Variable Natural Frequency Damper for Minimizing Response of Offshore Wind Turbine: Principle Verification through Analysis of Controllable Natural Frequencies

Abstract

Resonance causes extreme stress, acceleration of fatigue, and reduction in lifespan of offshore wind structures. The main factors that cause resonance are environmental loads such as wind and waves, and dynamic loads caused by rotor movement. Estimation of the natural frequency at the design stage is highly uncertain, and natural frequency changes occur due to various factors during long-term operation. Therefore, it is important to ensure structural safety from resonance through a vibration-monitoring system or an additional damper. In this study, the effect of seawater existing inside the substructure on the natural frequency of the structure was dealt with. The natural frequency estimation equation for a fixed offshore wind structure was derived with the “inner fluid simplification assumption”. The finite element modal analysis was performed to verify the principle of Variable Natural Frequency Damper (VNFD), a system that controls the natural frequency of offshore wind structures through a pump, and to find the range of natural frequency control. As a result, interior fluid affects the natural frequency of the wind turbine support structure. Specifically, the variable natural frequency range was very low, at about 0.027% for the monopile model at a depth of 10 m, but increased rapidly to about 3.66% at a depth of 70 m. Furthermore, when estimating the natural frequency of a fixed offshore wind turbine in deep water without consideration of interior fluid, the estimates can be higher than with consideration of it.

Variable Natural Frequency Damper for Minimizing Response of Offshore Wind Turbine: Effect on Dynamic Response According to Inner Water Level

“…The research on variable natural frequency damper (VNFD), which adjust the natural frequency of a structure based on its inner water level (IWL) and control the dynamic response of the structure due to resonance, is in its infancy. You [12] confirmed the change in the natural frequency of a structure as a function of its IWL but did not confirm the effect of this change on the dynamic response of the structure under environmental loads. They also investigated the change in the natural frequency of a linear tapered tower wind turbine structure, including the substructure, by introducing seawater into the structure.…”

“…H denotes the depth from the baseline in meters, and it should be considered by adding it to the center of the RNA and the inner water level. As a result, the "inner fluid simplifying assumption" allows for the calculation of the natural frequency considering the added mass of the IWL, as expressed in Equation (12).…”

“…Equation ( 14) expresses the displacement response of the structure to the dynamic loads applied to it as a function of frequency. Regulation of the water level inside the structure by using a VNFD affects the mass matrix M, which changes the natural frequency of the structure [12]. Resonance occurs when the input frequency matches the natural frequency of the structure and the response of the system is maximized when the −Mω 2 + K term in the denominator of H(ω) approaches zero, which shows that very large displacements occur when the structure is in resonance.…”

“…To ensure the reliability of the developed FE model, mesh convergence was first performed. [12]. In this process, convergence of the natural frequency was examined by and external fluids were modeled as 20-node 3D acoustic fluid elements (Fluid 220) to determine their acoustic properties.…”

“…Ko [22] obtained the natural frequencies of a linear tapered tower wind turbine structure with a rigid base substructure. You [12] investigated the change in the natural frequency of a linear tapered tower wind turbine structure, including the substructure, by introducing seawater into the structure. However, their study was focused solely on the influx of seawater into the monopile.…”