No abstract

There are global efforts in the offshore wind community to develop reliable floating wind turbine technologies that are capable of exploiting the abundant deepwater wind resource. These efforts require validated numerical simulation tools to predict the coupled aero-hydro-servo-elastic behavior of such systems. To date, little has been done in the public domain to validate floating wind turbine simulation tools. This work begins to address this problem by presenting the validation of a model constructed in the National Renewable Energy Laboratory (NREL) floating wind turbine simulator FAST with 1/50th-scale model test data for a semi-submersible floating wind turbine system. The test was conducted by the University of Maine DeepCwind program at Maritime Research Institute Netherlands' offshore wind/wave basin, located in the Netherlands. The floating wind turbine used in the tests was a 1/50th-scale model of the NREL 5-MW horizontal-axis reference wind turbine with a 126 m rotor diameter. This turbine was mounted to the DeepCwind semi-submersible floating platform. This paper first outlines the details of the floating system studied, including the wind turbine, tower, platform, and mooring components. Subsequently, the calibration procedures used for tuning the FAST floating wind turbine model are discussed. Following this calibration, comparisons of FAST predictions and test data are presented that focus on system global and structural response resulting from aerodynamic and hydrodynamic loads. The results indicate that FAST captures many of the pertinent physics in the coupled floating wind turbine dynamics problem. In addition, the results highlight potential areas of improvement for both FAST and experimentation procedures to ensure accurate numerical modeling of floating wind turbine systems.

The DeepCwind consortium is a group of universities, national labs, and companies funded under a research initiative by the U.S. Department of Energy (DOE) to support the research and development of floating offshore wind power. The two main objectives of the project are to better understand the complex dynamic behavior of floating offshore wind systems and to create experimental data for use in validating the tools used in modeling these systems. In support of these objectives, the DeepCwind consortium conducted a model test campaign in 2011 of three generic floating wind systems: a tension-leg platform (TLP), a spar-buoy (spar), and a semi-submersible (semi). Each of the three platforms was designed to support a 1/50th-scale model of a 5-MW wind turbine and was tested under a variety of wind/wave conditions. The focus of this paper is to summarize the work done by consortium members in analyzing the data obtained from the test campaign and its use for validating the offshore wind modeling tool, FAST.

To better access the abundant offshore wind resource, efforts are being made across the world to develop and improve floating offshore wind turbine technologies. A critical aspect of creating reliable, mature floating wind turbine technology is the development, verification, and validation of efficient computer-aided-engineering (CAE) tools. The National Renewable Energy Laboratory (NREL) has created FAST, a comprehensive, coupled analysis CAE tool for floating wind turbines, which has been verified and utilized in numerous floating wind turbine studies. Several efforts are underway to validate the floating platform functionality of FAST to complement its already validated aerodynamic and structural simulation capabilities. The research employs the 1/50th-scale DeepCwind wind/wave basin model test dataset, which was obtained at the Maritime Research Institute Netherlands (MARIN) in 2011. This paper describes further work being undertaken to continue this validation. These efforts focus on FAST’s ability to replicate global response behaviors associated with dynamic wind forces and second-order difference-frequency wave-diffraction forces separately and simultaneously. The first step is the construction of a FAST numerical model of the DeepCwind semi-submersible floating wind turbine that includes alterations for the addition of second-order difference-frequency wave-diffraction forces. The implementation of these second-order wave forces, which are not currently standard in FAST, are outlined and discussed. After construction of the FAST model, the calibration of the FAST model’s wind turbine aerodynamics, tower-bending dynamics, and platform hydrodynamic damping using select test data is discussed. Subsequently, select cases with coupled dynamic wind and irregular wave loading are simulated in FAST, and these results are compared to test data. Particular attention is paid to global motion and load responses associated with the interaction of the wind and wave environmental loads. These loads are most prevalent in the vicinity of the rigid-body motion natural frequencies for the DeepCwind semi-submersible, with dynamic wind forces and the second-order difference-frequency wave-diffraction forces driving the global system response at these low frequencies. Studies are also performed to investigate the impact of neglecting the second-order wave forces on the predictive capabilities of the FAST model. The comparisons of the simulation and test results highlight the ability of FAST to accurately capture many of the important coupled global response behaviors of the DeepCwind semi-submersible floating wind turbine.

Results of wave basin tests on three 1/50th scale floating wind turbine systems tested at the MARIN model basin are presented. The tests included a fully functional model wind turbine and a novel wind machine to produce swirl free inflow at a turbulence intensity of about 5%. Simultaneous stochastic wind and waves as well as multidirectional sea conditions were tested. This paper presents the experimental work as well as validation comparisons to NREL’s FAST floating offshore wind turbine dynamic modeling code. The paper also discusses the testing methodology and presents means to more closely match full scale performance at the low-Reynolds number operation regimes of the model test. Analyses presented include response amplitude operator and power spectral density plots for the spar-buoy, tensionleg platform and semi-submersible designs. The results presented for the systems highlight both turbine response effects and second-order wave diffraction forcing effects.