Experiments on responses of very large floating offshore structures in directional spectrum waves

Many plans to construct of very large offshore structures of floating type are proposed, such as floating cities, airports, and so on. The size of floating structures is expected to become larger and the natural frequencies of structural vibrations become lower. In these conditions, wave exciting force and moment induce structural vibration. The flexibility of structures must be considered in the phase of not only operation but also construction. Therefore, it is important to study the response including the structural deformation.This paper deals with the experiments on the responses of very large floating offshore structures in directional spectrum waves. To clarify the structural response in waves, response functions of flexible models for experiments are estimated. For this purpose, the response function is divided into two transfer functions. One shows the relation between the wave height and the wave exciting force, and the other evaluates the structural deformation due to the wave exciting force in a modal space. The natural frequencies and mode shapes are evaluated by structure analysis program for simple models on the condition of floating. The two transfer functions are verified by experiments respectively and utilized to estimate the response function in waves. The estimated response functions show that the position in the structure differs the amplitude of response. So, there are some frequency regions in that the larger response causes at the lee side than the weather one. This characteristics are also shown in the results of experiments in transient water waves.Three types of models are used for experiments in directional waves. Difference among the three models is only the structural flexibility. The structures are moored by a single buoy of a TLP type. The experiments are performed in many waves. The measured responses are displacements in the inertial coordinate, structural deformations and tensions of the mooring system. In the experiments, the most flexible structure is designed as follow. When the structural natural frequency is same as the wave one, a shape of the structural deformation agrees with the wave length. So the response of the large flexible offshore structures are easy to be affected by characteristics of waves, directional distributions and frequency properties. And, it is also shown that the characteristics of response for flexible structure is different from ones of rigid structures.