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. The applicability of analytical solutions for the SCR system was explored, combining catenary equations with a boundary layer method in order to estimate the shear force distribution and hence fatigue damage under the action of lifetime wave loading, comparing the results with those from the finite element analyses.

A hysteretic non-linear seabed model was then implemented within ABAQUS, in order to examine the effect of a non-linear seabed model on the calculated fatigue damage. The effects of different numbers of waves, and the ordering of wave packages of different amplitude, were explored in order to arrive at an appropriate strategy for conducting fatigue analysis with a non-linear seabed response. The investigations showed that the fatigue damage in a non-linear hysteretic seabed is independent of the number of representing wave cycles and the hierarchy of individual sea states, if the system experiences the most severe sea state in the beginning of the analysis. Therefore, the applicability of Miner’s rule for superposition of individual damages was proved for non-linear seabed with particular considerations. Parametric studies were undertaken to evaluate how different aspects of the seabed model, such as the shear strength profile, the degree of suction resistance mobilised during uplift of the riser and the maximum unloading stiffness, affected the fatigue damage.

The non-linear model was then used to examine the effect of gradual embedment of the riser, initially for moderate penetration into the seabed but then using modified parameters in the non-linear seabed model, to simulate the development of deep trenches. The effect of the trench depth on the stress distribution along the riser was studied, and consequently how the trench depth affected the fatigue performance in the touchdown zone.

The results show that in both linear elastic and non-linear hysteretic seabeds, the ultimate fatigue damage increases with increasing seabed stiffness and the peak damage point is moved slightly towards the anchor end of the riser. The gradual embedment of the riser into the non-linear hysteretic seabed under cyclic vessel excitation also increases the fatigue damage, slightly moving the peak damage point towards the vessel end of the SCR. The pre-trenching studies show that the development of deep trenches has significant effect on fatigue damage, considerably increasing the peak damage point with extreme relocation towards the vessel end of the SCR. The comparison of the shear force distribution and the ultimate fatigue damage for various trench depths shows the peak shear force in mean vessel position and the peak fatigue damage are relocated in opposite directions as the trench depth is increased. This suggests that the peak shear force in the vessel mean position is not a direct indicator of maximum fatigue damage. The fatigue damage is mostly driven by riser fluctuations from moderate waves, with a considerable number of cycles, at the trench edge towards the vessel end of the SCR.