The tension leg platform (TLP) is one of the compliant structures which are generally used for
deep water oil exploration. With respect to the horizontal degrees of freedom, it behaves like a floating
structure moored by vertical tethers which are pretension due to the excess buoyancy of the platform,
whereas with respect to the vertical degrees of freedom, it is stiff and resembles a fixed structure and is not
allowed to float freely. In the current study, a numerical study for square TLP using modified Morison
equation was carried out in the time domain with water particle kinematics using Airy’s linear wave theory
to investigate the effect of changing the tether tension force on the stiffness matrix of TLP's, the dynamic
behavior of TLP's; and on the fatigue stresses in the cables. The effect was investigated for different
parameters of the hydrodynamic forces such as wave periods, and wave heights. The numerical study takes
into consideration the effect of coupling between various degrees of freedom. The stiffness of the TLP was
derived from a combination of hydrostatic restoring forces and restoring forces due to cables. Nonlinear
equation was solved using Newmark’s beta integration method. Only uni-directional waves in the surge
direction was considered in the analysis. It was found that for short wave periods (i.e., 10 sec.), the surge
response consisted of small amplitude oscillations about a displaced position that is significantly dependent
on tether tension force, wave height; whereas for longer wave periods, the surge response showed high
amplitude oscillations that is significantly dependent on wave height, and that special attention should be
given to tethers fatigue because of their high tensile static and dynamic stress. |
