海底地層の影響を考慮した海洋構造物の横振動 （第１報）

Abstract

In order to analyze the lateral vibration of an offshore structure supported by the seabed layer, the effect of this layer on the vibration must be taken into consideration. In this study, firstly, the static stiffness of the seabed sand-layer was determined by equating the theoretical and experimental results on the force-deflection relationship at the top of an elastic pipe, which was partially supported by the sand layer. Then, the natural frequency of lateral vibration of the pipe was theoretically obtained by using the above-obtained static stiffness of the sand layer. Furthermore, experiments were carried out on the free lateral vibration of the pipe and the experimental natural frequency of the lateral vibration was obtained. Finally, the theoretical and experimental natural frequencies were compared to examine the validity and applicability of the evaluation method developed for the static stiffness of sand layer, and to observe the difference between its static and dynamic stiffness.The results obtained are as follows (1) The natural frequency of the pipe supported by sand layer is smaller by about 30-98% than that of the pipe fixed rigidly by the layer. Therefore, it is essential to consider the stiffness of the layer in the analysis of lateral vibration of an offshore structure.(2) The theoretical natural frequency calculated by using the static stiffness of sand layer obtained in this study is lower by about 10-22% than the experimental frequency, indicating that the dynamic stiffness of sand layer may be greater than the static one.(3) The method developed in this study for evaluating the static stiffness is easier and more useful than the existing method for practical application though about 20% error is involved in the analysis of the natural frequency.