Model Tests for a Floating Wind Turbine on Three Different Floaters

Transcription

1 Bonjun J. Koo 1 Technip USA, Inc., Katy Freeway, Suite 150, Houston, TX bkoo@technip.com Andrew J. Goupee Advanced Structures and Composites Center, University of Maine, 35 Flagstaff Road, Orono, ME Richard W. Kimball Maine Maritime Academy, 54 Pleasant Street, Castine, ME Kostas F. Lambrakos Technip USA, Inc., Katy Freeway, Suite 150, Houston, TX Model Tests for a Floating Wind Turbine on Three Different Floaters Wind energy is a promising alternate energy resource. However, the on-land wind farms are limited by space, noise, and visual pollution and, therefore, many countries build wind farms near the shore. Until now, most offshore wind farms have been built in relatively shallow water (less than 30 m) with fixed tower type wind turbines. Recently, several countries have planned to move wind farms to deep water offshore locations to find stronger and steadier wind fields as compared to near shore locations. For the wind farms in deeper water, floating platforms have been proposed to support the wind turbine. The model tests described in this paper were performed at MARIN (maritime research institute netherlands) with a model setup corresponding to a 1:50 Froude scaling. The wind turbine was a scaled model of the national renewable energy lab (NREL) 5 MW horizontal axis reference wind turbine supported by three different generic floating platforms: a spar, a semisubmersible, and a tension-leg platform (TLP). The wave environment used in the tests is representative of the offshore in the state of Maine. In order to capture coupling between the floating platform and the wind turbine, the 1st bending mode of the turbine tower was also modeled. The main purpose of the model tests was to generate data on coupled motions and loads between the three floating platforms and the same wind turbine for the operational, design, and survival seas states. The data are to be used for the calibration and improvement of the existing design analysis and performance numerical codes. An additional objective of the model tests was to establish the advantages and disadvantages among the three floating platform concepts on the basis of the test data. The paper gives details of the scaled model wind turbine and floating platforms, the setup configurations, and the instrumentation to measure motions, accelerations, and loads along with the wind turbine rpm, torque, and thrust for the three floating wind turbines. The data and data analysis results are discussed in the work of Goupee et al. (2012, Experimental Comparison of Three Floating Wind Turbine Concepts, OMAE ). [DOI: / ] Introduction Wind turbines can produce large quantities of clean electricity. Presently, plans exist to move wind farms to deep water offshore locations to take advantage of stronger and steadier winds. In the United States, the majority of the offshore wind resources are located between km offshore in water depths up to about 60 m [1]. The bottom-fixed wind turbine option presents challenges for deeper water depths and different floating platform concepts have been proposed to support the wind turbine. However, the coupled dynamics between the floating platform base and the wind turbine are not very well understood and theoretical developments and validation data are needed to advance this technology. The floating turbine response is rather complex and reflects the wind turbine aerodynamics, tower elasticity, wind turbine controls, incident waves, floating platform dynamic properties, and the mooring dynamics of the floater. Several coupled aeroelastic-hydrodynamic codes have been developed [2] for the analysis and design of floating wind turbines. However, these numerical tools need additional validation with model test data. Model tests for floating wind turbines have been performed [3,4]. However, the model tests thus far have not 1 Corresponding author. Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received January 14, 2013; final manuscript received May 2, 2013; published online March 24, Assoc. Editor: Krish Thiagarajan. been as comprehensive as the present tests in terms of the floating platform [3] or the turbine model [4]. The present model tests add to the technical knowledge base on offshore floating wind turbines in terms of the model test design, wind modeling, coupled response data for the floater and wind turbine, and the potential calibration of analysis tools. The paper describes the three floating turbine models, the instrumentation used and gives the wave and wind environments and related calibration results, and the results from the system identification tests. Table 1 Principal dimensions and mass properties Item Unit Designations Power MW 5.0 Blades mass kg 17,740 Blade length m 61.5 Hub mass kg 56,780 Nacelle mass kg 240,000 Tower top mass (hub, blades, and nacelle) kg 350,000 Hub radius m 1.5 Rotor diameter m Tower mass kg 249,718 Tower height m 77.6 Tower CG (% from tower base) % 43.0 Tower top diameter m 3.78 Tower base diameter m 6.5 Journal of Offshore Mechanics and Arctic Engineering MAY 2014, Vol. 136 / Copyright VC 2014 by ASME

2 Fig. 1 Wind turbine model Model Descriptions The model tests were carried out at MARIN (Maritime Research Institute, The Netherlands) with a model setup corresponding to a 1:50 Froude scaling. At this scale, the 200 m prototype water depth was fully modeled. The wind turbine was a scaled model of the National Renewable Energy Lab (NREL) 5 MW horizontal axis reference wind turbine. The principal dimensions are listed in Table 1. The model wind turbine was designed to simulate a prototype wind turbine and was also equipped with instrumentation in order to collect the appropriate data. A motor was made part of the wind turbine to allow control of the blade rpm. In the tests, the blade pitch angle and rotor rpm were set to prescribed values based on the wind velocity being tested. The torque induced by the difference between the rotor rpm and the prescribed rpm of the motor was measured and converted into power generation. The detailed description for the tested wind turbine can be found in Ref. [5]. In order to capture the coupling between the wind turbine and the floating platform, the fundamental tower bending mode was also modeled. The blades were constructed at the correct weight and were manufactured from carbon fiber. Figure 1 shows the wind turbine model. Three typical offshore floating structures are selected to support the wind turbine. The first concept is a spar-buoy, which is stabilized by the separation between the center of buoyancy and the center of gravity. The second concept is a semisubmersible which is stabilized by the water plane stiffness from the column Fig. 2 Selected platforms Fig. 3 Principal dimensions of the spar-buoy / Vol. 136, MAY 2014 Transactions of the ASME

3 Table 2 Mass properties Item TLP Spar-buoy Semisubmersible Draft (m) Mass (ton) ,444 Displacement (ton) ,265 KG (m) Roll gyration: k xx (m) Pitch gyration: k yy (m) Yaw gyration: k zz (m) a Note: with wind turbine and moorings. a Radius of gyration in air without wind turbine and moorings. Fig. 6 Schematic of the delta connection Table 3 Taut mooring system properties (spar-buoy) Item Unit Designations Fig. 4 Principal dimensions of the TLP (pontoon length m) Anchor radius m Anchor depth m Radius of fairlead m 5.2 Fairlead depth m 70.0 Unstretched line length A m Unstretched line lengths B and C m 30.0 Line A diameter m Lines B and C diameter m Mass per length line A (dry) kg/m 22.5 Mass per length lines B and C (dry) kg/m 12.6 Mass per length lines A, B, and C (wet) kg/m 0.0 Axial stiffness line A (EA) MN Axial stiffness lines B and C (EA) MN 68.0 Table 4 Catenary mooring system properties (semisubmersible) Item Unit Designations Anchor radius m Anchor depth m Radius of fairlead m 40.9 Fairlead depth m 14 Unstretched line length m Line diameter m Mass per length (dry) kg/m Mass per length (wet) kg/m Axial stiffness (EA) MN Table 5 TLP tendon properties Fig. 5 Principal dimensions of the semisubmersible Item Unit Designations Anchor radius m 30.0 Anchor depth m Radius of fairlead m 30.0 Tendon porch depth m 28.5 Unstretched tendon length m Tendon diameter m 0.6 Mass per length (dry) kg/m Mass per length (wet) kg/m 0.0 Axial stiffness (EA) MN Journal of Offshore Mechanics and Arctic Engineering MAY 2014, Vol. 136 /

4 Table 6 Instrumentation list Fig. 7 Wind generator Item Channel Remark Reference wave probes (2) 2 Reference ADV (2) 6 Three axes Optical measuring 6 Accelerometer top 3 Below nacelle Accelerometer mid 2 Middle at tower Accelerometer low 3 Bottom at tower 6 DOF load cell low 6 At base of tower 6 DOF load cell high 6 At nacelle Mooring: semi 3 Three mooring lines Tendon: TLP 3 Three tendons Mooring: spar 9 Three mooring lines þ six delta connections Rotor speed 1 Torque sensor 1 At main rotor shaft Fig. 8 Instrumentation on the wind turbine Fig. 10 NREL 5 MW wind turbine performance curve Table 7 Selected wind conditions Velocity (m/s) Rotor rpm Remarks Steady wind Steady wind Steady wind Rated wind Steady wind Steady wind Design maximum Steady wind a Survival Dynamic wind API spectrum Dynamic wind API spectrum Dynamic wind a API spectrum a Feathered turbine. Fig. 9 Instrumentation on the turbine tower and floating platform separation. The third concept is a tension leg platform (TLP). The TLP achieves stability through the use of tendon tension from the excessive buoyancy of the hull. Figure 2 shows the selected floating platform concepts. Figures 3 5 give the principal dimensions of the selected platforms and the mass properties of these platforms are listed and shown in Table 2. Since the prototype water depth is 200 m, in 1:50th scale ratio, the full length of the mooring system could be modeled. Three typical offshore mooring types are selected: taut mooring system caternary mooring system tendon mooring system The taut mooring system moored the spar-buoy with a d connection similar to the type employed in the Statoil Hywind [6]. Figure 6 illustrates a d connection and taut mooring line. The catenary mooring system was used for the semisubmersible platform and the TLP was moored by vertical tendons. Tables 3 5 summarize the prototype mooring system properties. The generation of a high quality wind field is of great importance for the correct coupling between the aerodynamic and / Vol. 136, MAY 2014 Transactions of the ASME

5 Table 8 Tested wave conditions JONSWAP Hs (m) Tp (s) Gamma Remarks Operation Operation year Design years Bi-directional 7.1/ / /6.0 Regular wave H (m) T (s) Remarks RG RG RG RG RG RG RG White noise Hs (m) T (s) Remarks WN WN Test types Table 9 Hammer tests Static offset tests Free decay tests Free decay þ steady wind Regular wave tests Regular wave þ steady wind White noise wave tests White noise wave þ steady wind Test types Table 10 Summary of system identification tests Measurements Structural natural periods Mooring stiffness System natural periods and total damping Damping contribution from wind Linear response characteristics (RAOs) Linear response characteristics include wind Linear response characteristics (RAOs) and nonlinear response characteristics (low frequency and high frequency) Wind damping contribution on system response Summary of station keeping test types Test description Wind only Wind tests for fixed wind turbine Wind tests for floating wind turbines Wave only Head seas Oblique seas Bi-directional seas (swell and local waves) Wind and wave Operation wave with wind speeds 1, 2, and 3 1 year storm wave with wind speeds 3, 4, and year storm wave with wind speed year storm wave and wind speed 6 Bi-directional wave þ steady wind 5 Bi-directional wave þ dynamic wind 2 hydrodynamic properties of the offshore floating wind turbines. A new high-quality wind generator was built for the tests. The wind generator consists of seven fans across and is five fans high. A dense honeycomb screen was installed in front of the wind fans to straighten the wind flow. Figure 7 shows the wind generator, honeycomb screen, wind fans, and acoustic Doppler velocimeters (ADVs). Details of the wind generator and the calibration procedure can be found in Ref. [7]. Instrumentation In order to measure the loads and motions of the floating wind turbines, a total of about 40 to 50 channels were used in the model tests, depending on the floater. The six degrees of freedom (DOF) motions of the floating wind turbine were measured by the optical tracking system. Three accelerometers were located at the base, middle, and top of the turbine tower to measure accelerations. The structural mode shapes and natural periods of the wind turbine tower were derived from these accelerometers. The nacelle was connected to the tower by means of a six component load cell that measured the six DOF forces and moments between the tower and nacelle. The global connection loads between the wind turbine and the platform were measured by another six component load cell between the tower base and platform top. The turbine performance was measured by the torque sensor between the motor and the blades. Figures 8 and 9 show the instrumentation for the wind turbine and floating platform. The mooring top tension was measured by a ring type transducer at the fairlead location. A z-shaped strain gauge was installed at each tendon porch to measure tendon top tensions. A total of three calibration probes and two reference probes were used for the wave calibration tests. The reference wave probes remained in place throughout the tests to ensure repeatability of the wave generation. A total of three ADVs were used for the wind calibration tests. During the tests, two reference ADVs were also deployed to measure the tested wind. Table 6 gives the summary of the test measurements. Environment The wind speeds were selected for the NREL 5 MW wind turbine power curve. Figure 10 shows the power curve of the NREL 5 MW wind turbine. Table 7 lists the tested wind speeds. During the model tests, six steady wind conditions were simulated. The steady wind speeds are defined at the hub height (i.e., 90 m above MW L) of the wind turbine. In addition to steady wind conditions, three dynamic wind conditions were also simulated to test realistic ocean wind conditions. The API (i.e., NPD) wind spectrum was used for the dynamic wind simulations [8]. The dynamic wind speeds are defined at 10 m above MW L. Table 8 summarizes the tested wave conditions. Three wave conditions were selected based on nine years worth of data measurement from the NERACOOS floating buoy system in the offshore Gulf of Maine. In addition to wind driven wave conditions, one swell condition was also selected to simulate a bi-directional sea state. The bi-directional sea state was simulated Table 11 Summary of load cases Operational wave 1 Operational wave 2 Design wave Bi-directional Steady wind 1 Operation low 1 Steady wind 2 Operation low 2 Steady wind 3 Operation low 3 Operation high 1 Steady wind 4 Operation high 2 Steady wind 5 Operation high 3 Design Swell 1 Steady wind 6 Survival Dynamic wind 1 Operation high 1 Dynamic wind 2 Operation high 2 Design Swell 2 Dynamic wind 3 Operation high 3 Survival Journal of Offshore Mechanics and Arctic Engineering MAY 2014, Vol. 136 /

6 Fig. 11 Wind field measurement locations Table 12 Fig. 12 by superposing the operational wave condition 2 from head seas (¼180 deg) and the swell waves from quartering seas (¼225 deg). Test Procedure Wind field measurement results Comparisons between target and measured waves Target Measured Difference (%) Operation 1 STD (m) Tz (s) Operation 2 STD (m) Tz (s) Design STD (m) Tz (s) Swell STD (m) Tz (s) The model tests started with the calibration of the selected environmental conditions. After completing the calibration tests, the Fig. 13 theory) Table 13 Operational wave 2 comparisons (measured versus Measured tower bending natural frequencies Natural frequency (Hz) 1st FA a 1st SS b 2nd FA a Fixed turbine TLP Spar-buoy Semisubmersible a Fore-after mode. b Side-side mode. system identification tests were conducted. The purposes of the system identification tests are to verify physical properties such as system stiffness, natural periods, total system damping, and linear response amplitude operators (RAOs) and nonlinear (low and high frequency) response characteristics of the floating wind turbine models. Table 9 summarizes the system identification tests / Vol. 136, MAY 2014 Transactions of the ASME

7 Fig. 16 The TLP setdown Fig. 14 Hammer test results After the system identification tests, the station keeping tests were carried out. In order to identify coupling between the wind turbine and the floating platform, four different in-place test phases were conducted. The first phase was wind only tests for the fixed wind turbine configuration. These tests served as the benchmark for calibrating and verifying the aerodynamic load coefficients such as the drag (CD), lift (CL), thrust, and torque of the wind turbine model. The second phase was wind only tests for the floating wind turbine configuration. In this phase, the wind turbine was responding to wind and to calm water. The test isolated the wind effects on the floating wind turbine response. The third phase was wave only tests for the floating wind turbine configuration. This phase served as the benchmark for calibrating and verifying the hydrodynamic coefficients and station keeping characteristics of the floating wind turbines. The final phase was wind and wave tests. These tests were carried out with steady and dynamic winds and regular and random wave environments. The station keeping test types and load cases are summarized in Tables 10 and 11, respectively. Since the current speed is low in the Gulf of Maine, currents were not simulated in these tests. Calibration of Environment Extensive wind calibrations were conducted. Three acoustic Doppler velocimeters (ADVs) were used for the wind calibrations. Each ADV measures velocity in three directions. The wind calibration procedure is summarized as follows: determine spatial distribution over a calibration grid calibrate all steady wind speeds calibrate dynamic wind velocity spectrum The wind field measurement range and locations are shown in Fig. 11 and the wind field measurement results are shown in Fig. 12. Details of the calibration results are discussed in another paper [7]. The waves were calibrated prior to installation of the model in the basin. The wave heights were measured at the location where the wind turbines will be located. In addition to a calibration probe, two more wave probes were deployed at expected mean offset positions (i.e., 9 m and 18 m) of the floating wind turbines. Two additional reference probes were also deployed. The calibration results show that the maximum difference in the standard deviation between the target waves and measured waves was less than 3%. Table 12 summarizes the comparison results between the target waves and measured waves. As shown in Fig. 13, the basin generated wave spectra shows good agreement with the theoretical JONSWAP spectrum. Fig. 15 Comparison of static offset test results Journal of Offshore Mechanics and Arctic Engineering MAY 2014, Vol. 136 /

8 Table 14 Comparisons of natural periods Unit (s) TLP Measured Measured Predicted DOF No wind Steady wind No wind Surge Sway Heave Roll /1.25 a Pitch /1.25 a Yaw Spar-buoy Measured Measured Predicted DOF No wind Steady wind No wind Surge Sway Heave Roll Pitch Yaw Semisubmersible Measured Measured Predicted DOF No wind Steady wind No wind Surge Sway Heave Roll Pitch Yaw a Without considering the elastic modes of the wind turbine. Fig. 18 Comparisons of the damping ratios (semisubmersible) Fig. 19 Comparisons of the damping ratios (spar-buoy) Fig. 17 Comparisons of the damping ratios (TLP) System Identification Test Results The turbine tower structural natural periods were measured with hammer tests. Hammer tests were executed by exciting the model with an impulse forces. The hammer test results are summarized and shown in Table 13 and Fig. 14. The hammer test results show that the floating platform characteristics significantly influence the bending frequencies of the turbine tower. As expected, the stiffer foundation, such as a TLP, provides a lower tower bending natural frequency than the compliant foundations such as a spar-buoy and semisubmersible. The stiffer foundations represent a free-fixed boundary condition such as a cantilever Fig. 20 Surge RAOs of the TLP beam, while a softer foundation represents the free-free boundary condition at the tower base. The total stiffness of the mooring system was measured by the static offset tests. The static offset test results are shown in / Vol. 136, MAY 2014 Transactions of the ASME

9 Fig. 21 The TLP surge response spectra Fig. 24 Surge RAOs of the spar-buoy Fig. 22 Pitch RAOs of the TLP Fig. 25 Heave RAOs of the spar-buoy Fig. 23 Pitch responses of the TLP Fig. 26 Pitch RAOs of the spar-buoy Journal of Offshore Mechanics and Arctic Engineering MAY 2014, Vol. 136 /

10 Fig. 27 Surge response of the spar-buoy Fig. 30 Heave RAOs of the semisubmersible Fig. 28 Pitch response of the spar-buoy Fig. 31 Pitch RAOs of the semisubmersible Fig. 29 Surge RAOs of the semisubmersible Fig. 15. As expected, softening was observed with the synthetic mooring system. On the contrary, hardening was observed with the catenary mooring system. The tendon system shows a linear stiffness trend. The setdown of the TLP was also measured during the horizontal static offset tests. Figure 16 shows the setdown measurement of the TLP. The natural periods and total damping of the floating platform system were obtained from free decay tests. In order to measure damping for the floating wind turbines, two types of free decay tests were carried out. The first type was a calm water free decay test that measures the system natural periods and hydrodynamic damping. The second type was a free decay test with steady wind that measures aerodynamic damping from the wind turbine. The six DOF natural periods of the floating wind turbines are listed in Table 14. The damping ratios with respect to motion amplitudes for all three floating wind turbines are shown in Figs The damping analysis results show that a steady wind substantially increases pitch damping of the spar-buoy and semisubmersible. Due to the slender and deep draft shape, relatively low heave, roll, and pitch damping were measured with the spar-buoy. The linear and nonlinear wave response characteristics of the floating wind turbines were measured by the regular and white noise wave tests. In order to identify wind effects on the global performance of the floating wind turbines, the regular wave and white noise tests were conducted with and without steady wind / Vol. 136, MAY 2014 Transactions of the ASME

11 Semisubmersible. Figures show the RAOs of the semisubmersible. Since the heave natural period of the semisubmersible is close to the linear wave energy, the heave RAO comparisons show the nonlinear damping effect near the heave natural period (¼ 17.5 s). Figures 32 and 33 show the surge and pitch response spectra of the semisubmersible. It is interesting to note that the surge and pitch linear wave frequency responses remained the same for both with and without wind, while the steady wind reduces the low frequency surge and pitch responses. Fig. 32 Surge response of the semisubmersible Conclusions This paper describes model tests for the global performance of three floating wind turbines and also presents the system identification test results. The hammer test results show that the tower structural natural frequencies are significantly influenced by the supporting floater. The stiffer foundation, such as a TLP, provides a lower bending frequency of the turbine tower than a compliant foundation such as a spar-buoy or a semisubmersible. The free decay test results show that the steady wind substantially increases pitch damping of the spar-buoy and semisubmersible. The white noise test results also show that the steady wind increases the surge and pitch damping for all three floating platforms. On the contrary, the wind load increases the wave frequency motion of the spar-buoy. Detailed data analysis results on the performance of the three floating wind turbines subjected to combined wind and irregular wave loading is discussed in the work of Goupee et al. [9]. Fig. 33 Pitch response of the semisubmersible TLP. Figure 20 shows the surge RAOs of the TLP. The surge natural period of the TLP is longer than the linear wave frequency range and, therefore, the damping effect from wind is not shown in the surge RAO. However, as shown in Fig. 21, the response spectra comparisons clearly show the wind damping effect at the TLP surge natural period (¼39.3 s) response. Figure 22 shows the pitch RAOs of the TLP. Since the pitch natural period of the TLP is near the linear wave excitation periods, the RAOs clearly show the wind damping effects on the TLP pitch response. As shown in Fig. 23, the response spectra comparison clearly shows the reduction of the TLP pitch natural period motion. Spar-Buoy. Figures show the surge, heave, and pitch RAOs of the spar-buoy, respectively. Since all six DOF motion natural periods of the spar-buoy are longer than the linear wave excitation period range, the damping effect from wind is not shown in the RAOs. On the contrary, Figs. 27 and 28 show the response spectra of the spar-buoy and both the surge and pitch responses show that the wind reduces the surge and pitch responses at the natural periods, while the wind increases the linear wave frequency responses of the spar-buoy in the surge and pitch modes. Acknowledgment The financial support from the Department of Energy through DeepCwind Grant Nos. DE-EE and DE-EE , the National Science Foundation through Grant No. IIP , and the University of Maine is gratefully acknowledged by the authors. In addition, the expertise and support of MARIN is greatly appreciated. References [1] Musial, W., Butterfield, S., and Ram, B., 2006, Energy From Offshore Wind, Offshore Technology Conference, Houston, TX. [2] Bae, Y., Kim, M. H., and Shin, Y. S., 2010, Rotor-Floater-Mooring Coupled Dynamic Analysis of Mini-TLP-Type Offshore Floating Wind Turbines, Paper No. OMAE [3] Skare, B., Hanson, T. D., Nielsen, F. G., Yttervik, R., Hansen, A. M., Thomesn, K. and Larsen, T. J., 2007, Integrated Dynamic Analysis of Floating Offshore Wind Turbines, European Wind Energy Conference, Milan, Italy. [4] Roddier, D., Cermelli, C., Aubault, A., and Weinstin, A., 2010, WindFloat: A Floating Foundation for Offshore Wind Turbines, J. Renewable Sustainable Energy, 2, p [5] Martin, H., 2011, Development of a Scale Model Wind Turbine for Testing of Offshore Floating Wind Turbine Systems, M.Sc. thesis, The University of Maine, Orono, ME. [6] Jonkman, J. M., 2010, Definition of the Floating System for Phase IV of OC3, Technical Report No. NREL/TP [7] Ridder, E., Koop, A., and Wilde, J., 2012, High Quality Wind Setup for Model Testing Floating Wind Turbines in an Offshore Basin, Paper No. OMAE [8] American Petroleum Institute, 2000, API Recommend and Practice 2A-WSD: Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms Working Stress Design, 21st ed, American Petroleum Institute, Washington, DC. [9] Goupee, A., Koo, B., Kimball, R., Lambrakos, K., and Dagher, H., 2012, Experimental Comparison of Three Floating Wind Turbine Concepts, Paper No. OMAE Journal of Offshore Mechanics and Arctic Engineering MAY 2014, Vol. 136 /