Abstract
The paper presents a comparison between experimental data and numerical results of the nonlinear wave induced heave and pitch motions and structural vertical shear forces and bending moments on a containership. The experimental data was obtained from tests with a model of a containership advancing in head regular waves with different steepness, ranging from small to large amplitude waves. Strong nonlinear effects were detected on the experimental data, identified by: the variation of the transfer functions with the wave amplitude, the existence of higher harmonics and the asymmetry of the response records with respect to the positive and negative peaks.
The experimental data is compared with numerical results from a nonlinear time domain strip method. The method assumes that the radiation and diffraction hydrodynamic forces are linear and the nonlinear contributions arise from the hydrostatics and Froude-Krilov forces and the effects of green water on deck. The effects of vertical viscous forces due to flow separation are also investigated. The associated forces are represented by a semi-empirical formulation and depend of cross flow drag and lift coefficients.
Nonlinear vertical motions and loads on ships advancing in waves have been investigated before, both experimentally and numerically however, the present paper focuses on several new aspects of the nonlinear vertical structural loads in regular waves. The new aspects include the identification and quantification of third harmonic amplitudes, the influence of the regular wave amplitude on the amplitudes of the first three harmonics and on the sagging and hogging peaks, and the influence of the steady structural loads, that exist also with the ship advancing in calm water, on the asymmetry of the vertical loads in waves. Finally, the influence of viscous forces associated with the large amplitude motions on the wave induced motions and structural loads are also investigated.