Motion instabilities in tethered buoy WECs

Research output: Chapter in Book/Conference paperConference paper

Abstract

Theoretical and experimental investigation of yaw
motion instability in a submerged axi-symmetric wave energy
converter is presented. The device is a truncated vertical cylinder
which is taut-moored via three tethers. Assuming linear hydrodynamics,
but retaining non-linear geometry associated with the
tethers, governing equations are derived in 6 degrees of freedom.
Due to the axi-symmetry 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 time-varying 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 three-tethered system is analogous
to sway motion in a single-tethered 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 languageEnglish
Title of host publicationProceedings of the 4th Asian Wave and Tidal Energy Conference, Taipei, Taiwan, September 2018
Publication statusPublished - 2018
Event4th Asian Wave and Tidal Energy Conference 2018 - Taipei, Taiwan, Province of China
Duration: 9 Sep 201813 Sep 2018

Conference

Conference4th Asian Wave and Tidal Energy Conference 2018
Abbreviated titleAWTEC 2018
CountryTaiwan, Province of China
CityTaipei
Period9/09/1813/09/18

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yaw
moments
retaining
floating
degrees of freedom
damping
hydrodynamics
perturbation
oscillations
symmetry
coefficients
geometry
predictions

Cite this

Orszaghova, J., Wolgamot, H., Draper, S., & Rafiee, A. (2018). Motion instabilities in tethered buoy WECs. In Proceedings of the 4th Asian Wave and Tidal Energy Conference, Taipei, Taiwan, September 2018
Orszaghova, Jana ; Wolgamot, Hugh ; Draper, Scott ; Rafiee, Ashkan. / Motion instabilities in tethered buoy WECs. Proceedings of the 4th Asian Wave and Tidal Energy Conference, Taipei, Taiwan, September 2018. 2018.
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abstract = "Theoretical and experimental investigation of yawmotion instability in a submerged axi-symmetric wave energyconverter is presented. The device is a truncated vertical cylinderwhich is taut-moored via three tethers. Assuming linear hydrodynamics,but retaining non-linear geometry associated with thetethers, governing equations are derived in 6 degrees of freedom.Due to the axi-symmetry of the system, there is no hydrodynamicexcitation moment in yaw. However, the yaw governingequation - correct to second order in buoy motions - revealsa time-varying restoring moment coefficient. Such systems canundergo large oscillations given a small initial perturbation,through the well known Mathieu instability. Targeted regularwave experiments were used to verify the model predictions onthe onset of yaw motion instability in the first two instabilitybranches. The yaw motion in a three-tethered system is analogousto sway motion in a single-tethered device. The yaw instabilityand the transverse/sway motion instability both arise due tocoupling with heave. Due to small damping, the instabilities canbe prevalent. The theoretical analysis presented is applicable toother floating WECs.",
author = "Jana Orszaghova and Hugh Wolgamot and Scott Draper and Ashkan Rafiee",
year = "2018",
language = "English",
booktitle = "Proceedings of the 4th Asian Wave and Tidal Energy Conference, Taipei, Taiwan, September 2018",

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Orszaghova, J, Wolgamot, H, Draper, S & Rafiee, A 2018, Motion instabilities in tethered buoy WECs. in Proceedings of the 4th Asian Wave and Tidal Energy Conference, Taipei, Taiwan, September 2018. 4th Asian Wave and Tidal Energy Conference 2018, Taipei, Taiwan, Province of China, 9/09/18.

Motion instabilities in tethered buoy WECs. / Orszaghova, Jana; Wolgamot, Hugh; Draper, Scott; Rafiee, Ashkan.

Proceedings of the 4th Asian Wave and Tidal Energy Conference, Taipei, Taiwan, September 2018. 2018.

Research output: Chapter in Book/Conference paperConference paper

TY - GEN

T1 - Motion instabilities in tethered buoy WECs

AU - Orszaghova, Jana

AU - Wolgamot, Hugh

AU - Draper, Scott

AU - Rafiee, Ashkan

PY - 2018

Y1 - 2018

N2 - Theoretical and experimental investigation of yawmotion instability in a submerged axi-symmetric wave energyconverter is presented. The device is a truncated vertical cylinderwhich is taut-moored via three tethers. Assuming linear hydrodynamics,but retaining non-linear geometry associated with thetethers, governing equations are derived in 6 degrees of freedom.Due to the axi-symmetry of the system, there is no hydrodynamicexcitation moment in yaw. However, the yaw governingequation - correct to second order in buoy motions - revealsa time-varying restoring moment coefficient. Such systems canundergo large oscillations given a small initial perturbation,through the well known Mathieu instability. Targeted regularwave experiments were used to verify the model predictions onthe onset of yaw motion instability in the first two instabilitybranches. The yaw motion in a three-tethered system is analogousto sway motion in a single-tethered device. The yaw instabilityand the transverse/sway motion instability both arise due tocoupling with heave. Due to small damping, the instabilities canbe prevalent. The theoretical analysis presented is applicable toother floating WECs.

AB - Theoretical and experimental investigation of yawmotion instability in a submerged axi-symmetric wave energyconverter is presented. The device is a truncated vertical cylinderwhich is taut-moored via three tethers. Assuming linear hydrodynamics,but retaining non-linear geometry associated with thetethers, governing equations are derived in 6 degrees of freedom.Due to the axi-symmetry of the system, there is no hydrodynamicexcitation moment in yaw. However, the yaw governingequation - correct to second order in buoy motions - revealsa time-varying restoring moment coefficient. Such systems canundergo large oscillations given a small initial perturbation,through the well known Mathieu instability. Targeted regularwave experiments were used to verify the model predictions onthe onset of yaw motion instability in the first two instabilitybranches. The yaw motion in a three-tethered system is analogousto sway motion in a single-tethered device. The yaw instabilityand the transverse/sway motion instability both arise due tocoupling with heave. Due to small damping, the instabilities canbe prevalent. The theoretical analysis presented is applicable toother floating WECs.

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M3 - Conference paper

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Orszaghova J, Wolgamot H, Draper S, Rafiee A. Motion instabilities in tethered buoy WECs. In Proceedings of the 4th Asian Wave and Tidal Energy Conference, Taipei, Taiwan, September 2018. 2018