| 1. |
- Engström, Jens, et al.
(author)
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Offshore Measurements and Numerical Validation of the Mooring Forces on a 1:5 Scale Buoy
- 2023
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In: Journal of Marine Science and Engineering. - : MDPI. - 2077-1312. ; 11:1
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Journal article (peer-reviewed)abstract
- Wave energy conversion is a renewable energy technology with a promising potential. Although it has been developed for more than 200 years, the technology is still far from mature. The survivability in extreme weather conditions is a key parameter halting its development. We present here results from two weeks of measurement with a force measurement buoy deployed at Uppsala University’s test site for wave energy research at the west coast of Sweden. The collected data have been used to investigate the reliability for two typical numerical wave energy converter models: one low fidelity model based on linear wave theory and one high fidelity Reynolds-Averaged Navier–Stokes model. The line force data is also analysed by extreme value theory using the peak-over-threshold method to study the statistical distribution of extreme forces and to predict the return period. The high fidelity model shows rather good agreement for the smaller waves, but overestimates the forces for larger waves, which can be attributed to uncertainties related to field measurements and numerical modelling uncertainties. The peak-over-threshold method gives a rather satisfying result for this data set. A significant deviation is observed in the measured force for sea states with the same significant wave height. This indicates that it will be difficult to calculate the force based on the significant wave height only, which points out the importance of more offshore experiments. © 2023 by the authors.
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| 2. |
- Eskilsson, Claes, et al.
(author)
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Solution verification of WECs: comparison of methods to estimate numerical uncertainties in the OES wave energy modelling task
- 2023
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In: Proceedings of the 15th European Wave and Tidal Energy Conference.
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Conference paper (peer-reviewed)abstract
- High-fidelity models become more and more used in the wave energy sector. They offer a fully nonlinear simulation tool that in theory should encompass all linear and nonlinear forces acting on a wave energy converter (WEC). The focus on the studies using are usually dealing with validation. However, a validated model does not necessarily give reliable solutions. Solution verification is the methodology to estimate the numerical uncertainties related to a simulation. In this work we test four different approaches: the classical grid convergence index (GCI); a least-square version (LS-GCI), a simplified version of the least-square method (SLS-GCI) and the ITTC rec- ommended practice. The LS-GCI requires four or more solutions whereas the other three methods only need three solutions. We apply these methods to four different high- fidelity models for the case of a heaving sphere. We tested two parameters in the time-domain and two parameters in the frequency domain. It was found that the GCI and ITTC were hard to use on the frequency domain parameters as they require monotonic convergence which sometimes does not happen due to the differences in the solutions being very small. The SLS-GCI performed almost as well as the SL-GCI method and will be further investigated.
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| 3. |
- Göteman, Malin, 1980-, et al.
(author)
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Resilience of wave energy farms using metocean dependent failure rates and repair operations
- 2023
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In: Ocean Engineering. - : Elsevier. - 0029-8018 .- 1873-5258. ; 280
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Journal article (peer-reviewed)abstract
- Emerging offshore renewable energy technologies are expected to become an important part of the futureenergy system, and reliability for these new technologies in different metocean scenarios must be guaranteed.This poses a challenge in extreme weather scenarios like storms, in particular for less mature technologiessuch as wave energy. Not only the offshore survivability must be controlled; the restoration after disruptiveevents and failures should be addressed and optimized. Offshore operations are costly and cannot be carriedout if the weather is too harsh, and the resulting downtime after failures may be financially devastating forprojects. In this paper, the resilience of large wave energy systems is studied with respect to wave conditions,metocean dependent failure rates, and weather windows available for offshore repair operations. A metocean-and time-dependent failure rate is derived based on a Weibull distribution, which is a novelty of the paper.The performance of the farm is assessed using the varying failure rates and metocean data at different offshoresites. Critical metocean thresholds for different offshore vessels are considered, and the resilience is quantifiedusing relevant measures such as unavailability and expected energy not supplied. The resilience analysis iscoupled to an economic assessment of the wave farm and different repair strategies. Our results show thatthe commonly used assumption of constant failure rates is seen to overestimate the annual energy productionthan when a more realistic varying failure rate is used. Two offshore sites are compared, and the availabilityis found to be higher at the calmer site. Most of the evaluated repair strategies cannot be considered to beeconomically justified, when compared to the cost of the energy not supplied.
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| 4. |
- Katsidoniotaki, Eirini
(author)
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Offshore renewable energy systems : Quantification of extreme loads using computational methods
- 2023
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Doctoral thesis (other academic/artistic)abstract
- This Ph.D. thesis investigates the dynamic response of offshore energy systems in extreme waves. The use of offshore energy technologies, such as wave energy systems and offshore wind turbines, is crucial for transitioning to clean energy and mitigating the effects of climate change. However, to design reliable systems, it is important to understand their behavior in harsh environmental conditions.The first part of the thesis focuses on classical Computational Fluid Dynamics (CFD) simulations for modeling the response of structures in extreme waves. Breaking waves are numerically reproduced and the corresponding slamming loads are estimated, as well as the maximal forces on critical components such as the mooring system. The thesis addresses the challenge of computational mesh deformation, which can lead to numerical instability and failure in simulating extreme structural responses. Dynamic mesh techniques are implemented to overcome the limitations of classical techniques. Additionally, the thesis explores alternative approaches to representing a sea state, such as equivalent regular waves and focused waves, to reduce the computational cost of full sea state simulations. A mid-fidelity numerical model is also employed, with its accuracy verified against a high-fidelity solution.The second part of the thesis advances the use of probabilistic machine learning to develop a surrogate model for the mapping between extreme waves and the corresponding forces on the structure. A Bayesian active learning method is employed to train the model with high prediction accuracy, especially in extreme events. The surrogate model is many orders of magnitude faster than classical modeling methods and enables efficient statistical quantification of the quantities of interest, such as loads in critical system components.Overall, this thesis provides a comprehensive examination of advanced computational methods for estimating the dynamic response of offshore energy systems in extreme waves and enables reliable and cost-effective design through the use of fast and accurate surrogate models.
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| 5. |
- Katsidoniotaki, Eirini, et al.
(author)
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Validation of a CFD model for wave energy system dynamics in extreme waves
- 2023
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In: Ocean Engineering. - : Elsevier Ltd. - 0029-8018 .- 1873-5258. ; 268
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Journal article (peer-reviewed)abstract
- The design of wave energy converters should rely on numerical models that are able to estimate accurately the dynamics and loads in extreme wave conditions. A high-fidelity CFD model of a 1:30 scale point-absorber is developed and validated on experimental data. This work constitutes beyond the state-of-the-art validation study as the system is subjected to 50-year return period waves. Additionally, a new methodology that addresses the well-known challenge in CFD codes of mesh deformation is successfully applied and validated. The CFD model is evaluated in different conditions: wave-only, free decay, and wave–structure interaction. The results show that the extreme waves and the experimental setup of the wave energy converter are simulated within an accuracy of 2%. The developed high-fidelity model is able to capture the motion of the system and the force in the mooring line under extreme waves with satisfactory accuracy. The deviation between the numerical and corresponding experimental RAOs is lower than 7% for waves with smaller steepness. In higher waves, the deviation increases up to 10% due to the inevitable wave reflections and complex dynamics. The pitch motion presents a larger deviation, however, the pitch is of secondary importance for a point-absorber wave energy converter. © 2022 The Author(s)
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| 6. |
- Mendoza, Victor, et al.
(author)
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Aerodynamic performance of a dual turbine concept characterized by a relatively close distance between rotors
- 2023
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In: Wind Energy. - : John Wiley & Sons. - 1095-4244 .- 1099-1824. ; 26:6, s. 521-537
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Journal article (peer-reviewed)abstract
- In this work, a closely spaced dual turbine concept is studied. The distance between the two side-by-side hubs is 1.05D$$ D $$, where D$$ D $$ is the rotor diameter. This configuration has a potential benefit for offshore wind developments in which power density can be maximized. The main goal is to evaluate the overall aerodynamic performance, blade loads, and wake structure of a reference wind turbine generator operating within this dual turbine configuration and to compare the effects against those for the typical single turbine configuration. For this purpose, an actuator line model has been employed together with the large eddy simulation approach for predicting the turbulence effects. This model was implemented by using the open-source computational fluid dynamics toolbox OpenFOAM. Results show a better performance for the dual turbine concept. Under same operating conditions, the aerodynamic power of each turbine within the dual concept is higher than the power of the stand alone turbine, particularly at lower operating wind speeds (approximately 2% to 3% of extra power per turbine). Comparison between the two configurations shows similar character of the tangential and normal forces acting on the blades in terms of magnitude and fluctuation, eliminating potential concerns regarding fatigue and blade design. The largest difference in the tangential and normal root bending moments are approximately 3% and 2%, respectively, between single and dual turbine configurations. Finally, wake recovery analysis shows a downwind velocity deficit that is not enhanced streamwise in the dual turbine configuration with no considerable difference after 7D$$ D $$.
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| 7. |
- Stavropoulou, Charitini, et al.
(author)
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Fast time-domain model for the preliminary design of a wave power farm
- 2023
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In: Renewable energy. - : Elsevier. ; 219
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Journal article (peer-reviewed)abstract
- This study presents a novel, fast time-domain model developed for an array of interacting point-absorber wave energy converters. The model is validated using experimental wave tank data. The point-absorbers, based on Uppsala University’s design, are arranged in a symmetric grid and interact with scattered and radiated waves while constrained to the heave motion. The model employs linear potential flow theory to solve the hydrodynamic coefficients in the frequency domain and employs Cummins’ formulation to solve the equations of motion in the time domain. Modeling an array of wave energy converters in the time domain yields a system of integro-differential equations, featuring convolution terms in the excitation and radiation forces. This implies that past waves radiated by the body continue to impact future dynamics. Irregular long-crested waves, generated from the Bretschneider spectrum, serve as the incident waves for the study. The model’s accuracy in capturing the dynamics and power absorption of the farm is demonstrated through validation against experimental data from a 1:10 scaled prototype of a six-point-absorber array. Despite inherent differences between the experimental and numerical set-ups, the model accurately represents the farm’s behavior. Furthermore, an efficiency test reveals that the numerical scheme approximates the performance of wave power farms comprising 6, 12, 24, 48, and 96 interacting devices within a maximum computational time of 20 s. Overall, this research presents a novel and accurate time-domain model for analyzing an array of point-absorber wave energy converters. The model’s ability to capture the dynamics and power absorption, along with its efficiency in simulating larger wave power farms, make it a valuable tool for the preliminary design stage.
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