Instability of a vertical riser in the wake of an upstream vertical riser

The static solution and its associated dynamic stability of a vertical riser situated in the wake of an upstream riser is investigated in the paper. Both the upstream and downstream risers are top-tensioned and pinned at the bottom ends on the seabed. Due to the wake effects, the downstream riser is subject to mean lift force as well as drag along its length. It is shown that under certain current conditions there exist multiple stable and unstable static solutions of the downstream riser. There also exists a critical current velocity and once this velocity is exceeded no static solutions of the downstream riser can be found. The disappearance of the static solutions is closely related to the loss of stability which is believed to be the main cause of ensuing clashing between the two risers. A further dimensional analysis reveals that the instability onset criteria can be described by three nondimensional parameters.