Floating wind turbines are novel renewable energy devices designed to operate in deep waters (>60 m), where most of the offshore wind energy resource is found. These floating turbines are structurally different from other existing offshore systems, and their structural stability in offshore conditions has not been extensively tested.
Intellectual Merit
This interdisciplinary research effort will make use of an experimental facility for earthquake engineering and state-of-the-art computational methods to study the dynamic structural responses of floating wind turbines in ocean waves. Two models, a frequency-domain model and a nonlinear time-domain model, will be developed to study the fluid-structure interaction problem. For simulations of wave-body interaction, the first model uses a linear boundary element model, while the second model applies a fully-nonlinear mixed-Eularian-Lagragian method. Both models include a comprehensive, fully-nonlinear dynamic model of the mooring system and a FEM model of the structural responses. The two-model approach enables the study of both long-term responses of the system (e.g. for fatigue studies) and transient response due to large waves (e.g. rogue waves). The research plan will focus on three areas: long-term structural response in wave fields, wave-structure resonances, and responses to large waves. For the model verification studies, full-scale shake table experiments will be carried out on a 65 kW system, and the recorded structural responses will be compared with model predictions. The models will then be used to predict the performance of a scaled-up 5 MW system.
This research is potentially transformative because it will provide new tools to evaluate the feasibility of current designs for floating wind turbines. This research could also be used to guide the development of future deep-water offshore wind farms.
Broader Impacts
A unique aspect of this work is that it uses earthquake engineering tools, specifically the Large High-Performance Outdoor Shake Table at University of California - San Diego, to study offshore wind energy structures.
In addition to training of undergraduate and graduate students directly involved in the research effort, learning materials based on their research will be incorporated into existing undergraduate and graduate structural and fluids mechanics courses. Other activities include using the Large High-Performance Outdoor Shake Table at University of California at San Diego as a demonstration to reach out to K-12 students from under-represented groups on topics related to renewable energy. Websites for this facility will be updated to include the floating wind turbine.
Wind is a key renewable energy resource. To date the focus of development is mostly on land-based wind farms. A major challenge of this technology is that the onshore sites with significant wind resource are usually located in areas far from densely populated regions. This causes difficulties and wastes in distribution of the generated electricity through transmission grids. On the other hand, these densely populated regions are mostly in the coastal areas. Therefore a straightforward solution is to exploit offshore wind energy. Another advantage of offshore wind farms is their easy access to steady wind so that there will be less wear on turbines. In addition, their impact upon human society is mitigated in comparison with their land-based counterparts. Among the existing offshore wind energy harvesting systems, a floating wind turbine is a novel design which greatly extends the deployable region to deep water (water depth larger than 60 meters). The design of floating wind turbines is tremendously challenging. On the one hand, these deep-water turbines work under completely different conditions from the land-based or shallow water ones. On the other hand, they are also structurally different from any existing offshore platforms. Therefore, existing design standards for land-based wind turbines or offshore structures cannot be directly applied. In this study we carried out a combined numerical and experimental studies to investigate the structural responses of floating wind turbines under the combined excitation of wind and water waves. On the numerical side, we have developed computational models to simulate wave-structure interactions, wind-blade interactions, as well as the structural mechanics of the mooring system (made of a system of cables) and the tower. On the experimental side, we have conducted full-scale experiments by using the UCSD Large High Performance Outdoor Shake Table (LHPOST) to test the responses of a 65KW wind turbine. The vibrations of the platform are based on simulation results of wave-induced motions so that with this setup we are able to duplicate the structural responses of the tower in waves. The accuracy of our structural model has also been validated by these experiments. In addition to the gained knowledge about the underlying physics of hydro-aero-elastic interactions, an important product of this research is a comprehensive numerical model capable of simulating the dynamics of floating wind turbines. The model will be a valuable tool in the future design and development of these systems.
