Experimental investigation of the effect of internal flow velocity on VIV characteristics of vertical top tension risers in a semi-uniform current

Abstract

An experiment of the effect of internal flow velocity on vortex induced vibration characteristics of vertical top tension risers was carried out in a large scale wind-wave-current flume. A transparent poly riser model with effective length of 1.5 m was tested with the upper 0.75 m in the air and the lower 0.75 m in a uniform flow field. By changing the internal flow velocity, the dynamic characteristics and dynamic responses of top tension risers, the effect of internal flow velocity on VIV amplitude of the risers under different top tensions, and the top tension variation of the risers under different internal flow velocities were investigated in a semi-uniform current. Results show that the natural frequency of risers decreases with the increase of internal flow velocity, and is subjected to significant mutual inhibition between internal flow and top tension; as internal flow velocity increases, the external flow velocity needed for the “lock in” state to occur decreases, which is negative to the safe service of risers; increase of internal flow velocity leads to gradual increase of riser displacement, though at a smaller rate of variation than that caused by top tension. Axial force measurement indicates that, under a modest initial top tension, internal flow velocity makes a great difference to the top tension of risers. This suggests that internal flow velocity can offset some of the riser responses induced by top tension except when the initial top tension is high, in which case internal flow velocity does not make much difference to top tension.