Wake-Induced Large-Amplitude Low-Frequency Motions of a Vertical Riser in the Wake of an Upstream Riser

It is concluded from our previous work that a vertical riser situated in the wake of an upstream riser can lose its stability when the current velocity exceeds a critical value. The ensuing irregular motion of the downstream riser is characterised by its large amplitude and low frequency. A numerical method is further developed in this paper to simulate the wake-induced large-amplitude low-frequency motions of the downstream riser as it wanders about in the wake. While an accurate structural model for riser clashing is still yet to be developed, the preliminary results presented here do indicate that in considering the riser impact velocity it is inadequate to use the VIV velocity alone as the wake-induced motion can have a significant contribution to the riser impact velocity.