Nonaxisymmetric analysis of porous soil under gravity structures

Among the wide variety of exploratory, drilling and production structures that are being used, the concept of gravity based structure is well established. These structures directly rest on natural seabed which can be visualized as a two phase porous medium consisting of soil particles and surrounding pore fluid. Though the geometry of these structures and the nature of the loading they encounter are three dimensional, it has been the general practice to idealize these structures as two dimensional in their analysis and design, for practical reasons. This thesis examines the influence of this assumption by comparing the static, transient and dynamic responses of such a foundation for two and three dimensional idealizations. -- The three dimensionality of the problem is idealized by an axisymmetric structure- soil medium subjected to environmental forces which are not axisymmetric, but whose resultant falls in a vertical plane passing through the axis of symmetry. The soil is represented by a two phase porous medium based on Biot's formulation. The governing equations are solved by using finite element technique. In the simplified three dimensional analysis, the non-axisymmetric nature of the loading and field variables is accounted by assuming their variation to be smooth in the circumferential direction (0) and expressing them as Fourier series in terms of 0. Material nonlinearity of the soil skeleton is considered by flow theory of plasticity, but the geometric nonlinearity is neglected. The validity of this approach is assessed by extensive comparisons with cases where analytical solutions are available. Comparisons are also made with more accurate three dimensional numerical solutions as well as with the results published by other researchers. -- Using the finite element program developed for this purpose, several element types and computational schemes suggested in the literature are evaluated for the simple cases and appropriate techniques have been selected for further analysis- A comparison of the results from full three dimensional, two dimensional (plane strain) and simplified three dimensional analyses for the simplest case of an elastic foundation indicated the beneficial aspects of the simplified method with reference to its efficiency and accuracy. A detailed parametric study on the effects of load eccentricity and inclination on the nonlinear response of undrained soil under plane strain conditions has shown that the collapse loads computed are in good agreement with empirical values used by the profession. Further comparisons between two dimensional and simplified method revealed that the three dimensional effects result in stiffer load-deformation response in general. Similar comparisons of the consolidation analysis and dynamic response analyses are also presented. It is concluded that the simplified method is very effective in terms of its accuracy and computational economy at working load level, but its accuracy decreases as collapse is approached