Towards a Risk-based Decision Support for Offshore Wind Turbine Installation and Operation & Maintenance

AbstractCosts of operation & maintenance, assembly, transport and installation of offshore wind turbines contribute significantly to the total cost of offshore wind farm. These operations are mostly carried out by specific ships that have to be hired for the operational phase and for duration of installation process, respectively. Duration, and therefore ship hiring costs is, among others, driven by waiting time for weather windows for weather-sensitive operations. Today, state of the art decision making criteria for weather-sensitive operations are restrictions to the significant wave height and the average wind velocity at reference height. However, actual limitations are physical, related to response of equipment used e.g. crane wire tension, rotor assembly motions while lifting, etc. Transition from weather condition limits to limits on physical equipment response in decision making would improve weather window predictions, potentially reducing cost of offshore wind energy. This paper presents a novel approach to weather window estimation using ensemble weather forecasts and statistical analysis of simulated installation equipment response

Aeroelastic Instability and Flutter for a 10 MW Wind Turbine

The goal of this thesis is to evaluate if flutter is a challenge to a 10 MW wind turbine. Flutter is an aeroelastic instability which occurs due to the interaction between the aerodynamic forces and the elasticity of the blade. Torsional motions of the blade lead to variations in the aerodynamic forces due to changes in the angle of attack of the airfoil. The variation in aerodynamic forces creates flapwise vibration of the blade. When the vibrations of the blades are in an unfavourable phase with the aerodynamic forces, flutter occurs. Flutter may lead to rapidly increasing vibrations of the blade and failure of the blade. The 10 MW reference turbine from NOWITECH, Norwegian Research Centre for Offshore Wind Technology, was studied. An aeroelastic stability analysis was performed using the aeroelastic stability tool HAWCStab2. It was found that this wind turbine becomes unstable at approximately twice the operational speed of the turbine. The turbine does not experience flutter in normal power producing operation. A simulation in the time domain was also performed, using the aeroelastic tool HAWC2. In a run-away situation, the turbine was found to become unstable with flutter before it reached the run-away speed. The turbine was then analysed with other blades. A softer blade and a stiffer blade were studied. The soft blade was found to become unstable at 1.8 times the operational speed of the turbine. The stiff blade was found to become unstable at around 2.5 times the operational speed. The stiff blade was the only blade where the turbine was able to reach the run-away speed without experiencing instabilities