Proceedings of the ASME nd International Conference on Ocean, Offshore and Arctic Engineering OMAE2013 June 9-14, 2013, Nantes, France

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1 Proceedings of the ASME nd International Conference on Ocean, Offshore and Arctic Engineering OMAE2013 June 9-14, 2013, Nantes, France OMAE MOTION OF OC4 5MW SEMI-SUBMERSIBLE OFFSHORE WIND TURBINE IN IRREGULAR WAVES Hyunkyoung Shin Ulsan, Korea Pham Thanh Dam Ulsan, Korea Byungcheol Kim Ulsan, korea Kwangjin Jung Ulsan, Korea ABSTRACT The interests in new and renewable energies increase sharply while our world suffers from environmental pollution and energy shortage. Governments and organizations throughout the world have tried to develop those energies to reduce pollution and solve energy crisis. In this study, we carry out a 1:80 scale model test and full scale numerical analysis of the OC4 5MW semi-submersible offshore wind turbine system designed by DeepCwind project. The purpose of this model test and numerical analysis is to predict and evaluate its motion in irregular waves. INTRODUCTION We have relied on fossil fuels such as oil, natural gas, and coal as a main source of energy. However, those fossil fuels have raised environmental problems and increased in price. To solve those problems, governments and organizations throughout the world have tried to find alternative energies which will not raise environmental problems. Of those energies, wind energy which has been developed since a long time ago shows high efficiency at lower cost. However, it is hard for wind turbines to be set and run on land anymore because of noise pollution and insufficient space to install wind turbines on land. So now we are turning to Floating Offshore Wind Turbine (FOWT) which can be installed and operated in deep sea. A few floating wind turbine model tests have been performed with coupled wind and wave environments. Hydro Oil & Energy has a linear scale 1/47th of a 5 MW spar-buoy floating wind turbine at Marintek s Ocean Basin Laboratory in Trondheim, Norway [1] with Froude scale was applied for hydrodynamic load and also aerodynamic load. Principal Power Inc. tested a 1/67th scale semi-submersible platform, windfloat [2], in this model test they used a disk instead of three blades to obtained aerodynamic thrust force. WindSea of Norway was performed at Force Technology on a 1/64th scale tri-wind turbine semi-submersible platform [3] which perform in wind tunnel by Reynolds scale and in basin by Froude scale. Another model tests were carried out at the (UOU) on a 1/128th scale OC3-Hywind [4], TLP with spring [5]. Especially, IEA Task30 OC4 Project, which NREL (National Renewable Energy Laboratory) has been responsible for, is currently underway [6]. The FOWT model is composed of both a semi-submersible type platform designed by DeepCwind project in the United States and NREL 5MW baseline wind turbine used in OC3 project [7]. These model tests have been carried with different techniques. In this study, to predict and evaluate the motions of OC4 semi-submersible offshore wind turbines system in irregular waves in deep sea, we performed a 1:80 scale model test of the OC4 semi-submersible offshore wind turbine system and its results are compared with the numerical simulation results. The numerical analysis tool is FAST (Fatigue, Aerodynamics, Structures, and Turbulence) with UOU In-house codes for hydrodynamic coefficients and mooring line forces [8]. Model test The model test to predict and evaluate the motion performance of OC4 Semi-submersible offshore wind turbine system was carried out in the Ocean Engineering Wide Tank(30m x 20m x 2.5m) in the, Korea. The scale model is shown in Fig. 1. Properties of the model are shown in Table 1 and its mooring system properties are shown in Table 2. Center of mass and radius of inertia were checked 1 Copyright 2013 by ASME

2 by both a modeling program, CATIA, and KG test. Differences between target from prototype and measured data from the model are shown in Table 3. Tension-excursion curve is used to check mooring line system. Measured data shows good agreement with target which is data obtained from UOU s mooring code in Fig. 2. Fig. 1 OC4 Semi-submersible type FOWT Model(1:80) Table 1 OC4 Semi-submersible offshore wind turbine system properties Item Full scale Model Scale ratio 1:1 1:80 Water Depth 200 m 2.5 m Turbine Power 5 MW - Mass 110,000 kg kg Diameter 126 m m Hub Mass 56,780 kg kg Blade Mass(1EA) 17,740 kg kg Nacelle Mass 240,000 kg kg Tower Height 77.6 m m Tower Mass 249,718 kg kg Tower Top Diameter 3.87 m m Tower Base Diameter 6.5 m m Platform Height 32 m 0.4 m Platform Mass 13,473,000 kg kg Upper Column Diameter 12 m 0.15 m Upper Column Height 26 m m Base Column Diameter 24 m 0.3 m Base Column Height 6 m m Pontoons Diameter 1.6 m 0.02 m Main Column Diameter 6.5 m m Columns Offset 50 m m Draft 20 m 0.25 m Table 2 Mooring system properties Item Full scale Model Scale ratio 1:1 1:80 Number of Mooring Lines 3EA 3EA Angle Between Adjacent Lines 120m 120m Depth to Anchors Below SWL 200m 2.5m Depth to Fairleads Below SWL 186m 2.325m Radius to Anchors 837.6m 10.47m Radius to Fairleads m 0.511m Unstretched Mooring Line Length 835.5m m Mooring Line Diameter m 0.001m Line Mass Density in Air kg/m 17.7 g/m Line Weight in Water kg/m 17.0 g/m Mooring Line Extensional Stiffness 753.6E3 kn N Fairlead Tension 1,099 kn 2.146N Degree of Mooring at Fairlead Table 3 Difference between target and model (1:80) Item Full scale Target Model Difference (%) Platform Mass (kg) E Center of Gravity (m) Roll Inertia about COG (kg*m 2 ) Pitch Inertia about COG (kg*m 2 ) Yaw Inertia about COG (kg*m 2 ) 6.788E E E Fig. 2 Tension-excursion comparison 2 Copyright 2013 by ASME

3 Two load cases are considered for this study. One is that FOWT with a locked rotor is under only irregular wave. The other is that FOWT with a rotating rotor is under irregular waves and uniform wind. Wind speed of model test is 2.4 m/s which is determined by fixed mount model test based on matching thrust force at the tower top. Hydrostatic coefficients and quadratic additional viscous damping from the OC4 project are shown in Table 4. UOU's mooring quasi-static code uses 4th order Runge Kutta method to solve quasi-static equations of two dimensions of a mooring line. NUMERICAL ANALYSIS FAST(Fatigue, Aerodynamics, Structures, and Turbulence) code developed by NREL and UOU in-house codes developed by the for hydrodynamic coefficients and mooring line forces are used for numerical analysis of OC4 semi-submersible offshore wind turbine in full scale. UOU Inhouse code including radiation solver and diffraction solver is used to calculate added mass, radiation damping and wave exciting forces which are input data for FAST. Fig. 5 Added Mass A15, A24 and Radiation Damping B15, B24 Fig. 3 Added Mass A11, A22, A33, A44, A55, A66 Fig. 6 Wave Excitation per unit amplitude (Modes 1~6) Table 4 Hydrostatic & Quadratic Additional Quadratic Damping Coefficients Hydrostatic restoring in heave Fig. 4 Radiation Damping B11, B22, B33, B44, B55, B66 Added mass matrix in frequency domain is shown in Figs. 3~5. Radiation damping coefficients in Figs. 4 and 5, and wave exciting force coefficients in Fig. 6. Where, modes 1~6 mean surge, sway, heave, roll, pitch and yaw modes in order E+06 N/m Hydrostatic restoring in roll E+08 N-m/rad Hydrostatic restoring in pitch E+08 N-m/rad Additional quadratic damping in surge 3.95E+05 Ns2/m2 Additional quadratic damping in sway 3.95E+05 Ns2/m2 Additional quadratic damping in heave 3.88E+06 Ns2/m2 Additional quadratic damping in roll 3.70E+10 Nms2/rad2 Additional quadratic damping in pitch 3.70E+10 Nms2/rad2 Additional quadratic in yaw 4.08E+09 Nms2/rad2 3 Copyright 2013 by ASME

4 LOAD CASES The load cases are divided into two big categories, LC1 and LC2. Both are shown in Table 5 and 6, respectively. Table 5 LC1, Only Irregular Waves Run (Sea state) Full scale Model Wind Wind Tp(s) Hs(m) Tp(s) Hs(m) 1(4) (5) (6) (7) FFT(Fast Fourier Transform) is used to check irregular waves. The basin generated irregular wave spectrum has a margin of error of 5% as compared with the theoretical JONSWAP wave spectrum in Figs. 7 and 8. Waves are recorded during model test by installing wave probe 1m apart from model installation point. Fig. 7 JONSWAP wave spectrum in sea state 4(Left) and sea state 5(Right) Table 6 LC2, Irregular Waves, Wind and Rotating Run (Sea state) Wind Full scale 11.4m/s 12.1rpm Wind Model 2.4m/s 108.2rpm Tp(s) Hs(m) Tp(s) Hs(m) 1(4) (5) (6) (7) Irregular waves for both model test and numerical analysis are obtained from JONSWAP wave spectrum. JONSWAP wave spectrum is hereunder: Fig. 8 JONSWAP wave spectrum in sea state 6(Left) and sea state 7(Right) RESULTS To predict and evaluate motions of OC4 semi-submersible offshore wind turbine system, we carried out model test and numerical analysis. Its motion in certain sea states is expressed in terms of a significant height as a representative value which is defined by the average of the 1/3 highest, that is, four times the square root of the zeroth-order of the response spectrum. To obtain a significant height from measured data, we use motion spectrum from FFT (Fast Fourier Transform). And results from model test are enlarged to full scale to compare with ones from numerical analysis. The units we used for comparison are meter(m) for translational motions such as surge, sway and heave and degree( ) for rotational motions such as roll, pitch and yaw motions. And we neglect sway and roll motions because waves we used for this model test and numerical analysis are only heading sea. Significant heights in LC1(Only regular waves) are shown in Figs. 9 and 10. There is a good agreement between model test results and numerical predictions of motion in sea states of irregular waves, except sea state 7. The motion increments 4 Copyright 2013 by ASME

5 from numerical analysis are bigger than ones from model test. An underestimation of viscous damping would be one possible explanation. We need to talk about the adjustment of additional damping coefficients from the OC4 project. Fig. 12 Significant Height of Pitch(Left) and Yaw(Right) in LC2 Fig. 9 Significant Height of Surge (left) and Heave (Right) in LC1 Significant heights in LC2 (Irregular waves with uniform wind and rotating rotor) are shown in Figs. 11 and 12. In numerical analysis, both transitional motions and rotational motions in LC2 show little differences with those in LC1, that is, the wind has a small influence on motions in numerical prediction. However, in model test, transitional motions in LC2 show little differences with those in LC1 but not in rotational motions. Based on model test, pitch motion in LC2 is higher than one in LC1. Yaw motion in model test is quite big because of a rotating rotor but this phenomenon does not occur in numerical analysis. It may be due to gyroscopic moment induced by the rotating rotor. Fig. 10 Significant Height of Pitch (Left) and Yaw (Right) in LC1 CONCLUSION The motion performance of the OC4 semi-submersible offshore wind turbine in irregular waves are predicted and evaluated by both model test and numerical simulation. From the numerical simulation, the responses in both LC1 and LC2 in sea states 4 ~7 are approximately equal. From the model test, the transitional responses in both LC1 and LC2 are approximately equal but there are differences in rotational responses. In the numerical simulation, the aerodynamic force due to wind having the rated wind speed(11.4 m/s) has a small influence on the motion of OC4 semi-submersible offshore wind turbine compared with the hydrodynamic force due to irregular waves. In model test, the rotating rotor may create a gyroscopic moment which induces yaw motion in LC2. However, the aerodynamic force has a small effect on the transitional motion of OC4 semi-submersible offshore wind turbine. The UOU-FAST code shows decent results for these load cases of LC1 and LC2 in predicting motions of semisubmersible type FOWT. Fig. 11 Significant Height of Surge (Left) and Heave(Right) in LC2 5 Copyright 2013 by ASME

6 Further model tests and numerical analyses in various load cases will be carried out to figure out parameters which affect motion of FOWT. ACKNOWLEDGEMENTS 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 Economy. (No and No ) REFERENCES [1] Skaare B., Hanson T.D., Nielsen F.G., Yttervik R., Hansen A.M., Thomsen K., Larsen T.J., 2007, Integrated dynamic analysis of floating offshore wind turbines, European Wind Energy Conference, Milan, Italy. [2] Roddier D, Cermelli C, Aubault A, Weinstin A, 2010, WindFloat: A floating foundation for offshore wind turbines, Journal of Renewable and Sustainable Energy [3] Windsea AS, 2013, Windsea Concept, [4] Kim K.M.,2011, Experimental and Numerical Study on Analysis of Motion of Floating Offshore Wind Turbine. Master s thesis, The graduate School of the University of Ulsan, School of Naval Architecture and Ocean Engineering. [5] Shin H., 2010, Model test of the OC3-Hywind floating offshore wind turbine. Proc. of 21st ISOPE, Maui, Hawaii, Vol.1, pp.361~366. [6] Robertson A., Jonkman J., Masciola M, Song H., Goupee A., Coulling A., and Luan C. 2012, Definition of the Semisubmersible Floating System for Phase II of OC4. [7] Jonkman J., Butterfield S., Musial W., and Scott G., 2009, Definition of a 5-MW Reference Wind Turbine for Offshore System Development, Technical Report NREL/TP [8] Jonkman J.M., Buhl Jr. M.L., 2005, FAST User s Guide, Technical Report NREL/EL Copyright 2013 by ASME