The OWEZ Meteorological Mast

The OWEZ Meteorological Mast Analysis of mast-top displacements P.J. Eecen E. Branlard ECN-E--08-067 OWEZ_R_121_mast_top_movement

Acknowledgement/Preface The Off Shore wind Farm Egmond aan Zee has a subsidy of the Ministry of Economic Affairs under the CO2 Reduction Scheme of the Netherlands Abstract NoordzeeWind carries out an extensive measurement and evaluation program as part of the OWEZ project. The technical part of the measurement and evaluation program considers topics as climate statistics, wind and wave loading, detailed performance monitoring of the wind turbines, etc. The meteorological measurements at the 116m high meteorological mast at the location of the wind farm are published on the project website and are widely used. Data at measurement sample frequency have been stored since May 2007 and recently have become available. From the acceleration measurements at the mast top, the displacements are reconstructed and the effect of these movements on the uncertainty of the wind speed measurements are estimated. From the acceleration measurements it is found that the influence of the mast-top movements on the wind speed measurements is limited and smaller than the flow distortion of the mast itself. The project is carried out under assignment of NoordzeeWind BV. Principal NoordzeeWind Henk Kouwenhoven p.a. Shell Wind Energy BV Postbus 38000 1030 BN Amsterdam Project information Contract number NZW-16-C-2-R01 ECN project number: 7.9433 2 ECN-E--08-067

Contents List of tables 4 List of figures 4 1. Introduction 5 2. Measured data 7 2.1 Meteorological mast 7 2.2 Sensor calibration 7 2.3 Data validation 7 3. Interpretation of the acceleration data 9 3.1 Selecting the data 9 3.2 Overview of the data 9 4. Analysis of the Acceleration Measurements 11 4.1 Estimate of the typical frequency 12 4.2 Estimate of the speed and displacement amplitudes 14 5. Results 16 5.1 Effects of the mast-top motion on the measurements 18 5.2 Comparison to other disturbances 19 6. Conclusions 21 7. References 21 ECN-E--08-067 3

List of tables Table 2.1 Coordinates of the meteorological mast at OWEZ... 7 Table 3.1 Date and time of the samples selected for each wind speed bin and for a wind direction around 235.... 9 Table 3.2 Date and time of the samples selected for each wind speed bin and for a wind direction around 15.... 9 Table 4.1 Frequency and maximum acceleration amplitude of the EW acceleration signal for each selected period are presented together with the maximum speed and displacement of the mast-top in the EW direction.... 15 Table 4.2 Frequency and maximum acceleration amplitude of the NS acceleration signal for each selected period are presented together with the maximum speed and displacement of the mast-top in the NS direction.... 15 Table 5.1 The maximum mast-top velocities and the ratio between the maximum mast-top velocity and the ambient wind speed.... 19 List of figures Figure 1.1 The meteorological mast (right) at the time of installation of the wind turbines of the OWEZ wind farm.... 5 Figure 1.2 The meteorological mast at the OWEZ wind farm... 6 Figure 2.1 Schematic drawing of the meteorological mast... 8 Figure 3.1 Acceleration measurements in the two directions EW and NS in the 10 minutes sample corresponding to a wind speed of 12 m/s and a wind direction of 206 (5-6-2007, 10:50)... 10 Figure 3.2 EW acceleration data during a 1-minute period. The main oscillation frequency of 4Hz (T=2.5s) is readily observed.... 10 Figure 4.1 Detailed fragment of the measured acceleration data and its sine estimate. The sine amplitude corresponds to the maximum amplitude of the acceleration signal in the 10 minutes period... 11 Figure 4.2 Typical Fourier spectrum of an acceleration signal... 12 Figure 4.3 Main frequency of the acceleration signal for the different samples where the wind direction is around 235 degrees... 13 Figure 4.4 Main frequency of the acceleration signal for the different samples where the wind direction is around 15 degrees... 13 Figure 4.5 Effect on integration on the amplitude of the signal with amplitude 0.06 m/s2 and frequency 0.4 Hz.... 14 Figure 5.1 : Maximum acceleration amplitude as function of wind speed for the EW and NS acceleration measurements for wind directions around 235 degrees.... 17 Figure 5.2 Maximum acceleration amplitude as function of wind speed for the EW and NS acceleration measurements for wind directions around 15 degrees.... 17 Figure 5.3 Amplitude of the speed obtained by integration of the maximum acceleration estimation... 18 Figure 5.4 Ratios between anemometer readings mounted on the South (S), North-West (NW) and North-East (NE) booms of the meteorological mast. The wind speed ratios NW/NE are indicated in green, the ratios NE/S are indicated in blue and the ratios S/NW are indicated in red. The indicated wind direction along the horizontal axis is the derived wind direction as described in section 2.2.2. Wind speeds above 4m/s have been selected... 20 4 ECN-E--08-067

1. Introduction NoordzeeWind carries out an extensive measurement and evaluation program (NSW-MEP) as part of the OWEZ project. NoordzeeWind contracted Bouwcombinatie Egmond (BCE) to build and operate an offshore meteorological mast at the location of the OWEZ wind farm. BCE contracted Mierij Meteo to deliver and install the instrumentation in the meteorological mast. After the data have been validated, BCE delivers the measured 10-minute statistics data to NoordzeeWind. ECN created a database under assignment of NoordzeeWind and fills the database with the delivered data. The technical part of the measurement and evaluation program considers topics as climate statistics, wind and wave loading, detailed performance monitoring of the wind turbines, etc. Before installation of the wind farm, a 116m high meteorological mast has been installed to measure the wind conditions. During this period, wind conditions are measured that are not disturbed by a nearby wind farm. This mast is in operation since the summer of 2005. After realisation of the wind farm, the mast has also been used to, among others, measure wind conditions in the wake of turbines and perform mechanical load and power performance measurements. The measurements at the 116m high mast are part of NSW-MEP tasks 1.2.1 and 1.8.1 and are reported in half-year reports [3-7]. An important design parameter of the meteorological mast is to limit the displacements of the mast-top. If the mast-top would have large deflections and associated speeds, the wind speed measurements could be disturbed. An earlier analysis has been performed [8, 9], but at that time, the data at measurement sample frequency were not available. Therefore, the analysis was limited. Since May 2007 the data at measurement sample frequency of the mast are stored and recently these have become available. These data are used to assess the displacements of the mast-top as function of wind speed. From the movement the effect on the accuracy of the wind speed measured at the mast-top is estimated. For this purpose the acceleration measurements are used. Figure 1.1 The meteorological mast (right) at the time of installation of the wind turbines of the OWEZ wind farm. ECN-E--08-067 5

Figure 1.2 The meteorological mast at the OWEZ wind farm. 6 ECN-E--08-067

2. Measured data 2.1 Meteorological mast The meteorological mast is a lattice tower with booms at three heights: 21m 70m and 116m above mean sea level (MSL). At each height, three booms are installed in the directions northeast (NE), south (S) and north-west (NW) [1]. Sensors attached to the meteorological mast are described in [2]. The location of the meteorological mast is given in Table 2.1. Table 2.1 Coordinates of the meteorological mast at OWEZ UTM31 ED50 WGS 84 x 594195 4º23'22,7'' EL y 5829600 52º36'22,9'' NB The instrumentation is described in a report [1]. The data are reported in half-year reports [3-7]. 2.2 Sensor calibration The applied sensors in the meteorological mast are calibrated according to maintenance schedules of BCE (Mierij Meteo). The cup anemometers are calibrated at DEWI Germany. BCE (Mierij Meteo) calibrates the other sensors. The calibration constants are applied to the data during the stage of data processing at BCE (Mierij Meteo). It must be noted that the 10-minute statistics are calibrated; the 4Hz data do not have all calibration constants applied. 2.3 Data validation In the measuring period, defective sensors or cables or other malfunctioning of the measurement system can corrupt the measured data. For this reason, BCE (Mierij Meteo) validates all measured data [4]. The quality and consistency of the data is assessed by means of manual check of the received data on 1. Consistency 2. Out of range numbers 3. Followed by marking of incorrect and unavailable records Corrupt or missing data fields are marked by error values (-999999). Mierij Meteo has specified that the acceleration measurements have a sample frequency of 32Hz. However, from the data it is found that the sample frequency for the acceleration data is 33Hz. In the analyses described in this report, the 33Hz sample frequency is used. ECN-E--08-067 7

Figure 2.1 Schematic drawing of the meteorological mast 8 ECN-E--08-067

3. Interpretation of the acceleration data 3.1 Selecting the data The analysis presented in this report is based on the data from the acceleration sensors in the top of the meteorological mast and from the anemometer and vane of the NW boom at 116m. Wind data are sampled at 4Hz, whereas the acceleration data are sampled at 33Hz. From the validated 10 minutes statistics values (that are also publicly available at www.noordzeewind.nl), interesting time series are selected for further analysis. Data from two wind directions (WD) and various values for the wind speed are selected. These wind directions are around 15 and 235 with respect to North and wind speed is selected based on 2m/s bin values. For each wind speed bin, and each wind direction, a single sample of 10 minutes length was selected. The selected 10-minute periods are presented in Table 3.1 and Table 3.2. Table 3.1 Date and time of the selected samples for each wind speed bin for a wind direction around 235. WS Date Time 4 May 7 2007 8:50 6 May 7 2007 11:20 8 May 7 2007 21:10 10 May 6 2007 20:30 12 May 6 2007 10:50 14 May 6 2007 12:50 16 May 10 2007 5:40 18 May 10 2007 0:20 20 May 10 2007 4:10 Table 3.2 Date and time of the selected samples for each wind speed bin for a wind direction around 15. WS Date Time 4 May 20 2007 12:30 6 May 20 2007 13:00 8 May 20 2007 14:50 10 May 20 2007 15:30 12 May 20 2007 22:10 3.2 Overview of the data After selecting the interesting dates and times from the 10 minutes statistics database, the relevant 33Hz acceleration data samples are extracted. The acceleration sensor provides two signals corresponding to the acceleration in the North-South (NS) direction and in the East-West (EW) direction. Examples of the data are presented in Figure 3.1. A first glance at the acceleration data (Figure 3.1) leads to the following observations: The sensors have an offset of about 0.3m/s 2. Since the mast is fixed, the data should be centred around zero and this is clearly an offset due to measurement uncertainties. This offset is removed by subtracting the mean to each data series. The acceleration data at 33Hz have discrete values. However, the accelerations are large enough to be able to perform the analyses. By zooming on the time series (cf. Figure 3.2), it clearly appears that the acceleration oscillates with a frequency almost constant with time. This main frequency is approximately 0.4Hz (T = 2.5s). ECN-E--08-067 9

Figure 3.1 Acceleration measurements in the two directions EW and NS in the 10 minutes sample corresponding to a wind speed of 12 m/s and a wind direction of 206 (5-6-2007, 10:50). Figure 3.2 EW acceleration data during a 1-minute period. The main oscillation frequency of 4Hz (T=2.5s) is readily observed. 10 ECN-E--08-067

4. Analysis of the Acceleration Measurements In order to asses the speed and movements of the mast-top for the various wind speeds and wind directions, a relatively straight-forward model is applied. The following procedure is applied to estimate the maximum amplitude of the mast-top oscillation. For each sample, the model of the acceleration consists of a sinusoidal function with the same frequency as the measured data and with an amplitude equal to the maximum of the acceleration signal in the entire 10 minute sample. This leads to an over-estimate of the values for the speed and displacement. This is the preferred approach. Figure 4.1 Detailed fragment of the measured acceleration data and its sine estimate. The sine amplitude corresponds to the maximum amplitude of the acceleration signal in the 10 minutes period. ECN-E--08-067 11

4.1 Estimate of the typical frequency For each 10-minte period (see Table 3.1 and Table 3.2) and for each signal (NS/EW), the main oscillation frequency is obtained by determining the abscissa of the maximum Fourier spectrum component of the measured acceleration data. Figure 4.2 Typical Fourier spectrum of an acceleration signal. The frequencies of each selected period are very similar as is illustrated in Figure 4.3 and Figure 4.4. For the different wind speeds and wind directions, the range of frequencies is [0.398 Hz; 0.415 Hz]. As is clearly shown, there is no obvious dependency of the mast-top movement frequency on the wind speed and wind direction. 12 ECN-E--08-067

0,42 Main frequency of the different acceleration samples Wind direction around 235 degrees Acceleration frequency [Hz] 0,41 0,40 EW main frequency NS main frequency 0,39 0 5 10 15 20 25 WS [m/s] Figure 4.3 Main frequency of the acceleration signal for the different samples where the wind direction is around 235 degrees. 0,42 Main frequency of the different acceleration samples Wind direction around 15 degrees Acceleration frequency [Hz] 0,41 EW main frequency NS main frequency 0,40 0 5 10 15 20 25 WS [m/s] Figure 4.4 Main frequency of the acceleration signal for the different samples where the wind direction is around 15 degrees. ECN-E--08-067 13

4.2 Estimate of the speed and displacement amplitudes From the acceleration measurements, the speed and position of the mast-top in each direction are obtained analytically by integrating the estimated acceleration. If A is the maximum acceleration amplitude in the x direction (EW direction), and f the main frequency of the acceleration for the studied sample, the acceleration & x&, the speed x& and displacement x are: && x = Asin(2πf t) A x& = cos(2πf t) 2πf A x = 4π f sin(2πf ) t 2 2 The same results with different frequency, amplitude and phase are obtained in the other direction (NS direction). For all the samples the frequency is close to 0.4 Hz, and as a result 1 / 2π f < 1, which leads to a decrease of amplitude at each integration. As a result of this, by defining A as the maximum amplitude of the acceleration, both the speed and the position of the mast are over-estimated. Figure 4.5 illustrates this decrease in amplitude for an acceleration signal of amplitude 0.06 m/s 2. Figure 4.5 Effect on integration on the amplitude of the signal with amplitude 0.06 m/s2 and frequency 0.4 Hz. 14 ECN-E--08-067

Table 4.1 Frequency and maximum acceleration amplitude of the EW acceleration signal for each selected period are presented together with the maximum speed and displacement of the mast-top in the EW direction. Wind Direction Wind Speed Acceleration frequency Acceleration Amplitude Speed Amplitude Displacement Amplitude [degree] [m/s] [Hz] [m/s/s] [m/s] [m] 17,0 4,4 0,415 0,057 0,022 0,008 20,2 6,3 0,414 0,055 0,022 0,009 16,1 8,0 0,408 0,086 0,034 0,013 28,7 10,2 0,411 0,122 0,048 0,019 357,0 12,0 0,408 0,148 0,058 0,022 246,2 6,1 0,408 0,111 0,043 0,017 248,6 8,0 0,398 0,126 0,050 0,020 257,2 10,2 0,399 0,206 0,082 0,032 206,6 12,2 0,414 0,144 0,056 0,022 216,6 14,3 0,405 0,164 0,064 0,025 257,1 16,3 0,407 0,326 0,128 0,050 233,3 18,3 0,410 0,252 0,098 0,038 233,5 20,0 0,407 0,377 0,151 0,060 Table 4.2 Frequency and maximum acceleration amplitude of the NS acceleration signal for each selected period are presented together with the maximum speed and displacement of the mast-top in the NS direction. Wind Direction Wind Speed Acceleration frequency Acceleration Amplitude Speed Amplitude Displacement Amplitude [degree] [m/s] [Hz] [m/s/s] [m/s] [m] 17,0 4,4 0,411 0,048 0,019 0,007 20,2 6,3 0,401 0,048 0,019 0,008 16,1 8,0 0,406 0,099 0,039 0,015 28,7 10,2 0,401 0,188 0,075 0,030 357,0 12,0 0,409 0,210 0,082 0,032 246,2 6,1 0,408 0,070 0,027 0,011 248,6 8,0 0,404 0,129 0,051 0,020 257,2 10,2 0,403 0,196 0,077 0,031 206,6 12,2 0,409 0,112 0,043 0,017 216,6 14,3 0,406 0,119 0,047 0,018 257,1 16,3 0,405 0,251 0,099 0,039 233,3 18,3 0,408 0,251 0,098 0,038 233,5 20,0 0,398 0,375 0,150 0,060 ECN-E--08-067 15

5. Results In the model previously described, the acceleration is fitted with a sine with amplitude equal to the maximum amplitude of the measured acceleration signal and with a fixed frequency. The latter is due to the fact that the frequency is not dependent on the wind speed and it can be assumed that the frequency of the mast-top movements is constant. Consequently, speed and position linearly depend on the acceleration. For this reason, if a correlation holds for acceleration, it also holds for the speed and the displacement of the mast-top. For each 10-minte period (see Table 3.1 and Table 3.2), the maximum amplitude of the measured acceleration is calculated. These amplitude values are presented in Figure 5.1 and Figure 5.2 as function of the averaged wind speed. Figure 5.1 and Figure 5.2 show that the maximum acceleration amplitude is a function of the ambient wind speed. With increasing wind speed, the frequency of the mast-top oscillating motion does not change, but the accelerations increase. Moreover, a similar trend is followed by both EW and NS components of the acceleration measurements. The two curves are quite similar. This means that the displacements in both directions are equally sensitive to a change of wind speed and thus evolve in the same order. In Figure 5.3 the maximum speed of the mast-top movements corresponding to the accelerations presented in Figure 5.1 and Figure 5.2 are presented. From Figure 5.3 it is concluded that at a given wind speed and direction, the mast speed (and thus, the mast displacement) in the EW direction is larger than in the NS direction. This might be correlated with the mast structure and main structural directions that play a role on the mast stiffness but it is not within the scope of this report. 16 ECN-E--08-067

Acceleration amplitude [m/s/s] - 0,40 0,35 0,30 0,25 0,20 0,15 0,10 0,05 0,00 Acceleration : maximum amplitude of the different samples Wind direction around 235 degrees EW acceleration NS Acceleration 0 5 10 15 20 25 WS [m/s] Figure 5.1 : Maximum acceleration amplitude as function of wind speed for the EW and NS acceleration measurements for wind directions around 235 degrees. Acceleration amplitude [m/s/s] - 0,40 0,35 0,30 0,25 0,20 0,15 0,10 0,05 0,00 Acceleration : maximum amplitude of the different samples Wind direction around 15 degrees EW Acceleration NS Acceleration 0 5 10 15 20 25 WS [m/s] Figure 5.2 Maximum acceleration amplitude as function of wind speed for the EW and NS acceleration measurements for wind directions around 15 degrees. ECN-E--08-067 17

Speed after integrating the estimated acceleration Speed amplitude [m/s] 0,16 0,14 0,12 0,10 0,08 0,06 0,04 0,02 0,00 EW-135 EW-235 NS-135 NS-235 0 5 10 15 20 25 WS [m/s] Figure 5.3 Amplitude of the speed obtained by integration of the maximum acceleration estimation. 5.1 Effects of the mast-top motion on the measurements The motion of the mast-top will have the largest effect on the measurements of wind speed. The motion of the mast-top will add an oscillating speed to the ambient wind speed. In this section, an estimate is presented of the uncertainty of the wind speed measurement due to the oscillating movements of the mast-top. For each 10-minute period that has been studied, the largest effects are selected, so that in a sense, an overestimate is presented here. The largest speeds of the mast-top are associated with the EW movements, at an ambient wind direction of 235 degrees (see Figure 5.3). An estimate of the effect on the scalar wind speed measurements is to assume the EW and NS components of the mast speed to be equal: V mast 2 EW 2 NS = V + V 2 V EW. The ratio is defined between the mast speed and the ambient wind speed, which is an indication for the relative influence of the mast-top movements on the wind speed measurements. The results are presented in Table 5.1. The result of the analysis presented here is that (shown in Table 5.1) the disturbance of the mast-top movement on the measured wind speed does not exceed 1.1%. 18 ECN-E--08-067

Table 5.1 The maximum mast-top velocities and the ratio between the maximum mast-top velocity and the ambient wind speed. Mast-top velocity EW (max) Mast-top velocity NS (max) Wind Speed [m/s] [m/s] [m/s] [-] 6,1 0,04 0,06 0,010 8,0 0,05 0,07 0,009 10,2 0,08 0,12 0,011 12,2 0,06 0,08 0,007 14,3 0,06 0,09 0,006 16,3 0,13 0,18 0,011 18,3 0,10 0,14 0,008 20,0 0,15 0,21 0,011 Speed Ratio V mast /WS 5.2 Comparison to other disturbances The largest other effect on the measured wind speed is the disturbance of the mast itself. From other reports it is known that the disturbance of the mast is significant. This is illustrated in Figure 5.4, where the ratios between anemometer readings mounted on the South (S), North- West (NW) and North-East (NE) booms of the meteorological mast are presented (taken from other report [7]). The typical disturbance at 116m height is 3% (excluding the anemometer measurements in the wake of the mast). In that respect, the mast motion will have a small contribution to the measurement uncertainty of the wind speed. ECN-E--08-067 19

Figure 5.4 Ratios between anemometer readings mounted on the South (S), North-West (NW) and North-East (NE) booms of the meteorological mast. The wind speed ratios NW/NE are indicated in green, the ratios NE/S are indicated in blue and the ratios S/NW are indicated in red. The indicated wind direction along the horizontal axis is the derived wind direction as described in section 2.2.2. Wind speeds above 4m/s have been selected. 20 ECN-E--08-067

6. Conclusions The data at higher sample rate have recently become available in the MEP-NSW project. The acceleration data available at 33Hz have been investigated to assess the displacements of the mast-top and the associated velocities. The effect of the mast movements on the measured wind speed has been assessed and it is found that a limited effect of maximum 1.1% is found. Compared to the influence of the flow distortion of the mast itself which is in the order of 3%, the mast movement effects are insignificant. 7. References 1. H.J. Kouwenhoven, User manual data files meteorological mast NoordzeeWind, Document code: NZW-16-S-4-R03, Date: 1 October 2007, 2. BCE (Mierij Meteo), sensor overview, OWEZ document 3672-OV 3. P.J. Eecen, L.A.H. Machielse, A.P.W.M. Curvers, Meteorological Measurements OWEZ, Half year report (01-07-2005-31-12-2005), ECN-E--07-073 4. P.J. Eecen, L.A.H. Machielse, A.P.W.M. Curvers, Meteorological Measurements OWEZ, Half year report (01-01-2006-30-06-2006), ECN-E--07-074 5. P.J. Eecen, L.A.H. Machielse, A.P.W.M. Curvers, Meteorological Measurements OWEZ, Half year report (01-07-2006-31-12-2006), ECN-E--07-075 6. P.J. Eecen, L.A.H. Machielse, A.P.W.M. Curvers, Meteorological Measurements OWEZ, Half year report (01-01-2007-30-06-2007), ECN-E--07-076 7. P.J. Eecen, Meteorological Measurements OWEZ, Half year report (01-07-2007-31- 12-2007), ECN-E--08-062 8. A.J. Brand and J.J. Heijdra, Kwantificeren invloed mastbeweging op gemeten windsnelheid, 17 oktober 2001, internal ECN communication 9. P.J. Eecen, Analysis of mast movements, note to project manager MEP-NSW, 07-8247 ECN-E--08-067 21