DeepStar study on effect of coupled horizontal and vertical interaction of Steel Catenary Risers with the seabed within the touchdown region
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Copyright 2017, Offshore Technology Conference. Steel Catenary Risers (SCR) are typically subjected to vertical and horizontal motions, as well as rotations, causing the SCR to interact with the seabed within an area described as the Touch Down Zone (TDZ). The interaction between the seabed and the riser at the TDZ induces bending stresses on the SCR that can have a significant impact on the fatigue life of the riser. ROV surveys of installed deepwater SCRs show that, in soft clayey seabeds, SCRs cut deep and wide trenches into the seabed that are up to four or five pipe diameters deep and three to four pipe diameters wide. Despite the observed lateral interaction between the seabed and the SCR, the current standard of practice for fatigue analysis of SCRs, through commonly used design software packages, only considers the vertical interaction of the SCR with the soil in the TDZ, thus neglecting the effect of the lateral interaction between the SCR and the seabed and its potentially detrimental effect on the fatigue life of the riser. A study was commissioned as part of DeepStar Phase X to evaluate the effect of horizontal interaction between the seabed and an SCR. Quasi-static analyses were conducted using a finite element method based program by simplifying the interaction of the SCR and the sea bed as that of a beam resting on, and surrounded by, linear and non-linear springs. The von Mises stress range was calculated for conditions in which the SCR was subjected to vertical motions and to vertical and horizontal motions while varying: (1) the amplitude of the motion; (2) the undrained shear strength of the sea bed; (3) the initial embedment of the riser in the seabed; and (4) the boundary conditions applied to the SCR. The results of our analyses indicate that, for the conditions that were studied, the effect of incorporating horizontal interaction between the SCR and the sidewalls of a trench on the von Mises stress range is typically less than 5%. The analyses also indicate that: (1) the effect of the lateral interaction of the SCR with the seabed increases as the amplitude of the vertical and horizontal motions increase; (2) the contribution of the lateral motion decreases as the soil strength increases and the soil stiffens; (3) the contribution of the lateral motion decreases as the embedment depth increases. This simple study showed that horizontal seabed-SCR interaction may not have a substantial effect on the fatigue life of the SCR. However, other sea states, soil conditions, and geometries may show substantially different results that should be investigated.
