Model Test of a 1:8 Scale Floating Wind Turbine Offshore in the Gulf of Maine

This dissertation presents the development and validation of the novel 1:8 scale floating turbine testing methodology that allows one to replicate full-scale design conditions in less than one year of offshore testing. This test method was carried out for a new floating wind turbine foundation design called VolturnUS developed by the University of Maine. The design uses innovations in materials, construction, and deployment technologies such as a concrete semi-submersible hull and a composite tower to reduce the costs of offshore wind. These novel characteristics require research and development prior to full-scale construction. This dissertation presents a unique offshore model testing effort aimed at de-risking a full-scale commercial project by providing properly scaled global motion data, allowing for implementation of full-scale structural materials, and demonstrating full-scale construction and deployment methods. This dissertation presents sample data and comparisons to numerical model predictions which confirm the validity of the testing methodology. The model is a 1:8-scale version of a 6MW semi-submersible floating wind turbine deployed far offshore Maine. The model was deployed offshore Castine, Maine in June, 2013 and supports a 20kW turbine. The model is the only grid-connected offshore wind turbine in the United States of America. In addition to presenting the testing effort, relevant data collected during the project are presented with the objective of confirming the applicability and usefulness of the 1:8 scale offshore model test methodology. The model testing effort includes careful treatment of the offshore test site, scaling methods, model design, and construction. A suitable test site was identified that provides the correct proportions of wind and waves in order to simulate scaled design load cases prescribed by the American Bureau of Shipping Guide for Building and Classing Floating Offshore Wind Turbines for a potential full-scale 6MW floating wind turbine design deployed far offshore in the Gulf of Maine. Combined wind, wave, and current design load cases were estimated using metocean buoy data collected in the Gulf of Maine by the University of Maine and National Oceanic and Atmospheric Administration. Design load cases include 500-year return period survival storm conditions with the turbine parked, 50-year extreme storm conditions with the turbine operating, and normal operational environments. A buoy deployed at the model test in Castine verified that the model test site can produce the desired scaled environmental conditions with a high probability. Data from the construction, deployment, and testing effort are presented which confirm the usefulness and applicability of the 1:8 scale offshore model testing approach. The construction and deployment successfully demonstrated the intended full-scale methods. Sample model test data is provided from environmental events representative of scaled design conditions. Model test data is directly compared to full-scale design predictions made using coupled aeroelastic/ hydrodynamic software. VolturnUS performance data from scaled extreme sea states show good agreement with predictive models. Model test data are also compared to a numerical representation of the physical model for the purposes of numerical code validation. The numerical model results compare very favorably with data collected from the physical model. These data confirm the appropriateness of the methodology used to develop the 1:8 scale model test. The effort has confirmed the suitability of the scaling laws, de-risked the new full-scale design, provided data usable for prediction of full-scale behavior, and generated data useful for numerical model validation