Alternative Empirical Procedures for Fatigue Damage Rate Estimation of Instrumented Risers Undergoing Vortex-Induced Vibration

Vortex-induced vibration (VIV) is a topic of great importance in fatigue damage assessment and life prediction for marine risers. In order to gain insight into riser motions and estimated fatigue damage due to VIV, data loggers such as strain sensors and/or accelerometers are sometimes installed on risers to monitor their motion in different current velocity conditions. Accurate reconstruction of the riser response and empirical estimation of fatigue damage rates over the entire riser length using measurements from a limited number of sensors is important for efficient utilization of the costly measurements recorded. In this study, different empirical methods are employed to analyze the VIV response of a long flexible cylinder subjected to uniform and sheared current profiles. The methods include weighted waveform analysis (WWA), proper orthogonal decomposition (POD), modal phase reconstruction (MPR), a modified WWA procedure, and a hybrid method which combines MPR and the modified WWA method. Fatigue damage rates estimated using these different empirical methods are compared and cross-validated against measurements. Formulations for each method are briefly presented and discussed with examples. Results show that all the empirical methods, despite different underlying assumptions in each of them, can be employed to estimate fatigue damage rates quite well from limited strain measurements.