Additional Wind/Wave Basin Testing of the DeepCwind Semi-Submersible With a Performance-Matched Wind Turbine

In 2011 the DeepCwind Consortium, led by the University of Maine (UMaine), performed an extensive series of floating wind turbine model tests at the Maritime Research Institute Netherlands (MARIN) offshore basin. These tests, which were conducted at 1/50 scale, investigated the response of three floating wind turbine concepts subjected to simultaneous wind and wave environments. The wind turbine blades utilized for the tests were geometrically-similar models of those found on the National Renewable Energy Laboratory (NREL) 5 MW reference wind turbine and performed poorly in the Froudescaled, low-Reynolds number wind environment. As such, the primary aerodynamic load produced by the wind turbine, thrust, was drastically lower than expected for a given Froude-scaled wind speed. In order to obtain appropriate mean thrust forces for conducting the global performance testing of the floating wind turbines, the winds speeds were substantially raised beyond the target Froude-scale values. While this correction yielded the desired mean thrust load, the sensitivities of the thrust force due to changes in the turbine inflow wind speed, whether due to wind gusts or platform motion, were not necessarily representative of the full-scale system. In hopes of rectifying the wind turbine performance issue for Froude-scale wind/wave basin testing, efforts have been made by UMaine, Maine Maritime Academy and MARIN to design performance-matched wind turbines that produce the correct thrust forces when subjected to Froude-scale wind environments. In this paper, an improved, performancematched wind turbine is mounted to the DeepCwind semisubmersible platform investigated in 2011 (also studied in the International Energy Association’s OC4 Phase II Project) and retested in MARIN’s offshore basin with two major objectives: 1) To demonstrate that the corrective wind speed adjustments made in the earlier DeepCwind tests produced realistic global performance behaviors and 2) To illustrate the increased capability for simulating full-scale floating wind turbine responses that a performance-matched turbine has over the earlier, geometrically-similar design tested. As an example of this last point, this paper presents select results for coupled wind/wave tests with active blade pitch control made possible with the use of a performance-matched wind turbine. The results of this paper show that the earlier DeepCwind tests produced meaningful data; however, this paper also illustrates the immense potential of using a performance-matched wind turbine in wind/wave basin model tests for floating wind turbines. INTRODUCTION In recent years, significant effort has been put forth researching, designing and testing floating wind turbine concepts in hopes of building an industry that can harness the strong, persistent winds off many of the world’s coasts [1-5]. An important part of this effort has been the development of coupled-aero-hydro-elastic floating wind turbine simulation tools [6, 7] and their subsequent validation with wind/wave basin model test data [8, 9]. Model testing of floating wind turbines in a wind/wave basin provides an opportunity to create valuable data in a controlled environment with minimal time, cost and risk. However, while many floating wind turbine model tests have been performed [4, 5, 10-12], it has proven difficult to properly model the aerodynamic loading of the wind turbine in the low-Reynolds number, Froude-scaled environment of a model wind/wave basin [13]. Even in the comprehensive 2011 DeepCwind model tests led by the University of Maine (UMaine) and performed at the Maritime Research Institute Netherlands (MARIN) of tension-leg platform, spar-buoy and semi-submersible floating wind turbines, poor wind turbine performance from the geometrically-similar blade design required increased wind speeds to create the correct mean thrust values [14]. This data has been used extensively for numerical model validation and it is important to understand the extent to which these test shortcomings distort the test data from the true full-scale response. Proceedings of the ASME 2014 33rd International Conference on Ocean, Offshore and Arctic Engineering OMAE2014 June 8-13, 2014, San Francisco, California, USA