University of Stavanger

uis.no

NORCOWE 2016

CONCLUDING CONFERENCE

Loads

Jasna Bogunović Jakobsen

1

Impact of refined metocean conditions on the offshore wind

turbine loads and wind energy production

1. Breaking wave loading on wind turbines in shallow waters(UiS, Profs. Gudmestad, Obhrai, Haver, PhD stud. J. Jose, PhD Choi)

2. Aerodynamic load variation with atmospheric stability and theassociated wind turbine fatigue damage

(UiS, Profs. Obhrai, Jakobsen, PhD stud. Eliassen, Cheynet, MSc stud. Putri )

3. Possible impact of wind-wave interaction on wind fields and windturbine loads

(UiS, Prof. Hjertager, PhD stud S. Kalvig, Acona)

4. Other topics: Alternative installation methods (UiS, Porf. Gudmestad, PhD A.Sarkar), O&M (UiS, UAA), Wind farm fatigue (UAA) ..

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1. Breaking wave loads on bottom fixed wind

turbines in shallow waters

Basic concepts:

3

Empirical values of slammingcoefficient by different researchers(all on ponopiles) show a widescatter.

Applicability of these values to a truss structure should be investigated.

Methodology for studying the slamming force

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Force on Jacket Structure

Experimental Analysis Numerical Analysis

Slamming Force,

Coefficients, Impact

Duration

Study the variation in maximum

slamming coefficients along the

length of the jacket members

Effect breaking wave

parameters on

Slamming Force

WaveSlam Experiment, 2013,

Large Wave Channel, University of Hannover

Experimental set-up on Large Wave Flume FZK

(Arntsen, 2013, www.fzk.uni-hannover.de).

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Simulation of plungingbreaking waves on a trussstructure (1:8 scale) inshallow water.

22 local force transducersto measure the responseof the structure.

Eight wave gauges along the wave flume, one at thefront pile of the structure,one in the middle and atthe back of the structure.

Unique data sets collectedand recorded.

Collaboration with NTNU, Statoil, DNV..

Total Force Analysis-EMD

128 128.2 128.4 128.6 128.8Time,s

129 129.2 129.4 129.6-5

25

20

15

10

5

0

30

35

Forc

e,

N.

Upper Envelope

Lower Envelope

Filtered total force(Exp)

Netbreaking wave force

Net Breaking wave force filtered out by

EMD Algorithm7

Total slamming force force

Slamming Force calculated by EMD and FRFMethod is compared.

Wave Slamming Force12

128 128.2 128.4 128.6 128.8Time,s

129 129.2 129.4 129.6-6

8

6

4

2

0

-2

-4

10

Forc

e,

kN

.

Slamming Force-FRF Method

Slamming Force-EMD Method

Comparison of Slamming Force

Local Force Analysis-FRF Method

Transfer functionscalculated for each localtransducers, by hammer impactappplied to local members of thestructure.

29Hammer Impact points on the bracings (Arntsen et al., 2013)

Slamming coefficients for different wave

heights and periods

Wave

Period

Wave

Height

Wave Breaking Position

T(sec) H(m) Front Middle Back

4.6 1.4 1401

1.5 1402

1.6 1404

1.7 1407

4.9 1.4 1411

1.5 1412

1.6 1413

1.7 1414

1.8 1416

5.2 1.4 1417

1.5 1419

1.6 1420

1.7 1421

1.8 1422

5.55 1.4 1427

1.5 1426

1.6 1423

1.7 1424

1.8 1425

0,000

1,000

2,000

3,000

4,000

5,000

6,000

7,000

1,5 1,6 1,7 1,8

Sla

mm

ing C

oeff

icein

t

Wave Height [m]

Mean

0,000

1,000

2,000

3,000

4,000

5,000

6,000

7,000

1,5 1,6 1,7 1,8

Sla

mm

ing C

oeff

icein

t

Wave Height [m]

Mean

T=5.55s

Slamming Coefficients for the bracings

T=5.2s

Experimental and Numerical Analysis

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0 1 2 3

Slamming Coefficient (Cs)

-0.3

0

0.3

0.6

0.9

1.2

1.5

Heig

ht

(m)

T=5.55s

T=5.2s

T=4.9s

T=4.6s

2 4 6 8

Slamming Coefficient (Cs)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Heig

ht

(m)

T=5.55s

T=5.2s

T=4.9s

T=4.6s

3D numerical model based on solving the viscous and incompressible Navier-

Stokes equations and the volume of fluid method (VOF) is used (Choi, 2014).

V1

B2

Slamming Coefficients from the CFD

simulations

Wave

Case

Slamming Coefficient, Cs

Breaking PositionB1 B2 B3 B4 B5 B6 V1 V2

b1 0.95 1.55 1.88 3.70 0.67 1.32 0.68 1.57 Behind the back leg

b2 1.63 3.03 3.06 5.33 0.88 1.45 1.72 2.21 Just behind the front leg

b3 2.40 7.87 0.39 5.90 0.74 0.13 2.81 1.15 At the front leg

c1 0.85 1.29 1.71 2.96 0.56 0.90 0.58 2.08 At the back leg

c2 1.79 4.12 0.41 4.22 1.00 0.24 2.19 2.31 Just in front of front leg

c3 1.90 5.17 0.30 4.53 0.82 0.13 2.09 1.10 Just in front of front leg

d1 0.95 1.16 2.36 3.59 0.57 0.41 0.86 2.63 Just in front of back leg

d2 1.36 2.58 1.96 3.87 0.64 1.11 1.50 1.92 Ahead of front leg

d3 1.71 3.87 0.39 4.13 0.45 0.19 2.15 1.40 Ahead of front leg

e1 0.70 0.81 0.92 2.22 0.45 0.26 0.51 0.93 Middle of the structure

e2 0.80 0.93 1.72 2.86 0.48 0.28 0.55 1.33 Ahead of front leg

e3 1.24 3.96 1.24 3.57 0.41 0.21 2.94 1.56 Ahead of front leg

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Summary of Slamming Coefficients for different members

Summary of the CFD breaking wave study

The maximum slamming coefficient for the bracing members of the jacket structure in

the wave impact zone is estimated as 7.87, which is similar to the value suggested by

Wienke and Oumeraci (2005). On the other hand, in the case of vertical member,

maximum slamming coefficient is obtained to be 2.96, which is slightly smaller than the

values suggested by Goda (1966).

In the design of OWT substructures, it is not advised to use the maximum value of

slamming coefficient along the entire member. A triangular distribution of force should be

adopted in the calculation of slamming forces on the members.

References:Jose, J, Podrażka, O, Obhrai, C, Gudmestad, OT, and Cieślikiewicz, W (2015). “Methods for Analysing Wave Slamming Loads on

Truss Structures used in Offshore Wind Applications based on Experimental Data,” Journal of Ocean and Wind Energy (JOWE).

Jose, J, Choi, SJ, Lee, KH, Gudmestad, OT (2016). “Breaking wave forces on an Offshore Wind turbine foundation (Jacket type)

in the Shallow water,” 26th International Ocean and Polar Engineering(ISOPE) Conference, Greece, Rhodes, 26 June - 2 July 2016.

Jose, J and Choi (2016). “Estimation of slamming coefficients on local members of offshore wind turbine foundation (jacket

type) under plunging breaker,” Journal of Naval Architecture and Offshore Engineering (submitted).13

2. Aerodynamic load variation with atmospheric stability

and the associated wind turbine fatigue damage

Assessment of atmospheric stabilityconditions on Fino 3 and Fino1 reserach platforms, using availablewind velocity and temperature data.

Vertical separations of 20m, 40m, and 60m.

First fase: comparison between themeasured coherence in neutralconditions and those in IEC-61400-1, Ref: Coherence of turbulent wind under neutral wind condition at FINO1, C.Obhrai, L. Eliassen, DeepWind2016 (SW wind)

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Example of analysis of the offshore turbulence data sets:

Fino 1 data, 10 min averages Jan 2008

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FINO 1 coherence analysis

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uu coherence for 20 m vertical separation.

Coherence for 40m and 20 m seperations

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Comparison with the coherence in IEC 61400-1

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Comparison with the coherence in IEC

61400-1, v-and w-components

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• Mann model show a good agreement with measured values at 40 m separation for

the uu and ww cocoherence, and tends to show a lower value at 20 m separation

for these cocoherences.

• Further work: Consider stability as a variable, Fit the manns model to the

measurements, Investigate the wind from a whole year

Parametric studies of the influence of different stability

conditions on the OC3 Hywind floating wind turbine response

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Different stability classes represented based the spectral

data observed at Havsøre fitted to the Mann spectral

model. The model does not account for temperature effects derectly.

(ref, work by A. Chougule, DTU/UiA )

A Study of the Coherences of Turbulent Wind on a Spar-Buoy Floating Wind Turbine,

Putri & Obhrai, submitted to Wind Energy.

Other studies on aerodynamic loads:Unsteady aerodynamic load anlysis:

Wind tubine rotor (OC3 Hywind) in axial harmonic motion

21Vortex panel code simulations (Eliassen PhD)

Aerodynamic Damping Ratio

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Reduction in damping when the motion

frequency is an integer factor of the blade

passing frequency

3. CFD simulation of a wind turbine exposed

to a wave-wind field

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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

Summary

Research work on the refined modelling of the environmental

forces and their influence on the design of offshore wind turbines

has been performed in NORCOWE and is ongoing!

Network of NORCOWE research partners has been of great value,

in particular linking the state-of-the-art offshore measurements of

environmental conditions and the associated load effects.

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