Frequency-domain prediction method for vortex/wake-induced vibrations of double flexible risers in tandem arrangements

A coupled flow-induced vibration response prediction method in the frequency domain for double flexible risers in tandem arrangements with wake interference is developed. The non-iterative vortex-induced vibration (VIV) prediction method for a single riser is applied to the upstream riser. Based on the VIV prediction results of the upstream riser, a wake-induced vibration (WIV) prediction method of the downstream riser is established by iterating the clearance between adjacent risers, the wake velocity, the WIV amplitude and the drag displacement to consider the wake interference. To predict the WIV frequency and amplitude of the downstream riser, the flow velocity input is the wake velocity of the upstream riser. In addition, if frequency capture occurs, the added mass coefficient of the downstream riser will be adjusted to obtain a natural frequency identical to the upstream dominant frequency. Otherwise, the added mass coefficient is still 1. The other procedures of WIV prediction are identical to the VIV prediction steps. Finally, compared to the experimental results, the coupled VIV-WIV prediction results can reach satisfactory precision in far-field wake interference, which is conducive to the practical engineering design for multiple-riser systems.