1,720,952 research outputs found
Rational Upscaling and Modelling of a Semi-Submersible Floating Offshore Wind Turbine
No full textFloating offshore wind turbines are taking on more and more prominence, as the industry moves towards larger turbines, farther offshore, in deeper water. The increase in turbine size can reduce the costs of offshore wind energy, but requires larger support structures. Rather than redesigning the structure completely, a rational methodology for upscaling an existing floating substructure can improve the efficiency of the design process. This work presents a guideline for the optimization and upscaling of a semi-submersible oating platform, addressing also special challenges related to changes in turbine technology, as well as design criteria for oating platforms. The OC4 semi-submersible platform is used as starting point of this study. Based on analysis results of an initial elementary upscaling procedure, the main criteria, when dealing with a semi-submersible oating platform, are specified. Optimization is then carried out, focusing on stability and eigenfrequencies. Reducing the upper column diameter and changing the ballast position within the columns, yields longer natural periods in heave and pitch, a lighter and cheaper platform, and a less over-conservative, but still stable and safe system. Based on this optimized design, a guideline for an upscaling procedure for any other turbine is given. The main scaling factor is determined from the mass ratio of the top structures, rather than the turbine rating. The main column is scaled, so that it fits the new tower base diameter. The scaling factor for the upper columns is computed, based on the ratio of the overturning moments, and considers the contribution of the different columns to the stiffness component in pitch. The mooring line length is scaled, such that it can yield a predefined stiffness. Finally, the controller gains are recalculated, based on the expected natural frequency in pitch. The optimization and upscaling process is carried out for Fraunhofer's offshore wind turbine IWT-7.5-164 and the DTU 10 MW reference wind turbine. The oating wind turbines are analyzed, modelled and simulated by means of simplified spreadsheet methods, linear frequencydomain calculations (in DNV's software HydroD), and detailed equation-based models (in Fraunhofer's software Modelica). The systems are evaluated regarding their eigenfrequencies, nominal pitch, stability and global performance in selected sea states, taking variable buoyancy and center of buoyancy into account, and adjusting the blade-pitch controller gains. The results, obtained from both computer programs and the initial hand calculations, are comparable and satisfying. The structural integrity is proved by a simplified approach, using tank pressure and sea pressure for computing the equivalent stress. More detailed strength checks for fatigue and ultimate limit states, as well as optimization of the mooring system, are left for future work.Mechanical, Maritime and Materials EngineeringOffshore and Dredging EngineeringEuropean Wind Energy Master - EWE
Gradient-based optimization of jacket support structure for offshore wind turbines with the local approach
No full textOffshore wind turbines are exposed to complex irregular and highly fluctuating environmental conditions. The analysis is, therefore commonly performed in the time domain, resulting in a time consuming and computationally demanding optimization process. In order to reduce the computational cost, an optimization approach using the principle of decomposition can be utilized. Here, it is assumed that when changing dimensions of a structural members, sectional response forces and moments remain constant. This assumption is made for both the actual changed member and for all other members of the structure. Additionally, this is assumed to be valid for a simultaneous change of all members all over the structure. It is the so-called local optimization approach and allows for an individual (local) optimization of each member of the structure. The objective of this thesis is the development of a gradient based optimization framework for the local optimization of structural members of an offshore wind turbine with jacket support structure and the implementation of an algorithm using the principle of decomposition. The development of the gradient based method for the local optimization necessitate the study of changes in global performance when modifying structural members locally and a method to receive these sensitivities in structural performance elementwise. Thereafter, the algorithms were used with the local optimization approach. The implementation was carried out using customized Matlab scripts for the automated load analysis with the finite element solver Fedem WindPower. The UpWind reference jacket used within the OC4-project and the generic NREL 5MW turbine atop were used as the case study for this work. In addition, as the objective function for the optimization process, a cost model that considers both manufacturing and material costs was developed. Finally the evaluation of the speed and the accuracy of the methods proposed is presented and the efficiency of the method is proved.Civil Engineering and GeosciencesHydraulic EngineeringOffshore & Dredging Engineerin
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Probabilistic estimation of fatigue loads on monopile-based offshore wind turbines
No full textOffshore wind energy faces three important trends: (1) wind farms grow in size, (2) monopiles are installed in deeper water, and (3) cost reduction remains the most important challenge. With wind farm size, the importance of variations in environmental site conditions across the wind farm increases. These site variations, e.g. water depth and soil conditions, can lead to significant differences of loads on support structures. For monopiles in deeper water, design is dominated by wave-induced fatigue loads. Since full fatigue load calculations are computationally demanding, they can typically not be performed for each turbine within large wind farms. Therefore, turbines must be grouped into clusters in early project phases, making time-efficient approaches essential. Optimization of design clustering is necessary to reduce design conservatism and the cost of offshore wind energy. Hence, the goal of this thesis is to investigate load site variations and clustering. Therefore, a probabilistic fatigue load estimation method is developed and verified with aero-elastic simulations in the time domain. Subsequently, the developed method is applied for an exemplary wind farm of 150 turbines in 30-40m water depth to perform – sensitivity studies of loads to changes in MSL, soil stiffness, and wave parameters, – probabilistic assessments of data, statistical and model uncertainties, and – deterministic and probabilistic design clustering. The estimation method is based on frequency domain analysis to calculate wave-induced fatigue loads, a scaling approach for wind loads, combination of wind-wave loads with quadratic superposition, and Monte-Carlo simulations to assess uncertainties. Verification confirms an accuracy of 95% for lifetime equivalent fatigue loads compared to time domain simulations. The computational speed is in the order of 100 times faster than typical time domain tools. Sensitivity studies show a significant influence of water depth and wave period on equivalent fatigue loads. The influence of soil on equivalent fatigue loads is minor for high soil stiffness but can increase significant for soils with low stiffness. Normal distributed input parameters in a probabilistic assessment yield a positively skewed probability distribution of equivalent fatigue loads. Design clustering is optimized based on site-specific fatigue loads using brute-force and discrete optimization algorithms. Results for the exemplary wind farm show a design load reduction of up to 13% compared to standardized design. Probabilistic clustering proved to be only relevant at cluster borders leading to a difference in allocation for 12 out of 150 turbines. Project results show that it is essential to account for load differences in large wind farms due to varying site conditions. This study improves clustering and provides a basis for design optimization and uncertainty analysis in large wind farms. Further work is needed to extend tool verification and formulate design clustering for cost optimization.Master Offshore and Dredging EngineeringStructural EngineeringCivil Engineering and Geoscience
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Fatigue reassessment for lifetime extension of offshore wind monopile substructures
Get PDFFatigue reassessment is required to decide about lifetime extension of aging offshore wind farms. This paper presents a methodology to identify important parameters to monitor during the operational phase of offshore wind turbines. An elementary effects method is applied to analyze the global sensitivity of residual fatigue lifetimes to environmental, structural and operational parameters. Therefore, renewed lifetime simulations are performed for a case study which consists of a 5 MW turbine with monopile substructure in 20 m water depth. Results show that corrosion, turbine availability, and turbulence intensity are the most influential parameters. This can vary strongly for other settings (water depth, turbine size, etc.) making case-specific assessments necessary.Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd
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