Dynamic response analysis of a semi-submersible floating wind turbines under combined ice and aerodynamic loads

Abstract

This thesis aims to analyze the dynamic response of a semi-submersible floating wind turbine under combined wind and ice loads through a coupled analysis. The importance of the topic is due to the increased interest and investments in the renewable energy and particularly in the wind energy. Lots of cold region have shown great potential in terms of wind energy and therefore the effect of ice loads on wind turbines should be investigated. Among the different types of floating wind turbine concepts, semi-submersible platform are probably the most suitable for operating in cold region due to the high stability, the depth flexibility and the possibility to fit the column with ice-breaking cones without compromising the hydrodynamic performances.

The analysis was performed using a aero-servo-hydro-elastic model developed on the software SIMA that was coupled with a semi-empirical ice module developed through a FORTRAN DLL. The loads acting on the platform and on the wind turbine are: ice loads, aerodynamic loads, mooring line loads and hydrodynamic loads. When evaluating the hydrodynamic loads, no waves are considered since they are assumed to be damped out by the ice.

First, we analyzed the ice load on a bottom fixed cylinder in order to test the ice load DLL and to understand how the different parameters affected the ice loads. After that, we performed different sets of coupled analysis on the floating wind turbine. For each set, we considered both ice and wind load and we kept one parameter constant while changing the others to understand how the ice characteristics affect the ice load and consequently the platform motion.

Our analysis showed that the ice load magnitude increases with both the ice thickness and the ice drifting speed, while the ice load frequency increases with the ice drifting speed and it decreases with increasing ice thickness. It also showed that the wind loads are dominant in wind direction, while the ice loads are dominant in the direction perpendicular to the wind direction. Regarding the ice load influence on the platform motion, we found out that roll and sway motion are the most affected by the ice loads while pitch and surge depend mainly on the wind loads. Moreover, the ice loads showed a damping effect on the platform motion in most of the cases, while only a few load cases resulted in a motion amplification effect which was related mainly to the roll motion.