Analysis Of Motion And Cable Tension For ALT-Tanker System

Research work indicates that the restoring stiffness of ALT-tanker system is piecewise-nonlinear. The movements of ALT and tanker are coupled through the cable. The alternation of tightening and slackening of the mooring line has induced discontinuity of stiffness and sudden change of cable tension; it can even cause break and invalidation of mooring line. Dynamic characteristics as well as cable tension changes characteristics of ALT-tanker system under wind, current and regular waves are investigated in this thesis. The main contents and results are as follows:The two degree of freedom analysis model of ALT-tanker system is established, corresponding to the ALT pitch and the tanker surge. The mooring line between the ALT and tanker is represented by an unsymmetrical, piecewise-nonlinear restoring function. The wave load on the tower is evaluated using Morison equation. The first order wave load acting on the tanker is calculated using linear diffraction theory based on the 2-D Helmholtz equation, and the near field approach of Pinkster is used to evaluate the second order drift force. The dynamic equation of motion is established based on Hamilton theory, and is solved using Runge-Kutta method. Dynamic response and cable tension of a mooring system composed of an 88.4-meter ALT and a 100,000 tonnage grade tanker are calculated, and the effects on dynamic characteristics and cable tension of different parameters are discussed.The results indicate that, slow drift oscillation could occur under harmonic excitations due to nonlinearities of the system. 1/2 subharmonic and harmonic motions co-exist for a wide range of frequency ratio, and both the ALT and tanker have the same order of subharmonics. Increasing the excitation amplitude or wind and current force can make the multiple solution region decreasing. Under large wind and current force, the steady-state equilibrium positions deviate from the initial equilibrium positions due to the effect of large amplitude drift of the tanker. At the same time, it changes more rapidly the mooring line from slackening to tightening by quickly drifting the tanker, so sudden change and increase of cable tension are induced.