Foundation Analysis of the Hibernia GBS

Abstract This paper presents the procedures followed in the analysis of the Hibernia GBS foundation. The Hibernia GBS is a cylindrically shaped offshore platform approximately 100 m in diameter which rests directly on the seabed. Soils at the GBS location consist mainly of dense to very dense sands, with varying content of fines, and with only two clay layers at large depths. The analysis incorporated pore pressure build up and dissipation during typical and extreme storms with the resulting soil cyclic degradation. Soil parameters were determined from in-situ tests performed in a geotechnical field investigation and from a comprehensive laboratory program which included testing under cyclic loading and under different stress paths. Resulting cyclic soil strengths used in design are presented as well as results from limit equilibrium analysis. Introduction The Hibernia GBS structure, located in the North Atlantic, will be subjected to severe environmental loads. As a result of this, several new aspects of analysis and design had to be resolved. The foundation engineering at Hibernia consisted mainly of four tasks:evaluation of site bathymetry and subbottom stratigraphy;site selection, in-situ testing and soil sampling;soil laboratory testing; andfoundation analysis which included interpretation nd definition of soil parameters and verification of GBS stability under environmental loads. This paper deals mainly with the last activity. The foundation analysis consisted primarily of the assessment of foundation stability when subjected to environmental forces such as waves, iceberg impact and earthquake. Limit equilibrium stability assessment was done for operating conditions and for different stages during the installation process. In addition, skirt penetration analysis and settlements were also estimated as well as equivalent soil spring stiffnesses and soil reactions, for use in GBS structural analysis. This paper will focus on the stability analysis for wave and iceberg loading, computation of equivalent soil spring stiffnesses for wave and iceberg loading and finally settlements. Soil Conditions at the Hibernia GBS Site The soil conditions at the GBS site, including the criteria used for site selection as well as details of the field work, have been presented elsewhere (1). However, due to the importance of this aspect, a summary will be presented here. Soil samples and in-situ testing were performed during a geotechnical investigation conducted in the summer of 1992. This information, together with a previous geotechnical investigation conducted nearby in 199 1, provided most of the soil information needed for soil characterization. At the final selected site, nearly continuous sampling and piezocone penetration test records were performed using the technique of overlapping contiguous sampling depths. The close sampling and testing interval was to assure that all possible weak zones were detected. Undisturbed and disturbed soil samples for testing in the laboratory were obtained; sampling was performed to depths up to 120 m below seabed. The GBS location is predominantly a sand site with only two clay layers at large depths. Large variations on fines content are found in the sands encountered, and some of the strata consist of layered sands and silts with clay intrusions.