Module 4: Offshore Wind Climate
1/64
OFFSHORE WIND CLIMATE
Wim Bierbooms
Wind Energy Research Group (Aerospace Eng.)
Module 4: Offshore Wind Climate
2/64
Overview
Wind climate
• in general• variation in height, space and time• offshore wind measurements• special offshore effects• offshore turbulence / extreme winds
Module 4: Offshore Wind Climate
3/64
Overview
Wind climate
• in general• variation in height, space and time• offshore wind measurements• special offshore effects• offshore turbulence / extreme winds
Module 4: Offshore Wind Climate
4/64
Introduction
Assessment of wind climate important:
• assessment of resources• determination of gross energy yield• site conditions at installation• accessibility for maintenance• loads on structures
mean wind speed
turbulence extreme wind
Module 4: Offshore Wind Climate
5/64
• caused by pressure differences (resulting from temperature differences)
• influenced by earth rotation and terrain
What is wind?
Module 4: Offshore Wind Climate
6/64
Overview
Wind climate
• in general
• variation in height, space and time• offshore wind measurements• special offshore effects• offshore turbulence / extreme winds
Module 4: Offshore Wind Climate
7/64
G : pressure gradient forceC : Coriolis forceW : drag forceUg: geostrophic windU : wind at the ground
Geostrophic windhighaltitude
lowaltitude
Module 4: Offshore Wind Climate
8/64
Coriolis effect
With respect tofixed frame
Fictitious force: just effected by rotating observe r
With respect torotating frame
Module 4: Offshore Wind Climate
9/64
Ekman layervariation of wind direction
Surface layervariation of wind speed
Atmospheric boundary layer
~ 1000 m
Module 4: Offshore Wind Climate
10/64
Wind profile in surface layer (~100 m)mechanical friction I
Wind profile (wind shear):
href is the reference height (10 m)z0 is the roughness length
=
)/ln(
)/ln()()(
0
0
zh
zhhVhV
refref
Module 4: Offshore Wind Climate
11/64
At land• z0 = 0.03- 0.25 mAt sea:• z0 ~ 0.0002 m
but dependent upon • wave height • fetch• wave age
Charnock relation:
; α = 0.0185g
uz
2*
0 α=
Wind profile in surface layer (~100 m)mechanical friction II
Module 4: Offshore Wind Climate
12/64
Standard height and roughness:
• 10 min. ( or 1 hour) average
• 10 m. height (= href)
• z0 = 0.03 m
Potential wind speedcorrect for local roughness
Module 4: Offshore Wind Climate
13/64
Wind profile in surface layer (~100 m)thermal effects I
Temperature
Hei
ght
• Parcel expands (pressure drops with height) → cools down• Adiabatic: no heat exchange
Module 4: Offshore Wind Climate
14/64
Wind profile in surface layer (~100 m)thermal effects II
Temperature
Hei
ght
Unstable atmosphere • Vertical movementsof air due to hotsurface
• Vertical velocity exchange
• Steeper gradient
Module 4: Offshore Wind Climate
15/64
Wind profile in surface layer (~100 m)thermal effects III
• stable (cold surface)• neutral• unstable (hot surface)
V
H
Module 4: Offshore Wind Climate
16/64
Temperature difference
Average wind - water temperature difference at North Sea location.
• Summer: stabilizing ∆T• Winter: destabilizing ∆T
Month
∆T
Tsea -Tair
Module 4: Offshore Wind Climate
17/64
Boundary layer stability at sea
Danish “offshore“ sites
Wind speed
% o
ccur
renc
e
Rødsand (DK)
% o
ccur
renc
e
Module 4: Offshore Wind Climate
18/64
Overview
Wind climate
• in general
• variation in height, space and time• offshore wind measurements• special offshore effects• offshore turbulence / extreme winds
Module 4: Offshore Wind Climate
19/64
Variability of the wind in time
year month day hour minute second
turbulence, related to
• terrain, obstacles
• storm fronts (gusts)
large scale weather systems
• high/low pressure systems
• seasonal variations time scale
contribution to wind speed
variability
daily patterns (often thermal driven):
• sea breeze
• mountain slope winds
“spectral gap”
Space: 100-1000 km < 1 km < 10 m
Module 4: Offshore Wind Climate
20/64
Basic wind statistics
number
V
V
time
V
time series of
10-min. (1-hour)
means
histogram probability density function
Weibull
Module 4: Offshore Wind Climate
21/64
Weibull distribution
probability of wind speed < V:
with k : shape parameter (k ≈ 2 in NW Europe coast)a : scale parameter
)1
1(
)(
k
Va yearavg
+Γ=
k
a
V
e)(
1−
−
∫∞
−−=Γ0
1)( ββα βα de
Module 4: Offshore Wind Climate
22/64
Fit of Weibull distribution – example I
Frequency of occurrence(wind rose)
Module 4: Offshore Wind Climate
23/64
Fit of Weibull distribution – example II
Weibull scale parameter Weibull shape parameter
Module 4: Offshore Wind Climate
24/64
Overview
Wind climate
• in general
• variation in height, space and time• offshore wind measurements• special offshore effects• offshore turbulence / extreme winds
Module 4: Offshore Wind Climate
25/64
Existing offshore wind map
• Evaluation ofoffshore potential
• Extrapolation fromland measurements
• Not suitable forenergy yield calculation
• Coastal effectscause main problems
Module 4: Offshore Wind Climate
26/64
How to obtain wind statistics for a given location ?
two basic
options
perform local
measurements
use existing data
not possible to
measure long
enough to obtain
good statistics
data not
representative
problem: solution:
correlate with other
sites which have
reliable long term
statistics
use of correction
methods (fetch,
obstacles on land)
“Measure
Correlate
Predict”
Windatlas
+ correction methods
WAsP
MCP
Other
sources
Module 4: Offshore Wind Climate
27/64
WASP offshore (Risø DK)
Determine Offshore wind atlas at 150 m height
• Calculate downward to hub height using local orography
• Account for coastaleffects
• Not yet fully operational
change inroughness
speed-upover hills
sheltermodel
Module 4: Offshore Wind Climate
28/64
Available offshore wind dataPlatform data (since ’80)• KNMI, RWS: Measuring Network
North Sea (limited w.r.t. land)• wind and wave data
Other sources• light ships• Voluntary Observing Ships• databases (e.g. NESS / NEXT
“hindcast” data)• pressure data (weather forecast)• remote sensing / satellite data
KNMI wind network (NL)
Module 4: Offshore Wind Climate
29/64
Remote sensing
Satelite sampling
• Spatial distribution
• Accuracy 2 m/s
• Time shift
• Satellite passages
• Diurnal cycles !?
Module 4: Offshore Wind Climate
30/64
Dutch Offshore wind map
Netherlands' Exclusive Economic Zone
Derived from pressure data (1997-2002)
Height: 120 m
Distribution available for 5 locations
© ECN
Module 4: Offshore Wind Climate
31/64
Measure Correlate Predict - example
Background MCP:time series annual wind speedslook similar; high and low valuesoccur at the same years
++
Long term correction factors2 years of measurements of asite near Schiphol+: 0.95x: 1.05
x, +: measurements
xx
Module 4: Offshore Wind Climate
32/64
Overview
Wind climate
• in general• variation in height, space and time
• offshore wind measurements• special offshore effects• offshore turbulence / extreme winds
Module 4: Offshore Wind Climate
33/64
MCP: dedicated measurements I
Risø/SEAS meteo masts at sea5 locations 50 m height in Danish seas
Module 4: Offshore Wind Climate
34/64
MCP: dedicated measurements II
“Fino” platform in German partof the North Sea
105 m height; full site assessment:Hydro, meteo and biological surveys
www.fino-offshore.de
Module 4: Offshore Wind Climate
35/64
MCP: dedicated measurements IIIPlatform OWEZ (116 m)• WEOM• 3 levels; 3 sides; booms of 12m• 2003• also metocean data
• “Monitoring- en evaluatieprogramma Near Shore Windpark” by the Dutch Government
Module 4: Offshore Wind Climate
36/64
MCP: dedicated measurements IVMeteo masts at sea:• Corrections for mast
shadow• Well proven but
expensive
cup sonic
Sodar / Lidar anemometers• Doppler shift of an acoustic
pulse / laser• Promising but expertise
required
Module 4: Offshore Wind Climate
37/64
Hourly variation of offshore wind speed
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
hour
10-m
win
d sp
eed
[m/s
]
AUK EUR K13 LEG MPN IJM
‚onshore site‘
**
Module 4: Offshore Wind Climate
38/64
Monthly variation of offshore wind speed
5
6
7
8
9
10
11
12
Jan Feb Mrt Apr Mei Jun Jul Aug Sep Okt Nov Dec
10-m
win
d sp
eed
[m/s
]
AUK EUR K13 LEG MPN IJM
Month
∆T
Tsea -Tair
Month
∆T
Tsea -Tair
Module 4: Offshore Wind Climate
39/64
Annual variation of offshore wind speed
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
10.5
1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997
10-m
win
d sp
eed
[m/s
]
AUK EUR K13 LEG MPN IJM
Consequences:
• Annual energyyield / income will vary from year to year !!
• Base energy yield estimation on long period (20-30y)
Module 4: Offshore Wind Climate
40/64
55.566.577.588.59
020
4060
80100
120
Offshore versus onshore: wind shear
V (m/s)
H
(m)
• larger V• smaller z 0• stability effects
more important• offshore verification
needed
Module 4: Offshore Wind Climate
41/64
Offshore versus onshore: Weibull distribution
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
wind speed at hub height [m/s]
prob
abili
ty [-
]
mean wind speed 6.5 m/s(onshore)
mean wind speed 8 m/s(offshore, Baltic)
mean wind speed 10.1 m/s(remote offshore, North Sea)
Module 4: Offshore Wind Climate
42/64
Wind climate offshore versus onshore
onshore offshore
diurnal pattern daily maximum uniform
seasonal pattern less pronounced more pronounced
stability diurnal pattern seasonal pattern
wind profile “unstable” on average “neutral” on average
mean wind speed decreasing inland higher than on land
Module 4: Offshore Wind Climate
43/64
Overview
Wind climate
• in general• variation in height, space and time• offshore wind measurements
• special offshore effects• offshore turbulence / extreme winds
Module 4: Offshore Wind Climate
44/64
III
up to ~30 km landsea
I
II
“windatlas” methods (zo depends on waves fetch age)
coastal discontinuity zone:
model improvement still underway
Coastal effects I
• strong variations in air/sea temperature gradient
(winter ~ summer onshore ~ offshore wind)
• resulting strong effect upon stability
• further complicated by internal boundary layer effects
Module 4: Offshore Wind Climate
45/64
Coastal effects IIchange in surface roughness
External boundary layer External boundary layer
z02
x z01
z
Land Sea
Internal boundary layer(IBL)
5 km
~150 m
land sea
External boundary layer External boundary layer
Internal boundarylayer (IBL)
5 km
~150 m
Module 4: Offshore Wind Climate
46/64
Overview
Wind climate
• in general• variation in height, space and time• offshore wind measurements• special offshore effects
• offshore turbulence / extreme winds
Module 4: Offshore Wind Climate
47/64
Basic wind statisticsdistinction between long and short term
t →
V →
30 min.
long term
short term
20 m/s
0 m/s
-5 m/s
5 m/s
0 m/s
20 m/s
10 min. 10 min. 10 min.
Module 4: Offshore Wind Climate
48/64
RECAP: Basic wind statistics – long term
number
V
V
time
V
time series of
10-min. (1-hour)
means
histogram probability density function
Weibull
Module 4: Offshore Wind Climate
49/64
Basic wind statistics - short term
number
V
V
time (s)
V
time series of
wind (turbulence)
histogram probability density function
Gauss
0 1 0 2 0 3 0 4 0 5 0 6 0- 5
- 4
- 3
- 2
- 1
0
1
2
3
4
5
Module 4: Offshore Wind Climate
50/64
Basic wave statistics - short term
number
η
η
Gauss
small
waves
narrow
banded
number
H
H
Rayleigh
Hs
0 1 0 2 0 3 0 4 0 5 0 6 0- 0 . 4
- 0 . 3
- 0 . 2
- 0 . 1
0
0 . 1
0 . 2
0 . 3
0 . 4
Time →
η→ time series of
water elevation
H
Module 4: Offshore Wind Climate
51/64
Basic wave statistics - long term
time (days)
time series of
Hs and Tz
(each 3-hour)
scatter diagram (‘2d histogram’)
0 2 4 6 8 1 0 1 2 1 40
1
2
3
4
5
6
Tz → Hs →
Num
ber
→
Module 4: Offshore Wind Climate
52/64
Turbulence - characteristics
• random variations around mean U• turbulence intensity TI= σσσσ/U• production:
– wind shear (mechanical)– buoyant (convective / thermal)
• loss:– dissipation (into heat)
• superposition of eddies, swirls(size from 2 km to 1 mm)
Module 4: Offshore Wind Climate
53/64
Intermezzo: spectrum
10-2 10-1 100 10110-4
10-3
10-2
10-1
100
101
102
Frequency →
Spe
ctru
m →
Power spectral density function• ‘distribution of energy content
over frequencies’• area below curve = σσσσ2 (variance)• largest eddies have most energy
(Integral) time scale• measure of time over which wind
speed is correlated
(Integral) length scales• characteristic size of eddy
Module 4: Offshore Wind Climate
54/64
Stochastic wind simulation; one-point I
Generation of a 10-min time seriesSummation of harmonics:
Sk: spectrum valueφφφφk: random (uniform 0-2 ππππ)
1
( ) cos( )K
k k kk
u t S f tω φ=
= ∆ +∑ }0 10 20 30 40 50 60
6
7
8
9
10
11
12
Time →
Win
d sp
eed
→
Module 4: Offshore Wind Climate
55/64
Choice of spectrum:• von Karman (isotropic)• KaimalSpectrum depends on:• mean wind speed• length scales• turbulence intensity
(offshore ~ 8%)
Generate wind field for several meanwind speeds• distribution: Weibull
10-2 10-1 100 10110-4
10-3
10-2
10-1
100
101
102
Frequency →
Spe
ctru
m →
0 5 10 15 200
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
Mean wind speed →pd
f→0 5 10 15 20
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
Mean wind speed →pd
f→
Stochastic wind simulation; one-point II
Module 4: Offshore Wind Climate
56/64
Stochastic wind field simulation -example
Wind speed →
Bladed; 3 velocity components:• u; longitudinal• v; lateral• w; vertical
Module 4: Offshore Wind Climate
57/64
Onshore vs. offshore turbulence
Module 4: Offshore Wind Climate
58/64
Measured offshore turbulence intensity T.I.
10 m30 m50 m
σUT.I. =U
T.I.Windspeed [m/s]
T.I. Height[m]
Module 4: Offshore Wind Climate
59/64
Rotational sampling of turbulence
eddies rotor
Module 4: Offshore Wind Climate
60/64
0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0- 3
- 2
- 1
0
1
2
3
4
Rotational turbulence spectrum
rotating observer
fixed observer
spectrum
turbulence
V
Time (s)
summation of harmonics(random phases){⇒⇒⇒⇒
⇓⇓⇓⇓
Module 4: Offshore Wind Climate
61/64
Extreme winds
• No offshore map (yet)
• Extrapolation of datacovering several years(extreme value theory)
Module 4: Offshore Wind Climate
62/64
Extreme value analysis - example
• determine yearly maxima U i
• order them to magnitude:
19.8, 20.7, …. 27.2, 27.5 m/s
• fit to extreme valuedistribution (different functions / methods available)
• read from graph: 50-years value
Module 4: Offshore Wind Climate
63/64
Extreme gusts
-2 0 2 4 6 8 10 1215
20
25
30
35
40
Time →
Wind speed →
IEC (included in Bladed):deterministic gustuniform over rotor planealso:
– extreme direction change– combination– extreme wind shear (vertical
and horizontal)
‘Real’• maximum amplitude• maximum rise time
Wind speed →
→
Wind speed →
Time →
Wind speed →
Time →
Wind speed →
Module 4: Offshore Wind Climate
64/64
Wind maximum at relativelow levels (~150 m);larger then geostrophic wind
Stable night time conditions
Common in Baltic Sea;Also in North Sea?
Sodar measurements
Low Level Jet