Summary
As the offshore oil industry designs platforms for its deep water lease sites, vortex induced vibrations (VIV) becomes a more important design concern. Deepwater platforms rely on flexible structural members, e.g. tendons or mooring lines, that provide less lateral restraint as the water depth increases. These structures might encounter near-surface sheared currents, reversing currents, or deep subsea currents along with waves at the ocean surface (Rijken, Niedzwecki, van de Lindt 1997). All of these environmental forces can cause VIV of these slender structural members. When fluid flows past a bluff body, such as a circular cylinder, the flow separated from the body as the flow velocity increases. Under certain conditions, vortices are shed and the resulting wake formation induces an alternating pressure loading on the cylinder surface (Belvins1990). This creates a transverse force that excites the cylinder at a certain frequency. When this frequency matches the natural frequency of the cylinder, it is know as lock-in and large transverse vibrations occur. Lock-in can occur over arrange of frequencies knows as the lock-in window. Another effect is increased current drag. A vibrating cylinder disrupts the flow of fluid to the structure, and hence more drag (Grant1977). Drag can significantly increase when a cylinder vibrates. These flow induced vibrations and increased drag forces are cause for concern since they can significantly influence the design fatigue life estimate of these slender structural members.
There has been a great deal of study of vortex-induced vibration in current flows. Some predictive tools and models have been developed which fairly accurately forecast VIV I uniform currents. However, very little research has been done on vortex-induced vibration in combined current and surface wave flows which are conditions more like the real ocean environment. Very little experimental data is reported in open literature for combined wave and current environments, and most of that data has been obtained at very small model scales (Dauchin 1996). Typically the model tests were performed using models only a few feet long and with very small diameters. Predicting VIV for the wave an current scenario is also important for a wide range of offshore engineering problems such as tendons, risers, mooring lines, towed pipelines pipeline free-spans, and structural members on towed jackets.
Preventing VIV with vortex suppression devices is also an area of interest to the offshore community (Nakamura and Koterayama 1992, Packwood 1990, Rogers 1983, Grant 1997). There are many different types of suppression devices, and some of the studies have focused upon which device is best for certain situations. A practical problem which is of great interest centers around the question of how much suppression device coverage is required to adequately minimize or eliminate vibrations. By gaining a greater understanding of this subject through experimental study, one could potentially minimize design cost to reduce or eliminate VIV response.
This research study focuses primarily on the 27 m (95 ft) long flexible cylinder submerged 0.6 m (2 ft) below still water level with variable speed towing. For this study, a test matrix of 139 tests was created. Details of the model design, instrument analysis, and interpretation are presented.
Related Publications: Niedzwecki, J.M., Chitwood, J.S. and Vandiver, J.K. (1998). “Vortex-Induced Vibration in Uniform Currents and Random Waves,” ASME Conference on Advances in the Understanding of Bluff Body Wakes and Vortex-Induced Vibration, Washington D.C. (Presentation)
Niedzwecki, J.M. and Chitwood, J.S. (1998). “Suppression of Vortex-Induced Motions Using Ribbon Fairing,” Marine Technology Society Conference, Baltimore MD, November. (Presentation)
