Distributions of nonlinear dynamics, stress and fatigue damage along a steel catenary riser in current affected by top-end surge

Abstract

A numerical model for computing the complex nonlinear dynamics and fatigue damage of steel catenary riser (SCR) system is proposed in this paper. The structural model of SCR is described by the absolute nodal coordinate formulation (ANCF), which is further coupled with Morrison equation to incorporate the hydrodynamic loads. The hysteretic riser-soil interaction model and Coulomb friction ‘bilinear’ model are applied to simulate the seabed reactions. After validation, the distributions of nonlinear motions along the SCR, the response characteristics of touchdown point (TDP) and the corresponding stress and fatigue are fully discussed. It is found that the motion trajectory in x-z plane presents various regimes along the SCR, relating to the phase difference between x- and z-motions induced by the variations of primary and harmonic frequencies. The secondary uplift of z-motion, accompanying by the sharp decline of vertical seabed force, is captured for TDP. The values of secondary uplift and seabed force decline increase or decrease with growing surge amplitude or period in step style. Both the amplitude and period of top-end surge affect the features of axial tension, bending moment, stress and fatigue damage, where the former plays a more significant role in the concerned parameter range.