Analysis of Lateral Vibration of an Offshore Structure with Consideration of the Effect of the Seabed Layer (2nd Report)

摘要

In order to analyze the lateral vibration of an offshore structure supported by the seabed layer, its effect on the vibration should be taken into consideration. The lateral deflection of an elastic vertical pipe, which was partially supported by the seabed layer and at the top of which a concentrated lateral force was applied, was theoretically analyzed by assuming the uniform and exponential distributions for the stiffness of the layer. Then, experiments were carried out on the force-deflection relationship of an elastic pipe under the same condition as mentioned above. In addition, the values of the stiffness for the various thickness of the sand layers under various conditions were determined by comparing the theoretical deflections with the experimental ones.Furthermore, the free lateral vibration of the above-mentioned pipe considered as a leg of an offshore structure was theoretically analyzed by using the above-obtained static stiffness of the sand layer. The result indicated that both the theoretical natural frequencies pertaining to the uniform and exponential distributions of the stiffness of sand layer fairly well coincided with the experimental frequency, and the latter distribution of stiffness gave a little better estimation than the former. Moreover, the nondimensional quantity CSD defined in this study among the natural frequency of lateral vibration of the pipe, the stiffness and linear damping coefficients of sand layer was found to be nearly constant in various cases of the sand layers without above water-layer, such as CASE 1 and CASE 2. Thus, the damping coefficients in these cases could be easily obtained by this CSD if the natural frequency of lateral vibration of the pipe and the stiffness of the sand layer were known in advance.Finally, in order to evaluate the damping coefficient of the sand layer, above which the water-layer exists, in CASE 3, corresponding to the practical case, the coefficient was calculated by multiplying the damping coefficient for CASE 2 of wetted sand layer without above water-layer by the frequency-ratio of ASE 3 to CASE 2, under the assumption that the damping coefficient was proportional to the natural frequency of lateral vibration of the pipe if the thickness of sand layer were constant. The obtained damping coefficient for CASE 3 fairly well coincided with the experimental one, identifying the validity of the trove-mentioned assumption and procedure. Hence, this procedure could be applicable for finding the above coefficient of seabed layer in practical cases.