  Floating wind turbines (FWTs) hold significant potential for the exploitation
of offshore renewable energy resources. Nevertheless, prior to the construction
of FWTs, it is imperative to tackle several critical challenges, especially the
issue of performance degradation under combined wind and wave loads. This study
initiates with the development of a simplified nonlinear dynamical model for a
semi-submersible FWT. In particular, both the rotor dynamics and the finite
rotations of the platform are considered in presented modeling approach,
thereby effectively capturing the complex interplay between the platform,
tower, nacelle, and rotor under combined wind and wave loads. Subsequently,
based on the developed FWT model, a novel adaptive nonlinear pitch controller
is formulated with the goal of striking a trade-off between regulating power
generation and reducing platform motion. Notably, the proposed control strategy
adopts a continuous control approach, strategically beneficial in circumventing
the chattering phenomenon commonly associated with sliding mode control.
Furthermore, the controller integrates an online approximator and a robust
integral of the sign of the tracking error, facilitating real-time learning of
system unknown dynamics while compensating for bounded disturbances. Finally,
both the accuracy of the established nonlinear FWT model in predicting key
dynamics and the superiority of the presented pitch controller are validated
through comprehensive comparative studies.
