This paper presents results of a series of experiments conducted on a 0.3m diameter fixed vertical cylinder in a wave tank. The experimental study was aimed to increase the drag forces on the cylinder without increasing the inertial forces by inducing flow-separation around the cylinder. For this purpose an innovative device was designed based on the hydrodynamic flow-behaviour at low Keulegan-Carpenter numbers. Both regular and random wave environments were simulated in the laboratory. The total wave forces on the cylinder were measured using a sophisticated experimental arrangement. The hydrodynamic coefficients were computed using least-squarefit technique. The drag coefficients increased to a maximum of five times due to the attachment of the device. The inertial coefficients are found to be insensitive to the device due to flow transparency. An example study is, also, included to show the usefulness of this increased drag forces in reducing the near resonant responses of a large diameter deep water structure.
Many offshore platforms proposed for the deep water applications are characterized by large diameter vertical cylinders piercing the free-surface (examples: Tension Leg Platform, Monotower Platform and Tripod Tower Platform). The member diameter for these type of structures is around 15m. For such large members the mechanism of wave loading is mainly from the inertial effect and the drag forces are very small or even negligible in case of moderate sea. This is because of the behavior of the oscillatory flow at low Keulegan-Carpenter (Kc) numbers. For waves of small amplitudes with high frequencies, which generally occur everyday in the North-Sea, the 15m diameter member will have Kc numbers less than or equal to 2. At low Kc numbers there is little time available for the wake to form/develop before the flow-reversal occurs. This physical phenomena had been confirmed by many researchers.