Large Deformation Finite Element Analysis of Riser-Soil Interactions with Strain-Softening Soils

Abstract

Dynamic analyses of steel catenary risers (SCRs) suspended from a floating platform usually show that the vicinity of the zone where the riser touches down on the seabed is important to fatigue assessments, and can be critical in realizing an economical SCR design or in assessing SCR design life extension. The motivations of the paper are to advance the understanding of the complex mechanism of SCR-soil interactions in a comprehensive yet efficient way, to supplement the extensive model test-based SCR-soil interaction studies in the literature, and to offer a numerical tool to predict the SCR-soil interaction in different clay types. The objective is to investigate the fundamental mechanism of SCR-soil interactions using advanced finite element (FE) analyses. The FE analysis is conducted in Abaqus using the coupled Euler-Lagrangian (CEL) technique to capture the local large deformation surrounding a riser. A strain-softening soil model for deepwater soft clays subject to cyclic degradation is developed based on cyclic direct simple shear (DSS) tests to avoid cycle-by-cycle simulations. The stress-displacement relations are proposed in the FE analysis to avoid the size effect associated with strain-softening soils. The FE analysis results are compared to the test data from the independently conducted model tests and the established results from the literature. Good agreements among different studies are observed.