Analysis of Offshore Foundations Subjected to Storm Loading

The potential for liquefaction of seabed sand underneath an offshore foundation subjected to cyclic storm loading is investigated. A semi-analytical approach has been adopted in the formulation of three dimensional finite element method for consolidation analysis. The effects of densification due to cyclic loading are also considered in the formulation. Results of experimental studies on sand samples are used to evaluate the effect of cyclic loading at different points in the soil. As an illustration of the method, the possibility of liquefaction of calcareous sand around a hypothetical offshore foundation under storm loading is investigated.

Introduction

The stability of foundations for offshore structures can be strongly affected if the seabed sediments have the potential to liquefy under wave-induced cyclic loading. The potential for liquefaction of seabed soils, particularly loose sands, is therefore a major issue that should be considered by the designers of offshore facilities on granular materials.

Foundations of marine structures are generally subjected to two kinds of loadings; ambient loads due to submerged weight and cyclic loads due to waves applied during a storm. Cyclic loads include a large number of cycles of short to medium periods (5 to 15sec.). Laboratory tests on sands have shown that the application of a large number of cyclic loads with moderate amplitude could produce a progressive degradation of the soil resistance and buildup of pore water pressures, which can alter the stability of marine structures founded on such soil. The pore pressure at various places within the soil profile may build up to a stage where it becomes equal to the mean effective stress resulting in cyclic liquefaction and leading to possible instability.