Study On 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, etc., 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.In this paper, seismic responses of jacket platforms are studied from three aspects based on three calculation methods of seismic responses as follows:First of all, 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.Secondly, 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- θ method and applying the FEM software ANSYS.Finally, 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 by using software MATLAB. 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 of a non-uniform beam, such as auto-PSD function, cross-PSD and higher spectral moments, can be solved directly avoiding the calculation of normal modes.