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Power Performance and Response Analysis of a Semi-Submersible Wind Turbine Combined with Wave Energy Converters in Intermediate and Deep Water
Full text linkRenewable energies are the forefront against environmental pollution and the leading technology in sustainable energy sources. Power produced by wind energy is a well established technology. There is a lot of space for expansion though, especially at offshore environment. Floating wind turbines can take advantage of the abundant wind energy available far out at the oceans. Increasing the power production of such structures and ensuring the efficient and safe position keeping of them in every depth is crucial. Two concepts that aim to tackle down these two issues are proposed. A combination of wind and wave energy converters and an intermediate water depth mooring arrangement. The installation of wave energy converters (WECs) at the floating wind turbine base will reduce the levelized cost of energy (LCoE). The use of the same power cables and mooring arrangement to deploy the WECs can prove beneficial financially wise. Furthermore, it can act as a boost to the development of wave energy in general. The transient depth between shallow and deep is a challenging field for the traditional mooring arrangements. A cost effective solution will help the deployment of floating wind turbines in this intermediate water depth fields.
A novel concept of combining a floating wind turbine with WECs is proposed. A semi submersible floater is used to support a 5MW wind turbine. Three flap typed WECs are deployed on the pontoons of the floater. A two-point type absorber called Torus is installed at the central column. The proposed concept is named STFC. A thorough analysis of the natural periods, regular and irregular wave tests is performed to evaluate the effect of the WECs to the floater’s behavior. Small to no effect is observed across all degrees of freedom except the reduction in pitch period due to increased hydrostatic stiffness. The irregular wave tests indicate that the absorbed power amplitude operator (RAO) of Torus has a wider excitation range than the flaps. The floater heave RAO is affected by the addition of Torus and the floater surge and pitch RAOs are affected by the additions of flaps. Irregular waves tests are carried out to evaluate the influence of the added WECs to the motions of the combined concept and the total power performance. The results indicate that the WECs have little to no effect to the motions of the structure except the expected reduction in pitch motions. This is preliminary indication that no changes should be made to the mooring due to the addition of WECs. The wind turbine power performance is not affected by the addition of WECs. There is small reduction in pitch standard deviation though, in loading condition EC3 that leads to slightly better wind quality. The Torus performance is satisfactory as it accounts for 9% of the total STFC power production. The total three flaps account only for 14% of the Torus power production thus their power performance is not satisfactory. The flaps rotation is out of phase with the wave excitation force so there is room for improvement if active damping and stiffness control is added.
Following, the STFC is moved to intermediate water depth z = -50m and a new hybrid mooring design is proposed. A brief explanation of the intermediate water mooring challenges is given and the basic design criteria are established. A step to step designing process is presented. The combination of studded chain, buoys, and clump weights is proposed. The name of the concept is CCCB. A restoring force test is carried out and the linearity of the restoring force response is verified. Irregular wave tests are carried out and the data indicate that the new mooring design is a feasible design. The maximum floater offset is restrained to 14% of the water depth. The pretension of the mooring system proves to be a significant factor for the total performance of the mooring arrangement. The maximum mooring line tension is kept within safe limits throughout the whole offset range. CCCB is compared with similar concepts and proves itself stiffer mainly because of its increased pretension. Points of interest are defined across the mooring line and spectra analysis is performed to evaluate the distribution of response frequencies across the chain length. The role of buoys as dampers of motions is established. CCCB utilization factor indicates that this mooring design can be used also in larger structures and there is room for cost reduction measures
Extreme response analysis of a floating offshore wind turbine and Fatigue analysis of the mooring lines
Full text linkIn this study, we evaluate the design of a floating offshore wind turbine mooring system for
intermediate water depths of 60 meters. We also analyzed extreme load cases and fatigue analysis
of the mooring lines. The mooring system proposed for this case is a 3-segment chain-rope-chain
configuration, with polyester rope as the intermediate rope segment. The mooring line's bottom
segment and middle segment lengths were modified to reach 10% of the mean breaking strength
(MBS) pretension in the polyester segment. The bottom part of the polyester was attached to a
buoy to prevent collision with the seabed. To model the nonlinearity of the polyester rope, we used
the Syrope method recommended by DNV. This method determines the dynamic stiffness of the
mooring line. The environmental conditions at the Sørlige Nordsjø II (SN II) site, located 140 km
off the coast of Norway, were analyzed using a joint distribution model from previous studies to
determine significant wave height, wave period, and wind speeds.
We proposed five different mooring configurations to be tested and compared. The results
suggest that while the mooring system 5 (MS5) configuration provides better platform stability,
the MS1 configuration is the preferable design due to its reduction of mooring line maximum
loads, standard deviation of the load, and hence fatigue damage. This makes MS1 the best choice
among the proposed line configurations for further analysis.
Extreme load cases (DLC16 at 25 m/s wind speed and DLC61 at 43 m/s wind speed) were
identified for detailed analysis. The results indicate that DLC61 is more critical for the design than
DLC16 due to higher extreme tension and larger platform surge displacement.
To evaluate fatigue, a maximum dissimilarity algorithm (MDA) was first implemented to
select representative sea states from a large database of generated sea states. This method resulted12
in a manageable number of simulations while trying to represent all of the sea conditions. Fatigue
analysis revealed that the chain segment at the fairlead should be the main focus of the fatigue
analysis as recommended by API which gives a lower estimated fatigue life. The fatigue life for
the chain at the fairlead was estimated to be around 280 years with a safety factor of 1.
In conclusion, this study tries to evaluate the mooring system design for the proposed site off
the coast of Norway in 60-meter water depths for wind farm development with a focus on mooring
line design and fatigue analysis. This study finds the MS1 to be the best of the five proposed
mooring configurations and the extreme response analysis of the mooring line suggests that even
at the 50-year return period sea state (DLC61) the extreme mooring line tension reaches around
35% of the MBS. The fatigue analysis also suggests that the chain segment at the fairlead has a
suitable estimated life of 280 years (safety factor of 1). These results suggest that MS1 can be a
reliable design for the development of a floating offshore wind farm at the proposed site
Mooring designs for a combined wind and wave energy system in intermediate water depth
Full text linkOver the past few decades, the renewable energy industry has expanded offshore in response to the growing demand for clean and sustainable energy. Larger structures with greater power output and less visual disturbance from shore are some of the reasons that motivate the development. Floating energy systems such as offshore wind turbines and wave energy converters (WECs) offer significant potential for producing clean, sustainable energy. The levelized cost of energy from wind and wave energy can potentially be reduced by developing combined wind and wave energy systems which allows for the co-sharing of infrastructure such as mooring systems, supporting platforms and power cables.
In the present study, different mooring systems of a combined wind and wave energy system – the semi-submersible torus flap combination (STFC) concept, deployed in intermediate water depths are developed. STFC consists of a torus WEC, three flap type WECs and a 5 MW NREL reference wind turbine. Fully coupled time-domain analysis of STFC under aligned wind and wave conditions has been performed using SIMA, a software for the analysis of marine operations. The tension-stretch relationship of polyester ropes is modelled using the Syrope model. Six environmental conditions at 50 m water depth representing operational conditions of STFC have been simulated.
The findings of this study have led to the production of two papers focusing on the mooring systems of STFC. The first paper compares catenary and taut mooring systems against a hybrid system consisting of chains, polyester ropes and a buoy. The results are presented in terms of system restoring forces, system natural periods, motion responses, mooring line tensions responses and the cost of mooring. Lastly, a design recommendation for the mooring system of STFC is given. The second paper focuses on a parametric study of the anchor radius of a polyester-based taut mooring system in order to reduce the mooring footprint and cost. The system restoring stiffness is kept constant while anchor spacing, mooring line length, cross-sectional area and minimum breaking load are proportionally reduced. Results are presented in terms of motion responses and mooring line tension responses.
In the first paper, the deployment of a hybrid mooring system is seen to decrease the non-linearity of tension responses, mooring stiffness, and the cost of mooring when compared to catenary and taut mooring systems. No slack line, vertical load on the anchor or contact between the polyester ropes and seabed is observed. Hybrid mooring systems offer solutions with the lowest cost as the combination of polyester ropes and drag-embedment anchors significantly reduce cost. In the second paper, a reduction in anchor radius while maintaining similar surge and sway restoring stiffnesses is seen to reduce pre-tension. Consequently, the yaw stiffness reduces, and a longer yaw natural period is observed between the largest and smallest anchor radius. The maximum tension increases with decreasing anchor radius due to increased surge resonant response. No event of slack line is observed. The minimum tension decreases at an increasing rate as the anchor radius decreases, suggesting that a further proportional reduction of the anchor radius would result in a slack line
Comparative dynamic analysis of semi-submersible floating two-rotor wind turbine on catenary and taut mooring in shallow water depth
Full text linkA floating offshore wind turbine (FOWT) comprising a semi-submersible platform and a two-rotor wind turbine installed in a water depth of 50 m is proposed in this thesis. The rotor used is the 5 MW Reference Wind Turbine from the National Renewable Energy Laboratory. Two mooring systems, under the same pre-tension, are proposed for comparison of their station-keeping capacity, tensions, and consequences to FOWT structure: a catenary system using steel chain and a taut system using polyester rope. The non-linear relationship between tension and stretch of the polyester rope is described using the Syrope model. A linear quadratic regulator (LQR), which main function is to limit the yaw motion, is used to design the blade-pitch controller for the two-rotor wind turbine configuration. Time-domain static and dynamic analyses are performed using a multi-body model of the FOWT in OpenModelica (OM) open-source software. The validation of the OM model is done by comparing the natural periods with a twin model created in SIMA, an industry recognized software for marine operations analyses. Six environmental conditions (EC) comprising turbulent wind and irregular waves are simulated. The two mooring systems are compared in terms of platform rigid body motions, mooring line tensions, and tower and blade moments. Mean, standard deviation and power spectral analysis of the time series are used for the comparison.
It is observed that the taut system presents larger surge offset, but similar or lower tensions on the mooring lines, when compared to the catenary system. In the case of heave, pitch and yaw motions the catenary system shows similar responses to the taut system for the less severe ECs and larger responses when close to or beyond the cut-out wind speed. The mooring system also does not greatly influence tower bending and twisting moments, and the blade flap-wise moment, except for the more severe conditions. Furthermore, it is shown that the LQR controller properly limits the yaw motion
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Variations on the Author
Full text link“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
Full text linkWe 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
Consequences of Load Mitigation Control Strategies for a Floating Wind Turbine
No full textWind turbines with floating support structures are blessed with their potential in generating high quality and affordable electricity due to the economics of scale. However, conventional land-based wind turbines blade pitch control system coupled with a floating structure present a problem known as control-induced resonance. The problem, if unresolved may lead to devastating implications such as structural and machinery failure due to fatigue.
This thesis started off by examining the working theory of a detuned version of conventional land-based controller intended for offshore floating wind turbines (FOWT) and subsequently using it as a baseline for performance comparison with a few alternative state of the art designs that also aim at reducing control-induced resonance. Rather than developing novel controller designs, the focus of this work has been on code implemen- tation and extending the performance evaluation criteria beyond structural. A decoupled approach is taken to separately account for the dynamics of different component levels. Modifications to the baseline control strategy are made through a java interface interacting with aero-hydro-servo-elastic code SIMA which is the environment where the global analysis is carried out. Finally the global response is then imported to MBS simulation software SIMPACK for drivetrain multi-body dynamics simulations.
Three alternative controllers are considered and implemented in this work. The first one is simple yet elegant variable speed variable pitch strategy proposed by Lackner in which platform surge velocity is being fed to the control loop with a constant gain as active damping term to augment the reference speed of the rotor [1]. The ’energy shaping’ filter developed by Pedersen in his doctoral thesis augment the reference speed through a function developed on the basis of energy conservation [2]. Lastly the energy shaping controller is complement with an individual pitch control mechanism to reduce blade flap- wise bending load described by Lackner [3] and Bossanyi [4].
The performance of controllers is evaluated based not only on their ability to mitigate structural loads but also loads on drivetrain component in which limited studies has been made. Frequency response analysis proves to be effective in identifying frequencies at which the controllers are behaving poorly and allows the possibility of filtering out of such frequencies at input. By looking at the frequency response of the component in the drivetrain, sources of excitation from global analysis can also be identified and thus giving a more complete picture with regards to the gains and losses using a particular controller. Finally, one hour fatigue damage comparison is used to justify that the mitigation of loads at certain frequencies leads to an improved fatigue performance
Consequences of Load Mitigation Control Strategies for a Floating Wind Turbine
No full textWind turbines with floating support structures are blessed with their potential in generating high quality and affordable electricity due to the economics of scale. However, conventional land-based wind turbines blade pitch control system coupled with a floating structure present a problem known as control-induced resonance. The problem, if unresolved may lead to devastating implications such as structural and machinery failure due to fatigue.
This thesis started off by examining the working theory of a detuned version of conventional land-based controller intended for offshore floating wind turbines (FOWT) and subsequently using it as a baseline for performance comparison with a few alternative state of the art designs that also aim at reducing control-induced resonance. Rather than developing novel controller designs, the focus of this work has been on code implemen- tation and extending the performance evaluation criteria beyond structural. A decoupled approach is taken to separately account for the dynamics of different component levels. Modifications to the baseline control strategy are made through a java interface interacting with aero-hydro-servo-elastic code SIMA which is the environment where the global analysis is carried out. Finally the global response is then imported to MBS simulation software SIMPACK for drivetrain multi-body dynamics simulations.
Three alternative controllers are considered and implemented in this work. The first one is simple yet elegant variable speed variable pitch strategy proposed by Lackner in which platform surge velocity is being fed to the control loop with a constant gain as active damping term to augment the reference speed of the rotor [1]. The ’energy shaping’ filter developed by Pedersen in his doctoral thesis augment the reference speed through a function developed on the basis of energy conservation [2]. Lastly the energy shaping controller is complement with an individual pitch control mechanism to reduce blade flap- wise bending load described by Lackner [3] and Bossanyi [4].
The performance of controllers is evaluated based not only on their ability to mitigate structural loads but also loads on drivetrain component in which limited studies has been made. Frequency response analysis proves to be effective in identifying frequencies at which the controllers are behaving poorly and allows the possibility of filtering out of such frequencies at input. By looking at the frequency response of the component in the drivetrain, sources of excitation from global analysis can also be identified and thus giving a more complete picture with regards to the gains and losses using a particular controller. Finally, one hour fatigue damage comparison is used to justify that the mitigation of loads at certain frequencies leads to an improved fatigue performance
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