SSI Damping MP Emergency Stops 279265

8/12/2019 SSI Damping MP Emergency Stops 279265

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Estimationof theVibration ecrementan

Offshore Wind TurbineSupport Structureausedby its Interaction with Soil

W . G . VERSTEIJLEN\ A . V . M E T R I K I N E 2 , J . s . HOV ING ,E. SM ID , W . E . DE VRIES -

AbstractI n today's cu t t ing costs environment in the offshore w i n d industry, significant achievements canbe made w i t h a bet ter assessment of dynamic soil-pile interaction.O f the main damping mechanisms active at an O WT , least is known about soil dampi ng. Thevalues for this contributionused in the industry today - mostly calculated analogously to a studype rf or me d i n 1980 [1] - are expe cte d t o be on the low side. Mo re research on the topic is recommended.Presence of more dampi ng tha n curre ntly assumed, wou ld sig nify that the (often) design dr iv ing fa tigue damage accumulation is lower than assumed. This would j u s t i f y designing more light-weights t ructures using less const ruct ion steel, or allowing for longer (insured) O W T Ufetimes the n thenow applied 20 years. B o t h these measures significantlydecrease costs of offshore w i n d .This paper evaluates measured signals of twelve 'rotor stop'- test on an OW T at Do ng E nergyowned - Burbo Banks w i n d farm. The v ibration decay was measured w i t h an accelerometer andstra in gauges along the tower. A simpli stic analytical model has been developed enabling analyseso f the measured signals.T wo main modal shapes were identified w i t h similar shape, but deviating amplitudes in the soilprofile. The large difference in dampi ng that exists between the vib rati ons ofthese modes is att r ibu ted to the difference i n influence th at the soil can have on these vibrati ons. T he fo und effecto f soil on the dam pi ng of this parti cular O W T is signi ficant ly larger than th e order of magn itudeused in the industry today.

^SiemensW i n d Power, Prinses Beatrixlaan 800, 2595BN The Hague, The Netherlands. Tel. +31 70 333 6920 /+3 1 6 175 16 437 email [email protected]^De l f t University of Technology - Faculty of C i v i l Engineering and Geosciences, Professor at the section ofStructural Mechanics, Stevinweg 1, 2628 CN De l f t ,The Netherlands


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1 Introduction

1.1 Background & MotivationC u t t i n g costs is the main p r i o r i t y i n today 's offshore w i n d in dus try. I n order to at least reach thesame levelized costs as coal power, n u m e r o u s areas exist where costs can be cut . One of theseareas is the usage of cons tru cti on steel. I n the design process of support s t r u c t u r e s for OWT ' sthe dynamic aspects of the stru ctu re - stiff ness and damp in g - are im po rt an t infl ue nci ng factor sfor det erm ini ng the diameter and w a l l th ickness of the tub ul ar s tructures , thus dete rmin in g theamount of steel usage. Th e cur ren tly most comm onl y applied monopile foun dati on mounts up toabout 20% of the CA PE X of the entire O W T structur e. Cons tru cti on costs and the weight of theapp l ied cons t ruc t ion s teel have a linear relation.

Damping has a posit ive inf luence in decreasing fa t igue damage accumula t ion du r i ng the OW T' sl i f e t i me . Each pe rcent ex tra dampii rg ra t io (of cri t ical) iircorporated in the loads an d de signprocess can have a s ignif icant cost savin g effect. Several damp in g mechan isms are acti ve at anO W T : f r o m top to bo tt om of the s tructur e) aerodynamic- , tun ed s loshing- , s truc tur al s teel- ,hydrodynamic- and damping caused by the influence of soil ( 'soil damp in g ' hereaf ter ) . O f these 5mechani sms , least is kn ow n of the mag ni tud e of soil damping.

The ma jor source for deter ir i in in g dam pi ng in the indust ry today is a research per form ed byM . F . Coo k [1] , [2] wh o assessed the sources of dairrping of different mode shapes of a single piledp l a t f o r m in the G u l f of Me xi co i n 1980. Th e magni tude s f or soil d a m p i n g which are es tim atedon t h e basis of this p a p e r are expec ted to be lower th an actu al and hence result in conservativedesigns, appl yi ng too much steel.

1.2 Description of ResearcliT h e basis of this research consists of the measurement of rotor stops of an OW T , and the develop-mei r t of a model, both discussed in the respective sections of this paper . T h e v i b r a t i o n decrementcaused by the rotor stops were measured by an accelerometer i n the nacelle and str ain gauges atthe top and bo tt om of the tower. Th e damp in g of the measured response was i denti f ied in b ot hthe freque ncy dom ai n and tim e dom ain , and an anal yti cal mod el was develope d to fu rt he r assessthe measurements .

2 Offshore Measurements2.1 Description ofSetup and Obtained DataFor this research it was chosen to per form ro tor s top tests to possibly gain more insight in thestr uct ural dynamic s of an O W T and the inf luence of the s o i l . Cost-, and comple xit y-wi se, a rot orstop test is a rela t ively low thres hold o pti on to pe rfo rm measru 'ements . D ONG -E ner gy was foundto be w i l l i i r g to avail thei r 'BB 16 ' O W T of thei r Burb o Banks w i n d fa rm , offshore Engl and's westcoast in the I r i s h sea. Thi s O W T is equippe d w i t h a Power L oa d M o n i t o r i n g ( PL M) system ofw h i c h the accelerometer in the nacelle and the str ain gauges at tower top and tower bottom werethe ma in sensors used for th is research. O n the 29*'' of Oc to be r 2010, n u m e r o u s ro tor stops wereper formed of w h i c h 12 tests p rov ided useable data. Fi gu re 2.1 gives a schemat i c vi ew on themeasurement se tup of BB 16 and th e power s p ec t r a at the 3 m ea s u r em en t loc ations of the first 6tests.


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Figure 1: Schematic view of the OW T'BB16' measurement setup and typical measured power spectra at thedifferent measurement locations.

Figure 2 gives a typical response of - in this case - the ben ding momen t at the tower bott omduring th e stages of tur bi ne pr oduc tio n, blade pitch-out (r otor stop) , blade feathe ring and bladepitch-in. The pi tch angle of the blades is pl ott ed i n the same graph. Th e red encircled par t of theresponse was used for frequency analysis and damping identification. Onl y th e linearly decayingoscillations after the rotor stop were considered.The re is a pat ter n noticeable i n the power spectra over the dif fer ent heights of the tower th atare depicted in figure . Each power spectrum contains a first ma jo r freq uenc y peak at 0.29 Hz,which corresponds to first natu ral bending mode shape of the stru ctur e. However, in the t opsecti on of the st ru ctu re (nacelle and tower top) , a second frequ enc y is present at 0.82 Hz. A checkw i t h th e aeroelastic BH aw C mod el and the developed mode l fo r th is research (see nex t section)indicated th at this frequen cy is linked to closelyspaced blade modes. I t is a rat her localiz ed mode:i t is hardl y measured i n the bo tt om section of the tower.


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Timeseries f.kjmentto.verbottoman dPitchAngle

b l a d e s infeathering b l a d e s pi tchinp o s i t i o n

b l a d e s infeatheringp o s i t i o n

Figure 2: Time series of test nr. 6. The red encircled part is of interest for this research: the decaying fore-aftmovement of the tower.

2.2 DampingIdentificationT l i e Qu al it y (Q) factor techni que is t l i e m a i n m e t l i o d used fo r iden tif> dng the d am pi ng of the 2m a i n freque ncies i n the measured sig nal in the freq uen cy domai n. I t is a rath er fast and accuratet echn ique which allows ide nti f ic atio n of dam pi ng of mul t i p l e f requencies i n a s ignal (as opposedt o for instance the loga r it hmi c decrement techir ique on raw t i me dom ain s ignals) . Th e Q- factoris a measiu'e for the skewness of f a f requenc y peak: the higher and narrower they are, the lesserthese frequencies are dampe d. As a check, also the logar it hmi c decrement technique was appli edi n t he t im e domain . N o wi ndo w fun ct i on was used ( to min i miz e energ j ' s h i f t ing i n the fre que ncydomain ) , and the raw t ime samples were zero-p added t o produc e enough sample points to getsmooth power spectra . A l l 1 2 tes ts were assessed, and f requency and damping wise, tests 4 an d 5were closest to the average values of all tests . The se tes ts were extra assessed (as a t h i r d check)by fitting their power spe ctr um w i t h an analyt ic al fit th at is der ive d i n an ite rat ive manne r. Th eairalyt ical power spectrum can be descr ibed w i t h the equ ati on gi ve n un de r figure 3, i n w h i c h Ais the amplitude, w the center frequency, Q the run ni ng f requenc y (a long the hor izon tal x-axis ) ,C the da mpi ng rat io, and N the amou nt of peri ods (cycles) in the ti me dom ai n sequence th at isFour ier tran sf or me d to the fre quen cy dom ai n. F igu re 3 shows this fit fo r the first na tu ra l bendingfre quen cy of the tower of the measured bendi irg nroment a t tower bo tt om of test 5. A 3 % ofc r i t i ca l damping gives the best fit.


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Figure 4 depicts t l i e part of t l i et ime series w l i i c l r was Fourier transformed to derive t l i e spectrum offigure 3. Difl:erent decre ment l ineshave been plotted, and the line w i t h3% of crit ical damping decrementseems to have the best correspondence. Th is sign al is cle arly dominated by a single fre que ncy: thesecond (blade induced) frequency isno t present in this signal whichis measured at the bo tt om of thetower.Similar fits as done i n figure 3 canalso be made on the second frequencypeak , as is done i n figure 5 i n whichthe tower top bending moment is assessed of test 4. F r om thi s gr aph wecan conclude that a 3 % of criticaldamping for the first natural bendi ng frequency and a 1.5 % of c r i t -ical dam pi ng for the second bl ade-induced frequency, are reasonable estimates.

Povverspeclnjmf. omenl ov^jrbottomtest5,f js t6periodsofinterestaftar rotorstop, 1stfrequency

Figure 3: Measured and analytical fitted power spectrum for first6 cycles of the bending moment at tower bottom fortest 5. Onlythe firstnaturalfrequency is present in thissignal at the tower bottom. The fit with 3 dampingratio is found to be the closest fi t.

PS = \Aw - ^2 _ 2if2a;C- 2 W i r ( C r u + in)

T i m e s e r i e s Momentto'.verbottomtest5.first6per iodsofinterestafter rotorstop

Figure 4: Measured time response of first 6 cycles of the bending moment at tower bottom fortest 5. Thesame fitted damping ratio's are plotted as logarithmic decrement. Again, the 3 damping ratio isfound to be the best fit. The firstnatural frequency clearly dominates the time response.


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Pov/efspectnim Momenttov/ertopl e s t4,first6periodsofinterestafter rotor stopfor1stfrequency,18periodsfor2ndfrequency- I F F T M e a s u r e d Signall-- Analyticalfit 3% ofcriticalof stfreq.-Analyt ica lfit 1.5%ofs e c o n d freq,-Analyt ica lfit2.5%ofs e c o n d freq.

K Frequency[Hz]Figure 5: Measured and analytical fitted power spectrum for first 6 cycles of 1st natural frequency and 18cycles for the second frequency of the bending moment at tower top fortest 4. Th e fit with 1.5damping ratio for the second frequency is found to be the closest fit. T he first natural frequency iswell fitted with 3 damping ratio.

A log ari thm ic decrement check on a s ignal w i t h t wo dom in an t frequ encies was not considered tobe very sensible, so figure 6 only depicts the t ime series that was Fourier t rans for med to derivethe spectrum of figure 5 to get an idea of the p at te rn i n the ti me doma in .

Timeseries foranalysesMomentl e v e rtoplest4, frst7 periorisofinterestafter rotorslopfor1s tfrequency

Figure 6: Time series taken for deriving the power spectrum for the first6 cycles of 1st natural frequency and18 cycles for the second frequency of the bending moment at tower top fortest 4.

As a summary, table 1 sums up the nam ed damp in g magnitu des . A not iceable difference ind a m p i n g value of 1.5 exists between the two mo da l frequencies.I t is belie ved th at th e ma in reason for the larg e differenc e i n id en t i f i ed damping can be foundi n the di ffer ent poss ible influ ence the soil can have on these tw o modes. Th e mot ion s afte r therotor stops seem to be domi na te d by two ma in frequencies: a glo bal be ndi ng mode over the enti rel eng th of the st ru ctu re, a nd a localiz ed mod e at the top of the s tru ctu re w h i c h is associated w i t hblade modes. These two modes have s imi la r shapes over the ve r t i ca l hei ght of the s t ru ctur e .However , the hor izon tal ampl i tude s of the blade-in duced mode in the lower par t of the s t r uct ure


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Table 1: Damping values of rectangular windowed initial vibration cycles oftests4 and 5.T e s t4 T e s t s

F r e q u e n c yIHz]

Damp ratio ?[%]ofcri t.

L o g d e e r[%]

F r e q u e n c y[Hz]

Damp ratio5[%] ofcrit.

L og d e e rI'MMtop 1st 0.293 3 18.86 0.292 3 18.86Mbot 1st 0.293 4 25.15 0.292 3 18.86Mtop 2nd 0.877 1.5 9.43 0.883 1.G 10.05Differencei ndamping betv/een twomain f r e q u e n c i e s : A = 1.5 9.43

are expected to be much smaller than those of the f i r s t global ben din g mode. T he glob al mode isdamp ed twic e as mu ch as th e localize d mode. As this global mode doesmobilize s o i l reactions andthe localized mode does not, th e difference i n identified damping - 1.5 % of c r i t i c a l - is believed tobe a measure for the mag nitu de of s o i l dampi ng that is mobil ized dur ing the vi br ati on amplitudesoccurr ing after a rot or stop. To put these numb ers i nt o perspecti ve; the design value for the to ta lexper ienced dam pi ng by th e stru ctu re inc orp orat ed in the design of BB 16 was 2.5 % log. deer.

3 Model DevelopmentI n order to further analyze the measured responses of the BB 16 structure, an analytic al modelhas been developed. The ' Euler -Ber nouUi' bea m bendi ng approach is used to deriv e the equ ationo f mot io n and bound ary condi tions . Figu re 7 displays a symb olic repr esentat ion of the model andpar t of its governin g equati ons (equ ation of mot io n and boun dar y conditi ons) to give an idea ofthe a nal yti cal way of solvi ng. T he model is fu rt her described by i n i t i a l conditions and interfaceconditions at waterline {x = a;; = 12.5m) an d mud li ne (a; = xî = Om), which are not printedhere bu t c an be fou nd in [3].

u(0)=V - ' ' * C ^ r

Will)

Wj(t)

p A

f c . . ; ^ - l ( = L =94 .98 n^ x = 93.25 IT

= \ . =-22n

Equation ofMo t ion

dw{x, tdt + ks{x w{x,t =

Boundary Conditions

dx^ dt^dxEI d\o{L,t) âm(At),3 ^ 8x

aû'(,t)..Hop-

d{u-w{L,tdt

dwjL.t)at= hi[n-w[L,t)) + Ca

Figure 7: Graphical representation of the analytical model developed to simulate the response after the performed rotorstops, x^l = 12.5 meter was the mean sealevel (waterline) during the tests. On theright side, some of the governing equations are given; the equation of motion and the 5 boundaryconditions.


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3.1 Parameter DiscussionTh e role of most of th e parameter s in the mod el can be deducte d b y lo o k ing at figure 7. T h ehere presen ted mod el is the en d-result of a streak of enhancement steps. I n the sim pli sti c way ofm o d e l i n g the s o i l , the research of P. Wegene r was inc orp ora te d for mod el in g the soil stiffness [4].I n hi s assessment on the appli cabi l i ty of the cur ren tly used PY-curves - w h i c h were developed forslender piles (h igh le ngth over diameter ( L / D ) ra t i o) - he suggested t o l i n k the stifl'ness kg of thed i s t r i b u t e d soil spr ings w i t h a fac tor (dependent on the L / D rati o) to the elast ic Young ' s module so f the saturated s o i l . As a second p a r a m e t e r to reach some what more reali stic mod el in g of thec u r r e n t l y used r i g i d ( low L / D ratio) behaving monopil e, he also suggested a p i l e - t ip correctionfac tor w h i c h is also dependent on the L / D ratio. For the L / D rati o of BB1 6, these two fact orsar e kg = lA8Es and a pil e ti p cor rec tio n fac tor of 11.5, w h i c h means that the spring at the end ofthe pile, kt, is a fa ct or 11.5 as s t i f f as the other dis tr i but ed spr ings. These tw o values are linkedt o the L / D ratio , and for BB1 6 th is is L / D ^ 5 . Th e values were der ive d w i t h a comp aris on s tud ybetwe en tw o models : t he stand ard F E M 1 dim ens ion al mod el of a be am on a W i n k l e r f o i m d a t i o nw i t h d is tr i but ed spr ings (used in the indu str y) , and a 3D elast ic F E M mode l w h i c h wa s assumedto be a mor e realisti c repr ese ntati on of the SPI process.T h e soil d a m p i n g , Cgwas l inked to the distributed spring stifl:ness kt as a factor. The SPI processis thu s govern ed by a thre e-pl ay ofparameter s : C j kt and ks and therefor also Es- Th is lat ter wastaken to be constant w i t h de pth and equal to 130 MPa . D et er mi ni ng the proper Young 's modu luso f offshore saturated soil is not entir el y st rai gh tf orw ard . Th is value is a conservativ e (n ot ver ys t i f f ) est imate for saturated sand ( the main soil type present at the BB1 6 locat ion) .

3.2 Model OutputA s most in te rest ing out pu t of the model , power s p ec t r a of the mome nt at tower to p and bo tt om ,and a t ime-domain response comparison are presented here below . F ig ur e 8, indic ates th at thesecond blade ind uc ed frequ enc y is most present at the to p of the st ruc tur e, and d a m p s out towa rdsthe bot tom.

Powerspectram Moment tower lop Powerspectrum Moment tov/erbottom

0.5 1Frequency [Hz]

0.5 1Frequency [Hz]

Figure 8r Model output: power spectra for the bending moment at the tower top and bottom.

Besides comparing the power spect ra of measured and modeled responses, it is also inter est in g tocompare a modeled t ime-domain response w i t h a measured one. Th is is done i n figure 9. T h esignals are the bending moments at the tower top, w h i c h is wh y the y are clearl y double fre que ncyd o m i n a t e d responses. T h e resemblance is considere d to be sati sfac tory for thi s sim pli sti c mod el .


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The attenuation is roughly the same, except for the last part, wl ier e the 'r eal' stru ctur e is prob ablysubject to some exte rnal exci tati on, lower ing the mean bendi ng momen t.

4 ConclusionsThis research was aimed at f inding the effect of soil on the vibr ati ona l decrement of an offshorew i n d tur bi ne suppor t structu re. Based on measurements at 3 diff eren t ver tic al locations pe rfo rme dafter 12 consecutive rotor stops on the 'B B16' O W T in Bur bo Banks, the f o l l o w i n g conclusionswere drawn:

1 . The es tim ation of the order of magni tude of vi br ati on decrement caused by equivalent linearviscous damping generated by soil-pile int erac tio n of the 'B B16 ' test turbine support structure lies in the range of 9.5 % logarithmic decrement, which equals 1.5 % ratio of crit icaldamping of the f i r s t bending mode.

2. Considering the fact that the average magnitude of the total identifi ed dampin g in themeasurements of the ' BB1 6' OW T is

19 % logari thm ic decrement (3 % ratio ofcritical) for the first natural bending frequencyo f 0.2969.5 % log. deer. (1.5 % rat io) for the second m ai n present frequency of 0.825 Hz

i t is concluded that, compared to the design value for BB16 of 2.5 % log. deer. (0.4 %ratio) of damping for the first natu ral frequency, the ide nti fie d value i n the measurements isrelatively high.

3. I n this research, a tool has been developed to simplisticallyassess the infiuence ofsoil on t hedamp ing of an O W T . A difference in displacement i n the soil profile between the v ibrati onso f two differe nt frequencies allows fori dent i fying the infiuence ofsoil on these vibrations.


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The mode shapes th at corre spond t o the first an d second measured frequencies have beeni den t i f iedv i a a combination of analyses of power spect ra of signals at diff ere ntm ea s u r em en t locations, th edevelopment of an analytic al model, and a conf irm ati on w i t h the Siemens-des ign model BHaw C.A dif ference in d i sp lacement s of these mode shapes in the soil prof il e and a diffe rence in damp i ng of the vib rat ion s of these t w o modes was identif ied. This a l lowed for assessing the inf l uenceo f the s t r u c t u r e ' s in te rac t ion w i t h soil on the tota l damp in g of the s tructur e. Th e second f r e que ncy i n the signal is associated w i t h a locali zed mode in the to p of the st ruc tur e caused b ythe closely spaced nat ura l modal f requencies of the blades. The ampli tudes i n the soil profi le ofth is moda l shape are snaaller t h a n those of the first bendi ng mode shape. Because of the var yi n g ampli tudes of the v ibrat ions of these t w o modes in the soil profile, the dif ference in identi f iedclamping is at tr ib ut ed to the diffe rence i n possible inf luenc e th at the soil can have on this damping.

Caut ion should be taken in generalizing the above stated order of mag ni tu de for the dam pi ngcaused by soil-p ile in te racti on. General izati on of the resul ts of this research is l i m i t e d by somefactors of which some are discussed here.T h e research is based on 12 rotor stops on a specific tu rb in e du ri ng one day. Exc ept for c hangin gsoil condit ions, also changing envir onment al condit ions ( in par t i cular w i n d speed) have inf iuenceon the magni tud e of dam pi ng. For instanc e, displ acement dependent dam pi ng is expec ted to beactive in the soil-p ile in t erac tion process, the i n i t i a l amp li tude of v i br ati on af ter a roto r s top hasan im por tan t inf luence on the exper ienced da mpi ng of the s tructure. I n th is respect , the damp in gassociated w i t h vibrations induced by a rotor stop, might not be representa t ive for the damp in gexperi enced dur in g most of the l i fe t ime of the O W T : the dampi ng occurr i ng dur in g v ib rati onswhi le the turbi ne is in produ cti on, dur i ng whi ch v i bra ti on ampli tudes are usually smaller tha nthose after a rotor stop.

Another f ac tor wh ich needs more inves tigat ioir is the un kn own effect of the slos hing d a m p e r on thedam pi ng of vi br ati ons after a rot or stop. A part of the ide nti fl ed differ ence i n dam pi ng betwee nthe two domi nant modes can possi bly be at tr ib ut ed to a differe nce i n sloshi ng dam pi ng inf luenc eo n the v ibrations of these modes.

References[1 ] M. F . Cook & J . K. Vandiver . Measure d and predicted dynami c response of a single pile platform

to random wave excita t ion, pages 637-64 3. Offshor e Tech nolog y Conferenc e Hous ton , Texas,M a y 1982.

[2 ] M. F . Cook. Damp in g es t ima t ion , response predi ct i on and fat igue calculati on of an operati onals ingle p ile p l atf orm . Master 's thes is , Massachuset t s Inst i t u te of Technology and Wood s H oleOceanographic Ins t i tu t ion , 1982.

[3 ] W . G. Ver s te i j len . Es t ima t ion of the v ib ra t i on dec rement of an offsh ore w i n d tu rb i ne suppor ts t ruc tu re caused by i ts in teraction w i t h s o i l . Master 's thesis. Techn ica l Un iver s i ty of D e l f t ,2011.

[4 ] P. Wegener . A c r i t i ca l evaluat ion of the curr ent design s t a n d a r d f or offshore w i n d tu rb inemonopil e foundations . Master 's thes is , Offshore Engineer ing , D e l f t Univ ers ity of Technology,2010.

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SSI Damping MP Emergency Stops 279265