Resonance Avoidance of Offshore Wind Turbines

Coincidence of structural resonances with wind turbine dynamic forces can lead to large amplitude stresses and subsequent accelerated fatigue. For this reason, the wind turbine system is designed to avoid resonance coincidence. In particular, the current practice is to design the wind turbine support structure such that its fundamental resonance does not coincide with the fundamental rotational and blade passing frequencies of the rotor. For offshore wind turbines, resonance avoidance is achieved by ensuring that the support structure fundamental resonant frequency lies in the frequency band between the rotor and blade passing frequencies over the operating range of the turbine. This strategy is referred to as “soft-stiff” and has major implications for the structural design of the wind turbine. This paper details the technical basis for the “soft-stiff” resonance avoidance design methodology, investigates potential vulnerabilities in this approach, and explores the sensitivity of the wind turbine structural response to different aspects of the system’s design. The assessment addresses the wind turbine forcing functions, the coupled dynamic responses and resonance characteristics of the wind turbine’s structural components, and the system’s susceptibility to fatigue failure. It is demonstrated that the design practices for offshore wind turbines should reflect the importance of aerodynamic damping for the suppression of deleterious vibrations, consider the possibility of foundation degradation and its influence on the support structure’s fatigue life, and include proper treatment of important ambient sources such as wave and gust loading. These insights inform potential vibration mitigation and resonance avoidance strategies, which are briefly discussed.© 2010 ASME