Nonlinear Dynamic Analysis and Fatigue Study of Steep Wave Risers Under Irregular Loads

As a reliable alternative option for traditional steel catenary risers (SCRs), steep wave risers (SWRs) have been widely applied to deepwater oil and gas production. However, the nonlinear dynamic analysis of SWRs is more complicated than that of traditional SCRs due to their special configuration and significant geometric nonlinearity. Moreover, SWRs are highly susceptible to fatigue failure under the combined excitation of irregular waves and top floater motions (TFMs). In this study, considering irregular waves and TFMs, a numerical SWR model with an internal flow is constructed based on the slender rod model and finite element method. The Newmark-β method is adopted to solve the dynamic behavior of SWR. Moreover, the Palmgren-Miner rule, a specified S-N curve, and rainflow counting method are applied to estimate the fatigue damage. An efficient numerical computation procedure, i.e. , DRSWR, is programmed with MATLAB in this study. Calculation results are compared with those of OrcaFlex to verify the accuracy of the DRSWR. The nonlinear dynamic response and fatigue damage of an SWR under the combined excitation of irregular waves and TFMs are obtained, and a comprehensive parametric analysis is then conducted. The analysis results show that the buoyancy section undergoes the highest level of stress and fatigue damage under the combined excitation of irregular waves and TFMs. An internal flow with high velocity and high density produces a high level of fatigue damage. The buoyancy factor and length of the buoyancy section should be set moderately to reconcile the reduction of the top tension with increased fatigue life. These results are expected to provide some reference significance for the engineering design of SWR.