FINO1 Mast Correction

Transcription

1 FINO1 Mast Correction A. Westerhellweg, T. Neunn; DEWI GmbH, Wilhelmshaven V. Riedel; DEWI North America Inc. A. Westerhellweg English Abstract Lateral speed-up effects, upwind flow retardation and downwind wake effects can be much more pronounced for offshore wind measurement sts than for onshore sts. Combined wind and wave loads require different st designs, which y result in denser lattices, larger footprints and shorter booms as compared to onshore sts. In order to use such offshore wind measurement data for energy yield assessments, the disturbances caused by the measure ment st need to be properly addressed. For this, DEWI has developed a st correction method based on the vanishing vertical wind gradients during very unstable situations, thus enabling a uniform ambient flow st correction (UAM). This st correction scheme was devised by DEWI in 2007 for the FINO1 offshore wind measurement platform [4]. In 2011, it has been re-assessed for a higher directional resolution (1 instead 10 ) and has been extended to all cup and sonic anemometer measurement heights. The st correction refers to the wind direction measured at FINO1 at 91.5 m LAT and the averaging interval of 10 minutes. During 2009 and 2010, a 1-year Lidar measurement campaign was performed on the FINO1 platform. The comparison with a st correction based on Lidar data showed high agreement and validated the perfornce of the UAM method [7]. The UAM method can be used for offshore sts if an undisturbed top anemometer is installed. The method, the correction results and the associated uncertainties for FINO1 are detailed below. Measurement Set-up The FINO1 st [2] has a square cross-section. Cup anemometers are installed on booms on the south-east side of the st (the in wind direction is south-west). Sonic anemometers and wind vanes are installed on the opposite north-west side of the st. The st is equipped with a top anemometer at the height of 103 m above LAT (lowest astronomical tide) located in a lightning protection cage. Temporarily, an additionally top anemometer was installed for correction purposes at a height level of m LAT. An offshore wind measurement st has to be designed to resist wind and wave loads which might not allow a measurement set-up according to IEC standards [3]. In the case of FINO1, the lattice st has a square footprint with a width that decreases linearly from 3.5 m (at 34 m LAT) to 1.4 m (at 91.5 m LAT). The boom lengths vary from 6.5 m to 3.0 m. The ratio of the boom 60 DEWI MAGAZIN NO. 40, FEBRUARY 2012

2 Tab. 1: Measurement set-up details of the boom mounted anemometers at FINO1. Anemometer, height LAT Mast width Boom length Orientation Ratio (distance to st centre)/ (st width) [m] [m] [m] [ ] [-] cup cup cup cup cup cup cup sonic sonic sonic Fig. 1: Offshore platform FINO1. Fig. 2: Left: Top anemometer at 103 m LAT in lightning cage and additional anemometer at m LAT. Right: Ratio of the wind speeds of the additional and the top anemometer lengths to the st width varies from 2.2 to 2.7 (Tab. 1) and is much sller than recommended in the IEC. Furthermore, the length of the mounting tube of the topanemometer does not conform to the IEC standards. In the immediate st wake, the wind speed reduction is very large and amounts to up to 40%. A st correction of the data is necessary. Correction of Top Anemometer Data A short-term measurement campaign with an additional anemometer, installed at 1.5 m height above the original top anemometer, was initiated. The purpose of this additional measurement was to gather wind data that would allow a correction of the flow disturbances at the original top anemometer. These disturbances are caused by the lightning protection cage and possibly by systetic speedup effects at the st top. While the original top anemometer is mounted at a height of 103 m above LAT, the additional top-mounted anemometer reaches a height of m above LAT. The vertical carbon fibre boom is sufficiently stiff. The anemometer is calibrated according to MEASNET standard. A sector-wise correlation has been established between the original top anemometer and the additional top-mounted anemometer. Data sets with a wind speed > 4 m/s have been evaluated. After the determination of the correction factors with 1 degree resolution (moving average over 2 degrees in wind direction), these factors have been applied to the data of the original top anemometer in order to transfer the data towards the additional top mounted anemometer. The directional plot of the ratios of the measured wind speeds (Fig. 2) shows the flow distortion of the booms of the lightning protection cage at the wind directions 0, 90, 180 and 270. In the wind direction sectors between the tubes of the lightning cage, systetic speed-up effects are present for the original top anemometer. The ratio of the two wind speed signals is not the same for all directions but is, on average, 0.98 for the north and 0.99 for the south wind directions. The difference between the north and south directions is caused by the position of the additional top anemometer on the north edge of the st. To avoid this effect the additional anemometer has to be placed higher, which would have had other drawbacks. A first measurement campaign was performed during an 8-month period during In 2008, the measurement has been re-installed with another anemometer but the identical measurement set-up. In the one-year period the previous correction functions were confirmed. With the application DEWI MAGAZIN NO. 40, FEBRUARY

3 Height LAT [m] Fig. 3: Wind profiles examined in direction sector [220 ;230 ] for unstable stratification. Fig. 4: 20 reduced data set (unstable stratification, wind direction sector [220 ;230 ]) wind profile between 91.5m and 41.5m corrected wind profile 0 95% 96% 97% 98% 99% 100% 101% 102% 103% Percentage deviation from wind speed measured at m height LAT [-] Wind profile for unstable stratification, the wind speed range above 4 m/s and the wind direction range [220 ;230 ], relative to the mean wind speed measured at 104.5m LAT. of the correction function, the reference height of these data changes from 103 m to a height level of m LAT. Uniform Ambient Flow Mast Correction Scheme The basic concept of the UAM method is to use a data set with no vertical wind gradient (uniform ambient wind flow), which allows to derive st correction functions by sector-wise comparison of the boom and top mounted anemo meters. In order to create a situation similar to putting the entire st into a very large wind tunnel, weather situations are isolated during which the vertical wind speed gradient approaches zero. The reining wind shear is corrected and the resulting wind speed ratios v top /v boom constitute the st correction functions. The correction method is explained below using the example of FINO1. The method consists of: a) Reduction of the measured wind data to periods where there is a strongly unstable stratification observed at the platform, determined by temperature measurements of the sea surface and of the air close above the sea. b) Correction of reining wind shear. c) Calculation of the ratios vtop/v boom as the st correction functions, separately for each height level. a) Reduction to periods with unstable stratification The wind data from the FINO1 platform have been reduced to periods where there was a strongly unstable stratification observed at the platform, i.e. whenever the air temperature at 30 m height is by more than 1 degree lower than the water temperature at -3 m depth. Although this is a very simple and pragtic method of describing the stratification, it was shown to be sufficiently precise for the present purpose. The required temperature measurements are not available for the whole measurement period. Only the periods March 2004 until February 2006 (2 years) and February 2010 until January 2011 (12 months) have been evaluated. b) Correction of reining wind shear Even in unstable conditions, the wind shear is not zero (Fig. 3). The wind shear for unstable stratification is therefore approxited and considered in the st correction functions. At FINO1, all boom-mounted cup anemometers are approxitely oriented towards the same direction. The resulting data set of step (a) is reduced to the wind speed range above 4 m/s and the wind direction range [220,230] degree, which is perpendicular to the boom orientation of the cup anemometers. For this wind direction sector the sllest disturbance caused by the st is expected theoretically. Fig. 3 shows the vertical wind profiles of the data sets examined in the evaluation period March 2004 February For this reduced data set, a mean wind profile has been determined (see Fig. 4, red line). Presubly, there is a sll lateral speed-up effect present at the st for this wind direction range, which leads to too high wind speeds measured with the boom mounted anemometers. The observed wind shear, calculated using the boom mounted cup anemometer measurements within the wind direction sector [220,230 ] (Fig. 4, blue line) has been assumed to be representative up to the height of the top anemometer. It is therefore used to calculate the corrected mean wind speed for the lower height levels based on the undisturbed measurement in m height. The resulting profile (see Fig. 4, yellow line), is called the corrected profile and is assumed to prevail at the FINO1 platform during unstable conditions. 62 DEWI MAGAZIN NO. 40, FEBRUARY 2012

4 Fig. 5: The ratio v top /v boom of the resulting data set serves as st correction for each boom mounted anemometer c) The Ratio v top /v boom represents the st correction for each height. The unstable data set is evaluated for the st correction. For the different heights, v boom is corrected according to the wind shear shown in Fig. 4 and the ratio v top /v boom serves as st correction for each boom mounted anemometer. The correction function is based on two different functional approxitions; the data have been divided into two parts (st wake and reining data). As an example, Fig. 5 shows all data and the data used for the st correction for 91.5 m LAT. In the four directions corresponding to the sections of the lightning protection cage, the variation was higher. These wind direction sectors have been cut out and excluded from the assessment of the st correction. Finally, st correction functions have been assessed for all boom mounted cup and sonic anemometers (Fig. 6). Validation with LIDAR Mast Correction A Leosphere Windcube Lidar device was installed on the FINO1 platform at about 10 m distance to the st. One year worth of data have been evaluated for a st correction ( ). A st correction function for the cup measurements at 71.5 m, 81.5 m and 91.5 m LAT has been assessed from the Lidar measurements and compared to the UAM results. For almost all wind directions, Lidar measurements are not influenced by st effects and can be considered as undisturbed wind data. The Lidar data (ratio v lidar /v cup ) have been used to assess a st correction function. The whole correction function has been assessed based on two separate functional approxitions; the data have been divided into two parts (st wake and reining data). In Fig. 7, the Lidar st correction is compared to the UAM results for the cup anemometer data at 91.5 m and 71.5 m. The correction functions show excellent agreement, especially for the lateral acceleration seen for the in wind direction sector SW, Within the st wake, around 315, the UAM method leads to sller corrections than the Lidar correction. Based on the Lidar measurements, it has been shown that the st correction function does not significantly depend on wind speed, stability or turbulence intensity [7]. Estition of the Mast Correction Uncertainty The uncertainty associated with the application of the UAM method varies with wind direction; this uncertainty is high for wind directions where anemometers are situated in or at the boundary of the st wake. From the input data and the correction procedure, different uncertainty sources can be identified. The in uncertainty sources are uncertainties relating to: wind direction (denoted in the following as u1) the correction of the top anemometer (u2) the identification of unstable atmospheric situations and the correction of the reining wind shear (u 3 ) stability effects and wind speed dependencies which have not been considered in the calculation of the correction functions (u 4 ) In the following the uncertainties of the st correction are estited. The uncertainties are described as standard uncertainty. Uncertainty in wind direction (u 1 ) The st correction functions for all measurement heights have been defined in terms of the wind direction measured at 91.5 m LAT. The wind direction at this height can differ from the wind directions at other height levels, inly due to two reasons: Turbulent fluctuations of the wind direction, measured by its standard deviation, and systetic wind direction changes with height associated with the Ekn spiral, most pronounced during stable atmospheric DEWI MAGAZIN NO. 40, FEBRUARY

5 ast correction [-] UAM st correction cup 41.5m LAT UAM st correction cup 51.5m LAT UAM st correction cup 61.5m LAT UAM st correction cup 71.5m LAT UAM st correction cup 81.5m LAT UAM st correction cup 91.5m LAT UAM st correction cup 34m LAT FINO1 cup anemometer ast correction [-] FINO1 sonic anemometer UAM st correction sonic 41.5m LAT UAM st correction sonic 61.5m LAT UAM st correction sonic 81.5m LAT Fig. 6: UAM st correction for all boom mounted cup anemometers (left) and sonic anemometers (right) at FINO1. FINO1 91.5m LAT FINO1 71.5m LAT ast correction [-] UAM st correction LIDAR st correction ction [-] ast correc UAM st correction LIDAR st correction Fig. 7: Lidar st correction and UAM st correction for the heights of 91.5m and 71.5m LAT at FINO1. stratification. These uncertainties associated with the wind direction have been estited to be in the range of u dir =2-6. The uncertainty of the st correction function with respect to the wind direction can be expressed as and are depicted in Fig. 8. Uncertainties concerning the top anemometer (u 2 ) The met st correction functions are based on simultaneous wind speed measurements with the boom and top mounted anemometers. The UAM method therefore relies on a high quality top anemometer installation. At FINO 1, the top anemometer wind data have been corrected for effects due to the lightning protection cage and systetic speed-up effects. The uncertainty of the st correction function due to this top anemometer correction has been estited to be ± Uncertainties regarding wind shear (u 3 ) The UAM method is based on vanishing vertical wind speed gradients during unstable atmospheric conditions. Sll reining wind shear is corrected. The uncertainty of this correction has been estited from profile measurements with Lidar shown in [7]. The average wind speed increase under unstable conditions shown in [7] varies from 0.03% to 0.06% per m height difference with mean value 0.05% and standard deviation of 0.02%. The standard deviation of 0.02% per m height difference has been used to estite the uncertainty of the st correction function with respect to wind shear for each measurement height as shown in Tab DEWI MAGAZIN NO. 40, FEBRUARY 2012

6 ion [-] espect to wind direct Uncert tainty in re 0,35 0,30 0,25 0,20 0,15 0,10 0,05 Uncertainty of st correction in respect to wind direction cup104.5 cup91.5 cup81.5 cup71.5 cup61.5 cup51.5 cup41.5 cup31.5 sonic81.5 sonic61.5 sonic41.5 0,00 Fig. 8: Uncertainty of st correction in respect to wind direction. Height above LAT [m] Height difference to 104.5m Uncertainty of st correction in respect to wind shear (0.02% per m height difference) Tab. 2: Uncertainty of st correction in respect to wind shear. Unc certainty of st correction [-] 0,16 0,12 0,08 0,04 Uncertainty of st correction 91.5 m correction [-] Mast c 1 Mast correction 91.5m +/- uncertainty st corr st corr + uncertainty st corr - uncertainty Fig. 9: 0, Left: Uncertainty of the st correction at FINO1, 91.5m LAT. Right: Mast correction at FINO1 91.5m LAT with doin of uncertainty Uncertainties due to stability effects and wind speed dependencies (u 4 ) The correction functions are assumed independent of wind speed, atmospheric stability and turbulence intensity. An additional uncertainty of 5% (in the st wake) and 0.5% for all other directions has been associated with the use of this assumption, because it was shown in [7] that these parameters have only minor influence on the st correction functions. Overall uncertainties of the st correction The uncertainty of the st correction is then combined from different uncertainty sources: The resulting uncertainties, depending on the wind direction, are shown in Fig. 9 as an example for the cup measurement at 91.5 m LAT. Fig. 10 shows the uncertainty of the corrected wind speeds in respect to the st correction as percentage for all cup and sonic measurements. The overall uncertainty is calculated as weighted mean over all wind direction bins with the long-term wind direction distribution at FINO1 from and is given in Tab. 3, separately for each height level. Optionally the data can be filtered for a data set that omits the wind directions in the st wake. If data are not filtered for a certain wind direction sector but the whole data set is used, the values of Tab. 3 apply for the uncertainty assessment. To calculate the overall uncertainty of the wind speed measurement after application of the st correction, ad- DEWI MAGAZIN NO. 40, FEBRUARY

7 t correctio on [%] Uncertain nty of s 30% 25% 20% 15% 10% Uncertainty of corrected wind speeds in respect to the st correction - FINO1 cup anemometers cup31.5 cup41.5 cup51.55 cup61.5 cup71.5 cup81.5 cup91.5 cup104.5 t correctio on [%] Uncertain nty of s 30% 25% 20% 15% 10% Uncertainty of corrected wind speeds in respect to the st correction - FINO1 - sonic anemometers sonic41.5 sonic61.5 sonic81.5 5% 5% 0% Fig. 10: Wind Direction [ ] 0% Wind Direction [ ] Uncertainty of st corrected wind speeds in respect to the st correction for the FINO1 cup and sonic wind speed measurements as percentage. Anemometer cup104.5 cup91.5 cup81.5 cup71.5 cup61.5 cup51.5 cup41.5 cup31.5 sonic81.5 sonic61.5 sonic41.5 Resulting overall uncertainty of st correction 2% 2% 2% 3% 3% 3% 4% 4% 2% 2% 3% Tab. 3: Overall uncertainty of st corrected wind speeds in respect to st correction for the FINO1 cup and sonic wind speed measurements. ditional sources of uncertainty like calibration uncertainty and operational characteristics of the anemometers have to be taken into account (like described in IEC [9]). According to ISO-IEC GUM [10], the uncertainty of the mounting effects can be replaced by the uncertainty of the st correction. References: [1] RAVE: Research at Alpha Ventus, [2] FINO: Forschungsplattformen in Nord- und Ostsee, [3] International Electrotechnical Commission (IEC): IEC Wind turbines - Part 12-1: Power perfornce measurements of electricity producing wind turbines, 1 st ed., 12/2005. [4] T. Neunn: FINO1 and the st shadow effect, 52nd IEA Topical Expert meeting: Wind and Wave Measurements at Offshore Locations, Berlin, Gerny, February [5] A. Beeken, T. Neunn, A. Westerhellweg: Five years of operation of the first offshore wind research platform in the Gern Bight FINO 1, DEWEK 2008, Bremen. [6] A. Westerhellweg, B. Canadillas, A. Beeken, T. Neunn: One Year of Lidar Measurements at FINO1-Platform: Comparison and Verification to Met-Mast Data. DEWEK 2010, Bremen. [7] A. Westerhellweg, V. Riedel, T. Neunn: Comparison of Lidar- and UAM-based offshore st effect corrections, EWEA 2011, Brussels. [8] F. Kinder, A. Westerhellweg, T. Neunn: Meteorological measurements at FINO1 during the existence of the wind farm Alpha Ventus, EOW 2011, Amsterdam. [9] International Electrotechnical Commission (IEC): IEC Wind turbines - Part 12-1: Power perfornce measurements of electricity producing wind turbines, 1 st ed., 12/2005. [10] ISO-IEC Guide 98-3: Uncertainty of measurement-part3: Guide to the expression of uncertainty in measurement (GUM: 1995), Switzerland, DEWI MAGAZIN NO. 40, FEBRUARY 2012