Modelling and dynamic analysis of a semi-submersible floating vertical axis wind turbine

Abstract

Wind turbines are mainly classified into horizontal axis wind turbines (HAWTs) and vertical axis wind turbines (VAWTs) based on different orientation of their axis of rotation. Ever-increasing demand for energy boosts the application of the wind turbines in the deep water. The applications of HAWTs in deep water using different floating support structures have led to an increasing and versatile research due to their commercial success. However, the application of the VAWTs in the offshore wind industry also has some potential due to its economical installation and maintenance. More and more efforts have been invested in developing floating vertical axis wind turbines (FVAWTs), but the research on the FVAWTs is still at an early stage. Although different concepts of the FVAWTs were proposed based on a combination of a rotor and a floater, an optimized design is still an open question. The rotor covers straight-blade rotor, Darrieus curved-blade type rotor and helical-blade rotor while a floater could be a spar, semi-submersible or tension leg platform (TLP). To evaluate a FVAWT, a simulation tool is needed to perform time domain numerical simulations. The simulation tool should have the capability to calculate aerodynamic loads on the rotor, hydrodynamic loads on the floater and structural dynamics of the rotor, and include a controller. Based on the calculated dynamic response, a response analysis is carried out to better understand the response characteristics of a FVAWT as a basis for design and safety criteria according to serviceability. The objective of this thesis has been the development of a coupled method for integrated dynamic analysis of the FVAWTs and application in a systematic study of a Darrieus rotor on a semi-submersible floating support structure. The aerodynamic analysis of a VAWT differs from that of a HAWT, especially when the VAWT is mounted on a floater. Thus, the aerodynamics of a VAWT is first addressed and a model improvement for evaluating the effect of tower tilting on the aerodynamics of a VAWT is performed. This improved model is validated against experimental data collected for an H-Darrieus wind turbine in skewed flow conditions. Based on the assumption that the velocity component parallel to the rotor shaft is small in the downstream part of the rotor, the effect of tower tilting is quantified with respect to power, rotor torque, thrust force and the normal force and tangential force coefficients on the blades. Secondly, a novel 5 MW FVAWT concept, based on a Darrieus rotor on a semi-submersible support structure, is proposed. An aero-hydro-servo-elastic tool Simo-Riflex-DMS is developed for modeling the dynamics of the FVAWT and validated. This integrated dynamic model takes into account the wind inflow, aerodynamics, hydrodynamics, structural dynamics (wind turbine, floating platform and the mooring lines) and a generator controller. Thirdly, the response characteristics of the 5 MW FVAWT are studied based on statistical analysis and spectral analysis of the response. The response characteristics of the FVAWT under steady wind condition and those under turbulent wind condition are compared to investigate the effect of the turbulent wind. The advantage in reducing the 2P effect on the FVAWT is identified by comparing with the equivalent land-based wind turbine. Furthermore, by comparing the FVAWT with a rigid FVAWT, the aspect of rigid versus flexible rotor is quantified, and thus the effect of the modeling method of the rotor on the responses is observed. Besides the normal operating condition, the global motions and structural responses of the FVAWT as a function of azimuthal angle are studied for the parked condition. To identify the effect of wind-wave misalignment on the platform motion, structural response and mooring lines, the dynamic response analysis of the FVAWT in selected misaligned wind and wave conditions are conducted. Moreover, it is also of great interest to compare the performance of a FVAWT with a FHAWT. A comparative study of the studied FVAWT and a FHAWT with the 5 MW NREL reference wind turbine mounted on the same semi-submersible is carried out. A set of time domain simulations with different conditions, i.e., the decay tests, wave only conditions, wind only conditions and the combined wind and wave conditions, are conducted. The dynamic responses of the FVAWT and the FHAWT, such as the global motions of the floater in six DOFs, the bending moment of the bottom of the tower and the tension at the fairleads of the mooring lines, are compared based on statistical results and power spectra. Lastly, a novel hydrodynamic brake installed in the FVAWT is proposed in this thesis. The FVAWTs with fixed-pitch blades experience large aerodynamic loads in high wind speed condition or in stormy weather. The blades may be deformed or broken, and the tower can collapse in more severe cases. Thus, initiating an emergency shutdown is one of the most important concerns for the FVAWTs. Therefore, a dynamic response analysis of the FVAWT with this installed hydrodynamic brake is studied for possible use in connection with emergency shutdown event. The effects of the hydrodynamic brake on the platform motions and structural loads under normal operating conditions and during the emergency shutdown events are evaluated. The use of both the hydrodynamic brake and mechanical brake is also investigated.