Fatigue damage assessment of steel catenary risers in the touchdown zone with incorporation of time-dependant seabed interaction effects

Steel catenary riser (SCR) design is heavily affected by fatigue performance in the touchdown zone (TDZ). Within the TDZ, the riser cyclically interacts with the seabed, resulting in the progressive generation of excess pore water pressure, leading to soil softening and remoulding. However, the soil may undergo a consolidation process during the intervening pause period, i.e., the calm weather or the inactive periods of SCRs oscillation, allowing the pore water pressure to dissipate. This process results in the regaining of effective stress and consequently achieving a higher soil strength, which is detrimental to the fatigue damage accumulation in the TDZ. The existing advanced hysteretic non-linear riser-seabed interaction models do not account for the consolidation effects. Besides, a secondary mechanism, i.e., the seabed erosion due to combined vortices generated by subsea currents and seawater entrapped between the oscillating riser and the trench may contribute to the cyclic riser embedment. The existing riser-seabed interaction models do not capture this secondary mechanism as well. In this research project, first, these key knowledge gaps were addressed by developing global and local riser-seabed-seawter interaction models and incorporation of the consolidation and erosion effects. Later, the developed models were used to propose novel methodologies for incorporation of the trench effects into the riser fatigue analysis. The consolidation effects was added by coding an effective stress framework into a user-defined subroutine (UEL) in ABAQUS. This was integrated with the global riser model to determine the long-term soil stiffness associated with damage accumulation during SCR cyclic motions and soil strength recovery during the intervening pause period. The seabed soil erosion effect on trench formation was investigated by developing a three-domain model of riser-soil-fluid interaction. The model was used to study the combined effect of soil erosion, soil fluidization, and cyclic riser oscillations on the plastic soil deformation and riser embedment. A Coupled Eulerian-Lagrangian (CEL) technique was employed and the strain rate and soil softening effects in an Eulerian domain were coded into the VUSDFLD subroutine of ABAQUS. The developed models were successfully verified against the experimental studies from the literature. As proven by subsea surveys, all of these riser-seabed-seawater interaction mechanisms result in a trench formation several riser diameter deep (3D to 7D) that can significantly affect the fatigue life in the TDZ. Although most of the studies in the literature show a beneficial effect of the trench on fatigue, there is still no coherent agreement amongst researchers on the beneficial or detrimental trench effects. To further investigate the trench effect on fatigue, first, two new methodologies were proposed, i.e., i) an alternative vessel excitation algorithm called the equivalent motion method (EMM) and ii) an equivalent soil stiffness approach called the hybrid trench model (HTM). The first method was investigated to predict the fatigue damage of the riser in the linear elastic seabed using the same riser on the rigid seabed but with a virtual vessel motion algorithm. An equation was extracted from a comprehensive set of analyses for a given riser resting on an elastic seabed to obtain an equivalent vessel motion amplitude on a rigid seabed with the same cyclic damage. As an alternative solution, the proposed EMM was found to be a promising basis for further extension into the non-linear riser-seabed interaction. The second methodology provides equivalent soil stiffness to simulate the target riser embedment, which is usually obtained from non-linear hysteretic riser-seabed interaction models. The capability of HTM in developing deep trenches, e.g., 5D, was examined along with perfect compatibility with the natural catenary shape of the riser to resolve any pressure hot spots and premature stabilization problems frequently reported in the literature. This novel methodology was integrated with the effective stress analysis developed earlier in this study to deeply investigate the trench effect on the fatigue performance of SCR in the TDZ, while considering the consolidation. Besides developing several advanced tools for enhanced analysis of SCR-seabed interacation, the study extended the insight into the fatigue performance of steel catenary risers in the touchdown zone that can be used by field operators and riser life extension authorities.

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