Random vibration analysis of gravity platforms subjected to wind-generated waves and earthquakes

Abstract

In this work a methodology is proposed to perform the dynamical analysis of a concrete gravity platform, e.g.p., subjected to wind generated waves and earthquakes. Particular emphasis is put on the prediction of the so-called long-term structural responses, i.e. the probable responses during and beyond the structure proposed lifetime. The uncertainties on the properties of the material of the structure, the ones related to the supporting soil, as well as the ones associated to the mentioned environmental loads, are taken into consideration in the proposed methodology. An important feature of the methodology is that it allows to carry out parametrical studies aiming to determine not only the structural responses of interest but also to define in a statistical sense the relative influence of each of the parameters in those responses. An application of the proposed methodology is presented. A parametrical study on the maximum responses of a concrete gravity platform located in the North Sea was carried out. The parameters included were: Youngs modulus and critical damping of the reinforced concrete, soil media idealization and soil shear modulus, wave and seismic excitation. A seismic risk analysis for the platform site was performed because of the lack of seismic information of the North Sea region. The responses considered here were the deck displacement, the base shear force and the overturning moment. The study showed that the uncertainties about the mentioned excitations provide the largest contribution in the dispersion values of the responses. The mean values of the long-term responses were about 30 % higher than the short-term ones for the considered case. This shows the importance of their computation as an important step in the assessment of the dynamical behaviour of this type of structures.

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