Ringing response of vertical small diameter cylinder under the action of freak waves is numerically simulated and analyzed in this paper. Spectrum analysis results of the responses of vertical small diameter cylinder under freak waves show that the responses of cylinder contain high-frequency components. When one of the high-frequency components is close to or is integral multiple to the natural frequency of the cylinder, ringing response may be induced. So the action of the freak waves which are strong nonlinear and asymmetry is one of the most important reasons of ringing of offshore structures.
In recent years, significant interest has been shown in identifying the mechanisms that induce ringing in offshore structural systems. This high frequency transient type response has been observed in offshore systems, particularly in tension leg platforms. By far the mechanism of ringing has not been fully understood yet. In the study of ringing of offshore structure, a need exists for clear identification of the system characteristics and environmental conditions that lead to its onset. Ringing is the strong transient response under severe loading conditions triggered, presumably, by the passage of a high, steep wave event. The transient response decays to steady state at a logarithmic rate that depends on the system damping. Ringing is a rare event, and has been unaccounted for in standard response analysis codes until recent experimental and full-scale observations brought it to right. This has been attributed to the higher-order loading mechanisms leading to its onset. Studies reported in the literature (e. g., Davies et al., 1994, Jefferys and Rainey, 1994, Natvig, 1994, Faltinsen et al., 1995) have focused on large volume structures and full scale and model test observations, (e. g., wave profiles and model behavior and validation of numerical procedures), are utilized in these studies.