The very low natural frequencies of tension leg platforms (TLP’s) have raised the concern
about the significance of the action of hydrodynamic wave forces on the response of such platforms. In
this paper, a numerical study using modified Morison equation was carried out in the time domain to
investigate the influence of nonlinearities due to hydrodynamic forces and the coupling effect between
surge, sway, heave, roll, pitch and yaw degrees of freedom on the dynamic behavior of TLP's. The
stiffness of the TLP was derived from a combination of hydrostatic restoring forces and restoring forces
due to cables and the nonlinear equations of motion were solved utilizing Newmark’s beta integration
scheme. The effect of wave characteristics such as wave period and wave height on the response of TLP's
was evaluated. Only uni-directional waves in the surge direction was considered in the analysis. It was
found that coupling between various degrees of freedom has insignificant effect on the displacement
responses. Moreover, for short wave periods (i.e., less than 10 sec.), the surge response consisted of small
amplitude oscillations about a displaced position that is significantly dependent on the wave height;
whereas for longer wave periods, the surge response showed high amplitude oscillations about its original
position. Also, for short wave periods, a higher mode contribution to the pitch response accompanied by
period doubling appeared to take place. For long wave periods, (12.5 and 15 sec.), this higher mode
contribution vanished after very few cycles. |
