Simulation of large-amplitude motion of floating wind turbines using conservation of momentum

Abstract

A new method is presented to directly derive the nonlinear equations of motion (EOMs) of a floating wind turbine system using the theorem of conservation of angular momentum and Newton's second law. The methodology considers the system as two rigid bodies: the tower and the rotor-nacelle assembly (RNA). The large-amplitude rotation of the tower is described by the 1-2-3 sequence Euler angles, which offer accurate nonlinear coupling between motions in 6 degrees of freedom (DOFs). Two additional DOFs of the RNA relative to the tower, nacelle yaw and rotor spin, are prescribed by mechanical control and are also included in the EOMs of the entire system. Results from the EOMs are transformed among different coordinate systems at every time-step for use in the computation of hydrodynamics, aerodynamics and restoring forces, which preserves the nonlinearity between external excitation and structural dynamics. The new method is verified by critical comparison of simulation results with those of the popular wind turbine dynamics software FAST. The concept of highly compliant floating wind turbines is introduced. The large-amplitude motions and gyroscopic moments of one of these smaller, lighter structures is simulated in an example.