Understanding the dynamics of an FWT (Floating Wind Turbine) is essential for its design and operation. Since a truss structure can reduce the wave load/resistance on the floating foundation, it becomes more popular in industrial applications. In this regard, knowing the effect of slender members of the truss structure on the motion response characteristics of such an FWT is vital. The present work develops a time-domain method for modeling the dynamics of a floating truss-structure wind turbine with multiple rotors on the deck of the platform. A hybrid panel-stick model is built up incorporating the potential flow theory to calculate the wave inertia force and a Morison strip method to calculate the wave drag force. A systematic methodology, and the corresponding efficient tool, have been developed to deal with the floating trussed structure consisting of a set of slender cylindrical members in arbitrary lengths, diameters, orientations, and locations. The Morison dynamic solver is incorporated into the time-domain solver for the FWT dynamics. The proposed model is validated against a model experiment of a semi-submersible FWT with a triangular-shaped truss-structured platform, which was carried out in RIAM (Research Institute for Applied Mechanics), Kyushu University. Good agreements between the simulation results and the experimental data confirm the validity of the developed method. Further numerical simulations are performed in a set of wind and wave conditions to investigate the effect of wave drag force on the FWT dynamics. It is found that without the fluid viscosity, resonant responses are excited in the platform motions at frequencies that are close to the natural frequencies of the FWT system. Via a comparison between the parked conditions and operating conditions of the FWT, it is found that in the presence of steady wind, the translational surge or sway motion is significantly excited at its resonance frequency. This may be attributed to the work done by the wind to the FWT, which enhances remarkably the total kinetic energy of the platform and consequently increases the translational surge or sway velocity of the platform at the equilibrium position. Applying a hybrid panel-stick model will be effective in reducing all these non-realistic large resonant responses.
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