| Deep-sea platforms are important equipments for the exploration and development of deep-sea oil and gas resources. These platforms are usually moored in the harsh marine environment by mooring system, subjected to the combined effects of wind, wave and current and other external loads. Therefore, to ensure the platforms'safety under operating condition and survival conditon, it will be of great significant to determine the tension of the mooring lines and the motion response of the platform under the combined effects of the environmental loads and the mooring force.For Deep-sea platforms, the inertia and damping of the cables have a significant impact to the motion response of the platform. Therefore, the analysis of the platform and its mooring system must be carried out by coupled dynamic method. Currently, the main methods for the coupled of the platform and mooring system are time domain method and frequency domain method.In this paper, the coupled dynamic analysis of the deep-sea platform and its mooring system is investigated. 3D Green's function and fourier transform are adopted to calculate the hydrodynamic characteristics of the floating structure. The mooring lines are simulated using non-linear finite element method. By solving the equations of motion of the entire floating system by employing full time domain coupled analysis method, the vessel's motion response and mooring lines'tension are obtained. The numerical simulation is carried out for a deepwater turret-moored FPSO. The time history of FPSO's motions and mooring line tensions and statistic results are compared to the results of the corresponding model test. The comparisons show that the theoretical results are in good agreement with the test results. It proves the reliability of the numerical method in the calculation of the dynamic response of the deep-sea moored floating structures.Compared with the time domain method, the frequency domain method needs less calculation, and can explain the mechanism of the mooring system. However, frequency domain method is normally only used in the weakly nonlinear case, and the studies based on the frequency method are mainly about the catenary mooring system. In this paper, taking the change of the static component of the mooring line tension due to the change of the equilibrium position during the oscillation into account, frequency domain perturbation method is adopted to expand the mooring line's nonlinear equation into the second order. The dynamic response of a taut mooring line which has a high elongation characteristic and suffers horizontal excitation on the top end is studied. The validity of the frequency domain method for the dynamic analysis of taut mooring lines is checked and the physical mechanism of taut mooring line is analysed. All of these can provide us some theoretical reference for the dynamic analysis of deep-sea taut mooring lines by using frequency method. |
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