Time Domain Nonlinear Analysis Of The Mooring System Of Deep-water Floating Structures

With the increasing need of the deep-water oil and gas resources, technologies related to deep-water exploitations have received continually improvement. As the center of exploitation, storage and even production, deep-water platforms are the focus of design and research. The motion performance of deep-water platforms in waves is the key target of the design. Mooring and riser systems play an important role in the platform positioning and resources exploitation respectively, and their dynamic response have received persistent attention. Within the process of positioning, the floating structure and it mooring system interact with each other. The restoring force induced by the mooring system influence the motions of the structure, and in return, the motion of the structure changes the amount of the restoring force. Similar interactions will occur between the floating structure and the riser system. Those three dominated the motion of structure and the dynamics of flexible parts together. In theoretical studies, time domain coupled dynamic analysis will provide more reliable results on structure motions and dynamic response of flexible parts, which will be of great significance on the reduction of project risks and improving of the accuracy of the reliability evaluation.In this paper, 6D time domain program of rigid body motions was developed based on the hydrodynamic theory of floating body in waves, time domain GREEN function, frequency domain GREEN function and FFT method. A finite element program was developed to simulate the flexible parts based on the theory of slender rods and mooring lines. The hydrodynamic coefficients, wave forces, motion responses of a floating buoy was calculated and compared with the literature results to verify the program. For flexible parts, the geometric nonlinearity, material nonlinearity were verified respectively, and in addition the dynamic problem of a single mooring line referring geometric nonlinearity, nonlinear boundary conditions and nonlinear external forces were conducted and compared with test result. For the coupling analysis, the computational results by the program of a FPSO in waves were compared with the corresponding test results, and the comparison shows the effectiveness of the program.Fiber lines are widely used in ocean engineering in recent years because of the light-weight and corruption-free properties. However conventional method can't predict the dynamic response of the fiber lines precisely because of the complexity of the material nonlinearity. In this paper, the elastic stiffness was treated as a constant in the process of derivation of the governing equations, and both the stiffness and the stain will be updated by the tensions and residual strain for each time step and each element.The material nonlinearity of the fiber lines reflects in the stress-strain relations, and specifically in cable dynamics, it reflects in the tension-strain relations. Accurate simulation of the tension-strain relations is the key problem of the dynamic calculation of the fiber lines. In this paper, it is found that the residual strain plays a role as important as the dynamic stiffness of the fiber lines in the process of simulating. An approximate method is proposed to estimate the dynamic stiffness and the residual strain. It is found that the method will greatly improve the accuracy of the calculation and reduce the number of iterations.In this paper, a comprehensive study of the mooring energy including potential energy, kinetic energy and mooring-induced damping was conducted. It is investigated that influence of different tension components under different initial tension levels, which will provide a rational reference for the design of the mooring system. In the design stage, it is very convenient to get the 2D/3D energy distributions of the mooring system from the 1D/2D energy distributions of the mooring lines. Afterwards, the distributions of restoring force and system stiffness can be obtained by the first and second order gradient of the system energy respectively in any directions, which can provide the designers with a whole understanding of the performance of a mooring system and give out an important reference for the optimal design.