| A Spar is considered as the excellent type for platforms in deep water and the mooring system of Spar is used to maintain the buoy's position,relating to the security of platform. When the moored platform moves with large amplitude, the mooring line will transfer from taut to slack, going with the tension increasing sharply, which may endanger the security and production. In this paper, the snap tension in mooring line of deep water induced by taut-slack condition is studied, and the main work and conclusions obtained are as following:(1) Based on the research of Mohammad Behbahani Negad, considering the arc coordinates and horizontal tension, the expressions of tension and radius of curvature are obtained with Taylor series expansion, and the continuum model including taut and slack state is established. The effects of tension in cable, wave number, coordinates of cable on the freely propagation characters of stress wave in cable are studied by stress wave theory and the rationality for the considered factors is testified.(2) Based on the stress wave theory and nonlinear dynamic theory, the motions and tensions of line under three different states of taut, slack and taut-slack are calculated, and the differences between the three states are compared. The results show that, the motion characters of line, the maximum tensions and the threats to structure corresponding to the three states are different. When the system exists in taut-slack state, the motion in normal direction jumps from a steady state to the other, spike and higher frequency components appear in the tension. The maximum tension in taut-slack state is 5 times higher than that in slack, and 2.4 times higher than that in taut. The taut-slack state can easily cause the damage of the structure and shorten fatigue life.(3) According to the impact theory, the analysis on Spar platform-mooring line coupled system is taken on, and the boundary curve of taut-slack region is given, and the influencing factors are discussed. The results show that, the alters of system parameters in moored Spar platform-mooring line coupled system, such as the dimensionless radio of displacement and damping, will cause the transition from the taut to slack state and cause the high tension in line. The maximum dynamic tension in line increases with the increasing of the wave height, wave frequency and pretension, decreases with the increasing of the length of line.(4) With experimental method, the snap loading of lines immersed in a quiescent fluid medium is carried out and the snap tension is observed, which proved the presence of snap tension and necessity of this studying. The results show that, the maximum tension in line increases with the increasing of the excitation frequency, excitation amplitude, stiffness, diameter and modulus, decrease with the increasing of the length of line. The experimental results prove the validity of the theory study mentioned above.(5) Comparative study on the experimental results in air and that in water are carried out, and the tension characters under snap loading are obtained as following: the snap loading appears at the exciting end, the snap tension increases sharply, harmonics of higher frequency components and negative tension appear in the tension. All of these can be regarded as the judgment basis for snap loading. In addition that, the snap tension appears at excitation end, which indicates that the mooring line near attaching point should be strengthen to ensure the security of platform.
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