Presentation of Large Rotor Study

8/11/2019 Presentation of Large Rotor Study

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Presentation toUniversal FoundationSeminar Hamburg25-26 February 2014

Grontmij Universal FoundationLoads from large Rotors


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Ambient Vibration TestsIn the period October - November 2012 a total of 100 ambient vibration tests were investigated.

Cross-wind damping of the lowest eigenmode is determined using:

The Enhanced Frequency Domain Decomposition (EFDD) technique.The Stochastic Subspace Identification (SSI) technique with an Unweighted Principal Component (UPC) and aWeighted Principal Component (PC) algorithm.

2 hours measurements with low variation of the blade pitch angle, the wind speed and the rotorspeed.


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Study content

The results presented are carried out by Grontmij through support byVestas under the Carbon Trust OWA study for large rotors.

The conclusions are made by Grontmij by means of simplified modelling.

They do not represent the point of views of the partners in OWA.

Further modelling is required for FEED


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Model basis

MODEL BASIS FOR TURBINE/WAVE SIMULATIONTurbine 8 MW selected

FLS: Fatigue loads based on min 5 rotor D distance (turbulence 11% insteadof 15 % at 15 m/s means reduced fatigue loads compared to IEC standardconditions)

Water depths: 45m

(Soil conditions (propose selected = 37 deg, cu > 80 kpa))

Metocean conditions (propose selected Exposed : IEC IB for turbines, Hs =9.4m, Tp = 14.2s)

INITIAL SIMPLIFIED STRUCTURAL MODEL:

Monopile D = 6.5 m restrained 8 m below seabed

Stiffness adjusted to obtain f0 = 0.20, 0.23 and 0.26 Hz

FINAL MODEL: 45 m structure supported on stiffness matrix a mudline


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Universal foundation geometries

.


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Mudline stiffness matrix


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First and second frequencies for UF

Concerns:Wave/wind interaction due to frequencies close to 0.2 HzEffect of misalignment as 0.2 Hz is corresponding to Tp,wave = 5 secEffect of fore-aft frequency down to 0.9 Hz


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Design Basis

FLS:Fatigue wind loads lower than standard IEC design by account to 5-7 D rotordiameter turbulence. This is included in the design of the leading turbineproducers.This reduces the fatigue loads relative to EIC 61.400-1 by say 20 to 25%.

All slimmer steel structures benefit from this.


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Design Basis

ULS:ULS load case 6.1 to be combined with Hmax wave loadULS load case 6.2 to be combined with 1.3Hs wave loadThe Turbine producers do not agree on ULS loads. Some consider only loadcase 6.1

load case 6.2 (part. coeff. on wind load 1.1), which can be combined with1.3Hs instead of with H max wave/current load (usually 1.86Hs), whichreduces the design ULS load


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Modelling

Vestas modeling:Simple model with 3 monopiles with different 1. mode frequency made itpossible to quantify wave/wind interaction versus 1. mode frequencyFurther, the modelling showed that it is important to model the dynamics due tothe effect of 2. mode fore-aft bending frequency (f1) being say 0.9 Hz as itincreases the fatigue loads in the bottom of the foundation significantly.Even though it was possible to obtain a safe design if f1 approximately equal to0.9 Hz to account for the dynamic amplification due to 2. mode fore-aft bendingby a limited number af simulations.In addition it was also possible to calibrate a misalignment correction model forUniversal Foundation


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Model basis: Only 13 load cases areconsidered (11 load cases for fatigue)

SIMPLIFIED CORRORATION WIND AND WAVESTable1.1 Correlated waves and wind

AverageUnacelle Probability Hs Tp

m/s delta p m s5 0,073 0,61 3,5

7 0,130 0,93 4,39 0,158 1,30 5,1

11 0,156 1,72 5,913 0,132 2,18 6,715 0,098 2,70 7,417 0,065 3,26 8,1

19 0,039 3,87 8,821 0,021 4,52 9,623 0,010 5,23 10,325 0,004 5,98 11,044 3h/5y 8,5 13,550 3h/50y 9,4 14,2


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Generic 164 m rotor

Conceptual turbine 164 m rotorBased on average of several large rotors

Preliminary estimates of design loadsIEC1B Average 10 m/s tip speed

U(3s,max) 50 m/s 90 m/s

Water depth(m)

Number ofblades

DiameterD (m)

Towerbottom

elevation(m)

Towerlength (m)

Minimumf0 (Hz)

-20 3 164 20 88 0,19

Generic turbine assumptions

ener c average m ro or w n oa s


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ener c average m ro or w n oa sexcl. all dynamic amplifications (notVestas loads)

.

Minimum sim with MULS IEC1B turbulence RotoWater depth f0 FULS MULS TULS Fe Me Te Fm Hz MN MNm MNm MN MNm MNm MN

-20 0,19 2,8 242 11 0,8 56 17 0,7 4555 2,8 449 11 0,8 119 17 0,7 96


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Required input from Designers to roughlyevaluate the structures

1. mode frequency and shapeMode frequency and shape corresponding to angle rotation of turbine axis in thevertical plane (bending fore-aft)

Lowest torsional mode (frequency and shape)

Deformations in the foundation for each of the following 3 specific static loads:

Shear force in el +20 m: 1 MN

Overturning moment in el +20: 50 MNm

Torsial moment in el +20m: 6 MNm)


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Required input from Designers to furtherevaluate the structures

Final simulationsVestas is able to run some simulation on the individual foundation types in orderto investigate the spectrum of the WTG loading being transferred to thefoundation to verify the load assumption and eventually produce time seriesload input and to update integrated design loads.

All that this will require is for the foundation designers to supply generalized(6x6) foundation mass and stiffness matrices at the tower/foundation interface(fx. via Guyan reduction) and appropriate damping levels for the 1st naturalfrequency of the complete support structure (WTG+foundation).


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Universal foundation wave loads

Max wave height used for ULS.Boussinesq non-linear time series used for fatigue loads (FLS).

Waves + C Incl 1,35 Incl 1,1Depth m 45 45 45

MULS MULS MeMudline Mmax MNm 394 264 115Mudline Fmax MN 14 9 4


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Key findings and conclusions

For fatigue loads it is required only to include a dynamic amplification factor(DLF) on wave load input to obtain the dynamic effect of the wave load.

This is depending upon the first mode frequency f0.

The dynamic factors are smaller than originally feared.

Maximum values occur for the lowest frequency.

The DLF for Fe is lower than for Me.


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The combined fatigue load for structures with f0 >0.23 Hz, where there is nosignificant misalignment is:

Meq(wind+wave) = (Meq(wind)^2+(DLF*Meq(wave))^2)^0.5

Feq(wind+wave) = (Feq(wind)^2+(DLF*Feq(wave))^2)^0.5

For Universal Foundation (f0 around 0.2 Hz) above equations needs themisalignment effects to be added.


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For monopile like structures with f0 = say 0.20 Hz the effect of misalignment hasto be included in terms of that the wind loads in the interface has to bemultiplied by a factor depending upon the actual misalignment and the ratio ofthe wave load to the wind load.

This is actual for Universal Foundation

Misalignment is no problem for the slimmer structures like jackets with higher f0.


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Key findings and conclusions too general

Vestas has found some dynamic amplification on the 2nd

fore-aft structure modedue to coincidence with mainly the 6P frequency.So it is required to set up a modelling (pure wind) to analyse the dynamic effectsfrom 2 mode fore-aft.The consequences are depending upon:

- Fore-aft bending frequency- Type of structure- Water depth (effects are increased with water depth)For Universal Foundation the specific analysis showed increasing the fatigueloads in the bottom of the foundation, depending upon the frequency.

The good thing is that it is only wind simulations on a structure withrepresentative characteristics, which is required to quantify the effect of the 2mode fore-aft dynamics.


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Options to solve design problems due to 2 Mode fore-aft:Two rather different options exist to improve a structure to absorb the increasedfatigue loads:a) Find an optimal configuration with 2. mode bending fore-aft frequency

above a certain frequency by stiffening the upper part of the structure

b) Absorb the increased loads by a stronger lower part of the structure

The 9P frequencies should not cause significant problems because of therelatively low energy content.

A further stiffening/strengthening was not required for Universal Foundationbecause the conceptual fatigue design was conservative by taking no accountto the directional spreading of wind and waves


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Account to Misalignment :The simple explanation for that misalignment is of importance:Wave loads initiate oscillations parallel to the rotor plane due to the lowdamping in this direction.

The general experience is that the effect of misalignment is a problem formonopole-like structures utilizing the minimum allowed first mode frequency,which in our case is say 0.20 Hz correponding to T(wave) = 5 sec..


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Misalignment : Results from the modeling (f0 = 0.20 Hz)

IIII


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Misalignment : Results from the modeling (f0 = 0.20 Hz)


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Misalignment Area 3MisalignmentAveragemisalignm. Factor 1 1,06 1,17U prob(45 deg Weightedm/s Factor

5,5 0,22 0,31 0,47 1,106,5 0,27 0,35 0,38 1,097,5 0,29 0,39 0,32 1,088,5 0,36 0,39 0,25 1,079,5 0,39 0,4 0,21 1,06

10,5 0,43 0,42 0,15 1,0511,5 0,48 0,4 0,12 1,0512,5 0,51 0,4 0,09 1,0413,5 0,54 0,39 0,07 1,0414,5 0,58 0,37 0,05 1,0315,5 0,62 0,34 0,04 1,0316,5 0,62 0,35 0,03 1,0317,5 0,65 0,32 0,03 1,03

18,5 0,68 0,3 0,02 1,0219,5 0,7 0,29 0,01 1,0220,5 0,74 0,26 0 1,0221,5 0,75 0,25 0 1,0222,5 0,77 0,23 0 1,0223,5 0,72 0,28 0 1,0224,5 0,78 0,22 0 1,02


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Example of misalignment calculation foran area in UK (45 m)

:45m Corrected

U Prob Mres,eq M*p^0,25 (M*p^0,25)^2 Misalignment Mres,eq M*p^0,25 (M*p^0,25)^2m/s delta p MNm Factor MNm11 0,156 31,3 19,7 387 1,06 33,1 20,8 43413 0,132 33,4 20,1 405 1,05 35,0 21,1 44615 0,098 37,6 21,0 443 1,04 39,1 21,9 480

sum 3097 sum 3397sum^0.5 56 sum^0.5 58

factor 1,0474,7%

Dominant wind velocities

Estimated increase of wind load due to misalignment for Me(waves)/Me(wind) = 0.80


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Final generic fatigue loads to UniversalFoundation

Final:Example 45 m:

At mudlineWind: Me(wind) = (Me +(20+45) Fe)( 1+x )(1+y ) MNm1+x = effect of misalignment

1+y = effect of f2 second mode bending fore-aft lower than specified limit (onlywind)Wave: Me = 112 ( 1+z ) MNm (dynamic interaction wave-wind)1+z = effect of f0 = 0.2 HzTotal: Me(total) = (Me(wind))^2 + Me(wave))^0.5

No account to directional distribution in above estimate

Through detailed but still simlified simulations (detailed turbine model) itis possible to determine x, y and z and then a safe design is obtained

l b l


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Conclusions 164m turbine on UniversalFoundation

UF is well suited for the V164-8.0 in a large range of water depth from 25-55m

For UF in deep (45-55 m) water the structure may be subject to significantdynamic amplification of the 2nd mode (fore-aft bending). Throughdetailed simulations it is possible to quantify the effect and obtain a safedesign.

UF is exposed to wind/wave misalignment due to low first naturalfrequency (around 0.20 Hz). Through detailed simulations it is possible toquantify the effect and obtain a safe design.

C l i 164 bi U i l


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Conclusions 164m turbine on UniversalFoundation

The study come from simplified design basis, thus cannot be 100% trustable. A complete FEED would be required to conclude that UF fits properly withV164.The study has taken place in the Carbon Trust OWA frameworkThe conclusions achieved are from Grontmij.

The conclusions do not represent the views from Partners in OWA.

Documents

Presentation of Large Rotor Study