Geotechnical challenge of offshore mudmat foundation stability: Combining analytical and finite element investigation of bearing capacity of sand overlying soft clay

Abstract

This paper presents how mudmat design of an offshore jacket at North Sea has taken on geotechnical challenge, where very soft clays underlying sand immediately underneath the mudmat prevents the design from directly using conventional analytical procedure. Soil conditions at the jacket site are briefly described and discussed. The paper presents FE modelling strategies adopted for taking on the geotechnical challenge and investigating the bearing capacity of the mudmat under such layered soils; it reveals the failure mechanism of the soils underneath the mudmats during loadings. The paper shows how the FE modelling, together with conventional analytical procedure, has led to the rational development of the bearing capacity envelope and facilitated finalising the design of the mudmats. The paper also briefly presents the skirt penetration analysis to demonstrate the target skirt penetration depth achievable. It has showcased how the geotechnical challenge has been met with the right strategies and holistic approaches. The practical design presented can serve as a reference case for determining the bearing capacity envelope and helping design mudmats, challenged by similar soil condition and mudmat/skirt make-up, for future offshore projects.