Effect of induced drag damping on the near resonant response of a deepwater platform

Srinivasan, Nagan (1991) Effect of induced drag damping on the near resonant response of a deepwater platform. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (35MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

Occurrence of resonance in the design of deepwater structures is unavoidable under all operating conditions. When the wave forces on the structure are dominated by the inertial component of loading, the hydrodynamic damping in the system is low. It is also known that the structural response can be reduced if energy can be dissipated through the fluid-media via the mechanism of flow-separation since it increases the fluid component of the total damping. Thus the resonant response could be suitably reduced by the generation of increased flow-separation. In view of certain new types of structures which possess one or more large diameter circular cylinders crossing the free surface, this dissertation describes an innovative technique to control the response at frequencies near resonance to moderate waves. -- When the Keulegan-Carpenter (KC) number for the flow is less than 4. the time available for the development of vortices behind a circular cylinder is less than adequate to form a stable wake. Consequently the drag forces on the structure are small. The first objective of this study was to increase the drag forces on a 0.3 m diameter circular vertical cylinder at low KC numbers (KC ≤ 2) for regular and irregular waves. The dissertation proposes a physical device to induce flow-separation that will increase the drag forces experienced by the circular cylinder. -- An experimental set-up was made to measure the wave forces on a simple vertical cylinder. The measured wave elevations and the wave forces were used to fit the Morison wave force formula, in the least squares sense, and the values of the inertial (Cm) and drag (Cd) force coefficients were determined. To study the effect of the physical device on the circular cylinder, the experiment was repeated by attaching the device to the cylinder. The experimental results revealed that, at low KC numbers, the value of Cd for the circular cylinder with the device increased by a factor of 4, irrespective of the wave height and wave period. The device did not significantly increase the value of the inertial coefficient of the main cylinder. -- The significant increase in the drag forces due to the attachment of the device could be successfully used to control the resonant response of certain deepwater structures. To investigate this, the dynamic response of an offshore platform model was tested in a wave tank, both with and without the device. The three-dimensional platform model was a 1/50th scale structure possessing the key features of a typical deep water tripod tower platform. The scaling considerations required for a hydroelastic model study in a wave tank were utilized to fabricate the 8.6 m tall hydroelastic model. The vibration properties of the model structure were determined both in air and water. To reliably estimate the modal parameters, modal testing and analysis were used. Parallel studies were conducted to analytically determine the natural frequencies and mode shapes, using a finite element method. The natural frequencies obtained by the two methods were in good agreement. -- The response of the physical model to resonant wave excitation was investigated for both regular and random waves and the experimental results were complemented by analytical results. Excellent correlation was obtained over the entire range of test conditions. In the analytical study concerning the effect of the device on the dynamic response of the structure to waves, a relative velocity formulation was used in the Morison forcing term and the nonlinear equations of motion were solved in the time domain. The experimental values of the Cm and Cd coefficients, with and without the device, were utilized in the analytical study. -- Both experimental and analytical results support the potential application of the device in controlling the resonant response of large diameter structures. Measured experimental results demonstrate that with the device the longitudinal acceleration response was reduced by a maximum of 32% for resonant wave conditions. Findings show that, as the wave height increases under resonant excitation, the percentage reduction due the induced drag damping also increases. -- Key words: Dynamics of offshore structures; Deepwater structures; Induced drag damping; Hydrodynamic damping; Hydroelastic model; Hydroelastic response

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