Dynamic analysis of tension-leg platforms

Abstract

The dynamic response of tension-leg platforms subjected to wave loading and ground motions was investigated using a deterministic dynamic analysis. The model employed in this study is based on derived coupled nonlinear stiffness coefficients and closed form inertia and drag forcing functions derived using Morison's equation. The forcing functions include relative motion between the fluid particles and the structure, and are integrated manually, thereby avoiding the need for expensive numerical integration. The set of coupled nonlinear differential equations was integrated sequentially in the time domain using the Newmark Beta method. A computer program was developed to simulate the time history response of the platform motion. With this program, a parametric study to identify parameters affecting the dynamic response of the platform was performed. Some of the parameters studied were wave period, wave height, water depth, initial tension, and cable stiffness. Horizontal and vertical ground motion components were used to study the effect of earthquakes on tension-leg platforms. Coupling between the six degrees of freedom (surge, sway, heave, pitch, roll and yaw) was found to have a significant effect on the structural response. The strongest coupling was that between heave and surge or heave and sway. Nonlinear drag forces also were found to be significant in that they represent the fluid damping and therefore result in response reductions with time. Stiffness nonlinearities were found to be important for large surge or sway. The displacement response to combined wave and earthquake loading was found to be dominated by waves; however, platform accelerations were significantly affected by the earthquake.