The dynamic response of an offshore platform with four pile legs in the 30m water depth is investigated theoretically to random waves of uni-direction by following two calculation methods.
1) Calculation by means of the spectral analysis method.
　A mathematical model is developed for the linearized wave-structure-foundation system and response characteristics of the platform displacement and the bending
stresses along the pile are calculated by means of the spectral analysis method for the Pierson-Moskowitz wave specrum of the significant wave height Hs=5.0m～0.5m.
2) Calculation by means of the wave simulation method.
　For the system of wave-structure, a time series model is developed and the wave simulation technique is applied to give a time series data of wave forces on piles including the non-linear drag force component. The displacement of the platform and stresses of the pile are calculated step by step for the trains of irregular wave simulated from the Pierson-Moskowitz spectrum of Hs=5.0m.