On the Long-Term Reliability Analysis of a Point Absorber Wave Energy Converter

Of interest in this study is the long-term response and performance of a two-body wave point absorber (“Reference Model 3”), which serves as a wave energy converter (WEC). In a previous study, the short-term uncertainty in this device’s response was studied for an extreme sea state. We now focus on the assessment of the long-term response of the device where we consider all possible sea states at a site of interest. We demonstrate how simulation tools may be used to evaluate the long-term response and consider key performance parameters of the WEC device, which are the heave and surge forces on the power take-off system and the power take-off extension. We employ environmental data at a designated deployment site in Northern California. Metocean information is generated using approximately 15 years of data from this site (National Data Buoy Center site no. 46022). For various sea states, a selected significant wave height and peak period are chosen to describe representative conditions. Then, using a public-domain simulation tool (Wave Energy Converter Simulator or WEC-Sim), we generate various short-term time-domain response measure for these sea states. Distribution fits to extreme response statistics are generated, for each bin that represents a cluster of sea states, using the open-source toolbox, WDRT (WEC Design Response Toolbox). Long-term distributions for each response variable of interest are estimated by weighting short-term distributions by the likelihood of the sea states; from these distributions, the 50-year response can be derived. The 50-year response is also estimated using an approximate but more efficient inverse reliability approach. Comparisons are made between the two approaches.

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Evaluation of long-term power capture performance of a bistable point absorber wave energy converter in South China Sea

Ocean Engineering ◽

10.1016/j.oceaneng.2021.109338 ◽

2021 ◽

Vol 237 ◽

pp. 109338

Author(s):

Nianfan Zhang ◽

Xiantao Zhang ◽

Longfei Xiao ◽

Handi Wei ◽

Weixing Chen

Keyword(s):

South China Sea ◽

South China ◽

Wave Energy ◽

Wave Energy Converter ◽

Energy Converter ◽

China Sea ◽

Point Absorber ◽

Power Capture

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Experimental Wave Tank Test for Reference Model 3 Floating-Point Absorber Wave Energy Converter Project

10.2172/1169792 ◽

2015 ◽

Cited By ~ 3

Author(s):

Y. H. Yu ◽

M. Lawson ◽

Y. Li ◽

M. Previsic ◽

J. Epler ◽

...

Keyword(s):

Wave Energy ◽

Reference Model ◽

Wave Energy Converter ◽

Floating Point ◽

Energy Converter ◽

Wave Tank ◽

Point Absorber ◽

Tank Test

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Preliminary Verification and Validation of WEC-Sim, an Open-Source Wave Energy Converter Design Tool

Volume 9B: Ocean Renewable Energy ◽

10.1115/omae2014-24312 ◽

2014 ◽

Cited By ~ 10

Author(s):

Kelley Ruehl ◽

Carlos Michelen ◽

Samuel Kanner ◽

Michael Lawson ◽

Yi-Hsiang Yu

Keyword(s):

Wave Energy ◽

Reference Model ◽

Design Tool ◽

Wave Energy Converter ◽

Energy Converter ◽

Point Absorber ◽

Preliminary Validation ◽

Model Project ◽

Converter Design ◽

Source Wave

To promote and support the wave energy industry, a wave energy converter (WEC) design tool, WEC-Sim, is being developed by Sandia National Laboratories and the National Renewable Energy Laboratory. In this paper, the WEC-Sim code is used to model a point absorber WEC designed by the U.S. Department of Energy’s reference model project. Preliminary verification was performed by comparing results of the WEC-Sim simulation through a code-to-code comparison, utilizing the commercial codes ANSYS-AQWA, WaveDyn, and OrcaFlex. A preliminary validation of the code was also performed by comparing WEC-Sim simulation results to experimental wave tank tests.

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Long-Term Stochastic Dynamic Analysis of a Combined Floating Spar-Type Wind Turbine and Wave Energy Converter (STC) System for Mooring Fatigue Damage and Power Prediction

Volume 9A: Ocean Renewable Energy ◽

10.1115/omae2014-23438 ◽

2014 ◽

Author(s):

Nianxin Ren ◽

Zhen Gao ◽

Torgeir Moan

Keyword(s):

Wind Turbine ◽

Fatigue Damage ◽

Wave Energy ◽

Wind Wave ◽

Wave Energy Converter ◽

Force Model ◽

Stochastic Dynamic ◽

Energy Converter ◽

Short Term

In this work, a combined concept called Spar-Toru-Combination (STC) involving a spar-type floating wind turbine (FWT) and an axi-symmetric two-body wave energy converter (WEC) is considered. From the views of both long-term fatigue damage prediction of the mooring lines and the annual energy production estimation, a coupled analysis of wind-wave induced long-term stochastic responses has been performed using the SIMO-TDHMILL code in the time domain, which includes 79200 one-hour short term cases (the combination of 22 selected mean wind speeds * 15 selected significant wave heights * 12 selected spectral peak wave periods * 20 random seeds). The hydrodynamic loads on the Spar and Torus are estimated using potential theory, while the aerodynamic loads on the wind rotor are calculated by the validated simplified thrust force model in the TDHMILL code. Considering the long-term wind-wave joint distribution in the northern North Sea, the annual fatigue damage of the mooring line for the STC system is obtained by using the S-N curve approach and Palmgren-Miner’s linear damage hypothesis. In addition, the annual wind and wave power productions are also obtained by using hourly mean output power for each short-term condition and the joint wind-wave distribution.

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The Effect of Environmental Contour Selection on Expected Wave Energy Converter Response

Journal of Offshore Mechanics and Arctic Engineering ◽

10.1115/1.4040834 ◽

2018 ◽

Vol 141 (1) ◽

Cited By ~ 4

Author(s):

Samuel J. Edwards ◽

Ryan G. Coe

Keyword(s):

Wave Energy ◽

Reference Model ◽

Wave Energy Converter ◽

Contour Method ◽

Response Analysis ◽

Extreme Conditions ◽

Force Response ◽

Energy Converter ◽

Extreme Response ◽

Insight Into

A wave energy converter must be designed to both maximize power production and to ensure survivability, which requires the prediction of future sea states. It follows that precision in the prediction of those sea states should be important in determining a final WEC design. One common method used to estimate extreme conditions employs environmental contours of extreme conditions. This report compares five environmental contour methods and their repercussions on the response analysis of Reference Model 3 (RM3). The most extreme power take-off (PTO) force is predicted for the RM3 via each contour and compared to identify the potential difference in WEC response due to contour selection. The analysis provides insight into the relative performance of each of the contour methods and demonstrates the importance of an environmental contour in predicting extreme response. Ideally, over-predictions should be avoided, as they can add to device cost. At the same time, any “exceedances,” that is to say sea states that exceed predictions of the contour, should be avoided so that the device does not fail. For the extreme PTO force response studied here, relatively little sensitivity to the contour method is shown due to the collocation of the device's resonance with a region of agreement between the contours. However, looking at the level of observed exceedances for each contour may still give a higher level of confidence to some methods.

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Passive Model Predictive Control on a Two-Body Self-Referenced Point Absorber Wave Energy Converter

Energies ◽

10.3390/en14061731 ◽

2021 ◽

Vol 14 (6) ◽

pp. 1731

Author(s):

Dan Montoya ◽

Elisabetta Tedeschi ◽

Luca Castellini ◽

Tiago Martins

Keyword(s):

Model Predictive Control ◽

Predictive Control ◽

Wave Energy ◽

Power Flow ◽

Linear Constraints ◽

Wave Energy Converter ◽

Control Technique ◽

Computational Time ◽

Energy Converter ◽

Point Absorber

Wave energy is nowadays one of the most promising renewable energy sources; however, wave energy technology has not reached the fully-commercial stage, yet. One key aspect to achieve this goal is to identify an effective control strategy for each selected Wave Energy Converter (WEC), in order to extract the maximum energy from the waves, while respecting the physical constraints of the device. Model Predictive Control (MPC) can inherently satisfy these requirements. Generally, MPC is formulated as a quadratic programming problem with linear constraints (e.g., on position, speed and Power Take-Off (PTO) force). Since, in the most general case, this control technique requires bidirectional power flow between the PTO system and the grid, it has similar characteristics as reactive control. This means that, under some operating conditions, the energy losses may be equivalent, or even larger, than the energy yielded. As many WECs are designed to only allow unidirectional power flow, it is necessary to set nonlinear constraints. This makes the optimization problem significantly more expensive in terms of computational time. This work proposes two MPC control strategies applied to a two-body point absorber that address this issue from two different perspectives: (a) adapting the MPC formulation to passive loading strategy; and (b) adapting linear constraints in the MPC in order to only allow an unidirectional power flow. The results show that the two alternative proposals have similar performance in terms of computational time compared to the regular MPC and obtain considerably more power than the linear passive control, thus proving to be a good option for unidirectional PTO systems.

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