Added Mass and Damping of a TLP Column Model

Abstract

SUMMARY The Tension Leg Platform (TLP) typically has a very high natural frequency in vertical (heave) motion. While the wave energy is quite small near this natural period of 2 to 4 seconds, the nonlinear forces can introduce excitation at the natural frequency, giving rise to large tendon loads. This phenomenon is commonly known as " springing". The damping values of the TLP leg in the vertical direction are very important in the computation of this load. Little is known about these damping characteristics. In order to investigate the hydrodynamic damping of a TLP, model testing of a vertical column of a typica1 TLP was performed. This test used a vertically moored cylinder to represent a TLP column at a scale of about 1:20. The mooring 1ines which restrained the cylinder were a pair of linear springs attached to the top and bottom ends of the TLP leg. The cylinder was oscillated in still water and in waves to record the decaying oscillations, and thus provided data to calculate the damping values. The cylinder was also mechanically oscillated with known sinusoidal motion, and the resulting loads were recorded. The hydrodynamic added mass and damping coefficients of the vertical caisson covering a wide range of natural periods and wave frequencies have been presented. Results of this investigation can be readily used and applied in the design of TLP's. The use of the damping values found in this study will reduce predict ions of TLP mot i on and tendon loads. For deep water TLP applications, the associated cost savings can be significant. INTRODUCTION Many offshore structures that are floating and moored to the bottom of the ocean respond to waves at two different frequencies. One of these frequencies is the wave frequency while the other is the frequency corresponding to the natural frequency of the system (if it is distinct from the wave frequency). For a ship-shaped floating body, the natural frequency in surge is low so that the second peak appears at the low end of the power spectrum plot. On the other hand, for a vertically moored TLP, this frequency in heave falls at the high end of the spectrum. In both these cases, the exciting force is of second-order and, thus, proportional to the square of the wave amplitude (in regular waves). The response of the floating vessel is increased near the natural frequency due to the dynamic amplification. The resonant response of the vessel is limited by the amount of damping present in the system, The damping generally appears from two natural sources. One of them comes from the springs themselves, called