A hybrid model to simulate the trench effect on the fatigue analysis of steel catenary risers in the touchdown zone

The gradual trench formation of steel catenary risers (SCRs) in the touchdown zone is known to significantly affect the SCR’s fatigue life. However, there is still no coherent agreement among researchers on the beneficial or detrimental effects of the trench on fatigue. Recent studies have shown that a potential source of contradictory fatigue results could be the methodology to incorporate the trench in the numerical simulations. Since the predefined mathematical trench profiles create non-realistic contact pressure hot spots in the seabed, and the nonlinear hysteretic seabed interaction models may cause premature trench stabilization, both methods distort the damage distribution. To resolve these problems, a new model called the Hybrid Trench Model (HTM) has been developed in this study by combining the linear soil stiffness and nonlinear hysteretic seabed interaction model. This hybrid model provides an equivalent stiffness distribution in the touchdown zone to simulate the trench profile obtained from a nonlinear riser–seabed interaction model. HTM’s capability in developing deep trenches, e.g., 5D, was examined along with perfect compatibility with the natural catenary shape of the riser, exhibiting the reliability of this method to incorporate the trench effect into the fatigue analysis.