Incorporation of the riser-seabed-seawater interaction effects into the trench formation and fatigue response of steel catenary risers in the touchdown zone

Abstract

The existing Steel Catenary Riser (SCR)-seabed interaction models do not account for the contribution of seawater as a key influential factor. The present study has examined the effect of SCR-seabed-seawater interaction on the trench enlargement and consequently on the fatigue performance of the SCR in the touchdown zone (TDZ). Coupled Eulerian-Lagrangian (CEL) analysis with three domains of SCR, seabed soil, and seawater were performed using ABAQUS. A parametric study was conducted to investigate the impact of seawater interaction along with incorporation of the shear strain rate and softening effects. The conventional bearing capacity approach for analysing pipe penetration into the seabed was also reviewed, and modifications were proposed to account for the effects of water entrainment into the trench. The results obtained from CEL analysis were integrated with a global SCR model to incorporate the SCR-seabed-seawater interaction effects into the fatigue analysis. The study showed that the entrainment of seawater into the trench can be coupled with SCR oscillations to further soften the seabed and erode the trench resulting in an enlarged trench profile affecting the fatigue damage in the TDZ.