The nonlinear viscous flow problem associated with the high frequency heave motion of a vertical column of a Tension Leg Platform (TLP) has been investigated numerically in a previous paper by the authors. It was found that the hydrodynamic damping force of a TLP column is very small. Adding appendages to the TLP columns is one of the choices to increase the heave damping force. The effect of an appendage in the form of a disk attached to the bottom of the vertical cylinder has been investigated in the present paper. Numerical simulation shows that the form drag component of the heave damping is changed dramatically compared to the skin friction component in the presence of the disk. The heave damping induced by the disk is linear with the amplitude of oscillation in the range of KC from 0.1 to 0.5. The form drag coefficient is found to increase two-fold due to the disk, in accordance with experimental results published in literature.
The resonant oscillations of a Tension Leg Platform (TLP) in heave, pitch and roll excited by sum-frequency wave forces are referred to as "springing" (Faltinsen 1990) and can contribute to fatigue in tethers. The oscillations are typically of small amplitude and high frequency. Previous investigations (Huse1990, Huse & Utnes 1994, Chakrabarti & Hanna 1991, Thiagarajan & Troesch 1994, Thiagarajan & Troesch 1998, Tao et al 1998) have shown that for a lightly damped offshore structure like the TLP, increasing the damping level of the system in order to control the resonant springing oscillations becomes critical for design. The two main governing parameters for small amplitude oscillatory flow induced by the vibration of a vertical cylinder are Keulegan- Carpenter number and the frequency parameter.