Study of the Fatigue Life of Steel Catenary Risers in Interaction with the Seabed

The fatigue life of a Steel Catenary Riser (SCR) near its touch-down zone is substantially affected by its interaction with the seabed. Hence, accurate estimate of its fatigue life requires the understanding and realistic modeling of the interaction between them. The interaction between SCR and the seabed depends on many factors, such as soil properties, riser characteristics, and the development of trenching at the seafloor. Existing approaches for modeling the seabed in interaction with a SCR approximate the seabed soil by a linear or nonlinear spring and a dashpot which respectively represent the stiffness and damping of the soil. However, they do not take into account certain phenomena resulting from plastic deformation or degradation of the seabed soil, such as trenching. In this study, a more realistic approach is developed for simulating the interaction between a SCR and the seabed soil. In addition to the use of a realistic P-y curve (where P stands for the supporting or resistance force of the seafloor and y for the vertical penetration of the riser into the soil) to simulate the soil deformation during its interaction with a riser, it considers the development of a trench caused by continuous poundings of a riser on the seabed and then its feedback effect on the variation of the bending moment along the riser. In this study, it has been found that trenching underneath a SCR may decrease the maximum variation of bending moment near its touch-down zone. Since the variation of the moment dictates the fatigue damage to the SCR, the results based on this approach indicate that the trenching development at the seabed may increase the fatigue life of a SCR and therefore, it may have important application to the design of a SCR.