Sensitivity of Fatigue Service Life of Steel Catenary Risers Using Nonlinear Riser-Soil-Interaction Model to Duration of Dynamic Simulation under Randomly Generated Waves-Paper

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The study examines how dynamic simulation duration affects fatigue-life estimates for steel catenary risers (SCRs), especially in the touchdown zone (TDZ), where the riser repeatedly contacts the seabed. It compares a constant-stiffness linear soil model with a nonlinear hysteretic model that reflects changing soil resistance and trench development.<br /><br />The authors modeled a 1,000 m-deep Gulf of Mexico riser under nine randomly generated wave conditions, using 30 wave realizations per condition and simulation durations of 0.5, 1, 3, and 10 hours. Fatigue damage was calculated using rainflow counting and a DNV Type D S–N curve.<br /><br />Short simulations substantially underestimated fatigue damage with the nonlinear soil model for most sea states. For example, 0.5-hour estimates were typically around 15% below 10-hour results, with differences reaching 27% in the calmest condition. Three-hour simulations still underestimated damage by about 8% on average. In contrast, linear-model estimates changed little with simulation duration once simulations reached three hours.<br /><br />The difference arises because nonlinear soil response evolves with loading history. Repeated riser movement softens the seabed and deepens the trench, shifting the critical riser location and increasing curvature and stress amplitudes. Trenching develops more gradually in mild seas, making fatigue estimates more sensitive to simulation length. In harsher seas, the trench stabilizes sooner, so shorter simulations are more representative. Longer simulations also reduce variability among random-wave realizations for both soil models.<br /><br />The study concludes that simulation durations adequate for linear soil analyses may be insufficient for nonlinear analyses. Fatigue assessments using nonlinear riser–soil interaction should account for trench development and the greater sensitivity of milder sea states, which can contribute substantially to cumulative fatigue damage.

Keywords

steel catenary risers

fatigue life

touchdown zone

dynamic simulation duration

nonlinear soil model

linear soil model

trench development

rainflow counting

DNV Type D S-N curve

riser-soil interaction