The Role of Friction in Vortex Induced Motion/Vibration

Many offshore structures subjected to cross currents may experience vortex induced vibration (VIV) or vortex induced motion (VIM). These two phenomena are related to the same mechanism, the release of regular Von-Karmen vortex sheets and their subsequent interaction with the body that produced them. Mitigation strategies usually involve fitting the structure with helical strakes which disrupt these sheets or fairing that push their production and subsequent release downstream away from the structure. Typically strakes are used for larger structures such as spars while strakes and fairings are used for long slender riser pipes. These devices suppress the positive lift coefficient, CLV, making the CLV versus amplitude ratio, A*, curve very flat. This means however that the response, A*, is now very sensitive to small changes in the lift coefficient so that a small amount of mechanical or hydrodynamic friction can significantly alter amplitude of the response. In testing such devices it is important that any external friction be minimized since true free response means CLV is equal to zero. Small amounts of external friction can greatly suppress the response. In the case of fairings small amounts of rotational friction can add dynamic stability to a fairing that would normally experience fluttering. In the case of model tests with spars it is important that the mooring system have low structural damping. Also damping in the prototype system can significantly lessen the platform's VIM. For testing of VIV mitigation devices it is important that the external friction is minimize. In the case of fairings the rotational friction can have a significant effect during the model test and also for the prototype units. This paper examines the role of friction and damping for two quite different structures undergoing vortex induced motion; (a) Spar fitted with strakes and (b) riser fitted with fairings.