Abstract
In order to make a simple analysis for the lateral vibration of an off-shore structure constructed with multilegs and a platform, more simplified method than the one used in the 1st or 2nd report was applied to the structure by replacing it with the new analytical model, evaluating its flexural rigidity and then by transforming the structure into the equivalent lumped mass method. The new model proposed in this study was the vertical cantilever beam, the lower end of which was fixed, and the upper end of which was loaded by the axial load and constrained not to rotate, even if it could be displaced horizontally by the small amount of displacement.Furthermore, the validity of this model was examined by the model experiments carried out on three kinds of multi-leg-structures which had 8, 10 and 12 legs.The main results obtained are as follows:(1) The values of natural frequency and displacement for each structure vibrating in water are independent of the direction of vibration within the ranges of the leg-intervals and the dimensions of legs used in this study.(2) The natural frequency obtained by the proposed method, which utilizes the Theory (II) or Theory (IV) defined in this paper, coincides very well with the experimental frequency within the error range that is about 0.00-1.36%. The value of this range is a little lower than the value (0.07-3.57%) estimated by the Theory (2) or Theory (4) defined in the 2nd report, and so the proposed method in this study gives a little more precise natural frequency than the other methods.(3) Furthermore, the method in this study gives the fairly well estimation with respect to the amplitude of the lateral vibration of the multi-leg-structures, since the error involved in the estimation is only about 9-14% until the fifth period of vibration.(4) The value of the frequency or displacement estimated by the Theory (II) gives the same degree of the error range as one by the Theory (IV).(5) Finally, it can be realized that the simple method proposed in this paper could be satisfactorily available for analyzing the lateral vibration of the practical off-shore structure with multi-legs.
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