Most of the existing mathematical models for analyzing the dynamic response of TLP are based on explicit or implicit assumptions that motions (translations and rotations) are small magnitude. However, when TLP works in severe adverse conditions, the a priori assumption on small displacements may be inadequate. In such situation, the motions should be regarded as finite magnitude. This paper will study stochastic nonlinear dynamic responses of TLP with finite displacements in random waves. The nonlinearities considered are: large amplitude motions, coupling the six degrees-of-freedom, instantaneous position, instantaneous wet surface, free surface effects and viscous drag force. The nonlinear dynamic responses are calculated by using numerical integration procedure in the time domain. After the time histories of the dynamic responses are obtained, we carry out cycle counting of the stress histories of the tethers with rain-flow counting method to get the stress range distribution.
In recent years, China's petroleum industries take great interest in deep water exploration and development just as their international counterparts do. The fixed exploitation platforms fit for shallow water are not suitable to deep sea because the construction cost increase greatly with the water depth. To meet the challenge, the concept of compliant platforms favorable for deep sea is thought over. Tension leg platform (TLP) is one of the candidate platforms for deep water exploitation. TLP can move with waves other than fixed at the initial position as the jacket platform does. It consists of hull, taut tethers and foundations, which allows motions of surge, sway, and yaw in the horizontal plane and heave, pitch, and roll in the vertical plane. Williams and Rangappa (1994) developed an approximate semi-analytical technique to calculate hydrodynamic loads, added mass and damping coefficients for idealized TLP consisting of arrays of circular cylinder.