Inferring Wave Properties from Acceleration Measurements on Offshore Substations

Wave conditions are an essential input for fatigue life design of offshore wind turbines and thus studying their properties can aid in the estimation of the historical fatigue loads measured on wind turbines. Particularly, waves interaction with the natural frequencies of the wind turbine can introduce resonance behavior, and as such accurate estimation of their properties is pivotal. In practice, wave conditions are monitored using wave radars by inferring the wave properties from statistical characteristics of the time domain heave signal or from its energy spectrum. However, those radar systems are not always positioned within close approximation of the wind farm, thus localized wave behavior on the wind turbines is not captured sufficiently well as the method of deriving those properties varies between different definitions and analysis techniques, introducing uncertainty in the reported wave characteristics. On the other hand, Operational modal analysis captures the wave frequency as non-white noise excitation. One can consider deriving those wave properties from an Offshore Sub Stations (OSS) as it experiences the same wave conditions as the wind turbines in the farm. Since the OSS natural frequencies are significantly higher (>0.55 Hz) than those of wind turbines (typically 0.2–0.35 Hz), wave loads are clearly visible in the low-frequency (<0.5 Hz) response spectrum of the OSS. This study proposes using long-term acceleration data from an OSS, alongside wave data from a nearby direction wave radar to investigate whether key wave properties, particularly wave frequency and energy, can be inferred by monitoring the low-frequency response of the OSS. The methodology can potentially be improved by factoring in variations in dynamic amplification as tidal levels change.