Non-Linear Time-Domain Analysis And Calculation For Moored Systems

Offshore structures are often maintained in position by a mooring system which resists all kinds of ocean environment loads from all directions. Therefore, the reliability analysis and motion simulation of a mooring system are of primary important in studying the dynamic behavior of offshore structures. Today, the motions of a mooring are analyzed by the coupled method between floating-body motion analysis and mooring-line tension calculation. By the analysis of current coupled approaches, new coupled methods are derived to solve a mooring system based on the non-linear finite element method.Because most mooring structures are non-slender bodies and the 3-dimensional effect can not be neglected, the 3D frequency-domain GREEN function method in the paper is adopted to calculate the hydrodynamic parameters of a moored floating-body. Considering the non-linear deformation effect of a mooring cable is not neglected under the condition of large deformation and large pretension, the non-linear finite element method is adopted to calculate this nonlinearity of a mooring cable. The non-linear tension-displacement response is calculated by Newmark method and mN-R method after the non-linear stiffness matrix of large deformation cables is derived by the principle of virtual work and non-linear continuous medium mechanics. In the process of analysis of the unit motion amplitude response, actual incident wave height is adopted to solve the floating motion, by the transformation, the motion responses function of mooring systems are gained. Comparing the method by which the tradition motion responses are solved, the method in the paper is most reasonable due to the nonlinearity of mooring cables.Based on the current time-domain coupled method, new time-domain coupled method is derived to calculate the motion of mooring systems. In this method, the non-linear finite element method is coupled with frequency-time domain transformation. By numerical analysis, the non-linear element method is not only fit for small deformation cables but also for large deformation cables.Because there are a lot of numerical errors in calculating floating-body hydrodynamic parameters due to complexity of floating-body form and the numerical method, the time-domain impulse response function can not be calculated perfectly. Based on above analysis, the floating body motions are simulated directly by 3D time-domain linear GREEN function method. Coupled with the non-linear finite element method, the motion time history of a moored body is calculated. In the time-domain motions simulation of a mooring system, bycalculating numerical examples, it is shown that the results of time-domain coupled method based on GREEN function method is more reasonable than the method based on frequency-time domain transformation in the peak values.