Modeling of riser-seabed-water interaction at touch down zone using computational fluid dynamics approach

Abstract

Steel catenary risers (SCR) are widely used in deepwater oil and gas production. Due to environmental loading the riser may be subject to six degrees of motion; however, in the touchdown zone (TDZ), the vertical penetration into the seabed and uplift are two of the main components. The riser-seabed-water interaction near the touchdown zone is one of the main concerns in fatigue life design of SCR. During upward displacement, suction develops under the riser and a trench might be formed when it separates from the seabed near the touchdown point (TDP). In subsequent downward movement, the riser penetrates through this trench to the seabed. Therefore, modeling of suction and trench formation is very important. In most of the existing models these factors are incorporated using empirical relationships. It is also recognized that the available finite element (FE) modeling techniques for this large deformation problem are computationally very expensive, although the penetration resistance can be simulated. In the present study, numerical modeling of riser-seabed-water interaction at the TDZ is conducted using ANSYS CFX software to evaluate the response of the riser during its penetration and uplift. A new model for undrained shear strength of soft clay is proposed that is applicable to a wide range of shear strain rates. The models for the effects of strain rate and strength degradation on undrained shear strength are incorporated properly in ANSYS CFX and simulations are performed for one penetration-uplift cycle. The CFX model developed in this study using the subdomain approach is computationally very efficient. It is found that the suction under the riser is the main source of uplift resistance for shallow embedments. The parametric study shows that the maximum uplift resistance and depth of trench depend on uplift velocity and undrained shear strength of clay.