Robust structural control of an underactuated floating wind turbine

This paper investigates the dynamic modeling and robust control of an underactuated floating wind turbine for vibration suppression. The offshore wind turbine is equipped with a tuned mass damper on the floating platform. The Lagrange's equation is employed to establish the limited degree-of-freedom dynamic model. A novel disturbance observer-based hierarchical sliding mode control system is developed for mitigating loads of the underactuated floating wind turbine. In the proposed control scheme, two prescribed performance nonlinear disturbance observers are developed to estimate and counteract unknown disturbances, where the load induced by wave is considered as a mismatched disturbance while the load caused by wind is treated as a matched disturbance. The hierarchical sliding mode controller regulates the states of such an underactuated nonlinear system. In particular, the first-order sliding mode differentiator is used to avoid the tedious analytic computation in the sliding mode control design. The stability of the whole closed-loop system is rigorously analyzed, and some sufficient conditions are derived to guarantee the convergence of the states for the considered system. Numerical simulations deployed on both the design model and the National Renewable Energy Laboratory 5-MW wind turbine model are provided, which demonstrate great effectiveness and strong robustness of the proposed control scheme.