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Offshore Measurements and Numerical Validation of the Mooring Forces on a 1:5 Scale Buoy
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.ORCID iD: 0000-0002-2031-8134
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.ORCID iD: 0000-0002-1165-5569
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity. Centre of Natural Hazards and Disaster Science (CNDS).ORCID iD: 0000-0001-5096-3559
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
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2023 (English)In: Journal of Marine Science and Engineering, E-ISSN 2077-1312, Vol. 11, no 1, article id 231Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
MDPI, 2023. Vol. 11, no 1, article id 231
Keywords [en]
wave energy conversion, point absorber, line force, offshore measurements
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:uu:diva-494290DOI: 10.3390/jmse11010231ISI: 000917599500001OAI: oai:DiVA.org:uu-494290DiVA, id: diva2:1727707
Funder
Swedish Energy Agency, 47264-1Swedish Research Council, 2015-04657Lars Hierta Memorial FoundationAvailable from: 2023-01-17 Created: 2023-01-17 Last updated: 2024-04-04Bibliographically approved
In thesis
1. Offshore renewable energy systems: Quantification of extreme loads using computational methods
Open this publication in new window or tab >>Offshore renewable energy systems: Quantification of extreme loads using computational methods
2023 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2023. p. 125
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 2233
Keywords
offshore systems, wave energy, CFD, extreme events, machine learning, Bayesian experimental design, active learning, surrogate modeling
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:uu:diva-492822 (URN)978-91-513-1701-4 (ISBN)
Public defence
2023-03-17, Häggsalen, Ångströmlaboratoriet. Lägerhyddsvägen 2, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2023-02-23 Created: 2023-01-28 Last updated: 2023-02-23
2. Time-domain modeling of a wave power farm
Open this publication in new window or tab >>Time-domain modeling of a wave power farm
2023 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

In this work, we model a point absorber wave energy converter (WEC) very similar to the device that originated and was developed at Uppsala University in Sweden. The device is simulated both as a one- and a two-body model. The one-body WEC model assumes a stiff connection line between the floating buoy on the sea surface and the translator inside the linear generator. The two-body model, on the other hand, allows for slack-line phenomena. This connection also acts as a mooring line for the buoy, and it is a crucial component in understanding and evaluating the survivability of WECs in the offshore environment, in particular during extreme wave conditions. The linear models are validated against experimental data, focusing on the buoy motion and the force in the mooring line. The two-body model captures the extreme dy-namics and force peaks well, whereas the one-body model predicts the WEC response with a few more inaccuracies. However, the one-body model still produces acceptable results in operational waves, and since one-body models are predominant due to their simplicity and nu-merical efficiency, we choose to extend the one-body model to simulate multiple interacting point-absorbers. Therefore, a novel, fast time-domain model is developed for an array of 6 up to 96 interacting point-absorbers. The WECs are placed in a symmetric grid, where each row contains one pair. The devices interact by scattered and radiated waves, while they are restricted to move in one degree of freedom, heave. Under the assumption of linear potential flow the-ory, the hydrodynamic coefficients for the excitation and radiation forces are obtained using an analytical multiple scattering method, as well as the boundary element software WAMIT. The equations of motion are solved following Cummins’ formulation. Modeling an array of WECs in the time domain comes down to solving a system of integro-differential equations, where convolution terms appear in the computation of the excitation and radiation forces. In the ma-jority of wave farm models, frequency-domain approaches are used to solve the equations of motion, since time-domain models are more computationally demanding and significantly more challenging to develop. This is not only because of the numerical integration involved but espe-cially due to the computation of the convolution term accounting for the radiated water waves on the free surface, implying that waves radiated by the body in the past continue to affect the dynamics in the future. Regardless of the computational effort associated with time-domain approaches, their use is required for realistic control applications and complex device dynam-ics, like non-linearities due to the power take-off configuration. It is of high interest to study whether the linear numerical model simulates accurately the performance of each interacting WEC. Therefore, the numerical results for the full array configuration are compared with ex-perimental data. The experimental results used were carried out in the COAST Lab at Plymouth University, UK, corresponding to a 1:10 scaled prototype of an array of point absorbers. De-spite the highly non-linear effects in the physical experiment, the free motion of the buoys in all directions, and the power take-off configuration, the numerical scheme is able to accurately capture the heaving motion of the buoys and their power absorption.

Place, publisher, year, edition, pages
Uppsala: Uppsala University, 2023. p. 70
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-514826 (URN)
Presentation
, Uppsala (English)
Available from: 2023-11-07 Created: 2023-10-23 Last updated: 2023-11-08Bibliographically approved

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Engström, JensShahroozi, ZahraKatsidoniotaki, EiriniStavropoulou, CharitiniGöteman, Malin

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