Flexible multibody dynamic modeling of a floating wind turbine

This paper presents a modeling approach for the nonlinear flexible dynamics simulation of a floating offshore wind turbine (FOWT). The system consists of a floating platform, the moorings, the wind turbine tower, nacelle and the rotor. The floating platform is modeled as a six-degrees-of-freedom (6-DOF) rigid body subject to buoyancy, hydrodynamic and mooring loads. The wind turbine tower is modeled as a three-dimensional (3D) damped tapered Euler–Bernoulli beam undergoing coupled general rigid body and elastic motions. The beam bending-bending and twist motions are considered. The nacelle is modeled as a rigid body attached to the tower tip. The rotor is also modeled as rigid body spinning around its axis and subject to aerodynamic load. The generator torque control is integrated into the model to capture the rotor spin dynamics. The equations of coupled rigid body and elastic motions are derived using Lagrange's Equation in terms of the generalized flexible coordinates and the platform quasi coordinates. The external loads are formulated and incorporated into the system equations of motion. The dynamic model is extensively validated against the most popular FOWT simulation tools with an excellent agreement. Finally, the influence of the tower flexibility on the FOWT dynamic behavior is investigated.