An Experimental Investigation on Vortex-Induced Vibration of Double Risers in Tandem Under Bidirectionally Sheared Flow

The interfering characteristics of vortex-induced vibration (VIV) responses for tandem double risers under the recently discovered bidirectionally sheared flow are unclear. To reveal the interfering VIV characteristics in bidirectionally sheared flow, a rotating test rig was built to simulate bidirectionally sheared flow in the ocean basin. A series of VIV experiments for two scaled riser models were carried out, which included a single riser and tandem double risers with two different center-to-center spacing (3D and 5D). Then the modal superposition method was used to reconstruct the VIV displacements in cross-flow (CF) direction and the drag displacement. Furthermore, the drag coefficient was identified and analyzed. Results indicate that the VIV frequency of tandem double risers remains consistent at same spacing. When the reduced velocity (Vr) is in range of 5.0 to 18.0, the VIV amplitude of tandem double risers is basically same, but slightly smaller than that of the single riser. However, the VIV amplitude of riser 2 is significantly higher than that of riser 1 at Vr > 18.0. Moreover, it is found that the overall average drag coefficient in tandem double risers is lower than 1.38 of single riser, with values of 1.25 (S = 3D) and 1.33 (S = 5D) respectively.