Stochastic response of coupled TLP-tether system under multi-directional seas.

摘要

This study concerns investigation of the dynamic behavior of tension leg platforms (TLPs) under simultaneous action of random wind and multi-directional wave fields in the presence of currents. Both time-domain and frequency-domain techniques are utilized in this analysis. Computationally efficient procedures to solve nonlinear problems arising from the nonlinearity due to the stiffness of the TLP and loads under multi-directional sea states are developed. The wave load effects under the directional seas are studied by representing the wave field by a coherence function instead of conventional approach that involves decomposition in directional components. In the time-domain analysis, a multi-dimensional ARMA model is developed to simulate efficiently the multi-directional wave field.; The dynamic analysis procedure divides a TLP into two coupled substructures. An analysis approach based on a parallel computation scheme is developed to solve the coupled substructure problem. The bottom tethers are modelled by a three-dimensional finite-element discretization. The stiffness formulation includes large deflections and varying tether tension that results from TLP motions. The top platform is modelled as a rigid six degree-of-freedom system exposed to environmental loads.; An example TLP is used to conduct a parameter study and to demonstrate the effectiveness of the proposed methodology. The TLP response under directional waves is significantly modified, e.g., the surge is reduced, whereas, yaw response is enhanced. The coupled tether-TLP response analysis shows that the tether dynamics affects primarily the TLP motions in the vertical plane, i.e., heave, pitch, and roll. These effects become more pronounced as the water depth increases.