On the Vortex-induced Vibration Response of a Model Riser and Location of Sensors for Fatigue Damage Prediction

This study is concerned with vortex-induced vibration (VIV) of deepwater marine risers. Riser response measurements from model tests on a densely instrumented long, flexible riser in uniform and sheared currents offer an almost ideal set-up for our work. Our objectives are two-fold: (i) we use the measured data to describe complexities inherent in riser motions accompanying VIV; and (ii) we discuss how such data sets (and even less spatially dense monitoring) can be used effectively in predicting fatigue damage rates which is of critical interest for deepwater risers. First, we use mathematical tools including Hilbert and wavelet transforms to estimate instantaneous amplitudes and phases of cross-flow (CF) and in-line (IL) displacements for the model riser as well as scalograms to understand time-frequency characteristics of the response; this work confirms that the motion of a long flexible cylinder is far more complex than that of a rigid cylinder, and that non-stationary characteristics, higher harmonics, and traveling waves are evident in the riser response. Second, a well-established empirical procedure, which we refer to as Weighted Waveform Analysis (WWA), is employed to estimate the fatigue damage rate at various locations along the length of the riser from strain measurements at only eight sensors. By iterating over numerous different combinations of these eight strain sensors as inputs (from among all the twenty-four available locations on the riser), optimal locations for the eight Corresponding Author sensors on the riser are identified by cross-validation, whereby predicted strains and fatigue damage rates at locations of instrumented sensors are compared with strains and fatigue damage rates based on actual recorded measurements there. We find that, if properly placed, as few as eight sensors can provide reasonably accurate estimates of the fatigue damage rate over the entire riser length. Finally, we demonstrate how more accurate fatigue damage prediction can result when non-stationary response characteristics are considered and a modified WWA method (that more effectively accounts for traveling waves than the WWA method alone does) is employed. INTRODUCTION Vortex-Induced Vibration (VIV) is of great interest in the context of fatigue design of marine risers as oil and gas production moves into deeper waters. Model tests provide valuable insight into the VIV phenomenon and associated riser response characteristics. Experiments, especially on long flexible cylinders, which better resemble real deepwater marine risers, help to study more complex response features than those conducted on short rigid cylinders under similar current/flow conditions [1]. In this study, measurements (strains and accelerations) available from the Norwegian Deepwater Programme (NDP) experiments on an instrumented riser are employed to examine the characteristics of the VIV response of a long flexible cylinder (model riser) placed in uniform or sheared currents.