Performance of Offshore Steel Jacket Platforms under Near-source Ground Motion Type Pulses

Abstract

Behaviour of jacket type offshore structures under ordinary ground excitations has already been addressed by a number of researchers. However, their performance under near-source ground excitations has not been particularly addressed in the literature. Records from recent earthquakes, such as the Kobe Japan (1995) and Chi-Chi Taiwan (1999), have revealed that the dynamic excitations in near-source are dominated by a large, narrow band and long period pulse caused by rupture directivity effects. Failure and sever damages were reported to occur in specific bridges, quay walls and multistory buildings near to the shaking fault. The dynamic characteristics of the damaged structures were close to the traits of the rupture directivity and fling pulses.This paper deals with the behaviour of jacket offshore structures under near-source strong ground excitations. A finite element approach has been chosen for this study. The numerical model has first been verified against available experimental data on tubular frames from other researchers. The verified model has then been used to examine the response of the jacket platform models, typical to those in the Persian Gulf, under harmonic, near-source and far-source excitations. To get an insight into both the pre and post failure zones of the structure response, an Incremental Dynamic Analysis (IDA) methodhas been employed.  In general, it has been found that with some jacket models the near-source excitations appear quite critical as compared with those from the corresponding far-sources. Conversely, with other some models, the far-source excitations have been found to be more unfavorable.

Keywords: Offshore Steel Platforms, Near-field Earthquakes, Incremental Dynamic Analysis (IDA)