Model test of an inverted conical cylinder floating offshore wind turbine moored by a spring-tensioned-leg

Int. J. Nav. Archit. Ocean Eng. (2014) ( 6:1~133 http p://dx.doi.org/110.2478/IJNAO OE-2013-01599 pISSN: 20092-6782, eISS SN: 2092-67900

ⓒ SNAK, 2 2014

Model testt of an inverted i d conicall cylinder floatinng offsho ore wind d turbin ne mooreed by a spring-te s ensionedd-leg Hyunky young Shin, Sangrai Cho o and Kwangj gjin Jung Schooll of Naval Arch hitecture and Ocean Engineering, University of Ulsann, Korea

ABSTRACT T: A new 5-M MW floating off ffshore wind tuurbine moored d by a spring-tensioned-legg was proposed d for installa-tion in aboutt 50m water depth. d Its subsstructure is a pplatform of th he inverted co onical cylinderr type with massive ballastt weight plate at the bottom m. A 1:128 scale model was built for the preliminary p en ngineering devvelopment. Th he model testss in waves andd wind were carried c out to estimate motiion characteristics of this platform p in thee Ocean Engiineering Widee Tank of the U University of Ulsan. Its mo otions were m measured and the RAOs werre compared. The proposed d floating off-shore wind tuurbine showedd a good stabiility and decennt responses in n waves, wind d and operatioon of the wind d turbine. KEY WORD DS: Spring-tennsioned-leg (STL); Floatingg offshore wind turbine (FOWT); 5-MW; Model test; RAO. R

ACRONYM MS CB CG FOWT MSL RAO

Center of buooyancy Center of graavity Floating offsshore wind turb bine Mean sea levvel Response am mplitude operattor

RNA R STL S TLP T UOU U

Rotor nacelle asseembly Sprring tensioned leg Ten nsion leg platfo form Un niversity of Ulssan

INTRODUC CTION Recently, some floatingg offshore win nd turbines (FO OWT) have been developed d and deployedd in deep sea, while a largee number of off ffshore wind tuurbines with fix xed foundationns have been in nstalled in wateer depths less tthan 50m water deep suppor-ting 3~5MW rotor nacelle assembly a (RNA). The installlation of two fixed offshore wind turbiness at 44m in waater depth wass made in the B Beatrice Wind Farm F Demonsttrator Project (W Wikipedia, 2013). Several reesearches on FO OWT have beeen made. Buldder et al. (2002) analyzed a tri-floater platfoorm wind turbin ne; Lee (2005)) studied a 1.5--MW wind turbbine; Wayman et al. (2006), S Sclavounos et al. (2007), Waayman (2006), Jonkman et al. (2009), Jonk-man (2010), JJensen et al. (2011) ( and Waang and Sweettman (2012) analyzed a variou us tension leg platform (TLP P), spar, semi-submersible aand barge subsstructures of FO OWT. Also a ffew floating wiind turbine mo odel tests have been performeed in wind andd waves. Hydroo Oil & Energyy performed a scale 1/47 moddel test of a 5M MW spar-buoy floating wind turbine at Marrintek’s Oceann Correspondinng author: Hyunnkyoung Shin, e-mail: [email protected] This is an Opeen-Access articcle distributed under u the termss of the Creative Commons Attribution Noon-Commerciall License (http:///creativecomm mons.org/licensees/by-nc/3.0) which permits unrestricted noon-commercial use, distributioon, and reprodu uction in any medium, proviided the originaal work is propeerly cited.

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Basin Laborat atory in Trondhheim, Norway (Skaare ( et al. 22007). Principaal Power Inc. caarried out a scaale 1/67 model test of a semi-submersible pplatform, WinddFloat (Roddieer et al. 2010)). In this modeel test, a disk was w used insteead of three bllades to obtainn aerodynamic thrust forces. WindSea of Norway N was ttested at Forcee Technology on a 1/64 scaale of tri-wind turbine semi-submersible pplatform (Winddsea, 2013). Reeynolds scale w was used in wiind tunnel and Froude scale iin basin, respectively. Modell tests of OC3--Hywind were carried out at the wide tankk, the Universitty of Ulsan (U UOU) on a 1/1228 scale and th he scale modell was moored bby 3 catenary mooring m lines (Shin, ( 2011) annd moored by a spring-tension ned-leg (STL) (Kim, 2011). To producce electricity with w higher effi ficiency at loweer costs in deeep sea, howeveer, it is necessar ary to consider building windd farms with pllenty of FOWT Ts, not a singlee FOWT. Thenn, FOWTs shou uld require nott only smaller ffoot prints to prevent p mutuall interferences among them, but b also installaation cost loweer than those off existing FOW WTs. In this papper, a new substructure of FO OWT satisfyingg both smaller foot prints and d lower installaation costs in 50m 5 of water iss suggested to ssupport a 5-MW W RNA, Jonkm man et al. (20099). Its characteeristics are as fo ollows: • An inverted d conical cylinnder type with heavy ballast w weight at its bottom to ensurre that center oof gravity (CG)) is lower thann center of buuoyancy (CB) • A tensioned d mooring line with a spring to t secure smalll foot print and d low dynamic tensions with a quick installaation. Model tessts with a 1/1228 scale ratio were w carried ouut in the Ocean n Engineering Wide Tank off UOU to pred dict the charac-teristics of mootions of the FOWT platform m in wind and w waves. Compaarisons are mad de between thee inverted coniical cylinder inn 50m of water and the OC3-H Hywind mooreed by a STL at 320m deep.

MODEL TE EST Floating offsh hore wind turrbine model Based on the OC3-Hyw wind moored by y a STL in 3200m of water (K Kim, 2011), th he scale 1/128 m model of 5MW W wind turbinee with an inverrted conical cyylinder platform m was designedd for shallow water applicatiions as shownn in Fig. 1. It iss a three bladee horizontal axiis reference wiind turbine witth 90m in hub height. The pllatform in Fig.. 2 consists of two parts. Its upper part is a cylinder to coonnect to the toower base and its lower one is an inverted conical cylind der with a largee ballast plate on o the bottom.. The inverted cconical cylindeer was selected d for achievingg high CB in sh hallow water an nd the heavy baallast plate at th he bottom wass designed for llow CG. The large l diameter of ballast bottoom plate increases yaw inertia of platform.. In this platforrm, CG locatess at 23.6m andd CB locates att 16.9m below w mean sea levvel (MSL). Th he model was moored by a SSTL. Also a spring s case forr w installed in nside the platfoorm. limited extenssion of spring was

Fig. 1 A FOWT modeel installed in wide tank.

Fig. 2 The inverted conical c cylindder type FOW WT platform.

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Fig. 2 illuminates its detailed drawing with a spring case inside. Principal particulars of the floating offshore wind turbine are given in Table 1. The model tests were performed in the Ocean Engineering Wide Tank, UOU (L × B × D × Dw = 30m × 20m × 3.0m × 2.5m). An artificial bottom plate (L × B = 18m × 2.0m) was manufactured to satisfy the shallow water boundary condition with 0.391m in water depth. The model was installed at 15m downstream of the wave generator and a wave probe was placed to measure the wave elevation as shown in Fig. 3. Four passive makers were mounted on the tower of the model in Fig.1 to measure motions in six degrees of freedom by eight VICON cameras (Fig. 3). Before the model test, wind speed was measured by 12 anemometers at the position where the model would be installed. The wind generator produces mean wind speeds up to 10m/s. Its dimension is 2.0m in height and 3.5m in width. Test data was recorded in 100 seconds for regular waves and in 256 seconds for irregular waves. Table 1 Principal particulars of FOWT. Item

Full-scale

Model (1:128)

Water depth

50m

0.3906m

Rotor mass

110,000kg

0.0525kg

126m

0.984375m

Nacelle mass

240,000kg

0.1144kg

Tower length

77.6m

0.6063m

Tower mass

249,718kg

0.12kg

Tower top diameter

3.87m

0.0302m

Tower base diameter

6.5m

0.051m

Platform length

45.6m

0.356m

Platform top cylinder diameter

9.4m

0.073m

Platform top conical cylinder diameter

28.67m

0.224m

Platform weight plate diameter

22.53m

0.176m

Platform mass including ballast

6,732,000kg

3.21kg

Total structure mass

7,319,000kg

3.49kg

Draft with mooring lines

36.4m

0.284m

Center of buoyancy (from MSL)

-16.9m

-0.132m

Center of mass (from MSL)

-23.6m

Rotor diameter

-0.184m

Roll inertia of whole system (about MSL)

11.132E9kgm

2

0.324kgm2

Pitch inertia of whole system (about MSL)

11.132E9kgm2

0.324kgm2

Yaw inertia of whole system (about MSL)

446.7E6kgm2

0.013kgm2

1

1

Tensioned leg length

13.6m

0.107m

Tensioned leg diameter

0.128m

0.001m

Pretension of tensioned leg

9458KN

4.51N

Mooring point (from MSL)

-36.4m

-0.284m

670KN/m

0.041N/m

Number of mooring lines

Spring constant

4

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Fig. 3 Test set in occean engineeriing wide tank,, UOU. The principal dimensionns are shown in n Fig. 4. Four tr triangle plates, which are mad de of light plast stic material, were w attached too the platform ffor reducing suurge and pitch motions of thee platform. Theese plates weree used to get laarge drag forcees at lower partt of platform.

Fig. 4 Principal dimensions d off model platforrm and appendage (unit = m mm). Load cases gular and irreguular waves witthout/with win nd and rotatingg rotor to obtain the responsee The modeel test was carrried out in reg amplitude opeerator (RAO). Wave generattor produces 133 regular wavees and 4 irregu ular waves as sshown in Tablees 2 and 3. Ann electricity mootor is used forr driving the ro otor. In this tesst, wind speed d and rotor speeed are based oon the rate win nd speed of thee NREL 5-MW W wind turbine. The 5MW win nd turbine rotorr operates with h rotor speed 12 2.1rpm at 11.4m 4m/s mean wind d speed. Usingg Froude scalinng, rotor speed and mean wind d speed of moddel test are deteermined as herreunder:

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• Rotor speed of scale model = 136.9rpm • Mean wind speed of scale model = 1.007m/s • Two load cases are defined as follow: • LC1: Regular waves, no wind, parked rotor • LC2: Regular waves, mean wind speed and rotating rotor

Table 2 Regular waves for LC1 and LC2. Full scale

Model test (1:128)

Run

Period (sec)

Freq. (rad/s)

Wave height (m)

Period (sec)

Freq. (rad/s)

Wave height (m)

1

7

0.90

2.56

0.62

10.15

0.02

2

9.42

0.67

2.56

0.83

7.55

0.02

3

10

0.63

2.56

0.88

7.11

0.02

4

12

0.52

2.56

1.06

5.92

0.02

5

14

0.45

2.56

1.24

5.08

0.02

6

17

0.37

2.56

1.5

4.18

0.02

7

21

0.30

2.56

1.86

3.39

0.02

8

24

0.26

2.56

2.12

2.96

0.02

9

26

0.24

2.56

2.3

2.73

0.02

10

28

0.22

2.56

2.47

2.54

0.02

11

30

0.21

2.56

2.65

2.37

0.02

12

32

0.20

2.56

2.83

2.22

0.02

13

34

0.18

2.56

3.005

2.09

0.02

ISSC wave spectrum was applied to 4 irregular waves as in Table 3. Based on these waves, 2 load cases were defined as follows: LC3: Irregular waves, no wind, parked rotor LC4: Irregular waves, mean wind speed, rotating rotor Table 3 Irregular wave conditions for LC3 and LC4. Irregular wave No

Sea state

1

Full scale model

Model test (1:128)

T (s)

H (m)

T (s)

H (m)

5

9.7

3.66

0.857

0.029

2

6

11.3

5.49

0.999

0.043

3

7

13.6

9.14

1.202

0.071

4

8

17

15.24

1.503

0.119

6

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Where, Hs is significant wave height and Tp is peak-spectral wave period. These irregular waves were produced during 256 seconds (about 48minutes at full scale). Comparisons between the theoretical wave spectra and the measured spectra are shown in Fig. 5. There is a good agreement between them.

Wave spectrum Sea State 6

Wave spectrum Sea State 5 3.E-04

1.E-04

Theoretical ISSC

Theoretical ISSC 8.E-05

3.E-04

Model test measured

Model test measured

m2/Hz

m2/Hz

2.E-04 6.E-05

4.E-05

2.E-04 1.E-04

2.E-05

5.E-05

0.E+00

0.E+00 0

2

4

6

8

10

0

2

4

Freq., Hz

6

8

10

Freq., Hz

Wave spectrum Sea State 7

Wave spectrum Sea State 8 3.E-03

7.E-04

Theoretical ISSC

Theoretical ISSC

6.E-04

2.E-03

Model test measured

Model test measured

4.E-04

m2/Hz

m2/Hz

5.E-04

3.E-04

2.E-03

1.E-03

2.E-04 5.E-04

1.E-04 0.E+00 0

2

4

6

8

10

0.E+00 0

2

Freq., Hz

4

6

8

10

Freq., Hz

Fig. 5 Theoretical and measured spectra for sea states 5-8.

RAO (Response Amplitude Operator) LC1 Fig. 6 shows the RAOs obtained from model tests of the OC3-Hywind moored by a long STL in 320m of water and the inverted conical cylinder moored by a short STL in 50m of water, respectively. The comparison is made to see how large the motion of inverted conical cylinder is. In surge and heave RAOs, both FOWTs show similar responses. The pitch RAO of the inverted conical cylinder is smaller than the one of the OC3-Hywind below 0.5rad/s. Both models have natural frequencies around 0.26rad/s in surge and pitch. The heave natural frequency of the inverted conical cylinder is not clearly shown in RAO because of large heave damping from the ballast weight bottom plate. Yaw restoring moment is very small because of one STL

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mooring line.. In real sites, both FOWTs need to emplooy a torque-baalanced tension ned-leg and/or a yaw controllling device too keep the turbiine operating inn upwind direcction.

Fig. 6 RA AOs of the inv verted conicall cylinder at 50m deep moored by a shortt STL and the OC3-Hywind O at a 320m deep m moored by a long l STL in reegular waves : LC1. LC2 onditions. The scaling law fo or aerodynamicc loads of FOW WT has not beeen establishedd LC2 incluudes both windd and wave co and still be inn dispute for appplication in baasin model tesst. Froude scaliing in this mod del test was appplied to produ uce wind speedd and the rotor--thrust might be b underestimatted. Both thrusst and torque can c be represen ntative of thosee in full scale with w both windd speeds and blade pitch anglees controlled properly (Martinn, 2011). n and oscillatess around the neew position. Ass Due to wiind and a rotatiing rotor, the platform drifts tto a new equiliibrium position can be seen inn Fig. 7, the invverted conical cylinder is driffted around 30mm (3.84m at full scale) in suurge and inclin ned about 4 inn pitch and thenn the wind turbbine system osccillates around the new positiion by wave ex xcitations. Fig. 8 preesents the RAO Os of two mod dels in regular waves, mean wind w speed an nd rotating rotoor. Responses of o the invertedd conical cylindder moored by a short STL att 50m deep aree smaller than those th of the OC C3-Hywind mooored by a long g STL at 320m m deep, except tthe peak in sur urge. Also, all modes m of the iinverted conicaal cylinder sho ow peak valuess around 0.25rrad/s. The new w proposed invverted conical cylinder with h appropriate aappendages moored m by a short STL shoows decent peerformances inn operation connditions, compaared with the OC3-Hywind O m moored by a lon ng STL at 320m m deep.

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F 7 An exam Fig. mple of surgee and pitch tim me histories in LC1 and LC22.

m by a sh hort STL and tthe OC3-Hyw wind at Fig. 8 RAOs of thee inverted conical cylinder aat 50m deep moored 320m deep moored m by a lon ng STL in reggular waves, mean m wind speeed and rotatinng rotor : LC2 2.

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Comparison between LC11 and LC2 Fig. 9 shoows comparisonn results betweeen RAOs in L LC1 and LC2. From F that comparison, RAOss in LC1 and LC2 L are similarr in most of frequencies. But, RAOs in LC2 2 are higher thaan those in LC1 1 near natural frequencies. f Thhis phenomeno on is caused byy f are not n adequatelyy applied in mo odel test. To verify this phennomenon, anotther model testt that damping near natural frequency with higher sccale ratio shoulld be performeed.

Fig. 9 Compaarison betweenn RAOs in reg gular waves only (LC1) andd in regular waves, meann wind speed and a rotating ro otor (LC2). Significant m motion The behavvior of offshorre structures in n irregular wavves may be deescribed in term ms of significaant amplitude of motion res-ponses at speecified sea statees. In order to o predict the sig ignificant ampllitude of motio on responses oof the floating offshore windd Turbine, moddel tests were carried c out in irregular i waves (sea states 5~ ~8). As shown n in Fig. 10, fro rom the captureed motion res-ponse, the mootion spectra were w obtained and a their signiificant heights were calculated.

Fig. 10 Process off calculation oof significant amplitude a of a motion respoonse.

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Int. J. Nav. Archit. Ocean Eng. (2014) 6:1~13

There is a small static drift motion in irregular waves because of the second-order drift force. The maximum static drift motion is shown in sea state 8 as shown in Table 4 and Fig. 11. Table 4 Static drift motion of 1:128 scale model in LC3 - only irregular waves. Static drift motion in mm (average values)

Time

Sea state 6

Sea state 7

Sea state 8

55~87 sec

-0.05

-0.294

-4.362

55~120 sec

-0.046

-0.32

-3.84

55~151 sec

-0.049

-0.34

-3.45

Model test - Sea state 6

Surge (mm)

100

55 sec

87 sec

120 sec

151 sec

50 0 0

20

40

60

80

100

120

140

160

180

200

120

140

160

180

200

120

140

160

180

200

-50 -100

Time sec

Model test - Sea state 7

Surge (mm)

100 50 0 0

20

40

60

80

100

-50 -100

Time sec

Model test - Sea state 8

Surge (mm)

100 50 0 -50 -100

0

20

40

60

80

100

Time sec

Fig. 11 Surge time history of platform in sea states 6, 7 and 8 of 1:128 scale model test (LC3).

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LC3 Fig. 12 shhows the signiificant motion height of the iinverted conicaal cylinder mo oored by a shoort STL in 50m m deep and thee OC3-Hywindd moored by a long STL at 32 20m deep in onnly irregular waves. w In sea states 5 and 6, thhe significant heights h of bothh models show a small differeence in all mod des. In sea state te 7, the signifiicant heights off the inverted cconical cylindeer moored by a STL in 50m ddeep are smalleer in both surgee and pitch, and nd a little largerr in heave, while, in sea state 8, larger in pittch and smallerr in both surge and heave thaan those of the OC3-Hywindd. The inverted d conical cylind der was mooreed by a short STL S due to thee nnot extend beecause the sprin ng is locked byy shallow waterr depth. In largger excursions, the mooring syystem of coniccal cylinder can the case show wn in Fig. 2. Thherefore the inv verted conical cylinder moorred by a short STL S has a smaall surge respon nse and a largee pitch responsee, compared with w the OC3-H Hywind mooreed by a long ST TL. And Respo onses in LC3 llooks too big compared c withh responses in LC1. However, these big responses are only shown in very severee wave condittions, especiallly sea state 8 (Hs = 15.24m,, Tp = 17s).

d Figg. 12 Significaant motion height of the inveerted conical cylinder moorred by a shortt STL at 50m deep and the OC C3-Hywind moored m by a lonng STL at 320 0m deep in only irregular w waves : LC3. LC4 Fig. 13 shhows the signiificant motion height of the iinverted conicaal cylinder mo oored by a shoort STL in 50m m deep and thee OC3-Hywindd moored by a long l STL at 32 20m deep in irrregular waves with w a rotating rotor under unniform rated wind. w Surge andd pitch responsee of an inverteed conical type FOWT are beetter, but not in n heave respon nse. The higherr surge responsse of the OC3-Hywind is caaused by a lonng STL. In sea state 5~7, an inverted coniccal type FOWT T’s heave respponse are a litttle higher thann OC3-Hywindd.

he inverted connical cylinder moored by a STL at 50m ddeep and the OC3-Hywind O Fig. 13 Signnificant motioon height of th m moored by a ST TL at 320m deeep in irregulaar waves, the mean wind sp peed and rotatiting rotor : LC C4.

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Int. J. Nav. Archit. Ocean Eng. (2014) 6:1~13

CONCLUSIONS A novel concept of FOWTs, the inverted conical cylinder moored by a STL at 50m deep, was proposed for shallow water applications. In order to estimate motion characteristics, scale model tests in regular, irregular waves and wind were performed in the Ocean Engineering Wide Tank of the UOU. Having the small responses and the small footprint, the inverted conical cylinder platform was designed with the ballast weight bottom plate for both low CG and large yaw inertia, the inverted conical cylinder for high CB, the STL with a spring case for both small dynamic tensions and the shift in natural frequency, and four triangle plates for small surge and pitch responses. An inverted conical cylinder type FOWT shows decent motions in most of load cases compared with the OC3-Hywind spar model moored by a STL at 320m deep. Natural frequencies of an inverted conical cylinder type FOWT are around 0.25rad/s in surge and pitch and around 0.19rad/s in heave. In LC1 (regular waves only), the RAOs of this model in natural frequency are 4.0m/m in surge, 0.7m/m in heave and 1.8deg/m in pitch. In LC2 (regular waves with wind and a rotating rotor), the effective RAOs of this model in natural frequency are 6.2m/m in surge, 0.9m/m in heave and 2.8deg/m in pitch. When compared LC1 with LC2, the surge and pitch responses in LC2 are bigger than the ones of LC1 non-universally. The reason why this non universal phenomenon is shown is that the inverted conical cylinder type FOWT has insufficient restoring moment in roll and pitch direction to countervail thrust force by wind and a rotating rotor. And from the results of both LC3 and LC4, the responses of an inverted conical type FOWT are small except for very severe wave condition, sea state 8. The inverted conical cylinder drifts in a new equilibrium position due to wind and/or a second order wave effect and oscillates around the new position. The thrust in basin model test was scaled in Froude number and may not be comparable to the one in full scale. In near future works, Reynolds number effects on the Froude scale FOWT should be clarified. In both real sites and model basins, a torque-balanced laying construction for STL is needed to secure large yaw restoring moments as well as yaw controlling devices.

ACKNOWLEGEMENTS This work was supported by the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea Government Ministry of Knowledge and Economy (No. 20124030200110).

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