Vortex-induced vibration of two parallel risers: Experimental test and numerical simulation

The vortex-induced vibration of two identical rigidly mounted risers in a parallel arrangement was studied using Ansys- CFX and model tests. The vortex shedding and force were recorded to determine the effect of spacing on the two-degree-of-freedom oscillation of the risers. CFX was used to study the single riser and two parallel risers in 2–8D spacing considering the coupling effect. Because of the limited width of water channel, only three different riser spacings, 2D, 3D, and 4D, were tested to validate the characteristics of the two parallel risers by comparing to the numerical simulation. The results indicate that the lift force changes significantly with the increase in spacing, and in the case of 3D spacing, the lift force of the two parallel risers reaches the maximum. The vortex shedding of the risers in 3D spacing shows that a variable velocity field with the same frequency as the vortex shedding is generated in the overlapped area, thus equalizing the period of drag force to that of lift force. It can be concluded that the interaction between the two parallel risers is significant when the risers are brought to a small distance between them because the trajectory of riser changes from oval to curve 8 as the spacing is increased. The phase difference of lift force between the two risers is also different as the spacing changes.