Realistic predictions of service life of steel catenary risers (SCR) require an accurate characterization of seafloor stiffness in the region where the riser contacts the seafloor, the so-called touchdown zone. This thesis presents the initial stage of development of a simplified seafloor support model. This model simulates the seafloor-pipe interaction as a flexible pipe supported on a bed of springs. Constants for the soil springs were derived from finite element studies performed in a separate, parallel investigation. These supports are comprised of elasto-plastic springs with spring constants being a function of soil stiffness and strength, and the geometry of the trench within the touchdown zone.
Deflections and bending stresses in the pipe are computed based on a finite element method and a finite difference formulation developed in this research project. The finite difference algorithm has capabilities for analyzing linear springs, non-linear springs, and springs having a tension cut-off. The latter feature simulates the effect of a pipe pulling out of contact with the soil.
The model is used to perform parametric studies to assess the effects of soil stiffness, soil strength, trench geometry, amplitude of pipe displacements, pipe stiffness, and length of touchdown zone on pipe deflections and bending stresses.
In conclusions, the seafloor stiffness (as characterized by the three spring parameters), the magnitude of pipe displacement, and the length of the touchdown zone all influence bending stresses in the pipe. Also, the tension cutoff effect, i.e., the pipe pulling away from the soil, can have a very large effect on bending stresses in the pipe. Neglecting this effect can lead to serious over-estimate of stress levels and excessive conservatism in design.
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