A multi-mode N2 (MN2) pushover procedure for ductility level seismic performance evaluation of jacket type offshore platforms

Abstract

The static pushover analysis can be employed to check the adequate ductility of jacket type offshore platforms (JTOPs) under the abnormal level earthquake (ALE) in the design phase. The conventional pushover analysis suffers from different inherent limitations such as disregarding the higher mode effects. In this paper, it is intended to develop an advanced pushover procedure for the seismic design of JTOPs. To this end, a multi-mode N2 (MN2) pushover procedure is proposed that modifies the SRSS modal combination rule in the extended N2 procedure. The proposed procedure utilizes several modes of vibration to increase the accuracy of the method. The presented procedure is applied to two case study JTOP structures for the ductility level seismic performance evaluation. The seismic demands obtained by the MN2 procedure is compared with the average results of the benchmark nonlinear response history (NRH) analyses conducted using a suite of far-field ground motion records scaled to the ALE design response spectrum. The results of a statistical study show that there is a strong correlation between the spectral values of the records at the shorter periods of JTOPs and the responses of the foundation and the lower braced story levels of the jacket, which emphasize the important effects of the higher modes on the seismic demands of JTOPs if considering soil-pile-structure interaction. The utilized formulation for the modal combination is shown to improve the accuracy of the MN2 method at the pile foundation and at the lower braced story levels of the jacket. Moreover, the seismic drift demands predicted by the proposed MN2 method are in excellent agreement with the benchmark NRH results along the height of the platforms. Considering the simplicity and ease of application, this method can be utilized as a useful tool for checking the design ductility level earthquake.