scholarly journals Transverse motion instability of a submerged moored buoy

2019 ◽  
Vol 475 (2221) ◽  
pp. 20180459 ◽  
Author(s):  
J. Orszaghova ◽  
H. Wolgamot ◽  
S. Draper ◽  
R. Eatock Taylor ◽  
P. H. Taylor ◽  
...  
Keyword(s):  
Wave Energy ◽  
Period Doubling ◽  
Vertical Cylinder ◽  
Mathieu Equation ◽  
Offshore Structures ◽  
Irregular Waves ◽  
Transverse Motion ◽  
Wave Direction ◽  
Wave Energy Converters ◽  
Energy Converters

Wave energy converters and other offshore structures may exhibit instability, in which one mode of motion is excited parametrically by motion in another. Here, theoretical results for the transverse motion instability (large sway oscillations perpendicular to the incident wave direction) of a submerged wave energy converter buoy are compared to an extensive experimental dataset. The device is axi-symmetric (resembling a truncated vertical cylinder) and is taut-moored via a single tether. The system is approximately a damped elastic pendulum. Assuming linear hydrodynamics, but retaining nonlinear tether geometry, governing equations are derived in six degrees of freedom. The natural frequencies in surge/sway (the pendulum frequency), heave (the springing motion frequency) and pitch/roll are derived from the linearized equations. When terms of second order in the buoy motions are retained, the sway equation can be written as a Mathieu equation. Careful analysis of 80 regular wave tests reveals a good agreement with the predictions of sub-harmonic (period-doubling) sway instability using the Mathieu equation stability diagram. As wave energy converters operate in real seas, a large number of irregular wave runs is also analysed. The measurements broadly agree with a criterion (derived elsewhere) for determining the presence of the instability in irregular waves, which depends on the level of damping and the amount of parametric excitation at twice the natural frequency.

Modelling extreme wave conditions for survivability tests of wave energy converters: CFD versus model tests

2021 ◽  
Author(s):  
Eric Gubesch ◽  
Nagi Abdussamie ◽  
Irene Penesis ◽  
Christopher Chin ◽  
Chien Ming Wang
Keyword(s):  
Water Column ◽  
Wave Energy ◽  
Offshore Structures ◽  
Surface Elevation ◽  
Experimental Sample ◽  
Oscillating Water Column ◽  
Experimental Conditions ◽  
Wave Energy Converters ◽  
Numerical Generation ◽  
Energy Converters

<p>This study investigates the experimental and numerical generation of realistic extreme waves in the Model Test Basin (MTB) at the Australian Maritime College, University of Tasmania, in order to test the survivability of offshore structures such as wave energy converters. The sea state and maximum wave height considered were collected during Tropical Cyclone Oma as it tracked down the Queensland Coast of Australia in February 2019. Upon successful generation of a repeatable experimental sample, the NewWave theory was used to regenerate the MTB surface elevation in a STAR-CCM+ computational fluid dynamics (CFD) numerical wave tank. The experimental surface elevation data was analysed with a fast Fourier transform to obtain the wave component amplitudes (a<sub>n</sub>) and phase angles (ε<sub>n</sub>).  These parameters were then used to generate a polychromatic wave in CFD. The 2D CFD simulations were extended to a 3D simulation that included an oscillating water column wave energy converter as per the experimental conditions. Results indicate that experimental focused wave groups can be replicated in CFD software with a similarity of 0.9407 for 2D simulations.  However, by applying an amplification factor to the crest amplitude of the focussed waves, one may further obtain improved accuracy in both 2D and 3D simulations. Further mesh resolution studies surrounding the oscillating water column may improve the accuracy of 3D fluid structure interaction simulations when investigating survivability.</p>

A Comparison of Selected Strategies for Adaptive Control of Wave Energy Converters

2011 ◽  
Vol 133 (3) ◽  
Author(s):  
Jørgen Hals ◽  
Johannes Falnes ◽  
Torgeir Moan
Keyword(s):  
Wave Energy ◽  
Reactive Power ◽  
Average Power ◽  
Volume Ratio ◽  
External Input ◽  
Irregular Waves ◽  
Optimal Velocity ◽  
Wave Energy Converters ◽  
Absorbed Power ◽  
Energy Converters

Wave-energy converters of the point-absorbing type (i.e., having small extension compared with the wavelength) are promising for achieving cost reductions and design improvements because of a high power-to-volume ratio and better possibilities for mass production of components and devices as compared with larger converter units. However, their frequency response tends to be narrow banded, which means that the performance in real seas (irregular waves) will be poor unless their motion is actively controlled. Only then the invested equipment can be fully exploited, bringing down the overall energy cost. In this work various control methods for point-absorbing devices are reviewed, and a representative selection of methods is investigated by numerical simulation in irregular waves, based on an idealized example of a heaving semisubmerged sphere. Methods include velocity-proportional control, approximate complex conjugated control, approximate optimal velocity tracking, phase control by latching and clutching, and model-predictive control, all assuming a wave pressure measurement as the only external input to the controller. The methods are applied for a single-degree-of-freedom heaving buoy. Suggestions are given on how to implement the controllers, including how to tune control parameters and handle amplitude constraints. Based on simulation results, comparisons are made on absorbed power, reactive power flow, peak-to-average power ratios, and implementation complexity. Identified strengths and weaknesses of each method are highlighted and explored. It is found that overall improvements in average absorbed power of about 100–330% are achieved for the investigated controllers as compared with a control strategy with velocity-proportional machinery force. One interesting finding is the low peak-to-average ratios resulting from clutching control for wave periods about 1.5 times the resonance period and above.

scholarly journals Review of Wave Energy Converter and Design of Mooring System

2020 ◽  
Vol 12 (19) ◽  
pp. 8251
Author(s):  
Dongsheng Qiao ◽  
Rizwan Haider ◽  
Jun Yan ◽  
Dezhi Ning ◽  
Binbin Li
Keyword(s):  
Wave Energy ◽  
Renewable Resources ◽  
Mooring System ◽  
Wave Direction ◽  
Research Focus ◽  
Energy Converter ◽  
Analysis And Design ◽  
Wave Energy Converters ◽  
Analysis Models ◽  
Energy Converters

In recent decades, the emphasis on renewable resources has grown considerably, leading to significant advances in the sector of wave energy. Nevertheless, the market cannot still be considered as commercialized, as there are still other obstacles in the mooring system for wave energy converters (WECs). The mooring system must be designed to not negatively impact the WEC’s efficiency and reduce the mooring loads. Firstly, the overview of the types of wave energy converters (WECs) are classified through operational principle, absorbing wave direction, location, and power take-off, respectively, and the power production analysis and design challenges of WECs are summarized. Then, the mooring materials, configurations, requirements, and the modeling approaches for WECs are introduced. Finally, the design of mooring systems, including the design considerations and standards, analysis models, software, current research focus, and challenges are discussed.

Constrained Optimal Control of a Heaving Buoy Wave-Energy Converter

2010 ◽  
Vol 133 (1) ◽  
Author(s):  
Jørgen Hals ◽  
Johannes Falnes ◽  
Torgeir Moan
Keyword(s):  
Energy Absorption ◽  
Wave Energy ◽  
Optimal Operation ◽  
Wave Energy Converter ◽  
Irregular Waves ◽  
Energy Converter ◽  
Model Predictive Controller ◽  
Wave Energy Converters ◽  
Predictive Controller ◽  
Energy Converters

The question of optimal operation of wave-energy converters has been a key issue since modern research on the topic emerged in the early 1970s, and criteria for maximum wave-energy absorption soon emerged from frequency domain analysis. However, constraints on motions and forces give the need for time-domain modeling, where numerical optimization must be used to exploit the full absorption potential of an installed converter. A heaving, semisubmerged sphere is used to study optimal constrained motion of wave-energy converters. Based on a linear model of the wave-body interactions, a procedure for the optimization of the machinery force is developed and demonstrated. Moreover, a model-predictive controller is defined and tested for irregular sea. It repeatedly solves the optimization problem online in order to compute the optimal constrained machinery force on a receding horizon. The wave excitation force is predicted by use of an augmented Kalman filter based on a damped harmonic oscillator model of the wave process. It is shown how constraints influence the optimal motion of the heaving wave-energy converter, and also how close it is possible to approach previously published theoretical upper bounds. The model-predictive controller is found to perform close to optimum in irregular waves, depending on the quality of the wave force predictions. An absorbed power equal to or larger than 90% of the ideal constrained optimum is achieved for a chosen range of realistic sea states. Under certain circumstances, the optimal wave-energy absorption may be better in irregular waves than for a corresponding regular wave having the same energy period and wave-power level. An argument is presented to explain this observation.

The dynamics and power extraction of bottom-hinged plate wave energy converters in regular and irregular waves

2015 ◽  
Vol 96 ◽  
pp. 86-99 ◽  
Author(s):  
R.P.F. Gomes ◽  
M.F.P. Lopes ◽  
J.C.C. Henriques ◽  
L.M.C. Gato ◽  
A.F.O. Falcão
Keyword(s):  
Wave Energy ◽  
Irregular Waves ◽  
Plate Wave ◽  
Power Extraction ◽  
Wave Energy Converters ◽  
Hinged Plate ◽  
Energy Converters

scholarly journals Array modeling and testing of fixed OWC type Wave Energy Converters

2020 ◽  
Vol 3 (3) ◽  
pp. 137-143
Author(s):  
Bret Bosma ◽  
Ted Brekken ◽  
Pedro Lomonaco ◽  
Bryony DuPont ◽  
Chris Sharp ◽  
...  
Keyword(s):  
Numerical Model ◽  
Wave Energy ◽  
Average Power ◽  
Irregular Waves ◽  
Regular Waves ◽  
Wave Energy Converters ◽  
Phase Errors ◽  
Array Optimization ◽  
Tank Test ◽  
Energy Converters

If wave energy technology is to mature to commercial success, array optimization could play a key role in that process. This paper outlines physical and numerical modeling of an array of five oscillating water column wave energy converters. Numerical model simulations are compared with experimental tank test data for a non-optimal and optimal array layout. Results show a max increase of 12% in average power for regular waves, and 7% for irregular waves between the non-optimized and optimized layouts. The numerical model matches well under many conditions; however, improvement is needed to adjust for phase errors. This paper outlines the process of numerical and physical array testing, providing methodology and results helpful for researchers and developers working with wave energy converter arrays.

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