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Abstract
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 axisymmetric (resembling a truncated vertical cylinder) and is tautmoored 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 subharmonic (perioddoubling) 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.
Original language  English 

Article number  20180459 
Pages (fromto)  121 
Number of pages  21 
Journal  Proceedings of the Royal Society AMathematical Physical and Engineering Sciences 
Volume  475 
Issue number  2221 
DOIs  
Publication status  Published  1 Jan 2019 
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Dive into the research topics of 'Transverse motion instability of a submerged moored buoy'. Together they form a unique fingerprint.Projects
 1 Finished

Novel Wave Energy Foundation Solutions to Survive Extreme Loads
Gaudin, C., Draper, S., Wolgamot, H., O'Loughlin, C., Rafiee, A. & Fievez, J.
Australian Research Council, Carnegie Wave Energy Ltd
1/01/15 → 30/04/19
Project: Research