Finite Element Analyses Applied in Design of Foundations and Anchors for Offshore Structures

Abstract

Offshore structures for oil and gas exploitation are designed to accommodate severe environments with large cyclic loads. These structures are either founded directly on the seabed, or they are moored to anchors installed in the seabed soil. The permanent and cyclic loading, the foundation or anchor geometry, and the nonlinear soil behavior may be very complex, and many interrelated aspects must be considered in the geotechnical design of the foundations. Finite-element analyses (FEAs) are used increasingly to deal with these complexities and offer the potential to increase accuracy, efficiency, and reliability and reduce the uncertainty of the design process. This paper presents the major geotechnical aspects in the design of foundations for offshore structures and examples from finite-element analyses carried out at the Norwegian Geotechnical Institute (NGI) to deal with these aspects. A brief review of the procedure used at NGI to obtain soil stress-strain-strength relationships from cyclic laboratory tests is included. It is demonstrated that FEA offers several benefits over classical methods, such as limiting equilibrium calculations.