Effect of Induced Drag Damping on the Near Resonant Responses of a Deepwater Tripod Tower Platform
Authors: Srinivasan N. , Swamidas A.S.J. , Pawlowski J.S. 
Published: October 2008

ABSTRACT
Near-resonant responses of a deepwater platform to wave excitation are controlled by inducing additional hydrodynamic damping in the system. The study used a device designed to induce flow-separation around a vertical column at low Kc numbers. The results of two experimental investigations are presented in this paper. In the first experiment, in-line forces on a 0.3m diameter vertical cylinder were measured without and with the attachment of the flow-separation device. At low Kc numbers the drag coefficients increased by five times due to the attached device to the cylinder, whereas the inertial coefficients were found to be insensitive to the device. The second experiment investigated the wave induced responses of a 8.6m high hydroelastic model of a deepwater tripod tower platform in a 200x 12x7m wave tank. A significant reduction in responses at and close to resonance, due to the induced drag damping, observed in the experimental measurements shows that the device is very effective for damping applications. The experimental findings are also supported by analytical results.

INTRODUCTION
It is not economically viable to completely avoid the occurrences of resonance at all operating conditions in the design of large offshore structures. In case of fixed deepwater structures the fundamental sway natural frequency of the structure falls within the significant part of the wave spectrum. This causes resonant vibration to these structures. For floating deepwater structures like tension leg platforms resonant excitations in heave and surge modes occur at the sum and difference frequencies of the components in the wave spectrum. Thus the resonant responses to wave excitation are unavoidable situation in the design of deepwater structures.
Many design concepts of deepwater structures are characterized by one or more large diameter vertical cylinders piercing the free-surface (example: Tension leg platform, Mono tower platform and Tripod tower platform). The member diameter for these type of structures is around 15m. For such large members the flow-separation characteristics are predominantly controlled by the low Kc numbers. The wave exciting forces are mainly inertial and drag forces are very small or even negligible for moderate seas. The behaviour of the oscillatory-flow at low Kc numbers is an unique problem. At low Kc numbers there is little time available for the flow-separation to initiate and develop behind a circular cylinder before the flow reversal occurs. This diminishes the chances for the formation of wakes on a circular cylinder. Considering the resonant oscillation of these large diameter deepwater structures, the damping associated with the flow-separation phenomena obviously does not contribute significantly to the structures (in-spite of a large structural response at resonance). Under such conditions this paper introduces an innovative method to induce flow-separation without significantly increasing the inertial forces on the structure. The viability of this technique in the application of introducing additional hydrodynamic damping to a fixed deepwater structure is studied through experimental and analytical means.
Numerous studies in the literature have confirmed that the presence of structural response velocities in the wave force formula acts as a response reduction mechanism [1].