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In connection with the elastic-plastic failure problem of offshore jacket platform subjected to a powerful earthquake, an improved modal pushover method based on performance design is proposed. The elastic-plastic seismic performance and failure modes of the jacket platform are obtained to solve the problem of elastic-plastic performance evaluation of the offshore jacket platform in strong earthquakes. The elastic-plastic seismic responses of the platform in 8-degree seismic fortification and rare intensity are calculated by using different methods, and the differences between these responses are compared. Besides, the influences of combined modes, mode shape vectors, and uncertainties of seismic are discussed. The results show that the high-order vibration modes and mode shape vectors have a great influence on the elastic-plastic seismic performance of the platform. The first 9 or higher order modes and mode shape vectors should be adopted. The seismic-resistant weak links are located at the top of the platform in 8-degree seismic fortification and rare intensity, which should be paid more attention to. The seismic responses of the platform show significant differences and discreteness in different seismic activities, which have the same peak seismic acceleration. The improved modal pushover method is suggested to be used to evaluate the elastic-plastic seismic performance of jacket platform.

Improving the knowledge about the effect of pipe-seabed-interaction on VIV is quite important for free spanning pipelines, which are often highly nonlinear due to interaction with the seabed. However, the implicit requirement for frequency domain procedures is structural linearity. In this study, a time domain analysis method for nonlinear pipe-seabed-interaction analysis of free spanning pipelines under VIV conditions has been developed. The flow is modeled by a Van-der-Pol wake oscillator developed by Facchinetti et al. And the stall parameter is taken into account in the general form of the VIV amplitude and the response frequency, which can successful model the VIV response amplitude comparing with a constant. After the free spanning pipeline discretized into finite elements, the average acceleration method is chosen to integrate the equations of motion. Three types of pipe-seabed-interaction (linear spring, perfectly plastic nonlinear spring, and tension cut-off nonlinear spring) are considered. The results show that the presented time domain method can capture the non-linear interaction between the pipeline and the seabed at the free span shoulders. And the maximum response amplitude of nonlinear case is lower than the one with linear spring.Copyright © 2009 by ASME