Dynamic response of Spar-type Floating Offshore Wind

Abstract

Floating offshore wind turbines (FOWTs) are an alternative technology to harness the abundant wind energy in open sea areas. A FOWT structure consists of a floating platform, a mooring system, and a wind turbine structure (tower and Rotor-Nacelle Assembly (RNA)). The FOWT numerical model integrates the structural dynamics and hydrostatic, hydrodynamic, aerodynamic and mooring loads. The main objective of this thesis is to investigate the dynamic responses of FOWT structure with large-amplitude waves. Special efforts are also devoted to studying the effects of large-amplitude waves- current interaction and the non-linear solitary waves-structure interaction. A coupled rigid-flexible multi-body model treats the blades, the tower as the flexible components and the platforms as rigid bodies. The model is derived with 10 degrees of freedom model to model the floating wind turbine. The equations of motions of the model based on energy formulation are derived using Euler-Lagrangian equations. The hydrodynamic forces are evaluated using Morison’s equations for a slender structure. The aerodynamic loads are estimated using the classical Blade Element Momentum (BEM) method. A dynamic mooring model is applied to determine the cable tensions of the mooring system proposed to anchor the platform to the seabed. The hydrodynamic effects of the large-amplitude waves are evaluated through derivation of the large-amplitude waves accelerations. The numerical continuum approach was used to compute the large-amplitude wave solutions. The formulations of the flow accelerations were proposed using the results from the numerical continuum approach. The accelerations, velocity, and pressure are required to evaluate the hydrodynamic forces and moments as a function of the platform displacements. An investigation into the flow kinematics under large-amplitude waves was carried out. Further, the FOWT responses achieved by applying large-amplitude wave theory were compared with the results of the linear wave theory. The application of large-amplitude waves significantly affected the FOWT displacement and the cable fairlead forces. The vertical displacement of the FOWT and the cable fairlead forces was observed to be most affected by the large-amplitude waves. A new combination of the large-amplitude waves with the uniformly underlying current was also proposed in this thesis. The impact of the current with a constant strength has been evaluated in two directions: following waves or against waves. It was shown that the current significantly modifies fluid horizontal velocity profiles and affects the FOWT. In addition, the current affected the static responses and the dynamic responses of the spar and the cables. The results also provided an understanding of the interconnectedness of the large-amplitude waves amplitude and the current strength. Finally, solitary waves were applied in the models to examine the FOWT system ability to an impulsive load. The solitary waves were considered as moving hump of water with high speed and long wavelength. Due to its high propagating speed, a short-duration response but extreme amplitude of the FOWT structure is caused by the solitary waves loads. Moreover, the interactions of the solitary waves-structure were also incorporated in the models to examine a more realistic model of a FOWT system. The interaction was represented by the modification of the stable solitary waves profiles. Therefore, a useful meshless method, Smoothed Particle Hydrodynamics (SPH), was used to capture the surface modification which is an input for a Finite Element Method approach to estimate the flow kinematics accounting for the waves-structure interaction. It was shown that including the wave-structure interaction amplified the platform displacements and the mooring forces.