Study On The Seismic Responses Of Jacket Platforms

Jacket platforms located in the sea not only are subjected to static loads, such as gravity, deck and equipment load, but also endure dynamic forces including wind, wave, current, earthquake, and collision, etc. Among those dynamic excitations, wave and earthquake are two typical loads. At present, there are many studies on responses of platforms under wave load, whereas the studies on earthquake responses of platforms are relatively fewer. Although the arisen frequency of earthquake is lower than that of wave, and its standing time is also shorter than that of wave, the destroy level of great earthquake is rather severe. The seismic activity is intense in our country, while the offshore platform is suffered from strong earthquake, the secondary disasters will happened, expecilly the pollution of ocean environment. Therefore, the aseismic analysis of offshore platforms is necessary.Comparing with the onshore structures, steel jacket platforms are complex structure-pile-soil-fliud systems, and both the structures and the circumstances have their characteristics. Therefore, it is a huge work to analyze the dynamic responses of the complex system, which conserns extensive theories, such as hydrodynamics, earthquake engineering, random vibration and soil dynamics, etc. It is a significant work to study appropriate calculational models and analytical methods whose precision are kept to some extend. The paper is concerned with the seismic responses of jacket platforms and some correlative researches as follows:First of all, the Ritz method is studied. Based on the principle of virtual displacement, the formulae of the functional of the present system are derived by using the characteristic for the pseudo-excitation method of reducing the structural stationary random response analysis to the analysis of structural harmonic response, and then the necessary conditions for the functional being the extreme value are applied to determine the coefficients in order to approximate the random seismic responses of structures. In addition, the evaluation criterion of Ritz vectors is proposed in order to analyze the reasons. According to the variational principle, among all the admissible functions which satisfy the essential boundary conditions, the exact solution will make the functional of the system obtain the extreme value. Therefore, for the whole system, it is advisable to consider the functional as the evaluation criterion. The one which makes the functional obtain the extreme value is the best.The success of the Ritz method depends on how well linear combinations of Ritz vectors can approximate the natural modes of vibration. In this paper, the effects of Ritz vectors on the response results are studied and the functional extreme values are compared. Results are presented for several cases, and show that the Ritz vectors which not only satisfy the geometrical boundary conditions but also satisfy the natural boundary conditions do not always lead to good results.Secondly, nowadays, with the widely use of well-known finite element software, interest in dynamic responses of jacket platforms was sparked on three-dimensional model. Theoretically speaking, the more complex and more precise are the calculation models; the more accurate are the computation results. However, these models are not easily accepted and achieved in engineering practice because of their low efficiency and complexity. Therefore, it is a significant work to study simplified structural calculation model whose precision are kept to some extent. In the papar, a simplified calculational model is brought forward for computing the random seismic responses of jacket platforms. Based on the Hamilton theory, the equation of bending motion is developed and solved by the classical Ritz method combined with the pseudo-excitation method, and then a program for computing random seismic responses is produced. Usually, random responses of this continuous structure are obtained by orthogonality of modes, and some normal modes of the structure are needed, causing inconvenience for the analysis of the non-uniform beam whose normal modes are not easy to be obtained. However, if the pseudo-excitation method is extended to calculate random responses by combining it with the classical Ritz method, the responses, such as auto-PSD function, cross-PSD and higher spectral moments, can be solved directly avoiding the calculation of normal modes. In order to illustrate the proposed approach, an example is presented, and three different types of Ritz functions are adopted which results are compared with the ones obtained from the well-known finite element software. It is shown that the present method is effective and useful in aseismic design of platforms.Thirdly, the dynamic responses of three-dimensional platforms subjected to earthquake load are compared by adopting different types of models, that is soil-pile-fliud interaction model and model with attached water and equivalent pile. The results and the numerical comparisons are provided in diagrams and tables. It is shown that the natural period changes long and the responses changes small for the soil-pile-fliud effect is considered. The natural frequencies are calculated by applying there-dimentional model and simplified beam model. Based on the comparison of the results, the frequency of simplified beam model is modified. The frequency modified beam model can be used to compute the vibration characteristics of platforms, which can save the time, promote the accuracy, and provide the reference for the vibration calculation of the jacket platforms. Fourthly, the response spectrum method and some related concepts are expatiated by combining with the Criterion for Classifying and Constructing Fixed Offshore Platforms, and the spectrum analysis is given to a jacket platform.Finally, some main contents on time-history analysis method are explained including the selection of parameter, structure model, and calculation process, especially the selection of earthquake wave which concerns the amount, shape, and intensity of seismic wave. Two illustrations are presented by programming based on the Wilson- 6 method and applying the FEM software.