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Wave Loads and Peak Forces on Moored Wave Energy Devices in Tsunamis and Extreme Waves
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Surface gravity waves carry enormous amounts of energy over our oceans, and if their energy could be harvested to generate electricity, it could make a significant contribution to the worlds power demand. But the survivability of wave energy devices in harsh operating conditions has proven challenging, and for wave energy to be a possibility, peak forces during storms and extreme waves must be studied and the devices behaviour understood. Although the wave power industry has benefited from research and development in traditional offshore industries, there are important differences. Traditional offshore structures are designed to minimize power absorption and to have small motion response, while wave power devices are designed to maximize power absorption and to have a high motion response. This increase the difficulty of the already challenging survivability issue. Further, nonlinear effects such as turbulence and overtopping can not be neglected in harsh operating conditions. In contrast to traditional offshore structures, it is also important to correctly account for the power take off system in a wave energy converter (WEC), as it is strongly coupled to the devices behaviour.

The focus in this thesis is the wave loads and the peak forces that occur when a WEC with a limited stroke length is operated in waves higher than the maximum stroke length. The studied WEC is developed at Uppsala University, Sweden, and consists of a linear generator at the seabed that is directly driven by a surface buoy. A fully nonlinear CFD model is developed in the finite volume software OpenFOAM, and validated with physical wave tank experiments. It is then used to study the motion and the forces on the WEC in extreme waves; high regular waves and during tsunami events, and how the WECs behaviour is influenced by different generator parameters, such as generator damping, friction and the length of the connection line. Further, physical experiments are performed on full scale linear generators, measuring the total speed dependent damping force that can be expected for different loads. The OpenFOAM model is used to study how the measured generator behaviour affects the force in the connection line.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. , p. 86
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1551
Keywords [en]
OpenFOAM, CFD, Wave power, Tsunami waves, Extreme waves, Offshore
National Category
Marine Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
URN: urn:nbn:se:uu:diva-328499ISBN: 978-91-513-0054-2 (print)OAI: oai:DiVA.org:uu-328499DiVA, id: diva2:1135809
Public defence
2017-10-20, Polhemsalen, 10134, Ångström, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2017-09-28 Created: 2017-08-24 Last updated: 2017-10-17
List of papers
1. Numerical models for the motion and forces of point-absorbing wave energy converters in extreme waves
Open this publication in new window or tab >>Numerical models for the motion and forces of point-absorbing wave energy converters in extreme waves
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2017 (English)In: Ocean Engineering, ISSN 0029-8018, E-ISSN 1873-5258, Vol. 145, p. 1-14Article in journal (Refereed) Published
Abstract [en]

Reliable simulation tools are necessary to study the performance and survivability of wave energy devices, since experiments are both expensive and difficult to implement. In particular, survivability in nonlinear, high waves is one of the largest challenges for wave energy, and since the wave loads and dynamics are largely model dependent, each device must be studied separately with validated tools. In this paper, two numerical methods based on fully nonlinear computational fluid dynamics (CFD) are presented and compared with a simpler linear method. All three methods are compared and validated against experimental data for a point-absorbing wave energy converter in nonlinear, high waves. The wave energy converter consists of a floating buoy attached to a linear generator situated on the seabed. The line forces and motion of the buoy are studied, and computational cost and accuracy are compared and discussed. Whereas the simpler linear method is very fast, its accuracy is not sufficient in high and extreme waves, where instead the computationally costly CFD methods are required. The OpenFOAM model showed the highest accuracy, but also a higher computational cost than the ANSYS Fluent model.

National Category
Marine Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-328485 (URN)10.1016/j.oceaneng.2017.08.061 (DOI)000414886600001 ()
Funder
Natural‐Disaster ScienceSwedish Research Council, 2015-04657
Available from: 2017-08-24 Created: 2017-08-24 Last updated: 2018-02-22Bibliographically approved
2. Buoy geometry and its influence on survivability for apoint absorbing wave energy converter: Scaleexperiment and CFD simulations
Open this publication in new window or tab >>Buoy geometry and its influence on survivability for apoint absorbing wave energy converter: Scaleexperiment and CFD simulations
Show others...
2017 (English)Conference paper, Oral presentation with published abstract (Refereed)
Abstract [en]

For wave energy to be an economically viable energysource, the technology has to withstand power levelsduring storms that can be close to 50 times higher thanduring normal operating conditions, and withstandmany years of wear. The impact of high wave loads isstudied not only within the field of wave energy, buthas long been a subject of study for ships, platformsand other offshore structures.To model the force on the device under extreme and/orovertopping waves is a difficult task. Experiments areexpensive and difficult to implement, and numerical meth-ods are either very computationally demanding CFD-methods, or less accurate approximative methods. Inaddition, the performance and experienced forces during extreme waves are model dependent, and differentoffshore structures must be studied independently.Here, a 1:20 scale model of the Uppsala Universitypoint-absorber type wave energy converter (WEC) has been tested in extreme wave conditions at the COASTLaboratory Ocean Basin at Plymouth University. The WEC consists of a linear generator connected to a buoyat the sea surface, and performance of two differentbuoys is studied: a cylinder and cylinder with moon-pool. Two types of wave sets have been used: focusedwaves embedded into regular waves, and irregular waves. The focus of this paperis on comparing the performance of the two buoys, and on analysing the experimental data using a numerical model. A fully non-linear computational fluid dynamics(CFD) model based on OpenFOAM is presented and validated.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-321807 (URN)
Conference
Marine Energy Technology Symposium, WATERPOWER WEEK IN WASHINGTON
Available from: 2017-05-11 Created: 2017-05-11 Last updated: 2017-08-24
3. The Effect of Overtopping Waves on Peak Forceson a Point Absorbing WEC
Open this publication in new window or tab >>The Effect of Overtopping Waves on Peak Forceson a Point Absorbing WEC
2016 (English)Conference paper, Published paper (Refereed)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:uu:diva-313253 (URN)
Conference
Asian Wave and Tidal Energy Conference Series, AWTEC, October 24-28 2016, Singapore, Singapore
Note

Title in Book of Abstracts: OpenFOAM Modelling of Point-absorbing WECs with Different Buoy Topologies

Available from: 2017-01-18 Created: 2017-01-18 Last updated: 2017-08-24Bibliographically approved
4. Peak forces on a point absorbing wave energy converter impacted by tsunami waves
Open this publication in new window or tab >>Peak forces on a point absorbing wave energy converter impacted by tsunami waves
2019 (English)In: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 133, p. 1024-1033Article in journal (Refereed) Published
Abstract [en]

Although a tsunami wave in deep sea can be simulated using linear shallow water theory, the wave dynamics of a tsunami running up a continental shelf is very complex, and different phenomena may occur, depending on the width and profile of the shelf, the topography of the coast, incident angle of the tsunami and other factors. How to simulate tsunami waves at an intermediate depth is studied in this paper by using three different simulation approaches for tsunamis, a soliton, a simulated high incident current and a dam-break approach. The surface wave profiles as well as the velocity- and pressure profiles for the undisturbed waves are compared. A regular Stokes 5th wave of the same amplitude is simulated for comparison. A wave energy converter model, previously validated with wave tank experiment, is then used to study the survivability of the Uppsala University wave energy device for the different waves. The force in the mooring line is studied together with the resulting force on a bottom mounted column, corresponding to the linear generator on the seabed.

Keywords
Wave energy, Extreme forces, Tsunami, OpenFOAM
National Category
Marine Engineering Energy Systems
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-328486 (URN)10.1016/j.renene.2018.10.092 (DOI)000456761300091 ()
Funder
Natural-Disaster ScienceSwedish Research Council, 2015-04657Swedish National Infrastructure for Computing (SNIC)
Available from: 2017-08-24 Created: 2017-08-24 Last updated: 2019-03-15Bibliographically approved
5. Survivability of a Point Absorbing Wave Energy Converter Impacted by Tsunami Waves
Open this publication in new window or tab >>Survivability of a Point Absorbing Wave Energy Converter Impacted by Tsunami Waves
2017 (English)In: 12th European Wave and Tidal Energy Conference Series, Cork, Ireland, 27 August - 1 September, 2017, 2017Conference paper, Oral presentation only (Refereed)
National Category
Marine Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-328487 (URN)
Conference
EWTEC2017
Available from: 2017-08-24 Created: 2017-08-24 Last updated: 2017-09-11
6. Peak Forces on Wave Energy Linear Generators in Tsunami and Extreme Waves
Open this publication in new window or tab >>Peak Forces on Wave Energy Linear Generators in Tsunami and Extreme Waves
2017 (English)In: Energies, E-ISSN 1996-1073, Vol. 10, no 9, article id 1323Article in journal (Refereed) Published
Abstract [en]

The focus of this paper is the survivability of wave energy converters (WECs) in extreme waves and tsunamis, using realistic WEC parameters. The impact of a generator damping factor has been studied, and the peak forces plotted as a function of wave height. The paper shows that an increased damping decreases the force in the endstop hit, which is in agreement with earlier studies. However, when analyzing this in more detail, we can show that friction damping and velocity dependent generator damping affect the performance of the device differently, and that friction can have a latching effect on devices in tsunami waves, leading to higher peak forces. In addition, we study the impact of different line lengths, and find that longer line lengths reduce the endstop forces in extreme regular waves, but on the contrary increase the forces in tsunami waves due to the different fluid velocity fields.

National Category
Marine Engineering
Research subject
Engineering Science with specialization in Science of Electricity
Identifiers
urn:nbn:se:uu:diva-328490 (URN)10.3390/en10091323 (DOI)000411225200079 ()
Funder
Natural‐Disaster ScienceSwedish Research Council, 2015-04657
Available from: 2017-08-24 Created: 2017-08-24 Last updated: 2023-08-28Bibliographically approved
7. Line Force and Damping at Full and Partial Stator Overlap in a Linear Generator for Wave Power
Open this publication in new window or tab >>Line Force and Damping at Full and Partial Stator Overlap in a Linear Generator for Wave Power
2016 (English)In: Journal of Marine Science and Engineering, E-ISSN 2077-1312, Vol. 4, no 4, article id 81Article in journal (Refereed) Published
Abstract [en]

A full scale linear generator for wave power has been experimentally evaluated bymeasuring the line force and translator position throughout the full translator stroke. The measuredline force, in relation to translator speed, generator damping and stator overlap, has been studied bycomparing the line force and the damping coefficient, γ, for multiple load cases along the translatorstroke length. The study also compares the generator ’s behavior during upward and downwardmotion, studies oscillations and determines the no load losses at two different speeds. The generatordamping factor, γ, was determined for five different load cases during both upward and downwardmotion. The γ value was found to be constant for full stator overlap and to decrease linearly witha decreasing overlap, as the translator moved towards the endstops. The decline varied with theexternal load case, as previously suggested but not shown. In addition, during partial stator overlap,a higher γ value was noted as the translator was leaving the stator, compared to when it was enteringthe stator. Finally, new insights were gained regarding how translator weight and generator dampingwill affect the translator downward motion during offshore operation. This is important for powerproduction and for avoiding damaging forces acting on the wave energy converter during operation.

Keywords
wave power, force measurement, line force, power take-off, damping
National Category
Electrical Engineering, Electronic Engineering, Information Engineering Oceanography, Hydrology and Water Resources
Identifiers
urn:nbn:se:uu:diva-313254 (URN)10.3390/jmse4040081 (DOI)000443616700018 ()
Note

De 2 första författarna delar förstaförfattarskapet.

Available from: 2017-01-18 Created: 2017-01-18 Last updated: 2020-01-07Bibliographically approved
8. Speed Dependent PTO Damping in a Linear Generator for Wave Power - Measured Damping and Simulated WEC Behaviour
Open this publication in new window or tab >>Speed Dependent PTO Damping in a Linear Generator for Wave Power - Measured Damping and Simulated WEC Behaviour
(English)Manuscript (preprint) (Other academic)
National Category
Marine Engineering
Identifiers
urn:nbn:se:uu:diva-328494 (URN)
Available from: 2017-08-24 Created: 2017-08-24 Last updated: 2017-09-21
9. Experimental study of generator damping at partial stator overlap in a linear generator for wave power
Open this publication in new window or tab >>Experimental study of generator damping at partial stator overlap in a linear generator for wave power
Show others...
2017 (English)In: 12th European Wave and Tidal Energy Conference Series, Cork, Ireland, 27 August - 1 September, 2017, 2017Conference paper, Oral presentation only (Refereed)
National Category
Marine Engineering
Identifiers
urn:nbn:se:uu:diva-328496 (URN)
Conference
EWTEC2017
Available from: 2017-08-24 Created: 2017-08-24 Last updated: 2021-10-17
10. On the Optimization of Point Absorber Buoys
Open this publication in new window or tab >>On the Optimization of Point Absorber Buoys
Show others...
(English)Article in journal (Refereed) Submitted
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-218218 (URN)
Available from: 2014-02-10 Created: 2014-02-10 Last updated: 2017-08-24Bibliographically approved
11. Calculating Buoy Response for a Wave Energy Converter - a Comparsion Between Two Computational Methods and Experimental Results
Open this publication in new window or tab >>Calculating Buoy Response for a Wave Energy Converter - a Comparsion Between Two Computational Methods and Experimental Results
Show others...
2017 (English)In: Theoretical and Applied Mechanics Letters, ISSN 2095-0349, Vol. 7, no 3, p. 164-168Article in journal, Letter (Refereed) Published
Abstract [en]

When designing a wave power plant, reliable and fast simulation tools are required. Computational fluid dynamics (CFD) software provides high accuracy but with a very high computational cost, and in operational, moderate sea states, linear potential flow theories may be sufficient to model the hydrodynamics. In this paper, a model is built in COMSOL Multiphysics to solve for the hydrodynamic parameters of a point-absorbing wave energy device. The results are compared with a linear model where the hydrodynamical parameters are computed using WAMIT, and to experimental results from the Lysekil research site. The agreement with experimental data is good for both numerical models.

National Category
Marine Engineering
Identifiers
urn:nbn:se:uu:diva-328498 (URN)10.1016/j.taml.2017.05.004 (DOI)000416966800008 ()
Funder
Natural‐Disaster ScienceSwedish Research Council, 2015-04657
Available from: 2017-08-24 Created: 2017-08-24 Last updated: 2018-03-07Bibliographically approved

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