Abstract

The good motion performance of Tension-leg platform (TLP) does credit to its tendons made of steel tube. The tendon system reliability is crucial for the safety of TLP platform, and the dynamic tension, as one of key issues, should be addressed carefully in design stage. The tendon tension is determined by redundant hull buoyancy, vertical motion at tendon top point on the hull, the set-down effect due to horizontal excursion etc. Moreover, the contributions of hull mean offset, wave frequency (WF) motion, nonlinear low frequency (LF) slow drift motion as well as high frequency (HF) springing effect need to be taken into account in the global performance analysis. The existing fully coupled approach models the tendon as discrete elements and solved by FEM; inertial and drag forces as well as interaction between hull and tendon system are able to be considered. However, on one hand, the computation consumes plenty of resource and time; on the other hand, inertial and drag effects of tendon may not be dominant factors. Therefore, the fully coupled approach is not the ideal solution, at least, in sea-state screening design phase when tremendous 3-hour time- domain simulations are required. Motivated by mentioned reasons, a quasi-dynamic method was initiated by Bureau. Decent results such as extreme tendon tension is obtained and compared to fully coupled results from third party software ORCAFLEX, and the discrepancy is limited within 5%. The tendon characteristics like relation of set-down and TLP offset, relation of horizontal restoring force and offset are pre- described by utilizing analytical method. The WF, LF and HF dynamic motions are assumed to be decoupled since their representative frequencies are far enough from each other. LF motions of horizontal 3-DOF i.e. surge, sway and yaw are solved through motion equation in time domain. 6-DOF WF motions and 3-DOF HF motions in vertical plane are re-constructed by using transfer function obtained from frequency domain. The tendon tension is post-processed based on the total tendon length taking into account vertical motion of tendon top point, set-down height and offset, at each time step. This paper represents and describes the methodology of this approach, assumptions, key steps, and discussions as well as conclusions are drew and conducted.

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