Responses of an articulated tower in waves and currents
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The responses of an articulated loading platform (ALP) in regular and irregular uni-directional waves ( with or without currents ) are investigated both i frequency and time domain. The first - and second-order wave diffractions-radiation are solved by the ring source boundary integral equation method, and the viscous drag forces are computed from the modified Morison equation. The two-term Volterra series and linearized drag force model are used for the spectral analyses in frequency domain, while the fourth-order Runge-Kutta method is used for the rime-domain integration of the nonlinear equation of motion. It is shown that the linear wave-body interaction theory combined with the linearized drag model appears to be satisfactory in computing wave-frequency responses in regular and irregular waves. However, the natural frequency of the ALP pitch motion is typically small, thus the linear theory alone is not adequate to evaluate the resonant slowly-varying pitch responses in random seas, for which the second-order difference-frequency wave loads play the most important role.
