Seismic Behavior of Jacket Offshore Platform Subjected to Near and Far Field Ground Motions

Offshore structures such as jacket platforms have to inevitably be designed against  sever  environmental actions. In seismically active areas these structures also become susceptible to earthquake excitations. Strong ground motions recorded  in recent earthquakes, including the 1995 Kobe, Japan, 1999 Chi-chi, Taiwan and 1999 Kocaeli, Turkey earthquakes, revealed that the dynamic motions in near fields are dominated by a large, narrow band, long period pulse caused by rupture directivity effects. Sever damages have been reported with specific bridges, quay walls and multistory buildings near to the shaking fault. It was been noticed that the dynamic characteristics of these structures were close to the characteristics of the rupture directivity and felling pulses.This paper deals with the behavior of existing jacket platform under near field earthquake loading. As an example seismic assessment of the existing 4 legged service platform placed in Persian Gulf is presented. A finite element approach utilizing “Opensees” Standard software has been chosen for this study. A dynamic non-linear direct integration analysis method has been used. Seven, near and far fault records have been examined.  In general it has been found that the far field excitations produce more critical consequence for the jacket offshore platform as compared with those from the corresponding near field excitations. It has been noticed that the correspondence between the dominant frequency of the record and the main natural frequency of the jacket structure can be used as a criteria to decide whether a far or near field Earthquake is more critical.