FATIGUE ANALYSIS OF STEEL CATENARY RISER AT TOUCHDOWN ZONE UNDER NONLINEAR RISER-SOIL INTERACTION

Using the P-y curve to simulate the interaction behavior between SCR and soil, the paper studies the riser-soil interaction response and performs fatigue analysis of SCR at TDZ under cyclic motions. The sag-bend and flow-line of SCR are modeled by flexible beam with large curvature and elastic foundation beam. SCR involves complex nonlinear dynamic behaviors, especially at the Touchdown Zone(TDZ) where the riser starts to touch the seabed. It is difficult to numerically simulate the riser-soil interaction due to the assocaited uncertainty in seabed model. A FORTRAN programming code modeling the riser-soil interaction is introduced into CABLE3D, and then CABLE3D RSI is obtained. The riser-soil interaction response and the global dynamic analysis are performed using the developed program, and the structural fatigue assessment is carried out by S-N approach. Results indicate: 1) SCR has been perturbed by 10 regular sinusoidal cycles and the responses show a number of features such as suction force mobilization, gradual increasing penetration depth, and gradually reducing mobilization of soil resistance at maximum penetration. The nonlinear riser-soil model accords with the actual seabed properties compared with linear and rigid seabed; 2) The comparative analysis shows that some points between TDP and the maximum embedment point are the key locations where maximum fatigue damage occurs. Parameters such as shear strength, suction factor play a vital role for the dynamic riser-soil interaction and fatigue behavior at TDZ.