| 1. |
- Engström, Jens, et al.
(författare)
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Offshore Measurements and Numerical Validation of the Mooring Forces on a 1:5 Scale Buoy
- 2023
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Ingår i: Journal of Marine Science and Engineering. - : MDPI. - 2077-1312. ; 11:1
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Tidskriftsartikel (refereegranskat)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. |
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| 3. |
- Eskilsson, Claes, et al.
(författare)
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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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Ingår i: Proceedings of the 15th European Wave and Tidal Energy Conference.
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Konferensbidrag (refereegranskat)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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| 4. |
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| 5. |
- Guth, Stephen, et al.
(författare)
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Statistical modeling of fully nonlinear hydrodynamic loads on offshore wind turbine monopile foundations using wave episodes and targeted CFD simulations through active sampling
- 2024
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Ingår i: Wind Energy. - : John Wiley & Sons. - 1095-4244 .- 1099-1824. ; 27:1, s. 75-100
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Tidskriftsartikel (refereegranskat)abstract
- Accurately determining hydrodynamic force statistics is crucial for designing offshore engineering structures, including offshore wind turbine foundations, due to the significant impact of nonlinear wave-structure interactions. However, obtaining precise load statistics often involves computationally intensive simulations. Furthermore, the estimation of statistics using current practices is subject to ongoing discussion due to the inherent uncertainty involved. To address these challenges, we present a novel machine learning framework that leverages data-driven surrogate modeling to predict hydrodynamic loads on monopile foundations while reducing reliance on costly simulations and facilitate the load statistics reconstruction. The primary advantage of our approach is the significant reduction in evaluation time compared to traditional modeling methods. The novelty of our framework lies in its efficient construction of the surrogate model, utilizing the Gaussian process regression machine learning technique and a Bayesian active learning method to sequentially sample wave episodes that contribute to accurate predictions of extreme hydrodynamic forces. Additionally, a spectrum transfer technique combines computational fluid dynamics (CFD) results from both quiescent and extreme waves, further reducing data requirements. This study focuses on reducing the dimensionality of stochastic irregular wave episodes and their associated hydrodynamic force time series. Although the dimensionality reduction is linear, Gaussian process regression successfully captures high-order correlations. Furthermore, our framework incorporates built-in uncertainty quantification capabilities, facilitating efficient parameter sampling using traditional CFD tools. This paper provides comprehensive implementation details and demonstrates the effectiveness of our approach in delivering reliable statistics for hydrodynamic loads while overcoming the computational cost constraints associated with classical modeling methods.
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| 6. |
- Göteman, Malin, 1980-, et al.
(författare)
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Resilience of wave energy farms using metocean dependent failure rates and repair operations
- 2023
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Ingår i: Ocean Engineering. - : Elsevier. - 0029-8018 .- 1873-5258. ; 280
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Tidskriftsartikel (refereegranskat)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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| 7. |
- Katsidoniotaki, Eirini, et al.
(författare)
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Comparison of dynamic mesh methods in OpenFOAM for a WEC in extreme waves
- 2020
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Ingår i: Developments in Renewable Energies Offshore. - 9781003134572
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Konferensbidrag (refereegranskat)abstract
- One of the big challenges for offshore wave energy systems is how to guarantee the survivability in harsh environmental conditions. Nonlinear and complex phenomena in steep and high amplitude waves can be captured by CFD methods. Mesh morphing is the most common used dynamic mesh method, yet the high and steep waves are a challenge since the large amplitude body motion leads to deterioration of the computational mesh quality. Advanced mesh methods have been developed to overcome this issue, such as overset method. The goal of the present paper is to compare morphing and overset methods implemented in OpenFOAM by modelling the response of a point-absorbing wave energy converter in extreme waves, identified along the 100-year extreme wave contour at the Humboldt Bay site, California. Simulations of two extreme sea states are conducted and the results from two different methods are compared and discussed.
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| 8. |
- Katsidoniotaki, Eirini, et al.
(författare)
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Digital Twin for the Prediction of Extreme Loads on a Wave Energy Conversion System
- 2022
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Ingår i: Energies. - : MDPI. - 1996-1073. ; 15:15
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Tidskriftsartikel (refereegranskat)abstract
- Wave energy is a renewable energy source with the potential to contribute to the global electricity demand, but a remaining challenge is the survivability of the wave energy converters in harsh offshore conditions. To understand the system dynamics and improve the reliability, experimental and numerical studies are usually conducted. However, these processes are costly and time-consuming. A statistical model able to provide equivalent results is a promising approach. In this study, the digital twin of the CFD solution is developed and implemented for the prediction of the force in the mooring system of a point-absorber wave energy converter during extreme wave conditions. The results show that the digital twin can predict the mooring force with 90.36% average accuracy. Moreover, the digital twin needs only a few seconds to provide the solution, while the CFD code requires up to several days. By creating a digital analog of a wave energy converter and showing that it is able to predict the load in critical components during extreme wave conditions, this work constitutes an innovative approach in the wave energy field. © 2022 by the authors.
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| 9. |
- Katsidoniotaki, Eirini
(författare)
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Extreme wave conditions and the impact on wave energy converters
- 2021
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The amount of energy enclosed in ocean waves has been classified as one of the most promising renewable energy sources. Nowadays, different wave energy conversion (WEC) systems are being investigated, but only a few concepts have been operated in a sea environment. One of the largest challenges is to guarantee the offshore survivability of the devices in extreme wave conditions. However, there are large uncertainties related to the prediction of extreme wave loads on WECs. Highfidelity computational fluid dynamics (CFD) simulations can resolve nonlinear hydrodynamic effects associated with wave-structure interaction (WSI).This thesis explores the point-absorbing WEC developed by Uppsala University in extreme wave conditions. The dynamic response and the forces on key components (mooring line, buoy, generator's end-stop spring) of the device are studied and compared. The high nonlinear phenomena accompany the steep and high waves, i.e., breaking behavior, slamming loads can be well-captured by the highfidelity CFD simulations. A commonly used methodology for extreme waves selection, recommended by technical specifications and guidelines, is the environmental contour approach. The 100-year contour in Hamboldt Bay site in California and the 50-year contour in the Dowsing site, outside the UK, are utilized to extract the extreme waves examined in the present thesis. Popular methodologies and data from different sources (observational and hindcast data) are examined for the environmental contour generation providing useful insights. Moreover, two popular approaches for the numerical representation of the extreme sea states, either as focused wave or as equivalent regular wave, were examined and compared. A midfidelity model of the WEC is successfully verified, as the utilization of lower fidelity tools in the design stage would reduce the computational cost. Last but not least, in CFD simulations the computational grid is sensitive in large motions, something often occurs during extreme-WSI. The solution of this issue for the open source CFD software OpenFOAM is provided here.
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