Fatigue design and dynamic analysis of Steel catenary risers at Touch down zone with nonlinear soil structure interaction model under randomly generated waves

The offshore industry is moving deeper and deeper into the ocean depths and a subsequent technological advancement are inevitable factors. Steel Catenary Risers (SCRs) are the most popular type of deep water risers and are very important part of any deep-water developments. Although the simplicity in installation and the associated economic advantage makes it widely popular, these highly slender structures are very sensitive to dynamic loads and fatigue. Hang-off point where the riser is connected to the floater and Touch Down Zone (TDZ) where the riser interacts with the seabed are the two-important fatigue critical regions in an SCR. Fatigue design at TDZ remains a challenging topic among riser engineers and the soil structure interaction here makes it extremely complex to design for fatigue. Linear soil structure models, which are popular among the design professionals due to its simplicity are extremely conservative and the industry is slowly moving towards the nonlinear soil models. This research investigates the effects of implementing the nonlinear soil model into the fatigue design of SCRs under randomly generated waves with detailed comparison with the linear soil model. The uncertainties related to estimation of fatigue damage at TDZ and the influence of duration of simulation are studied in detail and some significant observations and findings are presented in this research. A very extensive effort to understand the effect of different order of waves (of varying intensities) also formed a part of this study which yielded crucial results which opens new doors in this topic.