FINO1 Mast Correction
A. Westerhellweg, T. Neumann; 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 masts than for onshore masts.
Combined wind and wave loads require different mast designs, which may result in denser lattices, larger footprints
and shorter booms as compared to onshore masts. In order to use such offshore wind measurement data for ener­
gy yield assessments, the disturbances caused by the mea­
sure­ment mast need to be properly addressed.
For this, DEWI has developed a mast correction method
based on the vanishing vertical wind gradients during very
unstable situations, thus enabling a “uniform ambient flow
mast correction” (UAM). This mast correction scheme was
devised by DEWI in 2007 for the FINO1 offshore wind mea­
surement 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 mast 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 mast correction based on
60
DEWI MAGAZIN NO. 40, FEBRUARY 2012
Lidar data showed high agreement and validated the performance of the UAM method [7]. The UAM method can be
used for offshore masts 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 mast [2] has a square cross-section. Cup anemometers are installed on booms on the south-east side of
the mast (the main wind direction is south-west). Sonic
anemometers and wind vanes are installed on the opposite north-west side of the mast. The mast 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
104.5 m LAT. An offshore wind measurement mast has to
be designed to resist wind and wave loads which might not
allow a measurement set-up according to IEC61400-12-1
standards [3]. In the case of FINO1, the lattice mast 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
Anemometer,
height LAT
Tab. 1: Measurement set-up details of the boom
mounted anemometers at FINO1.
Fig. 1: Offshore platform FINO1.
Boom
length
Orientation
Ratio (distance to mast
centre)/ (mast width)
[m]
[m]
[m]
[°]
[-]
cup 91.5
1.375
3.0
135
2.7
cup 81.5
1.754
3.0
139
2.2
cup 71.5
2.124
4.0
143
2.4
cup 61.5
2.504
5.5
142
2.7
cup 51.5
2.875
5.5
140
2.4
cup 41.5
3.254
6.5
142
2.5
cup 34
3.532
6.5
143
2.3
sonic 81.5
1.754
3.0
311
2.2
sonic 61.5
2.504
5.5
308
2.7
sonic 41.5
3.254
6.5
308
2.5
Fig. 2: Left: Top anemometer at 103 m LAT in lightning cage and additional anemometer at 104.5 m LAT.
Right: Ratio of the wind speeds of the additional and the top anemometer
lengths to the mast width varies from 2.2 to 2.7 (Tab. 1)
and is much smaller 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 mast wake, the wind speed reduction is very
large and amounts to up to 40%. A mast 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 systematic speedup effects at the mast 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
104.5 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
Mast
width
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, systematic 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 mast. 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 2005-11-15 - 2006-07-12. In 2008,
the measurement has been re-installed with another anemometer but the identical measurement set-up. In the
one-year period 2008-12-28 – 2009-12-29 the previous
correction functions were confirmed. With the application
DEWI MAGAZIN NO. 40, FEBRUARY 2012
61
120
Height LAT [m]
100
80
60
40
20
reduced data set (unstable stratification, wind direction sector [220°;230°])
wind profile between 91.5m and 41.5m
0
95%
corrected wind profile
96%
97%
98%
99%
100%
101%
102%
103%
Percentage deviation from wind speed
measured at 104.5 m height LAT [-]
Fig. 3:
Wind profiles examined in direction sector [220°;230°] for unstable
stratification.
of the correction function, the reference height of these
data changes from 103 m to a height level of 104.5 m LAT.
Fig. 4:
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.
March 2004 until February 2006 (2 years) and February
2010 until January 2011 (12 months) have been evaluated.
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 mast correction functions by
sector-wise comparison of the boom and top mounted
anemo­meters. In order to create a situation similar to
putting the entire mast into a very large wind tunnel,
weather situations are isolated during which the vertical
wind speed gradient approaches zero. The remaining wind
shear is corrected and the resulting wind speed ratios
vtop/vboom constitute the mast 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 remaining wind shear.
c) Calculation of the ratios vtop/vboom as the mast 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 pragmatic 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
62
DEWI MAGAZIN NO. 40, FEBRUARY 2012
b) Correction of remaining wind shear
Even in unstable conditions, the wind shear is not zero
(Fig. 3). The wind shear for unstable stratification is therefore approximated and considered in the mast correction
functions. At FINO1, all boom-mounted cup anemometers
are approximately 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 smallest disturbance caused by the mast is
expected theoretically. Fig. 3 shows the vertical wind profiles of the data sets examined in the evaluation period
March 2004 – February 2006. For this reduced data set, a
mean wind profile has been determined (see Fig. 4, red
line). Presumably, there is a small lateral speed-up effect
present at the mast 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 104.5 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.
Fig. 5:
The ratio vtop/vboom of the resulting data set serves as mast correction for each boom mounted anemometer
c) The Ratio vtop/vboom represents the mast correction for
each height.
The unstable data set is evaluated for the mast correction.
For the different heights, vboom is corrected according to
the wind shear shown in Fig. 4 and the ratio vtop/vboom
serves as mast correction for each boom mounted anemometer. The correction function is based on two different functional approximations; the data have been divided
into two parts (mast wake and remaining data). As an
example, Fig. 5 shows all data and the data used for the
mast 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 mast correction. Finally, mast 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 mast. One
year worth of data have been evaluated for a mast correction (2009-08-01 – 2010-07-31). A mast 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
mast effects and can be considered as undisturbed wind
data. The Lidar data (ratio vlidar/vcup) have been used to
assess a mast correction function. The whole correction
function has been assessed based on two separate functional approximations; the data have been divided into
two parts (mast wake and remaining data).
In Fig. 7, the Lidar mast 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, espe-
cially for the lateral acceleration seen for the main wind
direction sector SW, 210°-270°. Within the mast wake,
around 315°, the UAM method leads to smaller corrections than the Lidar correction. Based on the Lidar measurements, it has been shown that the mast correction
function does not significantly depend on wind speed,
stability or turbulence intensity [7].
Estimation 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 mast wake. From the input
data and the correction procedure, different uncertainty
sources can be identified. The main 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 remaining wind shear (u3)
• stability effects and wind speed dependencies which
have not been considered in the calculation of the
correction functions (u4)
In the following the uncertainties of the mast correction
are estimated. The uncertainties are described as standard
uncertainty.
Uncertainty in wind direction (u1)
The mast 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, mainly due
to two reasons: Turbulent fluctuations of the wind direction, measured by its standard deviation, and systematic
wind direction changes with height associated with the
Ekman spiral, most pronounced during stable atmospheric
DEWI MAGAZIN NO. 40, FEBRUARY 2012
63
1,8
UAM mast correction cup 41.5m LAT
1,7
ma
ast correction [-]
UAM mast correction cup 71.5m LAT
1,4
UAM mast correction cup 81.5m LAT
1,3
UAM mast correction cup 91.5m LAT
1,2
UAM mastt correction
ti cup 34
34m LAT
1,1
1,3
1,2
1,1
0,9
0,9
Fig. 6:
30
60
90
120
150
180
210
wind direction [°]
240
270
300
330
360
0,8
60
1,8
FINO1 91.5m LAT
90
120
150
180
210
240
270
300
330
360
240
270
300
330
360
FINO1 71.5m LAT
1,7
1,6
1,6
UAM mast correction
1,5
ma
ast correc
ction [-]
ma
ast correction [-]
30
wind direction [°]
1,7
LIDAR mast correction
1,4
1,3
1,2
1,1
1,3
1,2
1,1
0,9
0,9
Fig. 7:
30
60
90
120
150
180
210
wind direction [°]
240
270
300
330
360
LIDAR mast correction
1,4
10
1,0
0
UAM mast correction
1,5
15
1,0
0,8
0
30
60
90
120
150
180
210
wind direction [°]
Lidar mast correction and UAM mast correction for the heights of 91.5m and 71.5m LAT at FINO1.
stratification. These uncertainties associated with the
wind direction have been estimated to be in the range of
udir=2°-6°. The uncertainty of the mast correction function
with respect to the wind direction can be expressed as
ο݉ܽ‫ݎݎ݋ܿݐݏ‬
߲݉ܽ‫ݎݎ݋ܿݐݏ‬
‫ݑ‬ଵ ሺ݀݅‫ݎ‬ሻ ൌ ฬ
‫ݑ‬ௗ௜௥ ฬ ൎ ฬ
‫ݑ‬ௗ௜௥ ฬ ο݀݅‫ݎ‬
߲݀݅‫ݎ‬
and are depicted in Fig. 8.
Uncertainties concerning the top anemometer (u2)
The met mast 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 systematic
speed-up effects. The uncertainty of the mast correction
64
0
UAM mast correction for all boom mounted cup anemometers (left) and sonic anemometers (right) at FINO1.
1,8
0,8
UAM mast correction sonic 81.5m LAT
1,4
1,0
0
UAM mast correction sonic 61.5m LAT
1,5
1,0
0,8
UAM mast correction sonic 41.5m LAT
1,6
UAM mast correction cup 61.5m LAT
1,5
FINO1 sonic anemometer
1,7
UAM mast correction cup 51.5m LAT
1,6
ma
ast correction [-]
1,8
FINO1 cup anemometer
DEWI MAGAZIN NO. 40, FEBRUARY 2012
function due to this top anemometer correction has been
estimated to be ± 0.0125.
Uncertainties regarding wind shear (u3)
The UAM method is based on vanishing vertical wind speed
gradients during unstable atmospheric conditions. Small
remaining wind shear is corrected. The uncertainty of this
correction has been estimated 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 estimate the uncertainty of the mast correction function with
respect to wind shear for each measurement height as
shown in Tab. 2.
0,35
Uncertainty of mast correction in respect to wind direction
Uncerttainty in re
espect to w
wind direction [-]
0,30
0,25
0 25
0,20
cup104.5
cup91.5
cup81.5
cup71.5
cup61.5
cup51.5
cup41.5
cup31.5
sonic81 5
sonic81.5
sonic61 5
sonic61.5
sonic41.5
0,15
0,10
0,05
0,00
0
30
60
90
120
150
180
210
wind direction [°]
240
270
300
330
360
Fig. 8: Uncertainty of mast correction in respect to wind
direction.
Height above LAT
[m]
Height difference
to 104.5m
Uncertainty of mast correction in respect to wind shear
(0.02% per m height difference)
104.5
91.5
81.5
71.5
61.5
51.5
41.5
34
0
13
23
33
43
53
63
70.5
0.000
0.002
0.004
0.006
0.008
0.010
0.012
0.013
Tab. 2: Uncertainty of mast correction
in respect to wind shear.
1,7
Mast correction 91.5m +/- uncertainty
1,6
Uncertainty of mast correction 91.5 m
Mast correction
c
[-]
Unccertainty of m
mast correction
n [-]
0,16
0,12
0,08
mast corr
15
1,5
mast corr + uncertainty
mast corr - uncertainty
1,4
1,3
1,2
1,1
1
0,04
0,9
0,8
0,00
0
Fig. 9:
90
180
i d direction
di ti [°]
wind
270
360
90
180
wind direction [°]
270
360
Left: Uncertainty of the mast correction at FINO1, 91.5m LAT. Right: Mast correction at FINO1 91.5m LAT with domain of uncertainty.
Uncertainties due to stability effects and wind speed
dependencies (u4)
The correction functions are assumed independent of wind
speed, atmospheric stability and turbulence intensity. An
additional uncertainty of 5% (in the mast 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 mast correction functions.
Overall uncertainties of the mast correction
The uncertainty of the mast correction is then combined
from different uncertainty sources:
‫ݑ‬ሺ݀݅‫ݎ‬ሻ ൌ ඥ‫ݑ‬ଵ ሺ݀݅‫ݎ‬ሻଶ ൅ ‫ݑ‬ଶ ሺ݀݅‫ݎ‬ሻଶ ൅ ‫ݑ‬ଷ ሺ݀݅‫ݎ‬ሻଶ ൅ ‫ݑ‬ସ ሺ݀݅‫ݎ‬ሻଶ 0
The resulting uncertainties, depending on the wind direction, are shown in Fig. 9 as an example for the cup mea­
surement at 91.5 m LAT. Fig. 10 shows the uncertainty of
the corrected wind speeds in respect to the mast 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 2004-2009 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 mast 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 mast correction, ad-
DEWI MAGAZIN NO. 40, FEBRUARY 2012
65
30%
30%
Uncertainty of corrected wind speeds in respect to the mast correction FINO1 cup
p anemometers
Uncertainty of corrected wind speeds in respect to the mast correction FINO1 - sonic anemometers
25%
20%
cup31.5
cup41.5
cup51.5
cup51 5
cup61.5
cup61 5
cup71.5
cup81.5
cup91.5
cup104.5
Uncertain
nty of mast correctio
on [%]
Uncertain
nty of mast correctio
on [%]
25%
15%
10%
sonic41.5
sonic61.5
sonic81.5
20%
15%
10%
5%
5%
0%
0%
0
30
60
90
120
150
180
210
Wind Direction [°]
240
270
300
330
360
0
30
60
90
120
150
180
210
Wind Direction [°]
240
270
300
330
360
Fig. 10: Uncertainty of mast corrected wind speeds in respect to the mast correction for the FINO1 cup and sonic wind speed measurements as
percentage.
Anemometer
Resulting overall uncertainty of mast correction
cup104.5 cup91.5 cup81.5 cup71.5 cup61.5 cup51.5 cup41.5 cup31.5 sonic81.5 sonic61.5 sonic41.5
2%
2%
2%
3%
3%
3%
4%
4%
2%
2%
3%
Tab. 3: Overall uncertainty of mast corrected wind speeds in respect to mast 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 61400-12-1
[9]). According to ISO-IEC GUM [10], the uncertainty of the
mounting effects can be replaced by the uncertainty of the
mast correction.
References:
[1] RAVE: Research at Alpha Ventus, www.rave-offshore.de.
[2] FINO: Forschungsplattformen in Nord- und Ostsee,
www.fino-offshore.de.
[3] International Electrotechnical Commission (IEC): IEC61400-12-1 Wind
turbines - Part 12-1: Power performance measurements of electricity
producing wind turbines, 1st ed., 12/2005.
[4] T. Neumann: FINO1 and the mast shadow effect, 52nd IEA Topical
Expert meeting: Wind and Wave Measurements at Offshore Locations, Berlin, Germany, February 2007.
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DEWI MAGAZIN NO. 40, FEBRUARY 2012
[5] A. Beeken, T. Neumann, A. Westerhellweg: Five years of operation of
the first offshore wind research platform in the German Bight – FINO
1, DEWEK 2008, Bremen.
[6] A. Westerhellweg, B. Canadillas, A. Beeken, T. Neumann: One Year of
Lidar Measurements at FINO1-Platform: Comparison and Verification
to Met-Mast Data. DEWEK 2010, Bremen.
[7] A. Westerhellweg, V. Riedel, T. Neumann: Comparison of Lidar- and
UAM-based offshore mast effect corrections, EWEA 2011, Brussels.
[8] F. Kinder, A. Westerhellweg, T. Neumann: Meteorological measurements at FINO1 during the existence of the wind farm Alpha Ventus,
EOW 2011, Amsterdam.
[9] International Electrotechnical Commission (IEC): IEC61400-12-1 Wind
turbines - Part 12-1: Power performance measurements of electricity
producing wind turbines, 1st ed., 12/2005.
[10]ISO-IEC Guide 98-3: Uncertainty of measurement-Part3: Guide to the
expression of uncertainty in measurement (GUM: 1995), Switzerland, 2008.