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, the concept of large offshore platforms with the buoyancy type large diameter member may become popular in the offshore industry. This paper investigates the dynamic response of such a structure under the action of sea waves, currents and earthquakes. The equation of motion of the structure is obtained by the substructure method. The response analysis is carried out using frequency domain random-vibration approach and the results are expressed using root-mean-square (rms) displacements and stresses. The long-term behavior of the offshore structure is examined with first passage probabilities of level crossing of extreme responses. It is suggested that reliable evaluation of the dynamic response can be carried out using the reliability method based on the second moment approach.
One of the possibilities for developing the ocean space effectively may be the construction of large-scale offshore structures. These structures with wide deck areas may be used as residential or commercial areas, airports, waste disposal sites and so on. Since large offshore structures have heavy dead loads, reaction forces on the foundation become severe and require very firm foundations(Kawano et al(l996)). If the reaction forces can be reduced by some method such as buoyancy-type large members, then large-scale offshore structures may become popular. In this study, the dynamic response analyses of offshore platform with buoyancy-type large members subjected to wave loads, current loads and earthquake loads are examined using random vibration approach.
