This paper presents the design, fabrication, deployment and testing of a grid-connected floating offshore wind turbine prototype, called VolturnUS, installed off the Castine, Maine coast in June 2013. The prototype is a 1:8 scale prototype and serves to de-risk the deployment of a full-scale turbine. VolturnUS utilizes innovations in materials, construction, and deployment technologies such as a concrete semi-submersible hull and an advanced composite tower to reduce the costs of offshore wind. The prototype unit was designed following the American Bureau of Shipping "Guide for Building and Classing Floating Offshore Wind Turbine Installations". Froude scaling was used in designing the 1:8 scale VolturnUS prototype so that the motions of the prototype in the relatively protected site represent those of the full-scale unit in an open site farther offshore. A metocean buoy which measured wind and wave properties verified the validity of the scaled environment at the selected site. The model was designed to replicate the geometry, materials, construction, tow-out and anchoring methods which will be used at full scale. A comprehensive instrumentation package monitors key performance characteristics of the platform to verify the design assumptions and coupled modeling software. Data collected include: wind speed, turbine power, rotor angular frequency, blade pitch, torque, acceleration; tower bending moment, 6 DOF accelerations at tower top and base, mooring line tensions, and wave elevation at the platform. Analysis of the data collected during 1:8 scale 50 year extreme events show that the design behaves as predicted by numerical coupled modeling software.

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