Influence of seabed response on fatigue performance of steel catenary risers in touchdown zone

[Truncated abstract] The influence of the riser-seabed interaction on fatigue performance of steel catenary risers (SCRs) is now widely accepted. Due to uncertainties associated with the complex nature of the riser-seabed interaction, existing analysis software and design recommendations have mostly been limited to consideration of a linear elastic seabed response, which is a significant over-simplification from a geotechnical point of view. Indeed, observations from ROV surveys have shown that trenches several diameters deep can develop in the touchdown zone of the SCR. Sophisticated non-linear hysteretic seabed models have recently been introduced, which are able to simulate the reduction in secant stiffness with increasing displacement of the riser, the gradual embedment of the riser into the seabed, and even trench development under cyclic perturbations of the floating vessel supporting the SCR. The current dissertation has focused on the effect of the seabed model in estimating fatigue damage in the touchdown zone of SCRs. The thesis starts with a review of analytical approaches for modelling the profile and stress distributions within risers. A generic Spar system, with a particular riser geometry and wave scatter diagram based on conditions in the Gulf of Mexico, were adopted to evaluate how the fatigue damage was affected by the seabed stiffness, initially for elastic response of the seabed. Finite element analyses were undertaken using the software package ABAQUS. All analyses were carried out as two-dimensional, quasi-static analyses, with the main focus being to explore the relative effect of different seabed responses rather than on assessment of the absolute fatigue damage.