Fatigue analysis of steel catenary risers under coupled cross-flow and in-line vortex-induced vibrations with oblique incoming flow

The paper presents a fatigue assessment method for offshore steel catenary risers (SCR) under vortex-induced vibrations (VIV) with multi-directional incoming flow. Firstly, the structural responses are obtained using the finite element method (FEM) in conjunction with a coupled cross-flow (CF) and in-line (IL) wake model. Subsequently, the cyclic stress associated with fatigue life is transformed into the frequency domain, and a corresponding Kriging model is constructed. Further, the structural fatigue life is correspondingly computed using the Miner-Palmgren damage rule and the structural fatigue reliability model is established. To validate the effectiveness of the FEM model and the frequency-domain-based Kriging model, comparisons between experimental and numerical results are conducted. Through the analysis of the structural response, the paper elucidates the mechanism underlying the impact of the incident angle on the fatigue distribution, while emphasizing the contribution of higher harmonics to structural fatigue. Moreover, a case study demonstrates the superiority of the proposed method in predicting the probability of failure and factor of safety compared to models that disregard the incident angle and IL responses. Overall, the proposed fatigue analysis methodology contributes to the reliability and availability of SCRs during the design stage, and to reduce potential fatigue-related failures.