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Real-Time Hybrid Model (ReaTHM) tests of a braceless semi-submersible wind turbine were carried out at MARINTEK’s Ocean Basin in 2015. The tests sought to evaluate the performance of the floating wind turbine (FWT) structure in environmental conditions representative of the Northern North Sea. In order to do so, the tests employed a new hybrid testing method, wherein simulated aerodynamic loads were applied to the physical structure in the laboratory. The test method was found to work well, and is documented in [1]. The present work describes some of the experimental results. The test results showed a high level of repeatability, and permitted accurate investigation of the coupled responses of a FWT, including unique conditions such as blade pitch faults. For example, the influence of the wind turbine controller can be seen in decay tests in pitch and surge. In regular waves, aerodynamic loads due to constant wind had little influence on the structure motions (except for the mean offsets). Tests in irregular waves with and without turbulent wind are compared directly, and the influence of the wave-frequency motions on the aerodynamic damping of wind-induced low-frequency motions can be observed. ∗Corresponding author: maxime.thys@marintek.sintef.no INTRODUCTION Floating wind turbines (FWTs) are an emerging technology which can be used to generate electricity from the significant wind resource in relatively deep water (>50 m). Scaled model tests are an important part of the qualification process for such novel concepts, and such tests may have many different objectives. Model tests can, for example, be used to confirm system behavior, evaluate nonlinear phenomena, assess extreme and detailed loads, validate computer codes, or convince decision makers of the feasibility of a concept [2]. There are, however, significant challenges related to carrying out scaled model tests of FWTs in an ocean basin. Hydrodynamic tests generally follow Froude scaling, but a consistent scaling of the wind turbine will then result in a reduced Reynolds number compared to the prototype, which leads to generally poor aerodynamic performance [3]. Furthermore, there are practical challenges related to generating (and measuring) constant and turbulent wind fields in a wave basin [4, 5]. In order to improve the aerodynamic load modeling in wave basin experiments, several researchers have attempted various forms of “non-geometrical” scaling of the wind turbine rotor. One form of non-geometrical scaling is to replace the wind turbine rotor with a drag disk (e.g. [6, 7]), which gives the correct mean thrust and provides some aerodynamic damping, and 1 Copyright c © 2016 by ASME can provide gyroscopic forces if spinning. A more sophisticated method of non-geometrical scaling is to modify the wind turbine airfoil shape and chord length in order to obtain improved performance at low Reynolds numbers. Improvements to the turbine performance in a wave basin have been documented, but it is not currently possible to simultaneously match the thrust, torque, and slope of the thrust curve adequately [8–10]. Numerical code validation using tests with non-geometrically scaled rotors has also proved challenging due to three-dimensional effects at low Reynolds numbers which are not accounted for by commonly used methods such as blade/element momentum [11]. In addition to the torque and thrust, accurate modeling of other aerodynamic forces and moments (such as the sway force and yaw moment), including the effects of the wind turbine control system, may be important. When the experimental goal is to qualify the global performance of the system, a so-called “realtime hybrid model testing” approach may be applied: in the case of the FWT in the wave basin, this implies that the aerodynamic forces are actuated upon the physical model according to simultaneous (real-time) simulations of the turbine rather than being generated by a small-scale physical turbine [1,12–14]. Real-time hybrid model testing will be referred to as ReaTHMTM testing (a trademark of MARINTEK) in the following. The platform motions are measured and passed to the numerical simulator, and actuators apply appropriate aerodynamic/generator forces and moments based on the results of the numerical simulations. Basin tests of FWTs with hybrid test techniques are relatively new, and previous tests have focused solely on the application of the thrust force [12]. The test campaign for NOWITECH extends the ReaTHM testing concept to 5 degree-of-freedom aerodynamic/generator loads, at a larger scale than previously tested, using a novel actuation system. The objectives of the test campaign were twofold: 1) to quantify the system behaviour in environmental conditions representative of the Northern North Sea, and 2) to prove the applicability of the hybrid test method. In addition, the results from these tests are expected to be used to validate numerical hydrodynamic and structural models of floating offshore wind turbines. Extensive documentation tests as well as several potentially design-driving load cases with turbulent wind, irregular waves, and current were therefore carried out. The test method is documented in Part I [1], while this work provides details of the test set-up and presents some of the interesting findings from the model test campaign. Calibration of a numerical model of the floating wind turbine is described in Part III [15]. MODEL DESCRIPTION The 5MW CSC platform, which supports the NREL 5MW wind turbine, was selected for testing [16, 17]. The physical model comprises the hull, tower, and rotor-nacelle-assembly (RNA) model for the hybrid test method. The Froude scaling law was applied based on a linear scale factor of λ = 30, and all values given in this paper are given in prototype (full) scale unless otherwise noted. The basin floor was positioned at a depth of 6.66 m which corresponds to a prototype scale depth of 200 m. Coordinate systems As shown in Fig. 1, the global Earth-fixed coordinate system is located at the centerline of the tower at the waterline at the platform’s resting static position. The z axis of the global coordinate system points downward and the system coincides with the location where waves were measured during calibration. This system is offset from the geometric center of the basin (extents shown in pink).

Real-Time Hybrid Model Testing of a Braceless Semi-Submersible Wind Turbine: Part II — Experimental Results