ABSTRACT:

The free bending vibrations of fixed offshore cylindrical structures partially or totally submerged in water are discussed. This investigation is necessary for the dynamic response analysis of offshore structures under wave or earthquake loadings. A general solution of the small harmonic vibration problem of a vertical cylindrical pile submerged in an ideal fluid of an infinite layer is given. The solution for hydrodynamic pressures on cylindrical piles including the effect of surface waves was obtained. The equation of fluid-structure interaction was solved by the variational Bubnov-Galerkin method. The added mass distribution profiles along the height of the fixed cylindrical structure depending on different values of the ratios of diameter to height and water depth are presented.

INTRODUCTION

The offshore exploration and the increase in the depth of fixed offshore structures with one or several cylindrical supports have led to the activization of fluid - structure study. In numerous theoretical works the fluid models with Various assumptions and boundary conditions are discussed. The most important assumptions are those ones which take into account the compressibility, viscosity, surface waves influence on structure vibrations and adoption about an identity of natural modes in the fluid and vacuum. The fluid compressibility and pressibiity influence on vertical cylindrical pile vibration is considered by Sheinin (1966, 1967a), Shulman and Gordeyeva (1973), Chandrasekaran et al.(1970), Liaw and Chopra (1970). The influence of the viscosity was studied in the works of Sheinin (1967a), Sheinin and Kalicheva (1983). The problem of small offshore - structure vibrations in fluid is usually solved on the assumption that the waves induced by the vibration of a structure on the free surface may be ignored. In reference to single cylindrical structures this assumption was studied by Sakurai (1964) and Liaw (1974).

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