Influence of attack angle on VIV suppression by multiple control rods for long flexible riser model

Experimental investigations are conducted to study the influence of attack angle of multiple control rods on the effectiveness of Vortex-Induced Vibration (VIV) suppression for deepwater riser. The riser model is made of UPVC pipe with length to diameter ratio L/D = 1750, and four control rods with diameter d = 0.25D are placed parallel to the riser model at a uniform angle interval of 90° around the riser circumference. The attack angle α, which is defined as the minimal angle between the incoming flow and the center-to-center direction of riser model and control rods, including 0°, 22.5° and 45°, are considered in this work. The experiments are carried out in a wave basin by means of towing tests, giving rise to the Reynolds numbers 1,400 < Re < 5,200. The VIV responses of riser model in both Cross-Flow (CF) and In-Line (IL) directions, together with the variation of end tension, are investigated. The experimental results show that the multiple control rods may significantly reduce the VIV responses of riser model in terms of the measured strain and fatigue damage. It is found that the attack angle adopted in this work has little influence on the VIV suppression in consideration of either CF or IL responses. In addition, the end tension of riser model is observed to increase slightly due to the presence of control rods, and the maximal increment of end tension corresponds to α = 0°.Copyright © 2011 by the International Society of Offshore and Polar Engineers (ISOPE).