Recent Advances in Nonlinear Soil Models for Fatigue Evaluation of Steel Catenary Risers SCRs

Abstract

Abstract The role of soil response on fatigue life of Steel Catenary Risers (SCRs) has been investigated through a scaled sectional SCR (cut-out section) in a geotechnical centrifuge. The SCR model simulated a section of the SCR starting at 16 ft (4.9 m) above the seafloor and extending approximately 360 ft (~110 m) through the touchdown region to the anchor point. The centrifuge tests were successfully performed using soils from the Gulf of Mexico (GoM) and offshore Angola. Past studies have demonstrated that fatigue damage in a full-scale SCR at touchdown zone can accurately be modeled using a scaled sectional SCR in a geotechnical centrifuge. The centrifuge results were compared to finite element simulations with the seabed response modeled using nonlinear springs derived based on unload-reload stiffness (secant stiffness) of degraded soil at steady-state conditions. This resulted in development of a simplified approach involving the nonlinear soil springs. Fatigue analyses indicated that this simplified approach can predict fatigue damage as measured in the SCR centrifuge tests. Centrifuge tests were also performed to determine the impact of trench depth on SCR fatigue. The tests involved a flat seabed and seabed with an initial prescribed trench. SCR fatigue damage in the test with an initial prescribed trench was similar to that of the flat seabed; however, the critical fatigue point had significantly shifted.