The soft settled type of offshore structures are different from their fixed counterparts in that they are simply sunk onto the sea bed without any anchoring, which allows for slip to occurs if a large horizontal external force exceeding the limit value of frictional resistance acts at the base. In this paper, a two dimensional rigid body of rectangular section is dealt with. Harmonic response analysis of the body subjected to surface sea waves and ground motion is carried out and the influence of the friction force and mooring stiffness on the response are investigated. It is concluded that although large displacement develop in soft settled type of structures they have the merit of developing relatively small restoring force (i.e. reaction force acting on the structure) compared to the fixed type of structures.
The floating type of offshore structures have the merit that the dynamic loading due to horizontal earthquake motion is reduced by the effect of base isolation, but motion as rigid body is generally large and a weak point that sinking could occur. On the other hand, fixed type of structures seldom sink and it has only relative displacement which is very small compared to the motion of the floating type of structure, while it is a demerit that the force acting on the structure due to earthquake motion is not dissipated. Fukusumi et al.(2000), compared the response characteristics of a an offshore cylindrical shell structure modelled both as floating type and fixed type structure. The soft settled structures are considered to have both the merits of not being endangered by sinking and being capable of reducing the dynamic load due to external dynamic excitation. The dynamic response of these structures due to ground motion was studied experimentally and theoretically by Sakuta et aL(1980).