1,721,045 research outputs found
Design and Analysis of Mooring System for Semi-submersible Floating Wind Turbines in Shallow Water
No full textThere has been a remarkable development in offshore wind energy. From the view of cost efficiency, floating wind turbine becomes more competitive than bottom fixed wind turbine when the water depth comes above 50 m. Semi-submersible floater concepts have been proposed to be deployed in shallow waters (50 m - 100 m) because of its smaller draft compared to other floater types.
However, mooring system design is extremely challenging for floating structures in shallow water. There is a transition from linearity to nonlinearity in the relationship between mooring line tension and offset at the fairlead, which indicates a potential for extremely large mooring line tension during harsh environment when the floater motions have large offset ranges. Moreover, the nonlinear tension increment in shallow water becomes more critical than deep water. In addition, the degree of tightness of mooring system will influence the tension increment trend as well.
In present thesis, two mooring system design concepts in 100 m and 50 m water depth (shallow water) have been proposed for the use of 5-MW-CSC semi-submersible floating wind turbine. Original mooring system design in 200 m water depth is taken as a reference. Initially, preliminary static design has been carried out in SIMA to determine mooring line properties, mooring system configurations and document static performances of mooring line. Afterwards, free decay tests are carried out to calculate the natural periods of the floating wind turbine with mooring system at six degrees of freedom in the three water depths. At last, fully coupled time-domain dynamic analysis in ultimate limit state and fatigue limit state were performed using Simo-Riflex-AeroDyn. Critical responses have been checked to verify if proposed design concepts satisfy design standards.
In order to achieve desired mooring line pretension and catenary shape in different water depths, studless chain link has been chosen for 50 m and spiral rope has been selected for 100 m. The corresponding clump weights have been enlarged to 60 tonnes for both cases. Moreover, in order to avoid large nonlinear tension increment, mooring system is designed to be soft on purpose in 50 m. Meanwhile, corresponding natural periods have been examined to be close to the reference model, larger than the periods of most relevant waves.
Extreme condition test results indicate that the maximum response of mooring line tension and floater motion occurs when wave is acting aligned with mooring line configuration. Despite, mooring line strengths are sufficient in the light of utilization factor calculations and floater motions have been successfully limited to a reasonable extent. In accordance with operational condition test, accumulated fatigue damage for chain link is larger than spiral rope, even though the stress ranges do not show big difference. This is because highly contact force between chain links produces significant fatigue damage, which does not apply to spiral rope. Furthermore, there is no protection treatment for chain link like plastic sheathing for wire rope. According to spectrum analysis, wave is dominating the responses during extreme condition, while wind contributes most to the responses by influencing low-frequency motion resonant responses in operational condition.
The strategy to fight against the nonlinear tension increment in 50 m is to design the mooring system to be relatively soft, which will lead to relatively large floater motion at the same time. Nevertheless, the compromise has achieved great effect: extreme mooring line tensions do not exceed the capacities, even though the tension has increased nonlinearly. Meanwhile the compromised floater motions are under reasonable ranges, without mooring line being totally lifted up. Mooring system design for 100 m shows great performance as well without extreme line tension or large floater motion, which could be regarded as a satisfactory design concept
Design and Analysis of Mooring System for Semi-submersible Floating Wind Turbines in Shallow Water
Get PDFThere has been a remarkable development in offshore wind energy. From the view of cost efficiency, floating wind turbine becomes more competitive than bottom fixed wind turbine when the water depth comes above 50 m. Semi-submersible floater concepts have been proposed to be deployed in shallow waters (50 m - 100 m) because of its smaller draft compared to other floater types.
However, mooring system design is extremely challenging for floating structures in shallow water. There is a transition from linearity to nonlinearity in the relationship between mooring line tension and offset at the fairlead, which indicates a potential for extremely large mooring line tension during harsh environment when the floater motions have large offset ranges. Moreover, the nonlinear tension increment in shallow water becomes more critical than deep water. In addition, the degree of tightness of mooring system will influence the tension increment trend as well.
In present thesis, two mooring system design concepts in 100 m and 50 m water depth (shallow water) have been proposed for the use of 5-MW-CSC semi-submersible floating wind turbine. Original mooring system design in 200 m water depth is taken as a reference. Initially, preliminary static design has been carried out in SIMA to determine mooring line properties, mooring system configurations and document static performances of mooring line. Afterwards, free decay tests are carried out to calculate the natural periods of the floating wind turbine with mooring system at six degrees of freedom in the three water depths. At last, fully coupled time-domain dynamic analysis in ultimate limit state and fatigue limit state were performed using Simo-Riflex-AeroDyn. Critical responses have been checked to verify if proposed design concepts satisfy design standards.
In order to achieve desired mooring line pretension and catenary shape in different water depths, studless chain link has been chosen for 50 m and spiral rope has been selected for 100 m. The corresponding clump weights have been enlarged to 60 tonnes for both cases. Moreover, in order to avoid large nonlinear tension increment, mooring system is designed to be soft on purpose in 50 m. Meanwhile, corresponding natural periods have been examined to be close to the reference model, larger than the periods of most relevant waves.
Extreme condition test results indicate that the maximum response of mooring line tension and floater motion occurs when wave is acting aligned with mooring line configuration. Despite, mooring line strengths are sufficient in the light of utilization factor calculations and floater motions have been successfully limited to a reasonable extent. In accordance with operational condition test, accumulated fatigue damage for chain link is larger than spiral rope, even though the stress ranges do not show big difference. This is because highly contact force between chain links produces significant fatigue damage, which does not apply to spiral rope. Furthermore, there is no protection treatment for chain link like plastic sheathing for wire rope. According to spectrum analysis, wave is dominating the responses during extreme condition, while wind contributes most to the responses by influencing low-frequency motion resonant responses in operational condition.
The strategy to fight against the nonlinear tension increment in 50 m is to design the mooring system to be relatively soft, which will lead to relatively large floater motion at the same time. Nevertheless, the compromise has achieved great effect: extreme mooring line tensions do not exceed the capacities, even though the tension has increased nonlinearly. Meanwhile the compromised floater motions are under reasonable ranges, without mooring line being totally lifted up. Mooring system design for 100 m shows great performance as well without extreme line tension or large floater motion, which could be regarded as a satisfactory design concept
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
Load assessment of optimally-arranged point absorbers arrays in front of a vertical wall
No full textIn this paper, we assess numerically the loads applied on PAs of optimally-arranged linear arrays in front of a bottom-mounted vertical wall of finite length under normal operating conditions. The arrays, maximizing the yearly absorbed energy, consist of five, identical, oblate spheroidal heaving PAs and are deployed at three near-shore sites in the Aegean Sea, Greece. The PAs are assumed to be attached on the wall via connection configurations restraining all rigid-body modes except heave. A spectral analysis is performed to quantify loads. The corresponding transfer functions are obtained from a frequency-based hydrodynamic model that solves the diffraction/radiation problem of the multi-body arrangement in the presence of the wall. Results, focusing on surge and sway restraining loads, are, initially, presented for regular waves and, then, for normal operating conditions (irregular waves), highlighting the effect of the arrays’ layouts and of the local wave conditions on the restraining loads. Comparison is also made with equally-spaced arrays to reveal potential positive effects of optimum layouts on structural integrity related issues.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Offshore Engineerin
Optimum Layouts of a Cluster of Heaving Point Absorbers in front of a Wall
No full textIn this paper, we determine optimum layouts of a cluster of oblate spheroidal heaving point absorbers in front of a wall, that maximize the annual averaged power absorbed by the cluster, while satisfying specific spatial constraints. An iterative optimization process is developed by coupling a hydrodynamic model with a genetic algorithms solver. Optimization is performed for three near-shore sites in the Aegean Sea, Greece. Optimum layouts are obtained considering part of or the whole wall length, available for the PAs’ sitting. The effect of the incident wave direction on the optimum layouts’ formation and the absorbed power is also assessed. Finally, the dependence of the maximized absorbed power upon the deployment site is illustrated.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Offshore Engineerin
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
Layout optimization of heaving Wave Energy Converters linear arrays in front of a vertical wall
No full textThe present paper focuses on the determination of optimum layouts for linear arrays of heaving Wave Energy Converters (WECs) in front of a vertical wall. Optimum layouts maximize the annual averaged absorbed energy at a given marine site and satisfy spatial constraints. For achieving this goal, we developed an efficient optimization numerical framework, where a genetic algorithm solver is appropriately coupled with a frequency-domain hydrodynamic model, while, furthermore, a numerical wave model is utilized to determine the local wave climate conditions at the site of interest. The context is applied for an array of five semi-immersed, oblate spheroidal heaving WECs deployed at five near-shore sites of mild wave environments in the Aegean Sea, Greece. For each site, different optimization cases are solved, facilitating the investigation of different aspects of the examined problem. The largest annual energy absorption ability is observed for optimum layouts, characterized by the placement of the array close to the wall and the formation of clusters of closely-positioned WECs near the wall edges. Compared to arrays employed at sites in south-eastern Aegean, optimally-arranged arrays at central Aegean locations showed reduced energy absorption ability due to milder local wave conditions and/or the existence of quite limited water depths.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Offshore Engineerin
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
- …
