Rotor-floater-tether coupled dynamics including second-order sum–frequency wave loads for a mono-column-TLP-type FOWT (floating offshore wind turbine)

In the present study, a numerical simulation tool has been developed for the rotor-floater-tether fully-coupled dynamic analysis of FOWTs in time domain including aero-blade-tower dynamics and control, mooring dynamics, and platform motions. In particular, the effects of second-order sum–frequency wave excitations on the coupled dynamic analysis are investigated. The fully coupled simulations are also compared with uncoupled simulations. For this purpose, a mono-column TLP with 5 MW turbine in 200 m water depth is selected as an example. The time histories and spectra of the FOWT motions and accelerations as well as tether top-tensions are presented for the given random collinear wind-wave condition. The shift of original floater natural frequencies due to the inclusion of tower flexural modes is demonstrated. The increase of wind pitch loading and damping due to the relative velocities between wind and platform motion is also explained through fully-coupled dynamic analysis. The second-order sum–frequency wave loading introduces high-frequency excitations near pitch-roll resonance frequencies or lowest tower flexural modes. Its effects are more clearly seen in uncoupled case due to the lack of aero damping. The increased high-frequency responses may significantly increase tower-top accelerations.

► We developed the fully coupled rotor-floater-tether dynamic analysis tool. ► We examine the effect of second-order wave force on the mono-column TLP-type FOWT. ► We examine the coupling effect of FOWT by comparing with uncoupled analysis. ► Complicated coupling effect can be analyzed by developed tool. ► The shift of pitch natural frequency and aero damping effect are examined.