Abstract
Turbulence spectral characteristics for various atmospheric stratifications are studied using the observations from an offshore mast at Vindeby wind farm. Measurement data at 6 m, 18 m and 45 m above the mean sea level are considered. At the lowest height, the normalized power spectral densities of the velocity components show deviations from Monin-Obukhov similarity theory (MOST). A significant co-coherence at the wave spectral peak frequency between the vertical velocity component and the velocity of the sea surface is observed, but only when the significant wave heights exceed 0.9 m. The turbulence spectra at 18 m generally follow MOST and are consistent with the empirical spectra established on the FINO1 offshore platform from an earlier study. The data at 45 m is associated with a high-frequency measurement noise which limits its analysis to strong wind conditions only. The estimated co-coherence of the along-wind component under near-neutral atmosphere matches remarkably well with those at FINO1. The turbulence characteristics estimated from the present dataset are valuable to better understand the structure of turbulence in the marine atmospheric boundary layer and are relevant for load estimations of offshore wind turbines. Yet, a direct application of the results to other offshore or coastal sites should be exercised with caution, since the dataset is collected in shallow waters and at heights lower than the hub height of the current and the future state-of-the-art offshore wind turbines.
Highlights
In the early 1990s, the first generations of offshore wind farms were commissioned to test the viability of extracting wind power in the marine atmospheric boundary layer (MABL)
The wave-induced stress may increase the friction velocity at 6 m amsl, and a lower-than-expected φw. 270 For an unstable atmosphere (ζ < 0), the values of φw estimated at 18 m remain under eq (5), which were not observed on FINO1 (Cheynet et al, 2018)
This study explores the turbulence spectral characteristics from wind records of a year duration on an offshore mast called South 480 Mast West (SMW) near the first offshore wind farm Vindeby
Summary
In the early 1990s, the first generations of offshore wind farms were commissioned to test the viability of extracting wind power in the marine atmospheric boundary layer (MABL). Was the Vindeby project the first offshore wind farm, but it provided precious information on meteorological conditions in the MABL using offshore and onshore meteorological masts. The data collected has been used to study the characteristics of the mean wind speed profile under various atmospheric conditions (Barthelmie et al, 1994; Barthelmie, 1999). Since the 2010s, several studies have indicated that diabatic wind conditions may significantly affect the fatigue life of offshore wind turbines (OWTs) components (Sathe et al, 2013; Hansen et al, 2014; Holtslag et al, 2016; Doubrawa et al, 2019; Nybø et al, 2020; Putri et al, 2020). To model properly the wind load for wind turbine designs, a better understanding of the spectral structure of turbulence in the MABL is necessary, which addresses partly the first of the three great challenges in the field of 35 wind energy (Veers et al, 2019)
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