Abstract:At normal operation condition, TTR (Top Tensioned Riser) is subject to a time-varying tension exerted by the top floating platform and it may lose stability in the lateral direction which is called parametric instability. As ocean exploration steps into deep-waters, ocean environment becomes more severe, and the flexibility of marine riser is enhanced due to the increased aspect ratio. It requires to figure out the new influential factors (which has been neglected or simplified) for parametric stability prediction with accuracy to ensure the integrity of riser system. In parametric stability prediction of a TTR, it typically adopts a linear spring model or a simplified engineering model for the DAT (Direct Acting Tensioner) system. In this study, a nonlinear model of the DAT is derived in a rigorous way accounting for the effects of gas pressure and volume, friction forces and pressure loss. The parametric stability of a 1000 m TTR is investigated employs the three tensioner models based on Floquet theory. It has found that the structural nonlinearity of the tensioner affects the calculations of the tension exerted on the riser top end and the natural frequencies of the riser system which may lead to the misprediction of parametric stability. The disparities of the predicted instability region mainly occur in the low frequency domain, and the instability area via the nonlinear model is the largest. It has also found that the structural nonlinearity of the tensioner becomes more profound with the increase of high-pressure gas volume and low-pressure nitrogen pressure. This study is of practical significance for revising the existing software before deep-water applications.
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