The significant influence of the riser-seabed interaction on the fatigue performance of steel catenary risers is now widely accepted. Most design, however, is still carried out using linear seabed springs, and assuming a flat seabed. Improved nonlinear hysteretic seabed models have recently been proposed, which automatically simulate the different stiffness in the seabed response through the touchdown zone. A further consideration, however, is the influence of the trench that forms at the seabed. ROV surveys have shown that trenches several diameters deep can develop beneath the riser in the early stages of the SCR life, and a critical question is how this affects the fatigue life. A non-linear soil hysteretic model has been used to model gradual trench development in the touchdown zone. Initially, the seabed model parameters are adjusted to allow trenches of varying depth to be developed over a moderate number of displacement cycles of the SCR. Design wave spectra are then applied, simulating a generic Spar system, after correcting the model parameters to more typical values normal range. The paper presents results that show the impact of trenches of different depths on the fatigue performance of SCRs in the touchdown zone.

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