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Abstract
Theoretical and experimental investigation of yaw
motion instability in a submerged axisymmetric wave energy
converter is presented. The device is a truncated vertical cylinder
which is tautmoored via three tethers. Assuming linear hydrodynamics,
but retaining nonlinear geometry associated with the
tethers, governing equations are derived in 6 degrees of freedom.
Due to the axisymmetry of the system, there is no hydrodynamic
excitation moment in yaw. However, the yaw governing
equation  correct to second order in buoy motions  reveals
a timevarying restoring moment coefficient. Such systems can
undergo large oscillations given a small initial perturbation,
through the well known Mathieu instability. Targeted regular
wave experiments were used to verify the model predictions on
the onset of yaw motion instability in the first two instability
branches. The yaw motion in a threetethered system is analogous
to sway motion in a singletethered device. The yaw instability
and the transverse/sway motion instability both arise due to
coupling with heave. Due to small damping, the instabilities can
be prevalent. The theoretical analysis presented is applicable to
other floating WECs.
motion instability in a submerged axisymmetric wave energy
converter is presented. The device is a truncated vertical cylinder
which is tautmoored via three tethers. Assuming linear hydrodynamics,
but retaining nonlinear geometry associated with the
tethers, governing equations are derived in 6 degrees of freedom.
Due to the axisymmetry of the system, there is no hydrodynamic
excitation moment in yaw. However, the yaw governing
equation  correct to second order in buoy motions  reveals
a timevarying restoring moment coefficient. Such systems can
undergo large oscillations given a small initial perturbation,
through the well known Mathieu instability. Targeted regular
wave experiments were used to verify the model predictions on
the onset of yaw motion instability in the first two instability
branches. The yaw motion in a threetethered system is analogous
to sway motion in a singletethered device. The yaw instability
and the transverse/sway motion instability both arise due to
coupling with heave. Due to small damping, the instabilities can
be prevalent. The theoretical analysis presented is applicable to
other floating WECs.
Original language  English 

Title of host publication  Proceedings of the 4th Asian Wave and Tidal Energy Conference, Taipei, Taiwan, September 2018 
Publication status  Published  2018 
Event  4th Asian Wave and Tidal Energy Conference 2018  Taipei, Taiwan, Province of China Duration: 9 Sept 2018 → 13 Sept 2018 
Conference
Conference  4th Asian Wave and Tidal Energy Conference 2018 

Abbreviated title  AWTEC 2018 
Country/Territory  Taiwan, Province of China 
City  Taipei 
Period  9/09/18 → 13/09/18 
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 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