Study On Dynamic Response Of Pile-Soil Interaction Offshore Platforms

With the development of ocean technology, more and more extremely large and long flexible offshore platforms used for various purposes are built in hostile ocean environments. Offshore platforms are very complex and the external wave force on the platform is uncertain. Thus, these platforms are prone to excessive wave-induced oscillations under both operating and extreme conditions. It results in injury of human health, structural fatigue and failure, reducing the accuracy and reliability of the equipments on the platforms, lowering the serviceability and survival of the platforms. Hence, the durability of the equipments and the safety for the structures, it is essential to predigest platforms and predict the dynamic response effectively.Pile-legs of fixed and jacket platform are usually embedded in complicated soils so that pile-soil interaction will strongly affect the dynamic characteristics of the offshore platforms. In the past, a model of fixed support at a depth of six times the pile leg diameter has been commonly used to analyze the dynamic characteristics of the platforms that maybe make big errors. In this paper, an effect coefficient determined by the depth of pile-leg embedding is calculated by using energy method. This finding demonstrates that under most conditions the usual fixed support model is unreasonable that should be given up.The dynamic response of simplified offshore platforms considering the soil-structure interaction is studied. The piles of the platforms are modeled as dynamic Winkler beams embedded in a deep homogeneous stratum through continuously distributed linear springs and frequency-dependent dashpots. Based on equivalent inertial moment, platforms are equivalent to variational cross-section Winkler-beams. The governing equilibrium equations pile-soil, structure-water and structure-air are given. Analytical solutions are developed for harmonic force in platforms laterally oscillating pile embedded in layers of homogeneous soil and excited at sea level. The dynamic characteristic influenced by the ratio of pile stiffness to soil stiffness is studied. It can be found that both pile-soil interaction and the masses on the top of platforms will strongly affect the dynamic characteristics. Compared with the usual three-dimensional FEM model, this study demonstrates that an equivalent model can be used to analysis dynamic characteristic at the initial stage. The analytical solution of pile dynamicresponse in layered soils made by far field earthquake Rayleigh Wave is present. The conclusion of this paper may be meaningful to the factual structures.