This paper is based on work conducted at the Offshore Technology Research Center to Investigate the relative importance of different types of nonlinearities on the dynamic response of tension leg platforms. A time domain solution using Monson's equation with several modifications and allowing to include or neglect various sources of nonlinearities was implemented and used in the dynamic analysis. This paper focuses on the nonlinear effect of evaluating the wave forces up to the free surface using different approximate methods. A TLP hull model with time varying tendon forces is subjected to regular and irregular waves with and without current. The effects on calculating the wave kinematics up to the mean water level or up to the actual free surface, making use of" various extrapolation or stretching techniques are then discussed.
As oil exploration proceeds into deeper water, the tension leg platform is one viable and promising structural solution. The characteristics of the TLPs are quite challenging and interesting at the same time. Despite the considerable amount of work which has already been done to understand their behavior, there is still a great deal more research which is required. A TLP can be modeled as a six degree of freedom system. The natural frequencies in surge, sway and yaw are well below the range of wave frequencies. On the other hand, the heave, pitch and roll natural frequencies are above the exciting wave frequencies. There are thus two different nonlinear aspects which need to be considered m the design of TLPs, one related to the amount of offset permitted, the other to the limits (i.e. minimum and maximum) of the tendon tension forces. The former is mainly influenced by the slowly varying drift forces (low frequency response) while the latter is greatly affected by the springing and ringing phenomena associated with high frequency response.