Swell impact on wind and OWT

Siri M. Kalvig, StormGeo, 16 april 2015

Outline: About swell and waves

A forecaster note

Better wave forecast

Wave induced wind

Implications for OWT

Future prospectives

Wind sea - waves generated by local wind

Swell - long period waves generated by distant storms

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Pierson, neumann and James, 1955

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Wave energy spectra

2D

Bølgeenergi per frekvens og retning

1D

Simple and uniform wave field

• 15 m/s vind from west, wind sea.

• Swell also heading east

Bølgeenergi per frekvens

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Wave energy spectra

Complex wave field;

• wind from southwest: wind sea heading

northeast.

• two swell systems; one from soutwest and

one from northeast.

1D

2D

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Silicon Valley

Anchorage

Houston

New York

Rio de Janeiro

Singapore

Tokyo

Seoul

Shanghai

Hong KongDubai

Baku

London

Aberdeen

Stavanger

Bergen

Oslo

Stockholm

Copenhagen

Hamburg

COMPETENCE - INSPIRATION - INNOVATION

25 Offices g 8 Forecast Centers g 1 Vision

Miami

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Moscow

Decision support – weather sensitive operations

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Managing RiskOne of the world’s leading suppliers

of Decision Support for Weather

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edge products together with our

customers.

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Solbjørg Apeland, Norne Statoil

Solbjørg Apeland, StormGeo office

Weather criteria for offshore wind often more critical than for offshore

oil/gas.

Sometime cultural differences? Dialogue with boat captains oil/gas

vs. dialog with engineer and technical

offshore wind…

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Tendency to overestimate Hs >> less working hours!!!

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Wave models at 300 meter resolution - SWAN

Hit 65%

ME -0.3

Hit 72%

ME -0.2

SWAN 300 m Broadhaven

ECMWF

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Another example – 2 locations – 7 days

Cyan – ERAGreen – SWAN no tidalRed – SWAN with tidal *)Blue – measurements

ECMWF expected to be between ERA and this SWAN set up

Promising accuracy

*) OSU tidal atlas

Høg-jæren vindpark, photo: Norsk Vindenergi

Høg-jæren vindpark, photo: Norsk Vindenergi

0

20

40

60

80

100

-1 1 3 5 7 9 11 13 15

Heig

ht

(m)

Mean wind speed (m/s)

Høg-jæren vindpark, photo: Norsk Vindenergi

0

20

40

60

80

100

-1 1 3 5 7 9 11 13 15

Heig

ht

(m)

Mean wind speed (m/s)

Sheringham Shoal, photo: Statkraft

“The offshore wind industry takes ‘old’

technology over land, and expect it to

work over sea...”

Illustration: Silje Kalvig Østdahl

A saying…. in 2009

A gap between best knowledge

and best practise ?

…Yes!

Neutral stratification (no buoyancy)

and

a flat, smooth sea surface

…are routinely used as assumptions in wind

energy calculations.

S.Kalvig, OT Gudmestad, N.Winther (Wiley, 2013)

The atmosphere and the

ocean interplay with

each other in various

ways

…an ocean of air where

the fishes are birds and

vice versa!

Model set up in StormGeo

ECMWF

ERA-Interim

80km / 3 hours

StormGeo in-

house

modeling

1-9 km / 1 hourSWAN

We need better

link between

wave models and

atmospheric

models!

From: Grand Valley State University

Need a new boundary

condition that take into

account the sinusoidal

movement of the

“ground”.

PhD project UiS

Supervisor group: B. Hjertager (UiS), E. Manger (Acona Flow Technology),

J.B. Jakobsen (UiS), N. Winther (StormGeo)

Different waves can be

superposed on eachother

𝝽 𝑥, 𝑡 = 𝑎 𝒂𝒏 𝑠𝑖𝑛 2𝜋(𝑥−𝑐𝑡

𝜆) + 𝒘𝒏 𝑐𝑜𝑠 2𝜋(

𝑥−𝑐𝑡

𝜆)

𝝽 is the total wave surface displacement, 𝒂𝒏 and 𝒘𝒏 are a unit vectors, x is the horizontal

position at a given time t, a is the wave amplitude, λ is the wavelength and c is the wave

speed.

Numerical wave simulations

Numerical wave simulations

Approximations used;

Newtonian fluid and incompressible flow

No coriolis force

No buoyancy

Turbulent closure; k-epsilon (Reynold’s stresses proportional

to the mean rates of deformation)

Waves seen as a solid – no deformation due to the wind

Domain: 1200m x 25 m x 400 m, Logarithmic wind at inlet with U400m =8 m/s, z0=0.0002 m.

Profiles are sampled from the middle of the domain (x=600 m) for every second between 251-300

seconds of simulations

Numerical wave simulations

wind aligned with wave

wind oppose wave

without wave

wind aligned with wave

wind oppose wave

without wave

Wave with a = 4 m, L = 50 m, c = 8.8 m/s Wave with a = 4 m, L = 100 m, c = 12.5 m/s

Domain: 1200m x 25 m x 400 m, Logarithmic wind at inlet with U400m =8 m/s, z0=0.0002 m.

Profiles are sampled from the middle of the domain (x=600 m) for every second between 251-300

seconds of simulations

Numerical wave simulations

wind aligned with wave

wind oppose wave

wind aligned with wave

wind oppose wave

Wave with a = 4 m, L = 50 m, c = 8.8 m/s Wave with a = 4 m, L = 100 m, c = 12.5 m/s

Domain: 1200m x 25 m x 400 m, Logarithmic wind at inlet with U400m =8 m/s, z0=0.0002 m.

Profiles are sampled from the middle of the domain (x=600 m) for every second between 251-300

seconds of simulations

Numerical wave simulations

wind aligned with wave

wind oppose wave

wind aligned with wave

wind oppose wave

Wave with a = 4 m, L = 50 m, c = 8.8 m/s Wave with a = 4 m, L = 100 m, c = 12.5 m/s

The horizontal component of the

wind speed (upper), the vertical

component of the wind speed,

(middle) and the turbulent

kinetic energy (lower) over a

wave with;

a=4 m, λ =70 m, c=10.5 m/s.

The domain size was 450 m x

400 m, only a close up is shown

her.

Direction between wind and waves very important

Master student:

Richard Kverneland at

UiS, IKM

Actuator line method in SOWFA

Actuator line method of Sørensen and Shen1 used in the Simulator for Offshore Wind

Farm Applications SOWFA2.

Manger and Kalvig visiting NREL, Boulder

1Sørensen, J. N., & Shen, W. Z. (2002). Numerical modeling of wind turbine wakes. Journal of Fluids Engineering2Churchfield MJ, Lee S and Moriarty P 2012 Overview of the simulator for offshore wind farm application (SOWFA) National

Renewable Energy Laboratory, Golden, CO, USA 03 May 2012

Actuator line method, wind tunnel test

Waves + Actuator Line (SOWFA) FAST

Wave simulations are combined with the actuator line simulations of SOWFA and

coupled with FAST. New set up: Wave Influenced Wind Turbine Simulations (WIWiTS)

New Method for direct study of:

Wave Influenced Wind Turbine Simulations

WIWiTS

WIWiTS

Using the NREL 5 MW turbine, reference turbine

(hub height 90 m, rotor diameter 126 m)

WIWiTS

WIWiTS domain with wave aligned (left) with the wind direction and wave opposing the wind

direction (right). The color contours showing the wind velocity in the x-direction.

Eirik Manger (Acona Flow technology)

WIWiTS

Generated rotor power per density (Wm3/kg) for the three different cases; wind and swell in the same

direction (blue), wind and swell in the opposite direction (red) and wind over a surface with low roughness

(black). L=100 m, a=4 m, c=12,5 m/s

wind aligned with wave

wind oppose wave

without wave

Inlet wind U400= 8m/s

WIWiTS – with FAST

Case (named after the period, Tp ) Wave parameters

6 sec, aligned and opposeda = 2 m, L = 56.2 m, c = (+/-) 9.4 m/s

7 sec, aligned and opposed a = 2 m, L = 76.4 m, c = (+/-) 10.9 m/s

8 sec, aligned and opposed a = 2 m, L = 100.0 m, c = (+/-) 12.5 m/s

10 sec, aligned and opposed a = 2 m, L = 155.9 m, c = (+/-) 15.6 m/s

No Wave, reference case a = 0 m, L = 0 m, c = 0 m/s

FAST : Aero-elastic code that can model the dynamic

response of horizontal-axis wind turbines

WIWiTS will replace the blade element momentum (BEM)

part that is usually used with FAST. FAST calculates the

structural response and feeds this back into the CFD

simulations. The wind field is then changed and new

structural responses are calculated with FAST.

WIWiTS – with FAST

Stress at the blade root due to the flapwise bending moment at the blade root.

Fatigue calculations by Lene Eliassen at NTNU.

WIWiTS – with FAST

𝐓𝐩 = 𝟔 𝐬 𝐓𝐩 = 𝟕 𝐬 𝐓𝐩 = 𝟖 𝐬 𝐓𝐩 = 𝟏𝟎 𝐬

No wave 1 1 1 1

Opposed 1.31 1.15 1.41 1.70

Aligned 1.01 0.98 1.06 1.19

Equivalent damage load at the blade root, considering flapwise bending

moment, based on a 100 seconds simulation.

𝐓𝐩 = 𝟔 𝐬 𝐓𝐩 = 𝟕 𝐬 𝐓𝐩 = 𝟖 𝐬 𝐓𝐩 = 𝟏𝟎 𝐬

No wave 1 1 1 1

Opposed 1.42 1.52 2.36 2.37

Aligned 1.17 1.74 2.12 1.53

Equivalent damage load at the tower base, based on a 100 seconds

simulation.

Borgholm Dolphin 10 Jan. 2015

Video: James Eaton

Solbjørg Apeland, RWE

Weather criteria:

Often between

1.0 - 1.5 meter Hs

46

Thank you!