WAKE FIELDS BEHIND RISERS UNDERGOING VORTEX-INDUCED VIBRATION

This paper presents measurements of the wake field behind three riser models obtained by using a Digital Particle Image Velocimetry (DPIV). The three riser models were a circular rigid cylinder, a cylinder fitted with fairings and a cylinder fitted with strakes. The models were free to vibrate in the cross-flow direction when towed in a uniform flow. The range of tested Reynolds number based on the diameter of the cylinder was from 3 x 10(4) to 2.5 x 10(5). The measurement results showed that the transverse vibration amplitudes for both the faired and straked cylinder were far less than those of the bare cylinder. Based on the wake flow field comparison between the models tested it was found that the modes of vortex shedding observed behind the bare cylinder did not occur behind either the faired or straked cylinder. This reveals that the vibrations responses of the cylinders are directly related to their wake modes. Strong, regularly shed vortices induce large amplitude vibration and weak, scattered vortices lead small amplitude or no vibration. The different wake patterns are presented for the three cylinders.