VIM (Vortex Induced Motion) is one of the important issues in the safety evaluation for cylindrical floating offshore structures. The VIM is basically placed as a phenomenon to occur in strong current, but that also appears in current and waves in the sea where offshore structures are installed.

The authors have recognized the phenomenon that the motion amplitude of a cylindrical floater in current and irregular waves together is larger than the sum of the motion amplitude in current and in irregular waves respectively in a VIM experiment. This VIM amplification phenomenon in the current and waves is remarkable when wave height is relatively low that has high occurrence frequency in the sea. It is, therefore, expected that the amplification phenomenon has large influence on the accumulative fatigue damages of the offshore structure’s mooring system.

In order to make clear this VIM amplification phenomenon, the authors have conducted detailed VIM experiment in waves using a circular cross sectional mono-column floater model. The results of the VIM experiment in current and waves are described in detail in this paper. The results of the experiment in irregular waves show different characteristics for VIM amplitude in current. The results in regular waves show the effect of wave height and wave period on VIM amplitude in waves. Using those results, the mechanisms of the VIM amplification in waves are investigated.

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