Dynami Responses of Floating Platform for Spar-type Offshore Wind Turbine: Numerial and Experimental *Jin-Rae Cho 1), Yang-Uk Cho ), Weui-Bong Jeong 3) Sin-Pyo Hong 3) and Ho-Hwan Chun 3) 1), ), 3) Shool of Mehanial Engineering, Pusan National University, Busan 609-735, Korea 1) jrho@pusan.a.kr & jrho@midasit.om ABSTRACT The station keeping and rotational stability are important for spar-type floating offshore wind turbine subjet to irregular wind and wave exitations. In this ontext, this paper addresses the numerial investigation of dynami response of a spar-type hollow ylindrial floating substruture moored by three mooring lines under irregular wave exitation. The wave-floating substruture and wave-mooring able interations are simulated by the oupled BEM-FEM methods in the staggered iterative manner. Through the numerial experiments, the frequeny responses of a rigid hollow spartype floating platform and the mooring able tension are investigated with respet to the total length and onnetion position of mooring ables. In order to verify the numerial results, a small-sale prototype (sale: 1/75) of ylindrial floating substruture is experimented in a wave aquati pool. The omparison of dynami responses of the floating substruture to one-diretional harmoni wave is also presented. 1. INTRODUCTION In order to seure the dynami stability of floating-type offshore wind turbine, the station keeping and the rotational vibration ontrol at sea beome ritial (Tong, 1998). Beause of this fundamental requirement, the design of floating-type offshore wind turbine requires extra tehnologies for floating platform, mooring lines or tension legs, anhors and antiorrosion when ompared to fixed-type wind turbine. Floating offshore wind turbines are lassified aording to how to keep the station position and to ontrol the vertial attitude, suh as submerged-, TLP (tension-leg platform)- and spar-types (Lee, 008; Jonkman, 009). However, all the types have some things in ommon from the fat that the station keeping and the vertial attitude are seured by a ombination of the buoyany fore, the mooring line tension and additional ontrol devie (Colwell and Basu, 009; Mostafa et al., 01). 1) Vie diretor of Midas IT. 3587
In ase of spar-type floating wind turbine, the buoyany fore produed by a long hollow ylindrial platform supports the whole offshore wind turbine and the tension of mooring lines keeps the station position of spar-type floating substruture. The rotational stability is haraterized by the pith and roll motions of floating substruture, and it is to a large extent obtained by the pith stiffness of spar-type floating substruture whih inreases in proportional to the metaetri height (Karimirad et al., 011). In addition, it is also influened by both the tension magnitude and the onnetion position of mooring lines. The dynami stability of floating offshore wind turbines subjet to wind and wave exitations has been studied by experimentally using sale models (Nielsen et al., 006; Goopee et al., 01; Mostafa et at., 01), by analytially/numerially with the simplified wind turbine geometry and the analytially derived wind/wave loads (Tray, 001; Karimirad, 010; Jensen et al., 011), or by the ombined use of CFD, hydro, FSI (fluid-struture interation) or/and MBD (multibody dynamis) odes (Zambrano et al., 006; Jonkman and Musial, 010; Wang and Sweetman, 01). Even though the researhes on floating offshore wind turbine have been atively onduted, the detailed useful results for the design of spar-type floating platform have not been published yet. In this ontext, the urrent study intends to numerially and experimentally investigate the dynami response of floating platform and the mooring tension using a 1/75 sale model of.5mw spar-type floating wind turbine. Three rotor blades, hub and naelle are exluded, and three mooring lines are pre-tensioned by means of linear springs. The wave-rigid body interation simulation of the mooring sale model is performed by the oupled BEM-FEM method. As well, the sale model is manufatured and experimented in wave tank by means of the speially designed sensor and data aquisition system. The response amplitude operators (RAO) of the sale model and the mooring tensions are obtained and ompared.. SPAR PLATFORM FOR FLOATING OFFSHORE WIND TURBINE The most important requirement of renewable energies is the effiieny and apaity, and in this regard wind power draws an intensive attention thanks to its potential to generate a huge amount of eletriity from plenty of winds around us (Hansen and Hansen, 007). Wind turbines in the early stage were designed for the installation on the ground and showed the rapid inrease in both the total installation number and the maximum power generation apaity. However, this worldwide trend enountered several obstales suh as the infringement of living environment and the limitation of being high-apaity and making large wind farm. This ritial situation naturally turned the attention to the offshore sites, a less restritive installation plae. Offshore wind turbines are lassified largely into two ategories, fixed- and floatingtype aording to how the wind turbine tower is supported. Differing from the fixed-type, the floating-type wind turbine is under the onept design stage beause several ore tehnologies are not fully settled down (Karimirad et al., 011). In partiular, the design of floating substruture beomes a ritial subjet beause it supports the entire wind turbine system and influenes the dynami stability. Currently, three types of floating substrutures are onsidered, barge, tension leg (TLP) and spar types. 3588
(a) (b) Fig. 1 (a) Spar-type floating offshore wind turbine, (b) 6-DOF rigid body motions. A typial spar-type floating offshore wind turbine is represented in Fig. 1, where the entire wind turbine is supported by the buoyany fore and the vertial position is adjusted by the weight at the bottom of platform. The dynami displaement of wind turbine whih is aused by wind, wave and urrent loads is restrited by the tension of mooring lines (Lefebvre and Collu, 01). The dynami stability of floating-type wind turbine is evaluated in terms of three translation (i.e., surge, sway and heave) and three rotation motions (i.e., pith, roll and yaw). These six degrees of freedom are oupled to eah other, and the pith angle is the most signifiant parameter to evaluate the dynami stability of wind turbine. The rigid body translational motions are haraterized by the stiffness and fairlead angle of mooring ables as well as the total mass of floating platform. The stiffness and fairlead angle are in turn influened by the speifi weight and total length of mooring ables. Meanwhile, the rigid body rotational motions are influened by the stiffness itself of floating platform and the fair lead position Z FL and angle. The stiffness of floating platform to the pith and roll motions is known to be proportional to the relative vertial distane Z CB Z CG between the enters of buoyany and gravity (Karimirad et al., 011) as well as the mass moments of platform. 3. RIGID BODY-FLUID INTERACTION 3 Referring to Fig. 3(a), let us be a semi-infinite unbounded flow domain with F the boundary F SF SB I and denote V be a ontinuous triple-vetor water veloity field, where S F, S and B I indiate the free surfae, seabed and flow-struture interfae respetively. Water is assumed to be invisid and inompressible and water flow is irrotational so that there exists a veloity potential funtion x;t satisfying x ;t :V. Then, the flow field is governed by the ontinuity equation 0, in F 0,tˆ (1) 3589
and the boundary onditions given by u n, on I () n g 0 z t, on S F (3) 0, z on S B (4) with tˆ being the time period of observation, g the gravity aeleration, n the outward unit vetor normal to the struture boundary. In addition, the potential funtion satisfies the radiation ondition: 0 as r at the far field. 3 The substruture oupying the material domain S with the boundary S D N I is assumed to be a rigid body. By denoting d, be its rigid body translation and rotation at the enter of mass, the dynami motion of the substruture is governed by the onservation of linear and angular momentums and the initial onditions given by md d kd F, in S 0,tˆ (5) I k M, in S 0,tˆ (6) d d,, d, d, (7), t 0 0 t 0 0 0 In whih m,,k, and k denote the total mass and the damping and stiffness oeffiients for the translational and rotational degrees of freedom, respetively. Meanwhile, I indiates the matrix of mass moments of inertia with respet to the enter of mass, and F and M are the external fore and moment vetors whih are alulated by F pn ds, M pr n ds (8) I Here, p and r are the hydrodynami pressure and the position vetor from the enter of mass, respetively. Meanwhile, atenary mooring able of length L is a slender flexible struture subjet to hydrodynami pressure, self weight, inertia fore and drag fore. Referring to Fig. 3(b), the nonlinear differential equations of motion (Goodman and Breslin, 1976; Aamo and Fossen, 000) for the differential able element d are governed by the equilibrium equations in translation and rotation, u T m ma 1 F t s I (9) M s r t 1 T (10) 3590
with the boundary onditions given by B u 0, at s 0 at seabed (11) P u d, at s L at onneting point (1) P In whih, m indiates the mass per unit ar length, m a the added mass of water, u the veloity vetor, s the ar length of unstressed able, and the engineering strain. In addition, r t is the vetor tangent to the able enter line, M the resultant internal moment, and F the external loading per unit ar length due to the self weight g, and F n, F and F q the normal, tangential and binormal drag fores (Morison et al., 1950). (a) (b) Fig. (a) A rigid floating body moored by atenary ables in irregular wave (b) fores ating on the able element. The potential flow is interpolated by the boundary element method while the dynami motions of the rigid floating substruture and mooring ables are approximated by the finite element method. The Euler-Lagrange oupling method is employed to deal with the interation between the rigid body struture motion and the water flow, and it with the lapse of time is numerially implemented in a staggered iterative manner (Sigrist and Abouri, 006; Cho et. al., 008). 4. NUMERICAL EXPERIMENTS A simplified 75:1 sale model of.5mw spar-type floating offshore wind turbine with three equal atenary mooring ables is taken for investigating the dynami response of the ylindrial floating platform and mooring ables. The geometry dimensions, masses and moments of inertia of the major omponents are given in Fig. 3(a) and Table 1. Three rotor blades, hub and naelle assembly are simplified as a lumped mass for both the numerial simulation and the wave tank experiment. The enter of buoyany (CB) and the enter of mass (CM) are measured from the bottom of platform and the relative vertial distane between two enters is set by 10.0m. 3591
Fig. 3 A simplified spar-type platform moored by three nylon ables (unit: m ) The width and depth of the water pool are set by 8 100m and the height from seabed is set by 35 m, respetively. The total length of eah mooring line and the o relative angles between two adjaent mooring lines are set by 4.343m and 10 respetively, and three mooring lines are equally pre-tensioned by 0.49kgf. The ross setional area A, the mass per unit length, the stiffness EA and the maximum max 9 7 allowable tension T are 0. 0m, 90 kg / m, 1. 0 10 N and 1. 0 10 N respetively, and the hydrodynami drag oeffiient C D is set by 0.05. Three mooring lines are aligned suh that mooring line 1 opposites to the inoming diretion of one-dimensional regular harmoni wave as represented in Fig. 3(a), and the amplitude and frequeny of regular harmoni wave are set by 1.0m and 0.01~ 0.5Hz, respetively. Table 1. Major speifiations of sale platform model. Components Items Values Diameter (upper & lower) 0.076m, 0.150m Platform Thikness (upper & lower).9m, 3.m Moments of inertia (pith, roll 9. 40kg m, 9. 37kg m & yaw) 0. 065kg m Figs. 4 and 5 represented the simplified sale platform model and the wave tank with the dimensions of 100 m in length, 8 m in width, and 3.5m in depth. On the right side of the wave tank, three ameras for the vision system and a data aquisition system are plaed. The sale platform model is moored by three nylon mooring lines, and tension sensors and springs are onneted between mooring lines and fairleads. A square plate is installed on the top of platform, where infrared light emit diodes (LEDs) are attahed to measure the dynami motion of platform. The light signals of LEDs are deteted by a trinoular vision system, and the six rigid body motions of the floating platform are monitored. In addition, a small-size attitude and heading referene system (AHRS) manufatured by Miro-Eletro-Mehanial-Systems (MEMS) Tehnology is attahed on the top of platform to measure the aeleration, angular veloity, and attitude of the floating platform. The sensor sends the motion signals to the omputer through USB (universal serial bus) port in RS-3 protool. 359
(a) (b) Fig. 4. Experimental setup for obtaining the dynami response of the moored sale model: (a) sale model (b) one-diretional wave tank. The response operators (RAOs) of surge, heave and pith motions obtained by simulation and experiment are ompared in Fig. 5. It is observed that the resonane frequenies predited by simulation are in exellent agreement with the experiment. Furthermore, it is verified that the pith amplitudes between simulation and experiment are in good agreement over all frequeny range. But, the amplitudes in surge and heave motions whih are predited by simulation show the disrepany. It is beause the damping effet is not appropriately refleted into the numerial simulation. 14 1 10 Naelle Surge Aeleration RAO Experiment AQWA 10 8 Naell Heave Aeleration RAO Experiment AQWA RAO(m/s /m) 8 6 RAO(m/s /m) 6 4 4 0 1 3 4 5 6 7 Frequeny (rad/s) (a) 100 1000 Pith RAO 0 1 3 4 5 6 7 Frequeny (rad/s) Experiment AQWA (b) RAO(deg/m) 800 600 400 00 0 1 3 4 5 6 7 Frequeny (rad/s) () Fig. 5. Response amplitude operators (RAOs): (a) surge, (b) heave, () pith. 3593
CONCLUSION Dynami response of spar-type floating platform has been investigated by numerial and experimental methods using a 1/75 sale model by exluding the detailed upper part. The numerial simulation of the rigid body-fluid interation was arried out by a staggered iterative BEM-FEM method, while the experiment was performed in a wave tank using a speially designed vision and data aquisition system. Through the omparison of the response amplitude operators (RAO) of surge, heave and pith motion to regular harmoni wave, it has been verified that the numerial results are in a good agreement with the experiment. ACKNOWLEDGEMENT This work was supported by the Human Resoures Development of the Korea Institute of Energy Tehnology Evaluation and Planning (KETEP) grant funded by the Korea Ministry of Knowledge Eonomy (No. 011403000070). REFERENCES Aamo, O.M., Fossen, T.I. (000) Finite element modeling of mooring lines, Mathematis of Computers in Simulation 53(4-6):415-4. Cho, J.R., Park, S.W., Kim, H.S., Rashed, S. (008) Hydroelasti analysis of insulation ontainment of LNG arrier by global-loal approah, International Journal for Numerial Methods in Engineering 76:749-774. Colwell, S., Basu, B. (009) Tuned liquid olumn dampers in offshore wind turbines for strutural ontrol, Engineering Strutures 31:358-368. Goodman, T.R., Breslin, J.P. (1976) Statis and dynamis of anhoring ables in waves, Journal of Hydronaut 10(4):113-10. Goopee, A..J., Koo, B..J., Lambrakos, K.F., Kimball, R.W. (01) Model tests for three floating wind turbine onepts, Proeedings of Offshore Tehnology Conferene, Houston, USA. Hansen, A.D., Hansen, L.H. (007) Wind turbine onept market penetration over 10 years (1995-004), Wind Energy 10:81-97. Jensen, J., Olsen, A., Mansour, A. (011) Extreme wave and wind response preditions, Oean Engineering 38:44-53. Jonkman, J. (009) Dynamis of offshore floating wind turbines-model development and verifiation, Wind Energy 1:459-49. Jonkman, J., Musial, W. (010) Offshore ode omparison ollaboration (OC3) for IEA task 3 offshore wind tehnology and development, Tehnial Report NREL/TP- 5000-48191, Colorado. Karimirad, M. (010) Dynami response of floating wind turbines, Transation B: Mehanial Engineering 17():146-156. Karimirad, M., Meissonnier, Q., Gao, Z., Moan, T. (011) Hydroelasti ode-to-ode omparison for a tension leg spar-type floating wind turbine, Marine Strutures 4:41-435. 3594
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