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  • 1.
    Engström, Jens
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    Shahroozi, Zahra
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    Katsidoniotaki, Eirini
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity. Centre of Natural Hazards and Disaster Science (CNDS).
    Stavropoulou, Charitini
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    Johannesson, Pär
    RISE Research Institutes of Sweden, Department of Applied Mechanics.
    Göteman, Malin
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity. Centre of Natural Hazards and Disaster Science (CNDS).
    Offshore Measurements and Numerical Validation of the Mooring Forces on a 1:5 Scale Buoy2023In: Journal of Marine Science and Engineering, E-ISSN 2077-1312, Vol. 11, no 1, article id 231Article in journal (Refereed)
    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.

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  • 2.
    Giannini, Gianmaria
    et al.
    Univ Porto FEUP, Dept Civil Engn, Fac Engn, P-4200465 Porto, Portugal.;Univ Porto CIIMAR, Interdisciplinary Ctr Marine & Environm Res, P-4200465 Porto, Portugal..
    Temiz, Irina
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    Rosa-Santos, Paulo
    Univ Porto FEUP, Dept Civil Engn, Fac Engn, P-4200465 Porto, Portugal.;Univ Porto CIIMAR, Interdisciplinary Ctr Marine & Environm Res, P-4200465 Porto, Portugal..
    Shahroozi, Zahra
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    Ramos, Victor
    Univ Porto FEUP, Dept Civil Engn, Fac Engn, P-4200465 Porto, Portugal.;Univ Porto CIIMAR, Interdisciplinary Ctr Marine & Environm Res, P-4200465 Porto, Portugal..
    Göteman, Malin
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    Engström, Jens
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    Day, Sandy
    Univ Strathclyde, Dept Naval Architecture Ocean & Marine Engn, Glasgow G4 0LZ, Lanark, Scotland..
    Taveira-Pinto, Francisco
    Univ Porto FEUP, Dept Civil Engn, Fac Engn, P-4200465 Porto, Portugal.;Univ Porto CIIMAR, Interdisciplinary Ctr Marine & Environm Res, P-4200465 Porto, Portugal..
    Wave Energy Converter Power Take-Off System Scaling and Physical Modelling2020In: Journal of Marine Science and Engineering, E-ISSN 2077-1312, Vol. 8, no 9, article id 632Article in journal (Refereed)
    Abstract [en]

    Absorbing wave power from oceans for producing a usable form of energy represents an attractive challenge, which for the most part concerns the development and integration, in a wave energy device, of a reliable, efficient and cost-effective power take-off mechanism. During the various stages of progress, for assessing a wave energy device, it is convenient to carry out experimental testing that, opportunely, takes into account the realistic behaviour of the power take-off mechanism at a small scale. To successfully replicate and assess the power take-off, good practices need to be implemented aiming to correctly scale and evaluate the power take-off mechanism and its behaviour. The present paper aims to explore and propose solutions that can be applied for reproducing and assessing the power take-off element during experimental studies, namely experimental set-ups enhancements, calibration practices, and error estimation methods. A series of recommendations on how to practically organize and carry out experiments were identified and three case studies are briefly covered. It was found that, despite specific options that can be strictly technology-dependent, various recommendations could be universally applicable.

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    fulltext
  • 3.
    Giassi, Marianna
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Thomas, Simon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Shahroozi, Zahra
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    Engström, Jens
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Isberg, Jan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Tosdevin, Tom
    Hann, Martyn
    Göteman, Malin
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Preliminary results from a scaled test of arrays of point-absorbers with 6 DOF2019In: Proceedings of the 13th European Wave and Tidal Energy Conference, 2019Conference paper (Refereed)
  • 4.
    Göteman, Malin
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity. Centre of Natural Hazards and Disaster Science (CNDS), Villavägen 16, Uppsala, 752 36, Sweden.
    Shahroozi, Zahra
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    Stavropoulou, Charitini
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    Katsidoniotaki, Eirini
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity. Centre of Natural Hazards and Disaster Science (CNDS), Villavägen 16, Uppsala, 752 36, Sweden.
    Engström, Jens
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    Resilience of wave energy farms using metocean dependent failure rates and repair operations2023In: Ocean Engineering, ISSN 0029-8018, E-ISSN 1873-5258, Vol. 280, article id 114678Article in journal (Refereed)
    Abstract [en]

    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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  • 5.
    Katsidoniotaki, Eirini
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity. Centre of Natural Hazards and Disaster Science (CNDS).
    Shahroozi, Zahra
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    Eskilsson, Claes
    Palm, Johannes
    Engström, Jens
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    Göteman, Malin
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity. Centre of Natural Hazards and Disaster Science (CNDS).
    Validation of a CFD model for wave energy system dynamics in extreme waves2023In: Ocean Engineering, ISSN 0029-8018, E-ISSN 1873-5258, Vol. 268, article id 113320Article in journal (Refereed)
    Abstract [en]

    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.

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  • 6.
    Shahroozi, Zahra
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering.
    Prediction horizon requirement  in control and extreme load analyses for survivability: Advancements to improve the performance of wave energy technologies2021Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The main objective of wave energy converters (WECs) is to ensure reliable electricity production at a competitive cost. Two challenges to achieving this are ensuring an efficient energy conversion and offshore survivability.        

    This thesis work is structured in three different sections: Control and maximum power optimization, forces and dynamics analysis in extreme wave conditions, and statistical modeling of extreme loads in reliability analysis.       

    The need for prediction and future knowledge of waves and wave forces is essential due to the non-causality of the optimal velocity relation for wave energy converters. Using generic concepts and modes of motion, the sensitivity of the prediction horizon to various parameters encountered in a real system is elaborated. The results show that through a realistic assumption of the dissipative losses, only a few seconds to about half a wave cycle is sufficient to predict the required future knowledge for the aim of maximizing the power absorption.         

    The results of a 1:30 scaled wave tank experiment are used to assess the line force and dynamic behaviour of a WEC during extreme wave events. Within the comparison of different wave type representations, i.e. irregular, regular and focused waves, of the same sea state, the results show that not all the wave types deliver the same maximum line forces. As a strategy of mitigating the line forces during extreme wave events, changing the power take-off (PTO) damping may be employed. With consideration of the whole PTO range, the results indicate an optimum damping value for each sea state in which the smallest maximum line force is obtained. Although wave breaking slamming and end-stop spring compression lead to high peak line forces, it is possible that they level out due to the overtopping effect. Waves with a long wavelength result in large surge motion and consequently higher and more damaging forces.        

    On the investigation of reliability assessment of the wave energy converter systems, computing the return period of the extreme forces is crucial. Using force measurement force data gathered at the west coast of Sweden, the extreme forces are statistically modelled with the peak-over-threshold method. Then, the return level of the extreme forces over 20 years for the calm season of the year is computed.

    List of papers
    1. Considerations on prediction horizon and dissipative losses for wave energy converters
    Open this publication in new window or tab >>Considerations on prediction horizon and dissipative losses for wave energy converters
    2021 (English)In: IET Renewable Power Generation, ISSN 1752-1416, E-ISSN 1752-1424, IET Renewable Power Generation, Vol. 15, no 14, p. 3434-3458Article in journal (Refereed) Published
    Abstract [en]

    The non-causal optimal control law for wave energy converters leads to a requirement of predicting waves and wave forces over a future horizon.  Using examples of generic body shapes and oscillation modes, we show through computations of the velocity reference trajectory how the length of prediction horizon required to reach the maximum power output depends on the level of dissipative losses in the conversion chain. The sensitivity to noise is discussed, and so is the use of filtering to improve performance when the available prediction horizon is short or predictions are inaccurate. Considerations are also made for amplitude constraints and other effects encountered in a real system.  With realistic assumptions for the level of dissipative losses, results indicate that the prediction horizon needed to approach the maximum achievable power output for real systems ranges from only a few seconds up to about half a wave period, which is shorter than has generally been assumed earlier.

    Place, publisher, year, edition, pages
    Institution of Engineering and TechnologyInstitution of Engineering and Technology (IET), 2021
    Keywords
    Wave energy converter, prediction horizon, dissipative losses, optimal velocity, useful power
    National Category
    Energy Systems Marine Engineering Control Engineering Ocean and River Engineering
    Identifiers
    urn:nbn:se:uu:diva-457294 (URN)10.1049/rpg2.12290 (DOI)000703141600001 ()
    Funder
    Swedish Research Council Formas, 2020-03634Swedish Energy Agency
    Available from: 2021-10-27 Created: 2021-10-27 Last updated: 2024-03-12Bibliographically approved
    2. Experimental investigation of a point-absorbing wave energy converter response in different wave types of extreme sea states
    Open this publication in new window or tab >>Experimental investigation of a point-absorbing wave energy converter response in different wave types of extreme sea states
    (English)In: Article in journal, Editorial material (Other academic) Submitted
    National Category
    Energy Systems Marine Engineering Ocean and River Engineering Energy Engineering
    Identifiers
    urn:nbn:se:uu:diva-457326 (URN)
    Available from: 2021-10-27 Created: 2021-10-27 Last updated: 2021-10-27
    3. Experimental results of force measurements from a scaled point absorbing wave energy converter subjected to extreme waves
    Open this publication in new window or tab >>Experimental results of force measurements from a scaled point absorbing wave energy converter subjected to extreme waves
    2021 (English)In: Proceedings of the Fourteenth European Wave and Tidal Energy Conference, European Wave and Tidal Energy Conference (EWTEC) , 2021Conference paper, Published paper (Refereed)
    Abstract [en]

    To achieve a high reliability and durability for wave energy technologies, the effect of extreme wave conditions on the system must be understood. Wave tank experiments are an essential tool to evaluate this, and provide also a foundation for validation of numerical and analytical methods. However, it is not straight-forward how to design such small scale experiments so that they realistically represent wave energy converters in the ocean. In this paper, wave tank experiments of a 1:30 scaled friction damping linear power take-off (PTO) and cylindrical buoy with ellipsoidal bottom are presented. The linear PTO includes a rod that moves vertically against a Teflon block which introduces friction damping. The damping can be adjusted by changing the spring length that provides the compressive force between the Teflon block and the rod. To study extreme forces and snap loads, two load cells measure the line force both directly beneath the buoy, and at the top of the PTO. The motion of the PTO and the buoy are measured with a wire draw line position sensor and Qualysis system, respectively, and a data acquisition system collects and synchronizes the data. The extreme wave conditions used in the experiments are sea states with 50 years return period at the Dowsing site, North Sea. The waves are modelled as regular, irregular and focused waves. Here, the experimental setup and dry testing experiments are presented, and results of the wave tank test experiment for extreme forces are evaluated and further compared with WEC-SIM, to evaluate the agreement of the numerical and experimental model.

    Place, publisher, year, edition, pages
    European Wave and Tidal Energy Conference (EWTEC), 2021
    Series
    Proceedings of the European Wave and Tidal Energy Conference, ISSN 2706-6932, E-ISSN 2706-6940
    National Category
    Marine Engineering Ocean and River Engineering
    Identifiers
    urn:nbn:se:uu:diva-457301 (URN)
    Conference
    Fourteenth European Wave and Tidal Energy Conference (EWTEC), 5-9 September, 2021, Plymouth, UK
    Available from: 2021-10-27 Created: 2021-10-27 Last updated: 2024-03-12
    4. Design and evaluation of linear and rotational generator scale models for wave tank testing
    Open this publication in new window or tab >>Design and evaluation of linear and rotational generator scale models for wave tank testing
    2019 (English)Conference paper, Published paper (Refereed)
    Place, publisher, year, edition, pages
    CRC Press, 2019
    National Category
    Energy Systems Ocean and River Engineering Marine Engineering Other Electrical Engineering, Electronic Engineering, Information Engineering
    Identifiers
    urn:nbn:se:uu:diva-457307 (URN)9780429505324 (ISBN)9781138585355 (ISBN)
    Conference
    3rd international conference on renewable energies offshore (renew 2018), 8–10 october 2018, Lisbon, Portugal
    Available from: 2021-10-27 Created: 2021-10-27 Last updated: 2024-03-12Bibliographically approved
    5. Wave Energy Converter Power Take-Off System Scaling and Physical Modelling
    Open this publication in new window or tab >>Wave Energy Converter Power Take-Off System Scaling and Physical Modelling
    Show others...
    2020 (English)In: Journal of Marine Science and Engineering, E-ISSN 2077-1312, Vol. 8, no 9, article id 632Article in journal (Refereed) Published
    Abstract [en]

    Absorbing wave power from oceans for producing a usable form of energy represents an attractive challenge, which for the most part concerns the development and integration, in a wave energy device, of a reliable, efficient and cost-effective power take-off mechanism. During the various stages of progress, for assessing a wave energy device, it is convenient to carry out experimental testing that, opportunely, takes into account the realistic behaviour of the power take-off mechanism at a small scale. To successfully replicate and assess the power take-off, good practices need to be implemented aiming to correctly scale and evaluate the power take-off mechanism and its behaviour. The present paper aims to explore and propose solutions that can be applied for reproducing and assessing the power take-off element during experimental studies, namely experimental set-ups enhancements, calibration practices, and error estimation methods. A series of recommendations on how to practically organize and carry out experiments were identified and three case studies are briefly covered. It was found that, despite specific options that can be strictly technology-dependent, various recommendations could be universally applicable.

    Place, publisher, year, edition, pages
    MDPI, 2020
    Keywords
    power take-off damping, wave power device, experimental testing, PTO simulator, uncertainty analysis, wave energy testing, experimental set-up, calibration
    National Category
    Other Electrical Engineering, Electronic Engineering, Information Engineering
    Identifiers
    urn:nbn:se:uu:diva-425480 (URN)10.3390/jmse8090632 (DOI)000582055100001 ()
    Funder
    EU, Horizon 2020, POCI-01-0145-FEDER-016882EU, Horizon 2020, PTDC/MAR-TEC/6984/2014StandUp, 47264-1
    Available from: 2020-11-19 Created: 2020-11-19 Last updated: 2024-03-12Bibliographically approved
    6. Offshore measurements of hydrodynamic forces on a 1:5 scale buoy
    Open this publication in new window or tab >>Offshore measurements of hydrodynamic forces on a 1:5 scale buoy
    Show others...
    (English)In: Article in journal, Editorial material (Other academic) Submitted
    National Category
    Energy Systems Marine Engineering Ocean and River Engineering Energy Engineering
    Identifiers
    urn:nbn:se:uu:diva-457327 (URN)
    Available from: 2021-10-27 Created: 2021-10-27 Last updated: 2021-10-27
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  • 7.
    Shahroozi, Zahra
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity. Uppsala University.
    Eriksson, Mikael
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    Göteman, Malin
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    Engström, Jens
    Design and evaluation of linear and rotational generator scale models for wave tank testing2019Conference paper (Refereed)
  • 8.
    Shahroozi, Zahra
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    Göteman, Malin
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    Engström, Jens
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    A Neural Network Approach To Minimize Line Forces In The Survivability Of The Point-Absorber Wave Energy Converters2023In: Proceedings of ASME 2023 42nd International Conference on Ocean, Offshore & Arctic Engineering (OMAE2023), ASME Press, 2023, Vol. 8, article id OMAE2023-102422Conference paper (Refereed)
    Abstract [en]

    One strategy for the survivability of wave energy converters(WECs) is to minimize the extreme forces on the structure by adjusting the system damping. In this paper, a neural network model is developed to predict the peak line force for a 1:30 scaled point-absorber WEC with a linear friction-damping power take-off (PTO). The algorithm trains over the wave tank experimental data and enables an update of the system damping based on the system state (i.e. position, velocity, and acceleration) and information on the incoming waves for the extreme sea states. The results show that the deep neural network (DNN) developed here is relatively fast and able to predict the peak line forces with a correlation of 88% when compared to the true (experimental)data. Then, the optimum damping for survivability purposes is found by minimizing the peak line force. It is shown that the optimum damping varies depending on the system state in each zero up-crossing episode.

  • 9.
    Shahroozi, Zahra
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    Göteman, Malin
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    Engström, Jens
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    Control of a point absorber wave energy converter in extreme wave conditions using a deep learning model in WEC-Sim2023In: OCEANS 2023 - LIMERICK, IEEE, 2023Conference paper (Refereed)
    Abstract [en]

    The survivability of wave energy converters (WECs) is one of the challenges that have a direct influence on their cost. To protect the WEC from the impact of extreme waves, it is often to over-dimension the components or adopt survivability modes e.g. by submerging or lifting the WEC if it is applicable. Here, a control strategy for adjusting the system damping is developed based on deep neural networks (DNN) to minimize the line (mooring) force exerted on a 1:30 scaled WEC. This DNN model is then implemented in a control system of a numerical WEC-Sim model to find the optimal power take-off (PTO) damping for every zero up-crossing episode of surface elevation which minimizes the peak line force. The WEC-Sim model was calibrated based on a 1:30 scaled wave tank experiment that was designed to investigate the WEC response in extreme sea states with a 50-year return period. It is shown that this survival strategy reduces the peak forces when compared with the response of a system that has been set to a constant PTO damping for the entire duration of the sea state.

  • 10.
    Shahroozi, Zahra
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    Göteman, Malin
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity. Centre of Natural Hazards and Disaster Science (CNDS), Uppsala, Sweden .
    Engström, Jens
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    Experimental investigation of a point-absorber wave energy converter response in different wave-type representations of extreme sea states2022In: Ocean Engineering, ISSN 0029-8018, E-ISSN 1873-5258, Vol. 248, article id 110693Article in journal (Refereed)
    Abstract [en]

    The experimental results of a 1:30 scaled wave tank experiment of a point-absorber in extreme sea states and intermediate water depth are studied. The effect of the PTO damping parameter and different non-linear phenomena in extreme wave conditions such as wave breaking and overtopping are investigated with the focus on the maximum line (mooring) force in the presence of an upper end-stop. In the comparison of different wave-type representations, i.e. irregular, regular, and focused waves, of the same sea state, not all the wave types necessarily yield to the same peak line force. Moreover, there exists an optimum damping value for each sea state in which the smallest peak force is achieved. Both end-stop spring compression and wave-breaking slamming result in peak line forces which may be compensated by overtopping water pressure. Large surge motion is obtained for waves with a long wavelength which can contribute to higher and more damaging line forces.

    Download full text (pdf)
    fulltext
  • 11.
    Shahroozi, Zahra
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    Göteman, Malin
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    Engström, Jens
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    Experimental results of force measurements from a scaled point absorbing wave energy converter subjected to extreme waves2021In: Proceedings of the Fourteenth European Wave and Tidal Energy Conference, European Wave and Tidal Energy Conference (EWTEC) , 2021Conference paper (Refereed)
    Abstract [en]

    To achieve a high reliability and durability for wave energy technologies, the effect of extreme wave conditions on the system must be understood. Wave tank experiments are an essential tool to evaluate this, and provide also a foundation for validation of numerical and analytical methods. However, it is not straight-forward how to design such small scale experiments so that they realistically represent wave energy converters in the ocean. In this paper, wave tank experiments of a 1:30 scaled friction damping linear power take-off (PTO) and cylindrical buoy with ellipsoidal bottom are presented. The linear PTO includes a rod that moves vertically against a Teflon block which introduces friction damping. The damping can be adjusted by changing the spring length that provides the compressive force between the Teflon block and the rod. To study extreme forces and snap loads, two load cells measure the line force both directly beneath the buoy, and at the top of the PTO. The motion of the PTO and the buoy are measured with a wire draw line position sensor and Qualysis system, respectively, and a data acquisition system collects and synchronizes the data. The extreme wave conditions used in the experiments are sea states with 50 years return period at the Dowsing site, North Sea. The waves are modelled as regular, irregular and focused waves. Here, the experimental setup and dry testing experiments are presented, and results of the wave tank test experiment for extreme forces are evaluated and further compared with WEC-SIM, to evaluate the agreement of the numerical and experimental model.

  • 12.
    Shahroozi, Zahra
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    Göteman, Malin
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    Engström, Jens
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Electrical Engineering, Electricity.
    Fatigue analysis of a point-absorber wave energy converter based on augmented data from a WEC-Sim model calibrated with experimental data2022In: Trends in Renewable Energies Offshore: Proceedings of the 5th International Conference on Renewable Energies Offshore, London: CRC Press, 2022Conference paper (Refereed)
    Abstract [en]

    To avoid over-designing wave energy converters (WECs), their reliability and survivability aspects need to be accurately addressed. The most common failure modes are: instantaneous failure due to high instantaneous loads, and fatigue failure due to the accumulated damage in the structure during years of operation. Here, we present a fatigue analysis of a point-absorber WEC in sea states corresponding to a 50-year environmental contour from the Dowsing site, UK. The data for this analysis is generated by a WEC-Sim model that is calibrated with a 1:30 scaled WEC from a wave tank experiment. In this study, the partial damage in each 1-hour sea state sample is calculated using the rainflow counting and Palmgren-Miner rule. Then, considering the joint probability density function of the sea states, the equivalent two-million cycle load is 2.42 MN for the full-scale system considering the accumulated damage in 50 years of operation. In a comparison of the fatigue limit state (FLS) and ultimate limit state (ULS), it was found that the ULS is the governing limit state in the design of the WEC system here.