Study On Seismic Fortification Of Offshore Platform Based On The Seismic Environment Of Bohai

Offshore platform is the main facility for exploitation of ocean resources,and it is a kind of structure that could lead to severe subsequent disasters if destroyed. Moreover, the seismic activity is much intensive in seas of China. No specific seismic fortification design code has been drafted for offshore platforms in China because of insufficient knowledge on the seismic characteristics of the coastal waters. The related codes, such as and the RP2A-WSD constituted by the American Petroleum Institute, are taken as reference for the determination of seismic design parameters of offshore platforms. But the direct citation of these codes leads to many problems, the consistency of design rule and the applicability exceeding probabilities are challenged.Compared with the Yellow Sea, East China Sea and South China Sea, the seismic intensity and frequency of Bohai are much stronger, and accordingly much more investigations have been done in this area. The present study on seismic fortification of offshore platform is based on the seismic environment of Bohai, such as the design rule and objective, the design reference period, probability of exceedance and design parameters. The main contents and technical route are as follows. Firstly, the research status and development on seismic fortification of related industries home and abroad, the structural characteristics of offshore platform and related earthquake engineering problems, the seismic environment of Bohai are analyzed, simultaneously, considering the economic level, the design rule and technical idea. Then, the seismic fortification objectives are determined based on the related experiences and the structural characteristics and seismic damage states of offshore platforms. The research results of seismic design parameters of offshore platforms are collected and analyzed, and the seismic fortification levels of offshore platforms are calibrated using analogy method. The seismic risk characteristics of Bohai are delineated and the seismic design parameters of offshore platforms are presented based on the seismic environment of Bohai. Finally, the structural safety is demonstrated by a numerical model of a platform built in Bohai. The main results are as follows:(1) The comprehensive analysis of seismicity, economic status and related seismic codes home and abroad prove that the exceeding probabilities of seismic design currently used during the design of offshore platforms in China are too conservative.(2) In view of the special flexibility and ductility of offshore platforms, the related seismic problems are presented, such as seismic fortification objective, long-period of ground motion, ground motion parameters and scenario earthquake, et al.(3) The seismic environments of Bohai are investigated by the analysis of historical earthquakes, spatio-temporal distribution of earthquakes. The result shows that Bozhong, that is the central area of Bohai, is the crowded area of strong earthquakes, 4 earthquakes of magnitude above 7 took place in this area. By the comparison of intensity of seismicity and seismic risk with Southern California, the seismic risk in Bohai is much lower, this provides a basis for the comparison of seismic design level between this study and API code to demonstrate the rationality of the recommended exceeding probabilities.(4) The two-stage seismic design objectives are determined, that is the serviceability limited state and deformation limited state. According to the existing seismic design parameters, the return periods of the corresponding earthquakes are determined as 200a and 1000 to 3000a by the analog analysis method.(5) The seismic risk characteristic of Bohai is delineated, and the seismic design parameters of different exceeding probabilities are presented based on the seismic risk zonation. A numerical analysis of a platform proves the rationality and reliability under the recommended seismic design level.The innovation of this study is that the two-stage seismic fortification objectives and seismic design parameter are firstly presented, and the seismic design parameters of different exceeding probabilities based on seismic risk delineation are determined.The research results provide the seismic design level and parameter for the design of offshore platforms, and provide basic data and method for the determination of seismic code for offshore platforms. Simultaneously this study fills up the blank of seismic design on offshore platforms in China, and it is an improvement for the seismic design codes of different structures in China.