Abstract: The piles of jacket platform move in cycles, so as to increase excess pore pressure of soil, decrease the lateral soil resistance and weaken pile foundation. The basic natural frequency that based on equivalent simplified pile theory not considering the degradation of pile foundation which fixing the pile at 6 pile diameters below mudline still has some difference with the test result. In a simulation experiments, cyclic environmental loading impose in jacket platform, a 1:10 scale model, and measure the vibration characteristics in x and y directions of jacket platform model after different cyclic loadings. The experiments aim to study the relation between pile foundation weakening and platform horizontal vibration characteristics. According to experiment results, lateral soil resistance show different degrees of decay in different cycle loadings; and the degradation of pile foundation is related to the amplitude and frequency of reciprocating motion of piles. Because of the effect of pile foundation weakening, the basic natural frequency of jacket platform decreases when lateral soil resistance decay. The test values of fundamental natural frequency are lower than the calculated values by 15.5% in x direction and 6% in y direction. And the differences increase to 22% and 13.14% after the degradation of pile foundation occurred. Assuming system damping is proportional damping, the degradation of pile foundation has a small impact on system damping.

Key words: degradation of pile foundation, cyclic loading, lateral soil resistance, fundamental natural frequency