| The stochastic nonlinear response analysis and dynamic reliability of offshore platforms on the extreme sea environments are studied in this paper. The research work includes: study on the sea environment load and its load effect, the dynamic ultimate capacity of offshore platforms, stochastic response analysis, and dynamic reliability assessment of offshore jacket platform. As an application of practical engineering, the structural dynamic reliability of Wei 11 -4C offshore platform in the south sea of China is evaluated on the extreme sea environments. The major contents are summarized as follows:1 ) On the serious sea environment, the static ultimate capacity analysis of offshore platform system is done, and the dynamic ultimate capacity is determined by double damage criterions. The dynamic ultimate capacity breaks through the category of the static ultimate capacity. It is important to the development of dynamic reliability theory. Then based on the stochastic vibration analysis of multi-degree system, combined with dynamic reliability theory, the calculation expression of dynamic reliability index is presented.2 ) The Winkler beam model is used to simulate pile-soil dynamic interaction. So the soil is treated as Winkler foundation. The pile is disposed as beam imbedded in the soil. The soil reaction is modeled as reaction spring and damper. Then according to t-z curve , Q-z curve and p-y curve measured in the sea filed geology, the effect of structure-pile-soil mutual interaction is systematically studied.3 ) The dynamic reliability of offshore jacket platforms to the sea wave has been studied. The dynamic reliability theory integrates the dynamic response theory and probability theory. Firstly, the random variables that mainly affected the response characteristic of offshore platforms are sampled, then the stochastic responses of offshore platforms are analyzed on the basic of those random variables, and the dynamic responses may be statisticated, so the dynamic reliability of offshore platforms is gained. Two structural models supported by the piles or fixed at end are adopted into the process of those analyses. Also, the effects of geology uncertainty, ocean environment parameter's uncertainty, different wave theories, structure-pile-soil mutual interaction, and fluid-structure interaction are considered on the dynamic response of offshore platforms. By means of the method proposedin the paper, the dynamic reliability of an offshore platform, named by Weil 1-4C, in the south sea of China is evaluated to the sea wave. The results show that the response of platform considering the pile-soil-structure mutual interaction is less than that of platform fixed in the six-time-diameter, the dynamic reliability index of Weill-4C offshore platforms is about 2.6, but the target reliability is about 2.5~2.8. So the Weil 1-4C platform is safe and suitable for practical application.4) The dynamic reliability of offshore jacket platforms to the seismic action has been studied in this paper. Many effects of uncertainties for environment, structure and earthquake are considered on the dynamic response of offshore platforms. Using the method proposed in the paper, the dynamic reliability of Weill-4C offshore platform in the south sea of China is evaluated to seismic action. The results show that the peak response of model considering the structure-pile-soil mutual interaction is less than that of fixed-model because soil has anti-seismic capacity in the structure-pile-soil model, but fixed-model cannot consider this effect. The response of X&Y a united-action (X&Y) seismic spectrum is larger than of single direction seismic spectra. The dynamic reliability index of weill-4C offshore platforms is about 1.6, less than target reliability index (1.5). So the Weill-4C platform is safe.5) A hybrid solution combined with analytical and numerical method is proposed to compute the stochastic response of the fixed offshore platforms to random wave, considering wave-structure interaction and non-linear drag forc...
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