Understanding vortex-induced vibration characteristics of a long flexible marine riser by a bidirectional fluid–structure coupling method

A bidirectional fluid–structure coupling method was developed to comprehensively understand the VIV characteristics of a 3D flexible marine riser. Through study VIV characteristics of the 3D riser in different flow conditions and by analysis of the structural characteristics of the riser’s responses, this study successfully captured the phenomena of switching of adjacent-order vibration modes and the characteristics of a “traveling wave” and a “standing wave” at both ends of the riser that cannot be obtained from 2D VIV studies. Analysis of the frequency of the structural vibration response characteristics found that the 3D flexible riser VIV showed multi-frequency vibration phenomena and the vibration response characteristics of a broadband spectrum in a high-speed flow condition. This research distinguished vortex forms at different locations of the riser, i.e., in the middle of the riser, mainly the “2P” or “P + S” forms occurred and at both ends of the riser, mainly the “2S” form appeared. After comparative analysis of the vortex forms of the riser wake in different speed flow conditions, this research identified that under a low flow velocity, the riser showed a 3D effect marginally, whereas under a high flow velocity, the riser showed a strong 3D effect.