Analysis of Gravity Offshore Structure Foundations
Authors: Prevost Jean H. , Hughes Thomas J.R. , Cohen Martin F. 
Published: December 2007

This paper shows that the interaction of an offshore structure with its soil foundation when subjected to complicated wave loading paths can be analyzed in a rigorous manner by taking into account the real three-dimensional, nonlinear, anisotropic, elastoplastic, path-dependent, stress-strain-strength properties of the material.

Introduction

Offshore structures must be designed for the worst expected environmental conditions, including a 100-year wave that may be as high as 30 m (98 ft). Under such extreme loading conditions a structure exerts enormous forces on its foundation. However, it is not sufficient to check its stability for the maximum forces introduced by, the largest possible wave acting as a static load. One also must consider that when this largest wave occurs, the structure already has been subjected to a large number of other, lighter waves causing cyclic loading of its soil foundation. Under such cyclic loading conditions, soils exhibit very significant dissipative and softening characteristics. Thus, the effect of cyclic wave forces on soil foundations is a type of fatigue problem, and it is important to determine to what extent the behavior of soils under cyclic loading may influence the safety of offshore structures.For the foundation design to be sound, the interaction between the structure and its soil foundation when subjected to both monotonic and cyclic, inclined, and eccentric force loading conditions must be analyzed. This set of circumstances is relatively new to geotechnical designers. Traditional methods of analysis no longer are applicable, and new techniques need to be developed.The complexity of the problem dictates that the method of analysis be numerical in nature. However, to make accurate and reliable predictions, a realistic three-dimensional analytical model of soil stress-strain-strength behavior is essential. Indeed, it must be emphasized that the solution provided by even the fanciest finite element program will never be better or more realistic than the constitutive equations adopted to model soil behavior. Because the cyclic wave forces subject the foundation materials to complicated three-dimensional stress-strain histories involving cycles of loading, unloading, and subsequent reloading, the dependence of the constitutive equations on their stress-strain history must be taken into account for the analysis of the wave/ structure/foundation interaction to be realistic. The general analytical soil model recently proposed by Prevost can cope adequately with such complex Prevost can cope adequately with such complex cyclic histories. The model formulation is general and incorporates both drained and undrained loading situations. It describes the anisotropic, elastoplastic, path-dependent, nonlinear stress-strain-strength properties of inviscid saturated soils subjected to complicated, and in particular to cyclic, three-dimensional loading paths. The model has been coded and incorporated into a finite element program. The purpose of this paper is to program. The purpose of this paper is to demonstrate the use of the formulation in analyzing offshore soil/structure/wave interaction problems.

JPT

P. 199