Numerical Analysis of a Gravity Substructure for 5 MW Offshore Wind Turbines Due to Soil Conditions

Transcription

1 Journal of Energy and Power Engineering 10 (2016) doi: / / D DAVID PUBLISHING Numerical Analyi of a Gravity Subtructure for 5 MW Offhore Wind Turbine Due to Soil Condition Min-Su Park, Youn-Ju Jeong and Young-Jun You Structural Engineering Reearch Intitute, Korea Intitute of Civil Engineering and Building Technology, Goyang-Si 10223, South Korea Received: January 12, 2016 / Acceted: January 21, 2016 / Publihed: March 31, Abtract: In order to increae the gro generation of wind turbine, the ize of a tower and a rotor-nacelle become larger. In other word, the ubtructure for offhore wind turbine i trongly influenced by the effect of wave force a the ize of ubtructure increae. In addition, ince a large offhore wind turbine ha a heavy dead load, the reaction force on the ubtructure become evere, thu very firm foundation hould be required. Therefore, the dynamic oil-tructure interaction ha to be fully conidered and the wave force acting on ubtructure accurately calculated. In the reent tudy, ANSYS AQWA i ued to evaluate the wave force. Moreover, the ubtructure method i alied to evaluate the effect of oil-tructure interaction. Uing the wave force and the tiffne and daming matrice obtained from thi tudy, the tructural analyi of the gravity ubtructure i carried out through ANSYS mechanical. The tructural behavior of the trength and deformation are evaluated to invetigate an ultimate tructural afety and erviceability of gravity ubtructure for variou oil condition. Alo, the modal analyi i carried out to invetigate the reonance between the wind turbine and the gravity ubtructure. Key word: Offhore wind energy, gravity ubtructure, uction bucket foundation, ubtructure method, tructural analyi. 1. Introduction Nowaday, the main ource of energy in the world i foil fuel. But the amount of foil fuel i limited and the ue of it caue environmental ollution and global warming. Thu, the tudie of renewable energy uch a hydro energy, wind energy, olar energy and geothermal energy are being carried out actively all over the world. The offhore wind energy ha gained attention from many countrie to find alternative and reliable energy ource, ince the otential of offhore wind energy ha been recognized for long and motly aociated with a nondetructive renewable energy. Therefore, many offhore wind farm are in the lanning hae. Variou tudie have Correonding author: Min-Su Park, enior reearcher PhD, reearch field: hydrodynamic of offhore tructure. It i noted that, thi aer i revied edition baed on Proceeding of the ASME th International Conference on Ocean, Offhore and Arctic Engineering, May 31-Jun. 5, 2015, Canada. been conducted on wind energy [1-3]. In order to contruct the offhore wind farm, the ubtructure uorting offhore wind turbine have to reit load from wind and wave. However, the ize of a ubtructure for wind turbine i gradually increaed ince the ize of a tower and a rotor-nacelle become larger with increment of gro generation. In other word, the ubtructure i trongly influenced by the effect of wave force and the afety of ubtructure i decreaed. Therefore, it i very imortant to accurately calculate the wave force acting on ubtructure. In addition, ince a large offhore wind turbine ha heavy dead load, the reaction force on the foundation become evere, thu very firm foundation hould be required. Therefore, the dynamic oil-tructure interaction ha to be fully conidered. There are two main method for the analyi of dynamic tructure-foundation interaction, direct and ubtructure method, a outlined by Wolf [4, 5].

2 Numerical Analyi of a Gravity Subtructure for 5 MW Offhore Wind Turbine Due to Soil Condition 151 In the reent tudy, ANSYS AQWA i ued to evaluate the wave force acting on the gravity ubtructure for 5 MW offhore wind turbine, ince Morion equation cannot accurately calculate the wave force in cae of the large ubtructure comared to a wave length. The ubtructure i ignificantly influenced by the wave teene and wave lamming effect but they are not conidered in the reent tudy, becaue the wave breaking i not occurred for the deign environmental wave condition. The wave force and wave run u on the ubtructure are reented for variou wave condition. Moreover, the ubtructure method i alied to evaluate the effect of oil-tructure interaction with imedance coefficient a daming and tiffne matrice. Uing the wave force and the tiffne and daming matrice obtained from thi tudy, the tructural analyi of the gravity ubtructure i carried out through ANSYS mechanical. The tructural behavior of the trength and deformation are evaluated to invetigate an ultimate tructural afety and erviceability of gravity ubtructure for variou oil condition. The firt few natural frequencie of ubtructure are heavily influenced by the wind turbine. Therefore, the firt natural frequency of ubtructure i to be within the oft-tiff range in between the 1 P and 3 P frequency range. The rotor frequency (1 P) range lie between Hz and Hz, and the blade aing frequency (3 P) range lie between 0.35 Hz and 0.61 Hz. A afety margin of 10% on the maximum and minimum rotor eed i adoted, which mean that, the allowable frequency i between Hz and 0.31 Hz [6]. Therefore, the modal analyi i carried out to invetigate the reonance between the wind turbine and the gravity ubtructure. It i found that, the uggeted gravity ubtructure can be an effective ubtructure for 5 MW offhore wind turbine. 2. Formulation The gravity ubtructure for 5 MW offhore wind turbine conited of a concrete con tye uertructure and a uction bucket foundation ytem a hown in Fig. 1. A the ubtructure ma increae, the interaction between the uertructure and foundation ytem intenifie, thu, the contribution to the tructural reone of the total ytem increae. An imedance function wa ued for analyi of the uction bucket-oil foundation ytem. Imedance coefficient a daming and tiffne matrice were alied to the equation of motion for the foundation ytem [7, 8]. Alying ubtructure method, the ile-oil foundation ytem can be exreed with the imedance coefficient for two-dimenion uch a the horizontal direction, the rotational direction and the vertical direction a hown in Fig. 2. Fig. 1 Geometrical definition of the gravity ubtructure: HAT (highet atronomical tide), MSL (mean ea level), LAT (lowet atronomical tide).

3 152 Numerical Analyi of a Gravity Subtructure for 5 MW Offhore Wind Turbine Due to Soil Condition Fig. 2 Pile-oil foundation ytem with ubtructure method. The imedance coefficient denote the relation between the dynamic reaction force and the correonding dilacement. The imedance coefficient related to the uction bucket ytem on relatively oft ground condition can be rereented a follow: K 0 K xx x K 0 K 0 zz K 0 K x (1) C 0 C xx x C 0 C 0 zz C 0 C x (2) in which: K K K xx x EI r K x EI 1.6 r EI r E A V K 50 zz l V D C 2.0K xx xx V G E G E D C C 1.5K x x x V D C 0.5K V C zz K zz 0.026l V 0.75 G E (3) (4) where, the uffix x, θ and z denote the comonent of horizontal direction, the comonent of rotational direction and the comonent of vertical direction, reectively. E, G, V, D and I are the Young modulu, the hear modulu of uburface ground, the hear wave velocity of uburface ground, the diameter of ile and the moment of inertia of ection, reectively. The imedance coefficient K and C denote the tiffne matrix and the daming matrix of foundation ytem. The relationhi between the hear trength modulu E, and the hear wave velocity V, i exreed a follow: G V (5) in which, υ and ρ denote the Poion ratio of oil and the denity of oil, reectively. In the reent tudy, the tiffne and daming coefficient of ingle ile, which ha ame thickne of uction bucket, i firtly obtained and it i aumed that, the around of uction bucket i conited of 107 ile to exre the uction bucket-oil foundation ytem, ince the general olution to calculate the tiffne and daming coefficient on the uction bucket foundation ytem i not well known. 3. Numerical Reult and Dicuion 3.1 Wave Force Evaluation Fig. 3 how the comarion of wave force on gravity ubtructure for variou water deth. The water deth of LAT, MSL and HAT i 17.2 m, m and m, reectively. In the comarion, the calculated total wave force are divided by incident wave amlitude (H/2). The wave force on the gravity ubtructure with the water deth LAT are larget comared to the other cae. Since the wave force i cloely related to the wetted urface of ubtructure and the water article velocity near free urface i larget, the wave force gradually decreae a the water deth become large. The eak wave force with water deth MSL and HAT decreae about 12% and 20% comared to the eak value of water deth LAT.

4 Numerical Analyi of a Gravity Subtructure for 5 MW Offhore Wind Turbine Due to Soil Condition 153 Wave force/wave amlitude (kn/m) 1,600 1,400 1,200 1, LAT MSL HAT Wave eriod () Fig. 3 Comarion of wave force on the gravity ubtructure for variou water deth. (a) Panel reure for LAT Fig. 4 and 5 how the anel reure and wave run u on gravity ubtructure. Since the ignificant wave eriod of outhern-wetern ea in Korea i 12.42, the comarion of thoe i made at the ame wave eriod. The anel reure near the free urface i larget and the attern of wave run u around the ubtructure i very imilar for all cae. 3.2 Wind Turbine Model and Environmental Load In the reent tudy, NREL (National Renewable Energy Laboratory) 5.0 MW wind turbine model i elected for the tructural afety analyi of gravity ubtructure. The detail of NREL 5.0 MW wind turbine are rovided in Table 1. The total weight of turbine and the tower model are about 350 ton and 348 ton, reectively. The hub height i m from MSL and the tower length i 68.0 m [9]. The height of gravity ubtructure i 32.7 m from eabed and the water deth (MSL) i m a hown in Fig. 1. At the tructural afety analyi, deign code IEC i adoted and tructure analyi i carried out according to the ultimate deign load condition 6.1a and 6.2a reented in Table 2. DLC (deign load cae) 6.1a and 6.2a reent ultimate limit tate for the arked (tanding till or idling) tate and grid lo tate, reectively. Environmental load of wind and wave for the (b) Panel reure for MSL (c) Panel reure for HAT Fig. 4 Panel reure on gravity ubtructure: (a) LAT, (b) MSL, (c) HAT. outhern-wetern ea of the Korea eninular are reented in Table 3. Extreme wind and wave load ubjected to the gravity ubtructure are calculated baed on the wind eed and the wave eriod of return eriod 50 year.

5 154 Numerical Analyi of a Gravity Subtructure for 5 MW Offhore Wind Turbine Due to Soil Condition Table 2 DLC for ultimate limit tate. DLC Condition Wave Wind IEC (6.1a) Parked tanding till or idling Extreme Extreme IEC (6.2a) Grid lo Extreme Extreme (a) Wave run u for LAT Table 3 Environmental load. Wind Wave Limit tate H Normal 7.7 m/ w = 1.52 m, FLS (fatigue load P w = 6.2 tate) Extreme 41.6 m/ H w = m, ULS (ultimate limit P w = tate) 3.3 Natural Frequency and Reonance (b) Wave run u for MSL (c) Wave run u for HAT Fig. 5 Wave run u on gravity ubtructure (a) LAT, (b) MSL, (c) HAT. Table 1 Detail of NREL 5 MW wind turbine model. Turbine arameter Unit Value Rating MW 5.0 Configuration - 3 blade Rotor, hub diameter m 126, 3 Cut-in, rated wind eed m/ 3, 11.4 Cut-in, rated rotor eed rm 6.9, 12.1 Cut-out wind eed m/ 25 Rated ti eed m/ 80 Rotor ma kg 110,000 Nacelle ma kg 240,000 Tower ma kg 347,460 Coordinate location of overall center m of ma (-0.2, 0, 64) In order to evaluate the reonance between the wind turbine and the gravity ubtructure, the modal analyi i carried out. The oil condition are exreed by uing the Caltran/NEHRP (national earthquake hazard reduction rogram) oil rofile a hown in Table 4 [10]. Table 5 how the tiffne and daming matrice on uction bucket-oil foundation ytem uing ubtructure method. The natural frequencie and the mode hae of gravity ubtructure reent in Table 6 and Fig 6, reectively. Since the uction bucket tiffne increae a the hear-wave velocity of oil increae, the natural frequency i exected to increae with the velocity increment (V 30 ). Therefore, the natural frequency of the total ytem i directly deendent on the oil condition of the foundation. The natural frequency of gravity ubtructure ytem obtained from modal analyi i lotted againt Table 4 Caltran/NEHRP oil rofile tye. Site Soil rofile cla name V 30 (m/) Standard enetration tet N-value (bf) Undrained hear trength (kpa) A Hard rock > 1, B Rock C Very dene oil and oft 360 > 50 > 100 rock D Stiff oil E Soft oil < 180 < 15 < 50 F Soil requiring ite ecific evaluation - - -

6 Numerical Analyi of a Gravity Subtructure for 5 MW Offhore Wind Turbine Due to Soil Condition 155 Table 5 Stiffne and daming matrice according to V 30. V m/ 270 m/ 560 m/ Stiffne coefficient (kn, kn m) Daming coefficient (kn, kn m ) K xx, K yy 1.21E+03 C xx, C yy 6.70E+00 K zz 1.12E+08 C zz 4.55E+05 KR xx, KR yy 6.21E+05 CR xx, CR yy 8.62E+02 KR xy, KR yx 1.83E+04 CR xy, CR yx 7.64E+01 K xx, K yy 6.26E+03 C xx, C yy 1.16E+01 K zz 1.12E+08 C zz 1.52E+05 KR xx, KR yy 1.08E+06 CR xx, CR yy 4.98E+02 KR xy, KR yx 5.50E+04 CR xy, CR yx 7.64E+01 K xx, K yy 1.87E+04 C xx, C yy 1.67E+01 K zz 1.12E+08 C zz 7.31E+04 KR xx, KR yy 1.55E+06 CR xx, CR yy 3.46E+02 KR xy, KR yx 1.14E+05 CR xy, CR yx 7.64E+01 K xx, K yy 5.36E+04 C xx, C yy 2.37E+01 K zz 1.12E+08 C zz 3.62E+04 1,130 m/ KR xx, KR yy 2.20E+06 CR xx, CR yy 2.43E+02 KR xy, KR yx 2.30E+05 CR xy, CR yx 7.64E+01 (a) Table 6 Natural frequencie of gravity ubtructure for variou oil condition (Hz). Mode 90 m/ 270 m/ 560 m/ 1,130 m/ Fixed to the ectrum denity of the turbine and the environmental load in Fig 7. It i found that, the natural frequency of gravity ubtructure ytem with more than V 30 = 270 m/ i located between the rotor frequency range (1 P) and the blade aing frequency range (3 P). Therefore, there i no oibility of reonance between the wind turbine and the gravity ubtructure with more than V 30 = 270 m/. 3.4 Structural Reult The load at TP (tranition iece) due to extreme (b) Fig. 6 Mode hae of gravity ubtructure for V 30 = 560 m/. Fig. 7 Comarion of gravity ubtructure on the ectrum denity of wind turbine and wave for variou oil condition. wind are reented in Table 7. In the reent tudy, the extreme value at TP for DLC 6.2a are ued ince they are the larget value comared to the other cae. The gravity ubtructure i comoed of concrete with 0.5 m thickne. The artial afety factor 1.35 i adated for all cae [11].

7 156 Numerical Analyi of a Gravity Subtructure for 5 MW Offhore Wind Turbine Due to Soil Condition Table 7 Extreme value at TP due to extreme wind. DLC F x (kn) F y (kn) F z (kn) M x (kn m) M y (kn m) M z (kn m) 6.1a ,04-21,48 3, a 1, ,32 39,827 82,343-2,610 Fig. 8 Structural dilacement of gravity ubtructure due to water deth condition (unit: m). reectively. Although the dilacement and the von-mie tre increae a the water deth become large, the bending tre decreae. The bending tre i concentrated at the connection art between circular cylinder and concrete cone. There i alo large variation of bending tre. Therefore, thi connection art hould be examined exlicitly for the reliable ubtructure deign. The tructural reult are ummarized at Table 8. It i found that, the ultimate trength of gravity ubtructure ytem atifie ULS deign condition for variou wave condition. Fig how the tructural dilacement, the bending tre and the von-mie tre with water deth HAT for variou oil condition, reectively. Although the dilacement decreae a the hear wave velocity of oil increae, the other reult have very imilar value. It mean that, the dilacement i trongly influenced by oil condition but the tructural Table 8 Structural reult due to water deth condition. LAT MSL HAT Fig. 9 Structural bending tre of gravity ubtructure due to water deth condition (unit: Pa). Dilacement (m) Bending tre (MPa) Von-Mie tre (MPa) Normal tre (x axi, MPa) Normal tre (y axi, MPa) Normal tre (z axi, MPa) Max Min Max Min 1.4E E E-16 Max Min 5.5E E E-14 Max Min Max Min Max Min Fig. 10 Structural von-mie tre of gravity ubtructure due to water deth condition (unit: Pa). Uing the wave force from ANSYS AQWA and the wind load at TP, the tructural analyi of gravity ubtructure i carried out through ANSYS mechanical. For variou water deth the tructural dilacement, the bending tre and the von-mie tre with V 30 = 560 m/ are lotted at Fig. 8-10, Fig. 11 Structural dilacement of gravity ubtructure due to oil condition (unit: m).

8 Numerical Analyi of a Gravity Subtructure for 5 MW Offhore Wind Turbine Due to Soil Condition Concluion Fig. 12 Structural bending tre of gravity ubtructure due to oil condition (unit: Pa). Fig. 13 Structural von-mie tre of gravity ubtructure due to oil condition (unit: Pa). Table 9 Structural reult due to oil condition. Dilacement (m) Bending tre (MPa) Von-Mie tre (MPa) Normal tre (x axi, MPa) Normal tre (y axi, MPa) Normal tre (z axi, MPa) 270 m/ 560 m/ 1,130 m/ Fixed Max Min Max Min 1.E-16 1.E-16 9.E Max Min 1.E-13 5.E-14 2.E Max Min Max Min Max Min tre i lightly influenced by oil condition. The maximum bending tre and the von-mie occur at the connection art between TP and circular cylinder. The tructural reult are ummarized at Table 9. For variou oil condition, the ultimate trength of gravity ubtructure ytem alo atifie ULS deign condition. The gravity ubtructure with uction bucket foundation ytem for 5 MW offhore wind turbine i uggeted to examine oil-tructure interaction. Uing the wave force and the tiffne and daming matrice obtained from the ubtructure method, the tructural analyi of the gravity ubtructure i carried out through ANSYS mechanical for variou oil condition. The reult obtained from the reent tudy are ummarized a follow: (1) Since the wave force i cloely related to the wetted urface of ubtructure and the water article velocity near free urface i larget, the total wave force on gravity ubtructure gradually decreae a the water deth increae. Moreover, although the tructural dilacement i trongly influenced by oil condition, the tructural tre i lightly influenced. (2) From the tructural analyi of gravity ubtructure, it i found that, the gravity ubtructure ytem atify tructural afety in reect of ULS. Alo, the reonance between the wind turbine and the gravity ubtructure i not occurred. (3) Conequently, the uggeted gravity ubtructure with uction bucket foundation ytem can be an effective ubtructure for reducing hydrodynamic effect and contruction cot in the outhern-wetern ea of the Korea eninular. (4) In the reent tudy, the tatic tructural analyi i only carried out. So the linearized initial tiffne i ued for the analyi. However, next te the dynamic tructural analyi will be erformed with the nonlinear foundation tiffne. Acknowledgment Thi reearch wa uorted by Korea Intitute of Marine Science and Technology Promotion through the reearch roject Safety evaluation of concrete ubtructure ytem for offhore wind ower (D H ) and Korea Intitute of Energy Technology Evaluation and Planning through the reearch roject Develoment of hybrid

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