Experimental Loads On a Flexibly Mounted Vertical Cylinder In Breaking Wave Groups

In order to better understand the physical nature of the ringing phenomenon, a thin-walled vertical cylinder, pivoted at the tank bottom and held by an adjustable vertical tension wire, was subjected to loadings by both non-breaking Stokes waves and by waves breaking in modulated wave groups. Both downtank displacement and acceleration were measured in time. Two frequencies, the rigid body and first bending, neither at the Stokes harmonics frequencies, were excited in the ringing style by the breaking waves, but not observably by the monochromatic waves. A strong correlation was found between the rigid body loading (deduced from measurements) and the local wave slope as measured by wave wires on the cylinder. The onset of the high frequency response, however, was correlated with breaking jet impact on the cylinder. Introduction In order to gain understanding of the physical mechanisms involved in the observed excitation of the structural response of large ocean structures by North Sea wind waves, experimental hydroelastic studies have been carried out in the OEL at UCSB. Experimental techniques continuously evolved through a series of three different tests beginning in 1996, and concluding in August 1997. Since this work was begun very similar experiments with results overlapping those found here have been conducted in the UK by Chaplin et al. (1997). The ringing phenomenon, first identified in the early eighties, can be described as a transient resonant response of the structure, and many studies of it have been undertaken. According to Davies et al. (1994), ringing is associated with asymmetric waves of maximum crest height, preceded by a shallow trough, which are found on the forward portion of wave groups. Jefferys & Rainey (1994) suggest that ringing is caused by a fast rise to the peak of the wave, implying large horizontal accelerations and velocities in the wave crest.