Frequency domain modelling of a coupled system of floating structure and mooring Lines: An application to a wave energy converter

In recent years, wave energy converters (WECs) have received considerable attention as an efficient way to harvest alternative energy sources. Within this class of systems, point-absorbers are popular and have become one of the most widely used renewable energy harvest designs all over the world, at least in the preliminary R&D stage, with many relevant research works having been published as well. However, unlike the single buoy and PTO systems which already have a comprehensive research basis, the connection cable has received little attention. The traditional taut cable analysis approach, initiated from the needs of the oil&gas industry, has been applied for WEC investigations. However, this approach utilizes an essential assumption that the oscillating term (PTO force) is much smaller than the static term of the cable force (pre-tension) and could be neglected, which may not be proper for WEC applications. In this work, a conventional frequency domain model is utilized to test and verify the validity of the previously mentioned assumption by presenting the ratio between two force terms. Then the ratio could be applied in combination with sea state contours to reveal the critical state of the cable. Then, a fully nonlinear time domain method of a numerical solution of the point-absorber wave energy converter is presented. According to the critical states obtained from the frequency domain analysis, an improved model of a slack cable is proposed. Its influence on the energy extraction performance is investigated using the open source code — WEC-Sim. This work provides insight into simulating a proper model of the cable and how the design of the cable influences the WEC performance.

Download Full-text

Analysis of a Two-Body Floating Wave Energy Converter With Particular Focus on the Effects of Power Take-Off and Mooring Systems on Energy Capture

Journal of Offshore Mechanics and Arctic Engineering ◽

10.1115/1.4023796 ◽

2013 ◽

Vol 135 (3) ◽

Cited By ~ 16

Author(s):

Made Jaya Muliawan ◽

Zhen Gao ◽

Torgeir Moan ◽

Aurelien Babarit

Keyword(s):

Wave Energy ◽

Wave Energy Converter ◽

Energy Converter ◽

Mooring Lines ◽

Energy Capture ◽

Irregular Wave ◽

Power Capture ◽

Actual Operation ◽

Linear Damping ◽

Two Bodies

The present paper summarizes analyses of a two-body floating wave energy converter (WEC) to determine the mooring tension and the effect of the mooring system on energy capture. Also, the effect of the power take-off (PTO) is assessed. An axisymmetric Wavebob-type WEC is chosen as the object of investigation. However, the PTO system is modeled in a simplified manner as ideal linear damping and spring terms that couple the motions of the two bodies. The analysis is performed using SIMO, which is a time domain simulation tool that accommodates the simulation of multibody systems with hydrodynamic interactions. In SIMO, docking cone features between the two bodies allow movement as per actual operation, and fenders are applied to represent end stops. Six alternative mooring configurations are applied to investigate the effect of mooring on power capture. Mooring analysis is performed to determine the necessary capacity of mooring lines for each configuration to carry the tension due to the WEC motion in extreme conditions. Hydrodynamic loads are determined using WAMIT. We assumed that the WEC will be operated to capture wave power at the Yeu site in France. The analysis is performed for several regular and irregular wave conditions according to wave data available for that site. Simulations are performed to study the effect of the PTO system, end stops settings and several mooring configurations on power capture.

Download Full-text

Mooring Analysis of a Floating OWC Wave Energy Converter

Journal of Marine Science and Engineering ◽

10.3390/jmse9020228 ◽

2021 ◽

Vol 9 (2) ◽

pp. 228

Author(s):

Alana Pols ◽

Eric Gubesch ◽

Nagi Abdussamie ◽

Irene Penesis ◽

Christopher Chin

Keyword(s):

Incident Wave ◽

Wave Energy ◽

Wave Energy Converter ◽

Numerical Code ◽

Scale Model ◽

Mooring Line ◽

Energy Converter ◽

Mooring Lines ◽

Restoring Force ◽

Acceptable Method

This investigation focuses on the modelling of a floating oscillating water column (FOWC) wave energy converter with a numerical code (ANSYS AQWA) based on potential flow theory. Free-floating motions predicted by the numerical model were validated against experimental data extrapolated from a 1:36 scale model device in regular and irregular sea states. Upon validation, an assessment of the device’s motions when dynamically coupled with a four-line catenary mooring arrangement was conducted at different incident wave angles and sea states ranging from operational to survivable conditions, including the simulation of the failure of a single mooring line. The lack of viscosity in the numerical modelling led to overpredicted motions in the vicinity of the resonant frequencies; however, the addition of an external linear damping coefficient was shown to be an acceptable method of mitigating these discrepancies. The incident wave angle was found to have a limited influence on the magnitudes of heave, pitch, and surge motions. Furthermore, the obtained results indicated that the mooring restoring force is controlled by the forward mooring lines under the tested conditions.

Download Full-text

An optimal arrangement of mooring lines for the three-tether submerged point-absorbing wave energy converter

Renewable Energy ◽

10.1016/j.renene.2016.02.048 ◽

2016 ◽

Vol 93 ◽

pp. 27-37 ◽

Cited By ~ 11

Author(s):

N.Y. Sergiienko ◽

B.S. Cazzolato ◽

B. Ding ◽

M. Arjomandi

Keyword(s):

Wave Energy ◽

Wave Energy Converter ◽

Energy Converter ◽

Mooring Lines ◽

Optimal Arrangement

Download Full-text

The performance of the Mocean M100 wave energy converter described through numerical and physical modelling

International Marine Energy Journal ◽

10.36688/imej.3.11-19 ◽

2020 ◽

Vol 3 (1) ◽

pp. 11-19

Author(s):

J. Cameron McNatt ◽

Christopher H. Retzler

Keyword(s):

Frequency Domain ◽

Wave Energy ◽

Time Domain ◽

Numerical Models ◽

Average Power ◽

Physical Modelling ◽

Wave Energy Converter ◽

Domain Model ◽

Energy Converter ◽

Cost Of Energy

Mocean Energy has designed a 100-kW hinged-raft wave energy converter (WEC), the M100, which has a novel geometry that reduces the cost of energy by improving the ratios of power per size and power per torque. The performance of the M100 is shown through the outputs of frequency-domain and time-domain numerical models, which are compared with those from 1/20th scale wave-tank testing. Results show that for the undamped, frequency-domain model, there are resonant peaks in the response at 6.6 and 9.6 s, corresponding to wavelengths that are 1.9 and 3.7 times longer than the machine. With the inclusion of power-take-off and viscous damping, the power response as a function of frequency shows a broad bandwidth and a hinge flex amplitude of 12-20 degrees per meter of wave amplitude. Comparison between the time-domain model and physical data in a variety of sea states, up to a significant wave height of 4.5 m, show agreements within 10% for average power absorption, which is notable because only simple, nonlinear, numerical models were used. The M100 geometry results in a broad-banded, large amplitude response due to its asymmetric shape, which induces coupling between modes of motion.

Download Full-text

Biofouling on mooring lines and power cables used in wave energy converter systems—Analysis of fatigue life and energy performance

Applied Ocean Research ◽

10.1016/j.apor.2017.04.002 ◽

2017 ◽

Vol 65 ◽

pp. 166-177 ◽

Cited By ~ 12

Author(s):

Shun-Han Yang ◽

Jonas W. Ringsberg ◽

Erland Johnson ◽

Zhiqiang Hu

Keyword(s):

Fatigue Life ◽

Wave Energy ◽

Systems Analysis ◽

Energy Performance ◽

Wave Energy Converter ◽

Power Cables ◽

Energy Converter ◽

Mooring Lines

Download Full-text

Design and Simulation of a Novel Mechanical Power Take-Off for a Two-Body Wave Energy Point Absorber

Volume 8: 30th Conference on Mechanical Vibration and Noise ◽

10.1115/detc2018-85873 ◽

2018 ◽

Cited By ~ 1

Author(s):

Xiaofan Li ◽

Chien-An Chen ◽

Qiuchi Xiong ◽

Robert Parker ◽

Lei Zuo

Keyword(s):

Frequency Domain ◽

Output Power ◽

Wave Energy ◽

Time Domain ◽

Wave Energy Converter ◽

Mechanical Power ◽

Total Output ◽

Test Machine ◽

Mechanical Motion ◽

Energy Converter

In this paper, a two-body self-react wave energy converter with a novel mechanical Power Take-off (PTO) is introduced. The PTO rectifies the mechanical motion and regulates the flow with a mechanism called Mechanical Motion Rectifier (MMR), which converts the reciprocating motion of the ocean wave into unidirectional rotation of the generator. The overall system is analyzed in both time and frequency domain. In time domain, the piecewise non-linear dynamic model of the MMR PTO is derived, and parameters that could significantly influence the MMR property is extracted. By building the model into WEC-Sim, a time domain wave energy converter (WEC) simulation tool, to simulate and evaluate the performance of the PTO. In addition, the system is modelled as a two-body vibration system for frequency domain analysis in order to further investigate and optimize the proposed wave energy converter. The tunable parameters within the system, including the equivalent mass, the equivalent damping coefficient, and the PTO stiffness, are discussed based on the criteria of maximization of the total output power. To verify the theoretical analysis, a bench test prototype is developed and tested on a hydraulic test machine. The experimental results in line with the derived model and can be used for reasonable estimation on the output power of the proposed system in real ocean conditions.

Download Full-text