Performance of non-linear seabed interaction models for steel catenary risers, part I: Nodal response

Non-linear hysteretic riser-seabed interaction models have been developed and implemented into business software packages within recent years to simulate the riser penetration into the seabed and its influence on fatigue life in the touchdown zone (TDZ). These models have shown significant impact on ultimate fatigue damage and users shall take caution while using the models paying particular attention to the selection of model parameters. The oscillation of steel catenary riser (SCR) in the touchdown zone can be quite complex, where neighbor nodes go under different episodes and magnitudes of penetration and uplift at the same time. Therefore, it is necessary to evaluate the non-linear soil models consistency in nodal performance. This paper comprehensively examined the nodal performance of a popular non-linear hysteretic riser-seabed interaction model through developing a global numerical riser model in ABAQUS and a user-defined subroutine (UEL). The model shows a dominantly strong nodal performance. However, nodal response violations and model malfunctioning were observed in the proximity of trench bottom towards the vessel, which is the most fatigue prone section of SCR in the touchdown zone. Also, it was identified that the model over-estimates the penetration and suction resistance and consequently the fatigue damage in the TDZ.