Gullfaks A Gravity Platform- Specialized 3-D Problems
For the project, Fugro developed an analytical procedure for dynamic soil-structure interaction analysis of offshore gravity structures. Fugro also developed guidelines for two and three-dimensional modeling of the foundation soil and the super-structure using FLUSH and SASSI computer programs.

The first gravity-based production platform was installed in the North Sea in the summer of 1975. Since then, platforms of this kind have been considered for use in regions associated with a high earthquake exposure. Numerous analytical procedures have been successfully adopted for seismic analysis of massive concrete structures such as nuclear power plants. However, because of the unique characteristics of gravity platforms (weight, size, flexibility of the superstructure, etc.), applicability of these procedures has not been established for those structures.

Therefore, a study was initiated by STATOIL to define a sequence of consistent acceptance criteria and realistic and practical seismic analysis procedures for sites in the North Sea experiencing both 100 and 10,000-year recurrence earthquakes. The analytical procedure included soil-structure interaction effects which are known to be significant in the seismic response calculation of gravity structures. The seismic, site, and structural data used in the study were the currently available data for the Gullfaks A platform.

By including the effects of soil-structure interaction, the calculated dynamic response of offshore gravity structures due to earthquake-induced ground motions can often be drastically reduced. In this study, structural response was calculated to investigate the effect of using different soil-structures interaction methods. Results from a linear and a nonlinear structural response analysis were compared in order to assess the applicability of the linear ductility ratio method to the platform structure.