Seismic behaviour of fixed off-shore platforms

This study is oriented to set up and verify numerical procedures suitable to investigate the behaviour of a typical offshore platform under seismic loads. In a preliminary phase the natural frequencies of the structure were studied in order to individuate the influence of the most important physical parameters, like added mass and conductors, on the dynamic behaviour of the platform. A model was thus individuated suitable for further investigations. In the second phase response spectrum analyses have been performed with different types of design spectrum: the API Rules spectrum, the Ground Motion Evaluation spectrum and two other spectra corresponding to the accelerograms adopted in the direct dynamic analysis. All the analyses have been referred to a Strength Level Earthquake. Finally transient analyses have been developed for two different time-histories and the results compared with the corresponding spectral results. This third investigation has lead to set up a procedure suitable to be applied in the non linear domain when the effects of RIE earthquake must be considered