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Reference wind turbines are an important component to the wind energy sector. They serve as publicly available benchmarks that can be openly used to explore new technologies and designs as well as aid in facilitating collaborative efforts between researchers and industry. Earlier this year, the International Energy Agency (IEA) 15-megawatt (MW) reference wind turbine was released and currently represents the largest publicly available reference machine (Gaertner et al. 2020). The size of the IEA 15-MW reference turbine mirrors the wind industry's trend of offshore machines with larger power ratings. According to the U.S. Department of Energy's "2018 Offshore Wind Technologies Market Report" and the American Wind Energy Association, significant development has occurred in the past few years that highlights the opportunity for targeted research investment in offshore wind (Musial et al. 2019). Several states including Massachusetts, New York, and Maryland have enacted new policies or bolstered their existing policies to support the development of over 4,000 MW of offshore wind energy. Looking to the near future, the U.S. offshore wind project development pipeline includes 25,824 MW of potential installed capacity (Musial et al. 2019). Though the total U.S. offshore wind energy potential is more than twice what the entire country currently uses, nearly 60% of the U.S .offshore wind resource is located in deep water, requiring floating foundation technologies (Schwartz et al. 2010). In most commercial wind farms in Europe, and more recently the United States, offshore wind turbines are supported on monopoles in water depths up to 30 meters (m) and steel jacket structures from 25 m to about 50 m. In water depths over 50 m, where a majority of the U.S. offshore wind power potential lies, the cost of jacket foundations becomes prohibitively expensive, requiring the use of floating offshore wind turbine technologies. This report serves as an addendum to "IEA Wind TCP Task 37: Definition of the IEA Wind 15-Megawatt Offshore Reference Wind Turbine" (Gaertner et al. 2020) and defines the University of Maine (UMaine) VolturnUS-S reference floating offshore wind turbine semisubmersible, designed to support the IEA 15-MW reference wind turbine. The design and arrangement described in this report are intended to generically represent future floating offshore wind turbine technology. In addition to the floating platform, this report also details the other floating-specific components of the floating offshore wind turbine including the mooring system, tower, and turbine controller.

Abstract. This paper describes the development of a new reference controller framework for fixed and floating offshore wind turbines that greatly facilitates controller tuning and represents standard industry practices. The reference wind turbine controllers that are most commonly cited in the literature have been developed to work with specific reference wind turbines. Although these controllers have provided standard control functionalities, they are often not easy to modify for use on other turbines, so it has been challenging for researchers to run representative, fully dynamic simulations of other wind turbine designs. The Reference Open-Source Controller (ROSCO) has been developed to provide a modular reference wind turbine controller that represents industry standards and performs comparably to or better than existing reference controllers. The formulation of the ROSCO controller logic and tuning processes is presented in this paper. Control capabilities such as tip speed ratio tracking generator torque control, minimum pitch saturation, wind speed estimation, and a smoothing algorithm at near-rated operation are included to provide modern controller features. A floating offshore wind turbine feedback module is also included to facilitate growing research in the floating offshore arena. All of the standard controller implementations and control modules are automatically tuned such that a non-controls engineer or automated optimization routine can easily improve the controller performance. This article provides the framework and theoretical basis for the ROSCO controller modules and generic tuning processes. Simulations of the National Renewable Energy Laboratory (NREL) 5 MW reference wind turbine and International Energy Agency 15 MW reference turbine on the University of Maine semisubmersible platform are analyzed to demonstrate the controller's performance in both fixed and floating configurations, respectively. The simulation results demonstrate ROSCO's peak shaving routine to reduce maximum rotor thrusts by over 10 % compared to the NREL 5 MW reference wind turbine controller on the land-based turbine and to reduce maximum platform pitch angles by nearly 30 % when using the platform feedback routine instead of a more traditional low-bandwidth controller.

Abstract. This paper describes the development of a new reference controller framework for fixed and floating offshore wind turbines that greatly facilitates controller tuning and represents standard industry practices. The reference wind turbine controllers that are most commonly cited in the literature have been developed to work with specific reference wind turbines. Although these controllers have provided standard control functionalities, they are often not easy to modify for use on other turbines, so it has been challenging for researchers to run representative, fully dynamic simulations of other wind turbine designs. The Reference Open-Source Controller (ROSCO) has been developed to provide a modular reference wind turbine controller that represents industry standards and performs comparably to or better than existing reference controllers. The formulation of the ROSCO controller logic and tuning processes is presented in this paper. Control capabilities such as tip-speed ratio tracking generator torque control, minimum pitch saturation, wind speed estimation, and a smoothing algorithm at near-rated operation are included to provide a controller that is comparable to industry standards. A floating offshore wind turbine feedback module is also included to facilitate growing research in the floating offshore arena. All the standard controller implementations and control modules are automatically tuned such that a non-controls engineer or automated optimization routine can easily improve the controller performance. This article provides the framework and theoretical basis for the ROSCO controller modules and generic tuning processes. Simulations of the National Renewable Energy Laboratory (NREL) 5-MW reference wind turbine and International Energy Agency 15-MW reference turbine on the University of Maine semisubmersible platform are analyzed to demonstrate the controller's performance in both fixed and floating configurations, respectively. The simulation results demonstrate ROSCO's peak shaving routine to reduce maximum rotor thrusts by nearly 14 % compared to the NREL 5-MW reference wind turbine controller on the land-based turbine and to reduce maximum platform pitch angles by slightly more than 35 % when using the platform feedback routine instead of a more traditional low-bandwidth controller.