Trenching effects on dynamic behavior of a steel catenary riser

Abstract

The fatigue life of a steel catenary riser (SCR) near its touch-down zone (TDZ) is substantially affected by its interaction with the seabed. Therefore, accurate estimate of the fatigue life of a SCR requires the understanding and realistic modeling of this interaction. The interaction depends on several factors, such as soil properties, riser characteristics, and the development of trenching at the seabed. Existing approaches for modeling the seabed in interaction with a SCR approximate the behavior of the seabed soil by linear or nonlinear spring and dashpot, which represent the stiffness and damping of the soil, respectively. However, these approaches do not account for certain phenomena resulting from the plastic deformation of soil, such as trenching development at the seabed. In this study, a more realistic approach is developed for simulating the interaction between a SCR and the seabed. In addition to the use of a realistic P–y curve (where P stands for the supporting force of the seabed and y for the vertical penetration of the riser into the seabed) to simulate the soil deformation during its interaction with the riser, it considers the development of a trench caused by continuous impact of a riser on the seabed and then its feedback effect on the variation of the bending moment along the riser. It is found that the trenching development on the seabed may decrease the maximum variation of bending moment of a riser near its TDZ. Since the variation of bending 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 the SCR and hence it may have important application to the design of a SCR.