Experimental studies on the effects of draft condition, current heading and mooring stiffness on vortex-induced motions of a Tension-Leg Platform

Abstract

The paper presents experimental studies on the effects of draft conditions, current headings and mooring stiffnesses on the Vortex-Induced Motion (VIM) responses of a Tension-Leg Platform (TLP) with four circular columns. The nominal transverse and yaw amplitudes, and the corresponding spectra obtained by fast Fourier Transform (FFT) and continuous wavelet transform (CWT) are analyzed to examine the characteristics of the motion responses. Four current headings in the range of 0° to 45° are adopted in the model test. The results show that the maximum response amplitude in both transverse and yaw directions are achieved at 0° current incidence. For the transverse response, the maximum nominal amplitude decreases and the lock-in range narrows as the current heading increases. The spectral analyses for the transverse and yaw motions at 0° current heading show that the dominant peak frequencies increase as the reduced velocity increases. It is observed that the low-frequency spectral peak of the in-line motion amplitude spectra increases as reduced velocity increases. A color band or a ribbon concentrates stably around the frequency of f<0.05 in the wavelet analysis. It is primarily caused by the initial acceleration of the model in the towing tank. The absolute values of correlations between the spring tensions and the transverse motion of the platform are much larger than those between the spring tensions and the in-line or yaw motions. Moreover, it is found that the VIM trajectories of the TLP are primarily always along the direction perpendicular to the current heading.