Experimental investigation on the suppression of vortex-induced vibration of long flexible riser by multiple control rods

Experimental investigations were conducted to evaluate the vortex-induced vibration (VIV) suppression for slender riser (length to diameter ratio L/D=1750) using multiple control rods. The uniform current was obtained by towing the riser model running in a wave basin, giving the Reynolds numbers ranging from 2400 to 7600. Four control rods with diameter d=0.25D were placed parallel to the riser model with uniform angle interval of 90 degrees. Different spacing ratios delta/D=0.187, 0.375 and 0.562 and coverage rates L-c/L=80%, 60%, 40% and 20% were considered, where delta is the spacing distance, L-c is the covered length. The strain responses in both cross-flow (CF) and in-line (IL) directions were measured by 56 Fiber Bragg Grating sensors. The experimental results show that the multiple control rods perform well in mitigating the VIV. In general the smaller spacing ratio and the larger coverage rates give rise to the better VIV suppression for the CF response, while the IL response is not sensitive to the delta/D and L-c/L. The optimal parameters are suggested to be delta/D <= 0.375 and L-c/L approximate to 60%. The monitored end tension of the riser model was observed to increase slightly due to the presence of control rods. (C) 2012 Elsevier Ltd. All rights reserved.