Responses of Articulated Loading Platform in Irregular Waves

Abstract

The wave frequency and slowly varying responses of an articulated loading platform (ALP) in unidirectional irregular waves are investigated both in frequency and time domain. The first- and second-order wave diffraction/radiation problems are solved by the ring-source boundary integral equation method, and the viscous drag forces are computed from the Morison drag formula based on the relative velocity squared. The two-term Volterra series are used to calculate the time series of nonlinear potential excitations in random seas. The linearized drag force model is used for the spectral analysis in the frequency domain, while a direct time-stepping integration method is used in the time domain to include the nonlinear drag. The relative importance of various excitation and damping sources for different-size ALPs is discussed. The second-order low-frequency potential excitation is in general much greater than drag-induced low-frequency forces. The low-frequency responses can be comparable to or even greater than the wave-frequency responses, depending on the available damping; hence, they need to be included for the reliable motion analysis of an ALP. Finally, it is shown that our computation agrees well with Sincock's experiment.