When analyzing the lateral vibration of an offshorestructure supported by seabed layer, the effect of this layer must betaken into consideration. In this study, first, the natural frequencyof lateral vibration of the offshore structure constructed with fourlegs and a platform was analyzed theoretically by replacing it withthe one-leg structure, the upper end of which was loaded by anaxial load corresponding to the equivalent mass of a platform perleg and was constrained to move in the horizontal direction only.Furthermore, the validity of this theory was examined by model experiment.The main results obtained are as follows:(1) The theoretical natural frequency calculated by the methodreplacing the 4-leg structure with one-leg structure coincideswith the experimental one within the difference of 7percent,and so this method is valid even in case that the effect of seabedlayer is taken into consideration.(2) In order to calculate the natural frequencies of practicaloffshore structures with good accuracy, the leg's axial forcedue to the weight of the platform must be considered, and theleg's own weight may be neglected in case of a shorter-legstructure (L<= 75 m in this study but must be taken intoaccount in case of a longer-leg one.
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