Dynamic response of a framed offshore tower to ice forces considering nonlinear soil-structure interaction

Haldar, Asim Kumar. (1977) Dynamic response of a framed offshore tower to ice forces considering nonlinear soil-structure interaction. Masters thesis, Memorial University of Newfoundland.

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Abstract

The dynamic response to ice forces of a pile-supported framed offshore structure is studied for two types of foundation conditions: i) Rigid base and ii) Semi-Rigid base. The semi-rigid base is idealized in two ways: i) lumped parameter (also called half-space, continuum and soil spring) foundation model with linear soil springs, and ii) finite elemented foundation model with linear and nonlinear soil behaviour. -- The structure analysed is an offshore tower supported by piles. The members are assumed to be rigidly connected and the added water mass is assumed equal to the mass of the water displaced. The structural modelling is based on a two-dimensional representation of the tower assuming a constant dimension equal to the base length perpendicular to the plane. The masses per unit length of the members in the plane of the frame are computed by summing up the structural mass, the amass of the water contained in the tube and the mass of the water displaced. The masses of the members perpendicular to the plane are assumed to be lumped at the horizontal cross-brace level joints. -- Linear analysis is carried out for the fixed base condition. For the lumped parameter foundation model, the foundation is idealized by linear horizontal and rotational springs at the mudline level and, for the finite elemented foundation model, by two-dimensional beam elements. The response is determined by modal superposition. The nonlinear behaviour due to shear deformations of the soil is handled by the equivalent linearization technique. Mode superposition facilitates an approximate solution in which the stiffnesses used are made compatible with effective shear strain amplitudes at the soil element centroids. The analysis uses the published data on strain-compatible soil properties for clays and sands, and the final values of the soil element stiffness properties are estimated by an iterative procedure. The investigation is carried out for two types of clay: a) Soft, and b) Stiff. In all the analyses, damping is expressed terms of the modal damping ratio which includes structural damping, viscous damping due to fluid drag, and pile damping due to interaction with the soil. -- The results of the study indicate that soil-structure interaction causes a complete redistribution of stresses in the structural members. For nonlinear soil behaviour, a typical member stress is 48% more than that for the rigid foundation model. Therefore, the study highlights the considerable need for including nonlinearity in soil-structure interaction studies. Recommendations are made to extend this investigation to other kinds of environmental forces on offshore structures, e.g. waves and currents.

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