NONLINEAR AND EXTREME WAVE GROUP INTERACTIONS WITH A CIRCULAR CYLINDER

The evaluation of extreme wave kinematics is decisive to determine design standards for marine structures. Offshore rogue waves in high wind areas can emerge as a consequence of unstable wave group focusing. Large-amplitude waves in such wave groups have different kinematic properties compared to those resulting from the superposition principle. This experimental study has been carried out to evaluate the wave load forces of highest amplitudes within unstable wave groups acting on a submerged, vertical, and circular cylinder. Different wave steepness parameters have been systematically adopted. It is shown that for moderate steepness values the wave loads show the presence of three harmonic components. Higher wave steepness parameters are responsible for the change of the cylinder's response in the form of a transient ringing. In fact, even though higher harmonics of the underlying regular wave trains do not resonate with the lowest resonant frequency of the cylinder, the severe broadening of the wave spectra in all bound wave components, resulting from the wave focusing within the unstable wave packet, triggers a ringing frequency excitation.