Experimental investigations on a passive control method for suppressing VIV of long flexible riser are presented in this work. A riser model with L/D (slenderness ratio) of 1750 was horizontally installed on a carriage and uniform current was simulated by towing the riser using the carriage in a wave tank, which leads to the Reynolds numbers ranging from 103 to 104. Four slender control rods with diameter d = 0.25D were placed parallel to the riser at uniform angle (i.e. at 90° intervals around the riser circumference). Different pitch distances l/D = 0.813, 1 and 1.188 were considered in this work, where l is the center-to-center distance between control rod and riser model. Strain along the riser and tension were measured by Fiber Bragg Grating and tensionmeter, respectively. The experimental results indicate that the response characteristics of a riser model with multiple control rods are quite distinct from that of a bare riser. Multiple control rods can suppress VIV response significantly, and their performance is mainly dependent on the pitch distance. It is found that the spatial maximum standard deviation of strain decreases about 84% due to the existence of control rods. The optimal pitch distance is found to be l/D ≤ 1. Moreover, spectrum analysis shows that the dominate frequency increases linearly with the increasing of flow velocity, for both the bare riser and the riser with control rods, while the frequency band of the suppressed riser becomes broader compared with the bare riser. It is also observed that the tension of the riser is slightly increased due to the presence of four rods, but not sensitive to the pitch distance.

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