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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.

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Two different parameters for the quantitative description of beam halo are discussed. Both are based on moments of the particle distribution and represent a convenient and model-independent method for quantifying the magnitude of beam halo observed in either spatial or phase-space projections. One parameter is a measure of spatial profile of the beam and has been defined by Wangler and Crandall previously. The current authors defined a new parameter using kinematic invariants to quantify halo formation in 2D phase space. Here we expand the development and present detailed numerical results. Although the spatial-profile parameter and the phase-space halo parameter both reduce to the same value when the distribution has the elliptical symmetry, in general these parameters are not equal. Halo in the 1D spatial profiles is relatively easily measured, but is variable as the beam distribution evolves and can hide as it rotates in phase space. The 2D phase-space halo is more difficult to measure, but it varies more smoothly as the halo evolves. It provides a more reliable characterization of the halo as an intrinsic property of the beam.

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