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Coupled Dynamic Analysis of Deepwater Floating Structure and the Mooring System

Author: WangXingGang
Tutor: SunZhaoChen;LiangShuXiu
School: Dalian University of Technology
Course: Port, Coastal and Offshore Engineering
Keywords: Deep water Floating structure Mooring system mooring line Green function Geometrically nonlinear finite element Coupled dynamic analysis Wave group Wave envelope spectrum
CLC: P75
Type: PhD thesis
Year: 2011
Downloads: 85
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Abstract


Ocean is rich in resources, and exploration of offshore oil and gas is boosted into deep water and even ultra-deep water. In order to ensure that offshore operations and other long-term floating structures can withstand the loads of the marine environment and work normally, accurate simulations of the time history of the mooring system and reliability analysis are very important. Boundary element method and Morison formulation are applied in this paper for hydrodynamic analysis, and a geometrically nonlinear finite element method is developed to solve the mooring line dynamics. The coupled dynamic analysis of floating structure and its mooring lines is executed in time domain. Wave groups occur in both deep and shallow water, and can cause severe loading on floating structures, especially at or close to natural motion frequencies. Hence, its influence has become an important factor considered in the design of the ocean structures. Wave elevations with different groupiness are simulated numerically, and the influence of wave grouping on the coupled analysis of the mooring system is studied here.Based on potential flow theory, the boundary integral equations are solved using the three-dimensional distributed source method in this paper. Green function satisfying both the free surface condition and the seabed condition is used, hence, only the body surface needs to be partitioned by meshes. At the same time, symmetry is adopted to reduce the amount and time cost in computation. The extended boundary integral equation method is adopted to remove the irregular frequency effect in frequency domain. According to Cummins’s theory, the results in frequency domain can be transformed into the time domain through the Fast Fourier Transform (FFT), then the motion equations of floating structure in time domain can be obtained.In deep water or ultra deep water, the inertia and damping of the mooring lines will have a significant impact on the upper structure, meanwhile, under the conditions of large deformation and large pretension, the nonlinear effects of deformation of the mooring lines cannot be ignored. Based on the total Lagrangian formulation, a geometrically nonlinear finite element method using isoparametric cable element is developed to solve the mooring line dynamics. The Newmark method is used for dynamic nonlinear analysis of mooring lines. Then, the numerical model above is applied to investigate the effects of some key factors, such as material, water depth, amplitude and frequency of the external excitation, pretension and apex angle, on mooring line tension.Using boundary element method/Morrision equation and geometrically nonlinear finite element method, a computing model is established for coupled dynamics of the deepwater mooring system in the action of wave, wind and current. The effects on the motion responses of floating structure and the mooring line tensions of water depth, wave height, wave period, overshoot parameter, velocity of current and wind, incidence direction of wind, wave and current are intensively researched.The influence of wave groupiness on the deepwater mooring system is studied for the first time. Ocean waves often appear in sequences of high wave elevations, which are known as wave groups. Groupiness is an important characteristic of wave field. It is not enough to ascertain the intention of wave groupiness merely by wave spectrum. The wave envelope is an important tool to describe the wave groupiness, the intension of which is inextricably linked to the shape of wave envelope spectrum. The Hilbert transform is used to calculate the wave envelope of measured data, and the wave envelope spectrum density is computed from the wave envelope by the usual Fourier transform. Given wave parameters, group height factor (GFH) and group length factor (GLF), wave elevations with different groupiness can be simulated accurately using wave spectrum combined with wave envelope spectrum. In the present study, coupled dynamics of the platform and the attached mooring lines under the action of wave groups are executed, and the effects of groupiness parameters on wave surface, motion responses and mooring line tensions are investigated.

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