farms. According to existing statistics, power converters are among the most critical components in offshore wind turbines, and suffer from a high failure rate. One efficient way to improve the reliability and availability of the converter system is by adding at least one independent redundant converter, which ensures that the system would still operate in case of a converter failure. However, the redundant converters will increase the system’s cost, volume, and weight. In this paper, we propose a cost-rate minimization model aiming to simultaneously determine the optimal allocation of redundant converters and the optimal number of the converters that are allowed to fail before sending a maintenance crew to the offshore platform. The optimal solution under system-level constraints is derived, and the conditions required to make using redundant converter system beneficial are discussed. Finally, the proposed model has been tested on data collected from an offshore wind farm database and the results are compared with a conventional wind turbine converter system.
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| 27. |
- Shafiee, Mahmood, 1982, et al.
(författare)
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An Optimal Number-Dependent Preventive Maintenance Strategy for Offshore Wind Turbine Blades Considering Logistics
- 2013
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Ingår i: Advances in Operations Research. - : Hindawi Limited. - 1687-9147 .- 1687-9155. ; Volume 2013 (2013)
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Tidskriftsartikel (refereegranskat)abstract
- In offshore wind turbines, the blades are among the most critical and expensive components that suffer from different types of damage due to the harsh maritime environment and high load. The blade damages can be categorized into two types: the minor damage, which only causes a loss in wind capture without resulting in any turbine stoppage, and the major (catastrophic) damage, which stops the wind turbine and can only be corrected by replacement. In this paper, we propose an optimal number-dependent preventive maintenance (NDPM) strategy, in which a maintenance team is transported with an ordinary or expedited lead time to the offshore platform at the occurrence of the N th minor damage or the first major damage, whichever comes first. The long-run expected cost of the maintenance strategy is derived, and the necessary conditions for an optimal solution are obtained. Finally, the proposed model is tested on real data collected from an offshore wind farm database. Also, a sensitivity analysis is conducted in order to evaluate the effect of changes in the model parameters on the optimal solution.
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| 28. |
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| 29. |
- Svensson, Elin, 1980, et al.
(författare)
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A model for optimization of process integration investments under uncertainty
- 2011
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Ingår i: Energy. - : Elsevier BV. - 0360-5442. ; 36:5, s. 2733-2746
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Tidskriftsartikel (refereegranskat)abstract
- The long-term economic outcome of energy-related industrial investment projects is difficult to evaluate because of uncertain energy market conditions. In this article, a general, multistage, stochastic programming model for the optimization of investments in process integration and industrial energy technologies is proposed. The problem is formulated as a mixed-binary linear programming model where uncertainties are modelled using a scenario-based approach. The objective is to maximize the expected net present value of the investments which enables heat savings and decreased energy imports or increased energy exports at an industrial plant. The proposed modelling approach enables a long-term planning of industrial, energy-related investments through the simultaneous optimization of immediate and later decisions. The stochastic programming approach is also suitable for modelling what is possibly complex process integration constraints. The general model formulation presented here is a suitable basis for more specialized case studies dealing with optimization of investments in energy efficiency.
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| 30. |
- Svensson, Elin, 1980, et al.
(författare)
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The value of flexibility for pulp mills investing in energy efficiency and future biorefinery concepts
- 2014
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Ingår i: International Journal of Energy Research. - : Hindawi Limited. - 0363-907X .- 1099-114X. ; 38:14, s. 1864-1878
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Tidskriftsartikel (refereegranskat)abstract
- Changing conditions in biomass and energy markets require the pulp and paper industry to improve energy efficiency and find new opportunities in biorefinery implementation. Considering the expected changes in the pulp mill environment and the variety of potential technology pathways, flexibility should be a strong advantage for pulp mills. In this context, flexibility is defined as the ability of the pulp mill to respond to changing conditions. The aim of this article is to show the potential value of flexibility in the planning of pulp mill energy and biorefinery projects and to demonstrate how this value can be incorporated into models for optimal strategic planning of such investments. The paper discusses the requirements on the optimization models in order to adequately capture the value of flexibility. It is suggested that key elements of the optimization model are multiple points in time where investment decisions can be made as well as multiple scenarios representing possible energy price changes over time. The use of a systematic optimization methodology that incorporates these model features is illustrated by a case study, which includes opportunities for district heating cooperation as well as for lignin extraction and valorization. A quantitative valuation of flexibility is provided for this case study. The study also demonstrates how optimal investment decisions for a pulp mill today are influenced by expected future changes in the markets for energy and bioproducts.
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| 31. |
- Thörnblad, Karin, 1971, et al.
(författare)
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A competitive iterative procedure using a time-indexed model for solving flexible job shop scheduling problems
- 2013
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We investigate the efficiency of a discretization procedure utilizing a time-indexed mathematical optimization model for finding accurate solutions to flexible job shop scheduling problems considering objectives comprising the makespan and the tardiness of jobs, respectively. The time-indexed model is used to find solutions to these problems by iteratively employing time steps of decreasing length. The solutions and computation times are compared with results from a known benchmark formulation and an alternative, slightly enhanced version of the same. For the largest instances---considering both objectives---the proposed method finds significantly better solutions than the other models within the same time frame, although there is a large difference in the performance of the models depending on which objective is considered. This implies that the evaluation of scheduling algorithms must be performed with respect to an objective that is suitable for the real application for which they are intended. The minimization of the makespan is no such objective, although it is the most widely used objective in research. We propose an objective incorporating tardiness. The iterative procedure for solving the time-indexed model outperforms the other models regarding the time to find the best feasible solution. We conclude that our iterative procedure with the time-indexed model is competitive with state-of-the-art mathematical optimization models. Since the proposed procedure quickly finds solutions of good quality to large instances, our findings imply that the new procedure is beneficially utilized for scheduling real flexible job shops.
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| 32. |
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| 33. |
- Thörnblad, Karin, 1971, et al.
(författare)
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An efficient algorithm for solving the flexible job shop scheduling problem
- 2013
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Ingår i: 25th NOFOMA conference proceedings, June 3-5 2013, Göteborg, Sweden.
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Konferensbidrag (refereegranskat)abstract
- Purpose: To investigate the efficiency of a discretization procedure utilizing a time-indexed mathe-matical optimization model for finding accurate solutions to flexible job shop scheduling problems considering objectives comprising makespan and tardiness, respectively. Design/methodology/approach: A time-indexed mixed integer programming model is used to find solutions by iteratively employing time steps of decreasing length. The solutions and computation times are compared with results from a known benchmark formulation and an alternative model. Findings: The proposed method finds significantly better solutions for the largest instances within the same time frame. Both the other models are better choices for some smaller instances, which is expected since the new method is designed for larger problems. Only our alter-native model is able to solve two of the largest instances when minimizing the tardiness. Research limitations/implications: Interesting future research topics include the introduction of constraints representing other relevant entities such as the availability of tools and fixtures, and the scheduling of maintenance activities and personnel. Practical implications: Real cases of flexible job shop problems typically yield very large models. Since the new procedure quickly finds solutions of good quality to such instances, our findings imply that the new procedure is beneficially utilized for scheduling real flexible job shops. Original/value: We show that real flexible job shop problems can be solved through the solution of a series of carefully formulated discretized mathematical optimization models.
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| 34. |
- Thörnblad, Karin, 1971, et al.
(författare)
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Mathematical modelling of a real flexible job shop in aero engine component manufacturing
- 2011
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Ingår i: Proceedings of 10th Workshop on Models and Algorithms for Planning and Scheduling Problems, June 2011, Nymburk, Czech Republic.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We formulate two mixed integer programming models stemming from a real flexible job shop problem with a total of ten resources and five main processing multipurpose machines. The models are compared w.r.t. memory usage, computation times, and accuracy.
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| 35. |
- Thörnblad, Karin, 1971, et al.
(författare)
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Mathematical optimization of a flexible job shop problem including preventive maintenance and availability of fixtures
- 2012
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Ingår i: Proceedings of the 4th World P&OM Conference / 19th International Annual EurOMA Conference, Amsterdam, Netherlands, July 2012.
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Konferensbidrag (refereegranskat)abstract
- The multitask cell at Volvo Aero Corporation is a flexible job shop containing ten resources aimed at being flexible with regard to product mix and processing types. Computing schedules for this type of job shop is an NP-hard problem. The computation times will therefore always be an issue, especially as the scheduling problem includes limited availability of fixtures and preventive maintenance planning. Computational results show that with the method developed we are able to produce optimal, or near-optimal, schedules for real data instances within an acceptable time frame. The scheduling principle proposed shortens lead times and minimizes tardiness.
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| 36. |
- Thörnblad, Karin, 1971, et al.
(författare)
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Optimering av scheman för en verklig produktionscell: tidsdiskretisering reducerar lösningstiden utan att lösningarnas kvalitet försämras
- 2011
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Ingår i: PLANs forsknings- och tillämpningskonferens, 31 aug-1 sep 2011, Norrköping, Sverige.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Vid optimering av industriella schemaläggningsproblem är kompromissen mellan beräkningstid och resultatnivå ofta kritisk, med risken att en optimeringsmodell förenklas så mycket att dess resultat inte är praktiskt relevant. I artikeln jämförs två matematiska optimeringsmodeller för optimal schemaläggning av en verklig produktionscell på Volvo Aero. I den ena modellen beskrivs bl. a. operationers starttider m.h.a. kontinuerliga variabler, medan tiden har diskretiserats i den andra modellen. Diskretiseringen innebär att planeringshorisonten indelas i ett antal tidssteg, här kallade diskreta tidsperioder. Det är en förenkling eftersom all indata då måste uttryckas i helt antal diskreta tidsperioder och varje operation endast kan schemaläggas vid början av en tidsperiod. Den senare modellen visade sig överlägsen vad gäller beräkningstider och bedömdes därför ha mycket stor potential, trots risken för approximeringsfel p.g.a. tidsdiskretiseringen. 2 I artikeln presenteras tester med verkliga indata, vilka visar att approximeringsfelen blir mycket små när den diskreta tidsperiodens längd är en timma eller mindre, trots att några operationstider är kortare än så. Det optimala schemat från den tidsdiskreta modellen har före jämförelsen justerats så att alla operationstider är de ursprungliga, detta utan att förändra vare sig maskinval eller valda sekvenser på maskinerna. Denna modell har goda förutsättningar att fungera praktiskt, eftersom den både är snabb och ger goda resultat.
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| 37. |
- Thörnblad, Karin, 1971, et al.
(författare)
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Optimization of schedules for a multitask production cell
- 2010
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Ingår i: Proceedings of The 22nd NOFOMA Conference in Kolding, Denmark, 10-11th of June 2010.
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Konferensbidrag (refereegranskat)abstract
- Purpose of this paper; To optimize production schedules of a real production cell containing multi-purpose machines working as a job-shop in a complex logistic environment producing aircraft engine components. The planning and control of the production in the so called multitask cell results in a complex combinatorial problem. Design/methodology/approach; A mathematical optimization model of the multitask cell has been developed and tested on real data instances. The current production planning prerequisites have been studied in order to choose an appropriate objective function. Findings; Production plans resulting from the optimization model, are compared with schedules formed by the First in First Out (FIFO) and Earliest Due Date (EDD) priority rules, which are similar to the current manual planning of the multitask cell. Practical implications; The proposed scheduling principle will shorten lead times, and provide a more efficient use of the resources of the multitask cell. The effects are most significant at times with high utilization of the cell. What is original/value of paper; The development of theory and practice of optimization, together with the development of computer hardware during the past decades, enable the utilization of optimization as a tool for computing efficient production schedules in a complex logistic environment.
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| 38. |
- Thörnblad, Karin, 1971, et al.
(författare)
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Scheduling optimization of a real flexible job shop including side constraints regarding maintenance, fixtures, and night shifts
- 2013
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We present a generic iterative scheduling procedure for the scheduling of a real flexible job shop, the so-called multitask cell at GKN Aerospace Engine Systems in Sweden. A time-indexed formulation of the problem is presented including side constraints regarding preventive maintenance, fixture availability, and unmanned night shifts. This paper continues the work in [35], with an improvement of the iterative solution procedure and the inclusion of constraints regarding night shifts during which only unmanned processing is allowed. The resulting schedules are compared with schedules constructed using two priority dispatching rules. Computational results show that the gain of including the night shifts constraints is significant and that the methodology developed is able to produce near-optimal schedules for industrial data instances for the coming shift within an acceptable practical time frame.
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