Wind-induced response of an offshore wind turbine under non-neutral conditions: A comparison with Hywind Scotland

In this study, aeroelastic simulations of a 5 MW spar wind turbine are performed by using simulated wind fields that are representative of surface layer marine atmospheric turbulence under different atmospheric stratifications. The spar floater's motion responses from the simulations are then compared with the observations from Hywind Scotland's 6 MW spar wind turbine. The platform's pitch and yaw motions from the simulations are consistent with the observations, in terms of mean wind speed and atmospheric stratification. The simulations and the observations show that a stable atmosphere induces the lowest platform pitch and yaw motions compared to neutral and unstable stratifications. Nonetheless, the discrepancy of platform motions between stable and unstable conditions is more pronounced from the observations than in the simulations. Uncertainties associated with the estimation of the atmospheric stability and the modelling of the turbulence's co-coherence for lateral separation may partly account for the discrepancies between the observed and the simulated motion responses of the spar wind turbine.publishedVersio

Numerical Analysis of the Effect of Offshore Turbulent Wind Inflow on the Response of a Spar Wind Turbine

Turbulent wind at offshore sites is known as the main cause for fatigue on offshore wind turbine components. Numerical simulations are commonly used to predict the loads and motions of floating offshore wind turbines; however, the definition of representative wind input conditions is necessary. In this study, the load and motion responses of a spar-type Offshore Code Comparison Collaboration (OC3) wind turbine under different turbulent wind conditions is studied and investigated by using SIMO-Riflex in Simulation Workbench for Marine Applications (SIMA) workbench. Using the two spectral models given in the International Electrotechnical Commission (IEC) standards, it is found that a lower wind lateral coherence under neutral atmospheric stability conditions results in an up to 27% higher tower base side–side bending moment and a 20% higher tower top torsional moment. Comparing different atmospheric stability conditions simulated using a spectral model based on FINO1 wind data measurement, the highest turbulent energy content under very unstable conditions yields a 26% higher tower base side–side bending moment and a 27% higher tower top torsional moment than neutral conditions, which have the lowest turbulent energy content and turbulent intensity. The yaw-mode of the OC3 wind turbine is found to be the most influenced component by assessing variations in both the lateral coherence and the atmospheric stability conditions.publishedVersio

Turbulence in a coastal environment: the case of Vindeby

The one-point and two-point power spectral densities of the wind velocity fluctuations are studied using the observations from an offshore mast at Vindeby Offshore Wind Farm, for a wide range of thermal stratifications of the atmosphere. A comparison with estimates from the FINO1 platform (North Sea) is made to identify shared spectral characteristics of turbulence between different offshore sites. The sonic anemometer measurement data at 6, 18, and 45 m a.m.s.l. (above mean sea level) are considered. These heights are lower than at the FINO1 platform, where the measurements were collected at heights between 40 and 80 m. Although the sonic anemometers are affected by transducer-flow distortion, the spectra of the along-wind velocity component are consistent with those from FINO1 when surface-layer scaling is used, for near-neutral and moderately diabatic conditions. The co-coherence of the along-wind component, estimated for vertical separations under near-neutral conditions, matches remarkably well with the results from the dataset at the FINO1 platform. These findings mark an important step toward more comprehensive coherence models for wind load calculation. The turbulence characteristics estimated from the present dataset are valuable for better understanding the structure of turbulence in the marine atmospheric boundary layer and are relevant for load estimations of offshore wind turbines. Yet, the datasets recorded at Vindeby and FINO1 cover only the lower part of the rotor of state-of-the-art offshore wind turbines. Further improvements in the characterisation of atmospheric turbulence for wind turbine design will require measurements at heights above 100 m a.m.s.l.publishedVersio

Spectral characteristics of surface-layer turbulence in the North Sea

According to IEC 61400-1 and IEC 61400-3 standards, the wind load on the rotor-nacelle assembly of offshore wind turbines should be estimated from the Kaimal or Mann spectral models, unless site-specific full-scale measurements are available. The Kaimal spectral model was developed in a flat and homogeneous onshore site and its applicability in offshore environment, e.g. the North Sea, where a number of wind turbines are in operation, is not thoroughly documented. The present paper utilizes the wind data recorded on the offshore platform FINO 1 in 2007 and 2008 to study the single-point auto-spectral and cross-spectral densities of wind turbulence. It investigates the validity of the Kaimal model, the Mann spectral model, the IEC Kaimal model and the one proposed in the NORSOK standard N-003. The latter standard is developed by the Norwegian petroleum industry for the design of offshore structures. Time series of 1 h duration are considered and a simple non-stationary wind model based on a time-varying fluctuating mean is used to increase the number of samples that can be treated as stationary. For wind velocities ranging from 14 m s-1 to 28 m s-1, a good agreement is observed between the Kaimal spectra and the measured ones, although the power spectral density of the wind fluctuations is larger than predicted for reduced frequencies below 0.04. The Mann spectral model showed a good agreement with the measured spectra. At the altitude of 80 m, we found in average Γ = 3.7, αϵ2/3 = 0.04 m4/3s2 and L = 70 m. Finally, the NORSOK spectrum agrees fairly well with the measured one if a Charnock coefficient of 0.011 is used.publishedVersio

Response sensitivity of a semisubmersible floating offshore wind turbine to different wind spectral models

Previous research on the OC3 spar floating offshore wind turbine (FOWT) has shown the sensitivity of the yaw and side-side modes' load and motion responses to different atmospheric conditions. Using the same baseline turbine of the OC3 spar wind turbine for a semisubmersible floater (OC4), this study investigates the load and motion responses of such offshore wind turbine for neutral and unstable atmospheric conditions. The effect of different levels of wind spatial coherence associated with two different wind spectral models for neutral conditions (Kaimal and Mann) are studied for the same turbulence intensity levels. An increase of 18% in the tower torsional moment fatigue damage equivalent load (DEL) is observed for the wind inflow with the weakest coherence (Mann spectral model), compared to the DELs under turbulent wind inflow with the highest coherence (Kaimal spectral model). Unstable atmospheric conditions are also simulated based on the Pointed-Blunt spectral model derived from FINO1 wind measurement. The yaw mode of the semisubmersible wind turbine is found to be the response component most affected by the variation in atmospheric stability conditions. A 28% higher fatigue DEL for the tower torsional moment is observed for very unstable atmosphere than the DELs under neutral atmospheric conditions.publishedVersio

Coherence of Turbulent Wind Under Neutral Wind Conditions at FINO1

AbstractCurrent wind turbine design standards allow for two different approaches to model the wind field used for engineering estimates; The Mann turbulence model and the Kaimal wind spectra combined with a coherence function. The point wind spectra generated, using the recommended parameters, are similar, but there are differences in the spatial distribution of the turbulence. Especially at larger separation distances the two models will have different coherence values. As the offshore wind turbines continue to increase in rotor size, it is getting more important to model the correct spatial distribution across the large rotor areas in order to calculate the fatigue damage. There are currently few measurements of the marine boundary layer, and one of the few offshore metrological masts is located at FINO1. In this study, the measurements of the wind velocities at three different heights at FINO1 have been used to study the vertical coherence. The study is limited to January 2008, and only neutral atmospheric conditions are considered. Based on the measurements, one can see that there are large differences between the measured values and the coherence estimated by the recommended values. However, the Mann turbulence model shows a trend that is more similar to the measured coherence values, than the coherence function used with the Kaimal spectra

Offshore Wind Turbine Loads and Motions in Unstable Atmospheric Conditions

Even though it is widely known that unstable atmospheric stability conditions can lead to higher turbulence, the use of proper turbulent wind models considering unstable conditions are not often used in the simulation of loads and motions of offshore wind turbines. For this reason, the Højstrup model, which was specifically developed for unstable conditions, is used to simulate a spar-buoy offshore wind turbine (OWT) and investigate the importance of unstable conditions in the design of floating offshore wind turbines. It is found that fatigue damage of a spar-buoy OWT is strongly influenced by unstable conditions, where very unstable condition gives 65% higher fatigue damage than neutral conditions for the tower top torsion, followed by 37% higher for tower base side-side bending and 24% higher for blade root flap-wise mode.publishedVersio

Turbulence in a coastal environment: the case of Vindeby

The one-point and two-point power spectral densities of the wind velocity fluctuations are studied using the observations from an offshore mast at Vindeby Offshore Wind Farm, for a wide range of thermal stratifications of the atmosphere. A comparison with estimates from the FINO1 platform (North Sea) is made to identify shared spectral characteristics of turbulence between different offshore sites. The sonic anemometer measurement data at 6, 18, and 45 m a.m.s.l. (above mean sea level) are considered. These heights are lower than at the FINO1 platform, where the measurements were collected at heights between 40 and 80 m. Although the sonic anemometers are affected by transducer-flow distortion, the spectra of the along-wind velocity component are consistent with those from FINO1 when surface-layer scaling is used, for near-neutral and moderately diabatic conditions. The co-coherence of the along-wind component, estimated for vertical separations under near-neutral conditions, matches remarkably well with the results from the dataset at the FINO1 platform. These findings mark an important step toward more comprehensive coherence models for wind load calculation. The turbulence characteristics estimated from the present dataset are valuable for better understanding the structure of turbulence in the marine atmospheric boundary layer and are relevant for load estimations of offshore wind turbines. Yet, the datasets recorded at Vindeby and FINO1 cover only the lower part of the rotor of state-of-the-art offshore wind turbines. Further improvements in the characterisation of atmospheric turbulence for wind turbine design will require measurements at heights above 100 m a.m.s.l