As the worlds first floating wind farm, one of the main technical challenges in the design phase of Hywind Scotland was the uncertainty related to wake interaction effects between the floating wind ...turbines. In this paper, we address this challenge by presenting an analysis of full‐scale measurements from the Hywind Scotland wind farm. Measurements of the floaters' roll, pitch and yaw motions are presented, both in the form of statistical data and response spectra. The floater responses of a turbine in free wind and a turbine in wake are compared, and the influence of different atmospheric stabilities on the floater motions is investigated. Despite the large distance between the turbines of 9 rotor diameters, wake effects in the measured floater motions are observed at the downstream turbine. Furthermore, both the wake responses and the free wind responses show a clear dependency on atmospheric stability. However, overall motions are small for all wind speeds, showing that the design performs satisfactorily in both free wind and wake conditions.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
In the design of offshore wind turbines, it is important to make a realistic estimate of the wind load. This is particularly important for floating wind turbines, having natural frequencies in a ...frequency range where the wind loads are high and large turbulent structures exist. This study shows that turbulence modelling greatly impacts the response of a 15‐MW floating wind turbine. The turbulence models recommended by the International Electrotechnical Commission (IEC) are challenged by considering two additional models: Large Eddy Simulations (LES) and an approach using input from offshore wind measurements (TIMESR). The two standard models, the Kaimal spectrum with IEC coherence model (Kaimal) and the Mann spectral tensor model (Mann), differ in their coherence formulation. This results in higher standard deviations for the surge and pitch motions, and lower for the yaw motion, when applying Kaimal in comparison to Mann. For the specific floater of this study, more damage is obtained in the mooring lines when applying Kaimal. Applying the more realistic models, LES and TIMESR, increases the range of response further, concluding that the two standard turbulence models may lead to incorrect estimations of the response of a floating wind turbine. LES and TIMESR take atmospheric stability into account, which is proven to alter the response significantly.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
In the design of offshore wind farms the simulated dynamic response of the wind turbine structure includes loading from turbulent wind. The International Electrotechnical Commission (IEC) standard ...for wind turbine design recommends both the Mann spectral tensor model and the Kaimal spectral model combined with an exponential coherence formulation. These models give deviating wind loads. This study compares these two models to a large eddy simulations model and a model based on offshore wind measurements. The comparisons are performed for three situations, covering unstable, neutral and stable atmospheric conditions. The impact of the differences in the wind fields on the quasi‐static response of a large bottom‐fixed wind turbine is investigated. The findings are supported by an assessment of the impact of individual wind characteristics on the turbine responses. The wind model based on measurements causes high tower bottom and blade root flapwise bending moments due to a high wind load at very low frequencies. Low and negative horizontal coherence is obtained using the Mann spectral tensor model. This causes relatively large yaw moments as compared to the results using the other wind models. The largest differences in response are seen in the stable situation. We furthermore show that the quasi‐static wind load has great impact on the total damage equivalent moments of the structure. From the results, we conclude that in the design of large offshore wind turbines one should carefully consider the structure of the turbulent wind. Further, longer simulations than recommended by the standards should be used to reduce uncertainty in estimated response.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
As the size of offshore wind turbines increases, a realistic representation of the spatiotemporal distribution of the incident wind field becomes crucial for modeling the dynamic response of the ...turbine. The International Electrotechnical Commission (IEC) standard for wind turbine design recommends two turbulence models for simulations of the incident wind field, the Mann spectral tensor model, and the Kaimal spectral and exponential coherence model. In particular, for floating wind turbines, these standard models are challenged by more sophisticated ones. The characteristics of the wind field depend on the stability conditions of the atmosphere, which neither of the standard turbulence models account for. The spatial and temporal distribution of the turbulence, represented by coherence, is not modeled consistently by the two standard models. In this study, the Mann spectral tensor model and the Kaimal spectral and exponential coherence model are compared with wind fields constructed from offshore measurements and obtained from large‐eddy simulations. Cross sections and durations relevant for offshore wind turbine design are considered. Coherent structures from the different simulators are studied across various stability conditions and wind speeds through coherence and proper orthogonal decomposition mode plots. As expected, the standard models represent neutral stratification better than they do stable and unstable. Depending upon the method used for generating the wind field, significant differences in the spatial and temporal distribution of coherence are found. Consequently, the computed structural design loads on a wind turbine are expected to vary significantly depending upon the employed turbulence model. The knowledge gained in this study will be used in future studies to quantify the effect of various turbulence models on the dynamic response of large offshore wind turbines.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Abstract
This work quantifies the relationship between the design tilt of an upstream rotor and the structural response of a spar floating offshore wind turbine located in its wake. Three wind speed ...scenarios are considered: below, at, and above rated operation. The inflow is generated with the Mann model and the wake and loads are simulated with FAST.Farm. As the upstream rotor tilt goes from 0° to 6°, we find that the mean wake is displaced upward by more than 0.2 rotor diameters (D) by a downstream distance of 7 D. The vertical velocities increase by up to 35 cm/s in the center of the wake. As a result, the downstream rotor is partially waked and experiences a rotated velocity vector. With a higher upstream rotor tilt, the velocities and moments on the downstream turbine increase their mean axial value and their lateral and vertical standard deviation. These changes affect the blade and tower loading and the floater motion primarily in the out-of-plane direction: the damage-equivalent loads for the tower pitch moment and blade-root moment increase by up to 10% because of higher variability at the first mode for the tower and at one blade passing frequency for the blade root. Lesser effects are observed for the roll moments and for floater sway and heave. When the joint effect of rotor tilt and platform motion is considered, the load response on the downstream system is amplified primarily for tower pitch and blade out-of-plane moment.
The paper presents the measurement strategy and data set collected during the COTUR (COherence of TURbulence with lidars) campaign. This field experiment took place from February 2019 to April 2020 ...on the southwestern coast of Norway. The coherence quantifies the spatial correlation of eddies and is little known in the marine atmospheric boundary layer. The study was motivated by the need to better characterize the lateral coherence, which partly governs the dynamic wind load on multi-megawatt offshore wind turbines. During the COTUR campaign, the coherence was studied using land-based remote sensing technology. The instrument setup consisted of three long-range scanning Doppler wind lidars, one Doppler wind lidar profiler and one passive microwave radiometer. Both the WindScanner software and LidarPlanner software were used jointly to simultaneously orient the three scanner heads into the mean wind direction, which was provided by the lidar wind profiler. The radiometer instrument complemented these measurements by providing temperature and humidity profiles in the atmospheric boundary layer. The scanning beams were pointed slightly upwards to record turbulence characteristics both within and above the surface layer, providing further insight on the applicability of surface-layer scaling to model the turbulent wind load on offshore wind turbines. The preliminary results show limited variations of the lateral coherence with the scanning distance. A slight increase in the identified Davenport decay coefficient with the height is partly due to the limited pointing accuracy of the instruments. These results underline the importance of achieving pointing errors under 0.1∘ to study properly the lateral coherence of turbulence at scanning distances of several kilometres.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
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