International Wind Engineering Conference IWEC 2014 IMPROVED TANK TEST PROCEDURES FOR SCALED FLOATING OFFSHORE WIND TURBINES KOLJA MÜLLER * , FRANK SANDNER * , HENRIK BREDMOSE † , JOSÉ AZCONA ‡ , ANDREAS MANJOCK § , RICARDO PEREIRA § * Stuttgart Wind Energy (SWE) - University of Stuttgart Allmandring 5B, 70569 Stuttgart, Germany e-mail: mueller@ifb.uni-stuttgart.de, sandner@ifb.uni-stuttgart.de † DTU Wind Energy - Technical University of Denmark Nils Koppels Alle - Building 403, 2800 Lyngby, Denmark e-mail: hbre@dtu.dk, www.vindenergi.dtu.dk ‡ Wind Energy Department - National Renewables Energy Centre (CENER) Ciudad de la Innovación 7, 31621 Sarriguren (Navarra), Spain e-mail: jazcona@cener.com, www.cener.com § DNV GL Energy - Renewables Certification Brooktorkai 18, 20457 Hamburg, Germany e-mail: andreas.manjock@dnvgl.com, ricardo.pereira@dnvgl.com Keywords: Floating Offshore Wind turbines, tank testing, scaled models, rotor model, representative aerodynamic forces, Software-in-the-Loop Summary: This study collects issues from previous tank test campaigns of scaled Floating Offshore Wind Turbines (FOWT), compares the different scaling methodologies, points out critical aspects and shows possible alternatives and recommendations for future tests depending on the specific objective. Furthermore, it gives practical recommendations for the modeling and construction of scaled rotors. The presented scaling procedure will be applied in tank tests within the EU Seventh Framework Program InnWind (ENERGY.2012.2.3.1 “Innovative wind conversion systems (10-20MW) for offshore applications”). 1 INTRODUCTION The numerical simulation tools for calculating motion and dynamics of FOWT are still under development. Many existing codes of the offshore and oil & gas industry provide approved and validated routines describing the dynamics and loading from waves and currents to floating structures. But they lack a sufficient consideration of the aerodynamic loads on the rotor caused by turbulent wind, complex rotor aerodynamics and an own control system on top of the structure. This introduces a fully non-linear loading source to the entire system. Additionally, the influence of second order hydrodynamics and mooring line behavior is not always modeled in sufficient detail. K. Müller, F. Sandner, H. Bredmose, J. Azcona, A. Manjock, R. Pereira 2 Currently many research programs like OC3/OC4 [1] compare the results of different simulation codes together with measurement data from model tests. Today only a few full scale prototypes are in operation and measurements of existing floating concepts are rare. Therefore experiments with scaled floating models in wave tanks in combination with wind modelling are a cost effective approach to gather data for referencing and validating the dynamics and loads of FOWT. This might also be a necessary step for the certification of a prototype. 2 LITERATURE REVIEW For more than twenty years the idea of installing wind power generators offshore on floating foundations is pursued in research and industry. Within many of the past and present projects the challenge of experimental tests with combined wind and wave loading has been addressed. Overviews of these projects can be found in [2], [3], [4] and [5]. Useful practical information on scale model building can be found in the theses of the University of Worcester, see [6] and [7] and [8]. Wave tank tests of FOWT can be conducted with two major purposes: Firstly for validation of numerical models and secondly in order to obtain the full system dynamics in all occurring load situations for full-scale prototypes, which are going to be built. There have been various commercial projects for the implementation of floating wind energy systems. The projects Hywind [9], [10] and WindFloat [11], [12], [13] and [14] were among the first to gain commercial experience in the years 2009 and 2011, respectively. Gicon [15], [16] and GustoMSC with their “Tri-Floater” [17], [18], [19], [20] followed in the years after with multiple test campaigns. In Japan the Fukushima project [21] with various phases includes model and prototype tests for a conventional spar concept, a “hybrid spar” and a semi- submersible. The wave tank tests for the spar concept are documented in [22]. Innovative large-scale offshore floating structures are investigated by Kyushu University, [23]. Ishihara has developed an innovative multi-turbine foundation and presented model tests in [24] and [25]. The “Japan Marine United Advance Spar” was scaled for wave tank tests, [4]. The large-scale prototype projects in Europe, of which tank tests are known, are HiPRWind with a semi-submersible [26], BlueH with a two-bladed rotor and semi-submersible foundation [27] and the Norwegian Sway, which is a tension-leg spar [28]. A comparable concept is the Nautica wind power layout, for which some information on testing can be found online in [29]. The multi-turbine concept WindSea offers some data on the wave tank tests online, see [30] and [31]. Tests were also conducted for the multi-purpose foundation Poseidon, see [32]. This concept was tested in various wave tanks with and without wind and also on site with different prototype scales. The Pelastar TLP by Glosten Associates [33] was tested but very little information is published. The Vertiwind concept by Technip with a vertical-axis wind turbine was tested in a scale of 1:2, see [34]. Recently, the Nautilus semi-submersible concept, which has also been tested in the Cork wave tank in Ireland, was published in [35]. Iberdrola is developing a TLP concept and has presented the wave tank test layout in [36] with a publication on the most recent test campaign, see [37]. The University of Maine is involved in a project building the first scaled FOWT prototype in the US called VolturnUS, see [38]. All of the described model tests have used a scale between 1:130 and 1:2. K. Müller, F. Sandner, H. Bredmose, J. Azcona, A. Manjock, R. Pereira 3 Various publicly funded research projects have included FOWT wave tank tests. As part of the Marinet project [39], state of the art scaling routines were revised and the dynamic-elastic scaling proposed as adequate for scaled FOWT-tests. The DeepWind project aimed at an integrated research on vertical-axis floating wind turbine (VFOWT) systems including tests [40]. A numerical model development of VFOWT together with a wave tank validation is given by [41]. Within the Winflo project [42], scaled models were developed and tested in order to understand the dynamic behavior of the concept design and to validate the floater and mooring design. The models included an adapted rotor design. The DeepCwind [43] project to this point delivers the most throughout insight available on the capabilities of numerical and experimental investigation of the three major FOWT systems: tension leg platform (TLP), semi-submersible and spar buoy. Major parts of the research were the assessment of a proper scaling methodology [44] and its numerical verification [45], the realization of a model wind turbine for wave basin testing [46] and the development of thrust scaled blades [47]. Also in the focus were the analysis of the interaction of the mooring dynamics with the global response of the FOWT [48] and the calibration and validation of a full scale simulation model within the simulation software FAST [49]. In 2013 a new test campaign with the same semi-submersible platform has been performed under improved testing conditions in the MARIN facility in the Netherlands, see [50]. The results together with a comparison to the previous DeepCwind campaign and a model of a smaller scale (1:130) for a reliability assessment can be found in [51]. A code validation of the MLTSIM hydrodynamics code coupled with the FAST software with the same test data can be found in [52]. A discussion on the differences of the behavior of the three different platform concepts is available in [53]. A detailed test session description aiming at the realization of a full scale turbine has been given by Utsunomiya. His tests cover simplified scale model tests for extreme condition dynamic analysis, see [54], and simulation validation, see [55]. They reach towards scaled prototype tests in full-scale at-sea-environment [56] and [57]. The derivation of the numerical model is given in [58]. A semi-submersible with a single-point mooring has been tested by Osaka University see [59]. Shin et al. published test results to study the motions of the OC3- Hywind spar [60] and a new spar-type FOWT model in 2D and 3D wave tanks, see [61], [62]. Another investigation of a “stepped spar”-model in a curved wave tank has been presented by Sethuraman et al. with a focus on the validation of the software OrcaFlex and nonlinear mooring line behavior comparing the RAOs for pitch, surge and heave motion in regular and irregular waves, see [63]. Philippe et al. [64] tested a model of the Dutch Trifloater in order to validate their numerical model. The influence of storm waves on a simplified TLP model was analyzed by Wehmeyer et al. [65] with varying tower stiffnesses, putting large effort in high wave modeling quality and showing that the inclusion of second order hydrodynamics is essential, especially regarding TLP-specific responses such as ringing and slack line events. Another validation of numerical results for a TLP has been done by Ren et al., [66] and Olinger et al. [67], who collected information on the dynamic behavior in pitch, surge and heave direction using simplified models of TLP and spar type FOWT. They found that surge motion of the platform dominates over other motions for TLP & Spar and that varying tether K. Müller, F. Sandner, H. Bredmose, J. Azcona, A. Manjock, R. Pereira 4 pretension has little effect on RAO values. Within the AFOSP project a monolithic concrete spar design of a FOWT has been developed and wave basin tests presented in [68]. The impact of the rotor dynamics of the wind turbine rotor or the gyroscopic effect on the coupled system dynamics has been studied in [69]. In [70] an instability of floating wind turbines has been reported, yielding large amplitudes in yaw together with large amplitudes in pitch-motion coupled by the gyroscopic effect. The presence of two different phases of the surrounding medium (air and water) poses a general problem to the geometrical scaling of the considered system. A description of the general loads acting on offshore structures within classical marine technology and the underlying physics can be found in [71] and [72]. For hydrodynamic similitude, Froude scaling needs to be applied, but inevitably leads to an error in the scaling of the Reynolds number for both hydro- and aerodynamics. While testing experience indicates that on the hydrodynamic side, the KC number is more significant [73], the aerodynamic Reynolds number is of high importance due to its influence on the flow condition around the airfoil. The geometric downsizing of the wind turbine effectively alters the aerodynamics of the scaled turbine, i.e. the Reynolds number and the associated forces on the turbine [74]. In order to keep a correct relationship of the forces, different solutions have been used in the past. Rotor models range from concentrated masses with added point forces [55], over drag disks with a rotating body [12] up to the redesign of the rotor blades in order to correctly scale the aerodynamics of the rotor, see [50], [75] and [76]. More optimization of this state-of-the-art scaled blade design through an alternative pitch angle distribution is possible [77]. In order to avoid the discussed scaling problem, a part of the environment can be imitated by real-time controlled actuators as has been done for hydrodynamics [78], [79] and [80] and for aerodynamics [81]. Building on the given experiences, two alternative procedures for rotor modeling are presented and evaluated below. The aims are to optimize the quality of experimental results when using an adjusted blade design and to reduce the costs of future experiments. 3 AERODYNAMIC FORCES FROM RE-DESIGNED MODEL SCALE ROTOR The scaling of the aerodynamic forces is sketched in the following. Further the establishment of these loads through direct wind forcing is discussed and the requirement of a tailored model scale rotor design is detailed, following the work of [39], [44], [51], [77] and [76]. As the wind generation system may limit the range of wind speeds that can be generated, a free parameter is introduced, which modifies the scaled inflow speed at the expense of changing the tip speed ratio. This approach is denoted the Froude- scaling, see [82]. The limitations of the redesigned rotor approach and the open questions subject to current research are outlined. a. Froude scaling and the need for a redesigned rotor As mentioned above, testing floating wind turbines in wave tanks leads to application of Froude scaling in order to fulfill the requirement of a true scaling of the wave force. This follows from the role of gravity as the governing restoring effect for the waves. Given a length scale ratio of (1) K. Müller, F. Sandner, H. Bredmose, J. Azcona, A. Manjock, R. Pereira 5 the conservation of gravitational acceleration g between prototype and model scale locks the time scale ratio to be (2) Here are representative length and time scales and subscripts denote prototype and model respectively. Due to the requirement of a conserved ratio between structural mass and water mass, mass scales according to (3) where is the density of water. The above scaling of length, time and mass leads to a scaling of water particle velocities as (4) Hence, if the ratio of air and water velocities is to be preserved from prototype to model scale, a similar relation for the air-velocities follows (5) Further, forces, , scale as (6) Rotor thrust is given as (7) where is the thrust coefficient, is the rotor area and is the air density. Given that the rotor area scales as and that the air velocities scale by , the requirement of force to scale as (8) yields the desired scaling of the thrust coefficient as (9) Although this result of a conserved thrust coefficient may suggest that the prototype rotor design can be geometrically down-scaled to lab scale, the lowered lab-scale Reynolds number which results from the scaling of air-velocity leads to insufficient thrust levels. At the smaller Reynolds number, the lift-to-drag ratio is reduced and an increased chord-length is therefore needed for the model-scale blade to deliver the same thrust-coefficient as at full scale [39], [44], [51], [77] and [76]. For this reason a re-design of the blades at model scale is needed. In this context it is natural to base the design on low-Reynolds number air foils. The redesigned model scale rotor will thus have blades with increased chord-length relative to the prototype scale blades, see e.g. [77]. b. Adjustment of air-velocity scaling The changed Reynolds number and blade geometry implies that the similarity of the aerodynamic flow between prototype and model scale is broken. Further, for the above scaling, a large scale ratio leads to very small air-velocities at lab scale – in some cases so K. Müller, F. Sandner, H. Bredmose, J. Azcona, A. Manjock, R. Pereira 6 small that perturbations from heat convection and other motion in the lab can cause significant distortion. On the other hand, the wind generation system may also have limited capacity such that at larger scale ratios, certain wind speeds cannot be obtained. Note here that the wind generation is very different from the controlled conditions in a usual wind tunnel. The model scale rotor area may be 3m in diameter and the wind must be generated over water. For the above reasons, an adjustment of air-velocity scaling may be needed. This can be described through a free parameter , such that (10) For = 1, Froude scaling is obtained, while < 1 implies larger air velocities than for Froude scaling. With this modified air-scaling, the thrust coefficient must scale as (12) Further, the tip speed ratio ( ) will not be conserved since (13) Although conservation of is generally preferred, the lack of aerodynamic similarity is likely to justify such a change. Note that with the factor , the Reynolds number scales as (14) where is the kinematic viscosity. The Froude- scaling is detailed in [82]. c. Pros and cons of the scaled rotor approach The approach of a re-designed rotor at model scale is able to deliver the correct thrust, see e.g. [51], [76]. Further, given that the mass distribution and rotational speed are scaled correctly, the correct gyroscopic forces and 1P, 3P forcing frequencies will be reproduced [39], [77]. If also the structural stiffness is scaled correctly, the structural frequencies and deflection to the loads will scale correctly [39], [77]. Further dimensionless numbers which will be conserved are the Keulegan-Carpenter number (KC) and the Lock number. This is conserved in the Froude- scaling, both for the aerodynamic and hydrodynamic forces. Other dimensionless numbers are not conserved. These are the Reynolds number in the air and in the water which leads to changed hydrodynamic force coefficients and the need for a re-designed rotor; the Weber number which measures the balance of hydrodynamic surface tension to inertial loads (not expected to have importance, except at very small scales); the Strouhal number in water and air - for preserved Strouhal number and strict Froude scaling , though, the vortex shedding frequency will scale as √ which is consistent with the time scaling; the Mach number in water and air (not considered to be important); and the tip speed ratio, which is only conserved for . Besides the dimensionless numbers, a couple of effects will also not scale automatically:  the aerodynamic torque  the aerodynamic power  the generator torque and its contribution to roll-forcing  the magnitude of the 3P forcing from the tower shadow K. Müller, F. Sandner, H. Bredmose, J. Azcona, A. Manjock, R. Pereira 7 However, a careful rotor and nacelle design may help on this. An accurate reproduction of the above mentioned properties is part of current active research. 4 AERODYNAMIC FORCES FROM SOFTWARE-IN-THE-LOOP METHOD Another method to include a realistic force to represent the aerodynamic thrust in combined wind and wave scaled tests is based on the use of a ducted fan substituting the wind turbine scaled rotor. The fan thrust is controlled by the fan rotational speed set by the controller, which again depends on a computer real time simulation of the full scale rotor in the wind field. The real time simulation considers the platform motions measured in real time in the wave tank test. Therefore, the aerodynamic damping is modelled by the fan force. We refer to the described method as Software-in-the-Loop (SIL). The layout of the SIL system is shown in Figure 1. The left side describes the simulation part of the system, which works in full scale, and the right side represents the wave tank scaled test. The different magnitudes that are interchanged between both blocks are transformed by the appropriated scaling laws based on the factor scale . Figure 1: Software-in-the-Loop Method Diagram The simulation tool provides the total aerodynamic force on the shaft from integration of all the aerodynamic loading at the blade elements. This force in full scale is transformed to the model scale ( ) and the pulse width of the PWM (Pulse-Width Modulation) signal needed to produce the force in the ducted fan is provided by a calibration curve previously obtained. The control system regulates the fan speed that introduces the desired force at the model’s hub height. The waves produced by the wave maker are also acting over the platform and, together with the aerodynamic thrust, inducing motions. The acquisition system measures the positions and velocities for the 6 degrees of freedom of the platform at a certain sampling period. These measurements are sent to the simulation tool that is waiting for the data to advance one time step and calculate the new value of the aerodynamic thrust. For this K. Müller, F. Sandner, H. Bredmose, J. Azcona, A. Manjock, R. Pereira 8 reason, the sampling period t and the simulation time step T have to be set accordingly (with a factor 0.5 ). The methodology is explained in more detail in [83], where some experimental results obtained in a test campaign of a 6 MW semi-submersible platform at the ECN (Ècole Centrale de Nantes) wave tank are also presented. These tests results using the SIL method are compared with computer simulations to show the performance and validate the method. a. Pros and cons of the SIL approach The SIL approach can provide a realistic aerodynamic thrust on the scaled model. As the computation of the force takes into consideration the motion of the platform, the effect of the aerodynamic damping is included. In addition, the control actions, the different types of wind (turbulent, constant, gusts) and the operating condition (idling, power production, etc.) are taken into account for the calculation of the thrust. Conditions where the waves and wind are misaligned are sometimes not easy to be reproduced in wave tanks with wind generation systems. With this method, they can be easily achieved by changing the fan orientation at the tower top. Furthermore, the SIL system allows performing test cases including wind at wave basins where the wind generation system is not available. In addition, the simplicity of the method makes it cost effective and flexible because the material is not specific for a certain wind turbine model and it could be used in different tests for different models. Effects that are not scaled correctly with this procedure are:  the aerodynamic torque  the gyroscopic momentum. Alternatively the use of a rotating scaled mass to represent the rotor inertia can be used to match the gyroscopic effects. Active research on the response of different fan units depending on the size of the wind turbine, scale factor, etc. is being conducted with the aim to explore the limits of the methodology. 5 CONCLUSIONS To this point, numerous model scaled tests have been run and extensive experience on the dynamics of FOWT systems could thus be collected and have been summarized in this paper. The complex physics of the coupled wind-wave environment pose a strong challenge toward the scaling procedures. Past research projects have shown that various approaches with different levels of complexity for the combined Froude-scaled model test exist. As testing procedures are likely to become part of the certification process, two alternatives to the state- of-the-art procedure with blades modified according to Froude-scaling have been presented here in order to reduce the complexity of future test procedures. Whereas the SIL approach reduces the aerodynamics to a correctly reproduced thrust force by a fan the re-designed rotor aims at a match of power and thrust coefficients. Thus, the first solution will answer questions related to the full system dynamics whereas the latter allows also more detailed studies of the rotor aerodynamics and special effects therein. Eventually, the testing procedures for FOWT are significantly more complex than the common tests of naval architecture. It will be therefore essential to thoroughly select the adequate setup for each test and exclude features and effects where possible. K. Müller, F. Sandner, H. Bredmose, J. Azcona, A. Manjock, R. Pereira 9 6 BIBLIOGRAPHY [1] A. Robertson, J. Jonkman, M. 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