Investigation of Vortex Induced Vibrations on Wind Turbine Towers

Abstract

Vortex Induced Vibrations (VIV) is a well-known and explored topic. It is a quite complex phenomenon as it is part of different disciplines including fluid mechanics, structural mechanincs, vibrations and computational dynamics. It can be found in many engineering applications such as bridges, industrial chimneys, transmission lines, marine risers in petroleum production and in other hydrodynamics applications. Despite it is a well-known and widely explored topic, the modelling of vortex induced vibrations for engineering purposes still presents a lot of issues due to the complexity of the phenomena involved. These oscillations are of great interest for structural engineers, not only because of the large oscillation amplitude but also due to the long term cyclic loads which can cause significant fatigue damage to the wind turbine towers.

In this thesis the phenomenon of VIV on wind turbine towers is examined. Before the commissioning of the towers in the offshore wind farms, there is a number of different load cases when these structures are susceptible to VIV. These cases are summarized in the following: (i) towers standing on the quay-side, (ii) on the vessel during their transportation offshore and (iii) as installed towers on the foundation (e.g. monopile or jacket) before the installation of the rotor-nacelle assembly (RNA). The main objective is to formulate a realistic model for the prediction of wind turbine towers response due to VIV. The tower geometry includes a tapered section at the top of the structure complicating the phenomenon. The thesis approach starts with a literature study and continues with investigating the different approaches proposed from the Standards (e.g. Eurocode) and the researchers over the years. Investigation of the theoretical background of each analytical model, sensitivity analysis of the main influencing parameters and comparison of the computed vibration amplitudes contributed to the identification of advantages and limitations of these approaches.
Consequently, knowledge gained from this research and data extracted from forced oscillation experiments performed in a wind tunnel contributed to the development of a more accurate, reliable and realistic model for the prediction of the tower response due to VIV. The recommended design procedure models the fluid structure interaction through a negative aerodynamic damping and takes into account the tapered section of the tower, the effect of turbulence intensity and the shear exponent factor of the wind. Finally, fatigue analysis of the wind turbine towers is performed in order to assess the effect of VIV on the lifetime of these structures.
Concluding, through the literature study and the review of the approaches proposed by the design codes and various researchers, a design procedure for the prediction of the tower response over a range of wind velocities is developed. Simplifications of existing methodologies proposed by the Standards limit their accuracy for design purposes. Finally, further experimental investigations and CFD simulations regarding the effect of group arrangements of the towers and the effectiveness of suppression measures are proposed for future research.