### Abstract

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 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 Sep 2018 → 13 Sep 2018 |

### Conference

Conference | 4th Asian Wave and Tidal Energy Conference 2018 |
---|---|

Abbreviated title | AWTEC 2018 |

Country | Taiwan, Province of China |

City | Taipei |

Period | 9/09/18 → 13/09/18 |

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### Cite this

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

}

*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.

Research output: Chapter in Book/Conference paper › Conference 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.

UR - https://www.awtec2018.com/

UR - https://drive.google.com/drive/folders/1Fy1NPbRbDbKIZnDEDcvdywn4r_XgAHzB

UR - https://drive.google.com/file/d/1GwiTSWaPeMqfXINvnBuPMR-5GmlXXA0p/view?usp=sharing

M3 - Conference paper

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

ER -