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Abstract
In this paper the dynamics of a submerged axisymmetric wave energy converter are studied, through mathematical models and wave basin experiments. The device is diskshaped and tautmoored via three inclined tethers which also act as a power takeoff. We focus on parasitic yaw motion, which is excited parametrically due to coupling with heave. Assuming linear hydrodynamics throughout, but considering both linear and nonlinear tether geometry, governing equations are derived in 6 degrees of freedom (DOF). From the linearized equations, all motions, apart from yaw, are shown to be contributing to the overall power absorption. At higher orders, the yaw governing equation can be recast into a classical Mathieu equation (linear in yaw), or a nonlinear Mathieu equation with cubic damping and stiffness terms. The wellknown stability diagram for the classical Mathieu equation allows prediction of onset/occurrence of yaw instability. From the nonlinear Mathieu equation, we develop an approximate analytical solution for the amplitude of the unstable motions. Comparison with regular wave experiments confirms the utility of both models for making relevant predictions. Additionally, irregular wave tests are analysed whereby yaw instability is successfully correlated to the amount of parametric excitation and linear damping. This study demonstrates the importance of considering all modes of motion in design, not just the powerproducing ones. Our simplified 1 DOF yaw model provides fundamental understanding of the presence and severity of the instability. The methodology could be applied to other waveactivated devices.
Original language  English 

Article number  20190762 
Pages (fromto)  20190762 
Journal  Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 
Volume  476 
Issue number  2235 
DOIs  
Publication status  Published  1 Mar 2020 
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Dive into the research topics of 'Onset and limiting amplitude of yaw instability of a submerged threetethered 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., Fievez, J. & Rafiee, A.
1/01/15 → 31/12/17
Project: Research