In order to develop the ocean space effectively, it is required to construct large scale offshore structures. Offshore platform with large deck area may provide great possibilities to develop ocean spaces which may be used as resident areas, airports, power stations, etc.. Since these structures have very heavy dead loads, reaction forces on the base foundation become severe and necessitate very firm foundation system. If these reaction forces can be reduced by some method such as large diameter members of buoyancy-type(semi float type). The concept of using large offshore platforms of semi float type can become popular in the offshore industry. In this study, it is suggested that in order to implement the proper selection of buoyancy type member, it is important to clarify the relations between the effective loads, the external exciting forces, and allowable displacements of the soil-foundation system. Some parameters, which are used for the evaluation of wave force and the dynamic characteristics of the structure, give important effects on the response evaluation. It is shown that since these effects are evaluated with the perturbation method, the reliability index provides good indications of the dynamic response evaluation.
Offshore platform with large deck area may provide great possibilities to develop ocean spaces which may be used as resident areas, airports, power stations, etc. Since these structures have very heavy dead loads, the reaction forces on the base foundation become severe and require very firm foundation system(Sigbjomsson et al (1978)). If these reaction forces on the base foundation system can be reduced using the buoyancy type large members, important contributions on developing the large ocean spaces can be provided by these offshore structures which experience more severe loads than those on land structures.
