Experimental Research on Vortex-Induced Force Characteristics of Flexible Riser with Buoyancy Module and Strakes

Abstract

In this paper, the vortex-induced force characteristics of a flexible riser with buoyancy modules and strakes were experimentally investigated. The strain information caused by vortex-induced vibration (VIV) at different cross-sections on the flexible cylinder was measured in the model test. Combined with the modal superposition method and inverse analysis method for vortex-induced force, the vortex-induced forces on the flexible cylinder were inverted, including the excitation force in phase with velocity and added mass force in phase with acceleration. Then, the excitation coefficients and added mass coefficients at each time step were identified via a least-squares method. The results show that the added mass coefficient and excitation coefficient of the flexible riser covering buoyancy modules present a “sawtooth” distribution along the length of the pipe, while the total mass (sum of structural mass and added mass) is consistent along the pipe. The strake has little effect on the added mass coefficient of the buoyancy module and the bare pipe section, but when it covers the bare pipe section, the excitation coefficient of the bare pipe section decreases to a negative value, showing a damping effect. In addition, when the bare pipe section of the flexible riser is completely covered with helical strakes, only the vortex-shedding frequency component of the buoyancy modules remains in the VIV response of the pipe. The reason for this phenomenon is that helical strakes completely suppress the vortex shedding of the bare pipe section, resulting in the reduction in the VIV excitation energy of the original bare pipe section to 0.