Dynamic Curvature of a Steel Catenary Riser on Elastic Seabed Considering Trench Shoulder Effects: an Analytical Model

Prediction of the fatigue life of steel catenary risers (SCR) in the touchdown zone is a challenging engineering design aspect of these popular elements. It is publically accepted that the gradual trench formation underneath the SCR due to cyclic oscillations may affect the fatigue life of the riser. However, due to the complex nature of the several mechanisms involving three different domains of the riser, seabed soil, and seawater, there is still no strong agreement on the beneficial or detrimental effects of the trench on the riser fatigue. Seabed soil stiffness and trench geometry play crucial roles in the accumulation of fatigue damage in the touchdown zone. There are several studies about the effect of seabed soil stiffness on fatigue. However, recent studies have proven the significance of trench geometry and identified the touchdown point oscillation amplitude as a key factor. In this study, a boundary layer solution was adapted to obtain the dynamic curvature oscillation of the riser in the touchdown zone (TDZ) on different areas of seabed trenches with a range of seabed stiffness. The proposed analytical model was validated against advanced finite element analysis using a commercial software. A range of seabed stiffness was examined and the corresponding fatigue responses were compared. It was observed that in the elastic seabed, the effect of soil stiffness is attributed to the curvature oscillation amplitude and to the minimum local dynamic curvature that SCR can take in the touchdown zone. The proposed analytical model was found to be a simple and reliable tool for riser configuration studies with trench effects, particularly at the early stages of riser engineering design practice.