Performance Study Of A Reinforced Concrete Floating Platform For Offshore Wind Turbines

The bottom-fixed foundations that have been widely deployed in the offshore wind farms are economically limited to a maximum water depth of 50 m. In such cases, the floating offshore wind turbine(FOWT) can serve as an economically viable option for further exploration of deep-water wind resources. Recent years have seen many conceptual designs and a few pilot projects of the FOWT, such as Hywind in Norway, Wind Float in Portugal and Compact Semi-Sub in Japan. However, there is still no floating wind farm so far all around the world, which may attribute to the high cost of the floating platforms for offshore wind turbines that were mainly made of steel structures.Reinforced concrete structures have the potential to be an alternative to the floating platform for offshore wind turbines since it can reduce the construction costs, increase the corrosion resistance, and simplify the construction procedure compared with the steel structures widely used currently. This thesis presents a conceptual design of a reinforced concrete semi-submersible floating platform for a 5 MW wind turbine, and the numerical simulations have been carried out to analyze the performance of the proposed floating platform, including the stability, hydrodynamics and strength characteristics. The results show that the conceptual design satisfies the code-specified requirements in different loadin g conditions, indicating the feasibility of a reinforced concrete floating platform for offshore wind turbines. The main works were summarized as follows:(1) The state-of-the-art development of the semi-submersible floating wind turbines has been investigated. Based on the summary of existing achievement, a flow chart for the conceptual design and the directions for design optimization have been proposed. Research on the performance of the semi-submersible floating platform including stability, hydrodynamics and strength analysis has been reviewed. Finally, the most critical issues that need to be addressed are identified.(2) A design formula for setting the limit of metacentric height of a semi-submersible platform for floating wind turbines has been proposed according to steady-state heel angles and rolling periods based on the stability theory in marine engineering, then the most critical heeling axle of the floater has been analyzed theoretically. Finally, numerical simulations have been performed using the SESAM software to analyze the intact and damaged stability of the proposed conceptual design.(3) Based on the relevant theoretical basis of wave force calculation, the motion equation of a floater in the frequency domain under regular wave has been derived. Then, the hydrodynamic model of FOWT has been simulated to compute and analyze the Response Amplitude Operators(RAOs) of the platform, and the influence of the heave plates to heave motion has been analysed.(4) The overall finite element model of FOWT has been established. The wind and current loads were computed according to the empirical formulas, while the design wave method based on spectral analysis has been used to identify the worst-case wave directions and periods, and response spectra of structural internal forces on four critical sections and one critical point have been calculated to determine the characteristics of the worst-case wave loads. Furthermore, the internal forces of the platform under wave, wind, and curr ent loads have been calculated accordingly and combined using the load factors. The global strength of the platform, especially the connection nodes between columns and beams, has been investigated.