| Ocean oil exploration has been going deeper and deeper to satisfy the increasing energy demand. The platform type has changed from fixed to floating, of which, marine riser system is a very important and flexible element for drilling or producing. Flexible riser can be induced to vibrate with large amplitude under certain conditions by vortices shedding behind it, which is the so-called vortex-induced vibration (VIV) and is the main source of riser's fatigue damage. Great saving in cost is possible if the VIV response on flexible risers can be predicted more accurately, considering the huge investment put into resisting VIV.The present research presented in this dissertation is part of the project"Numerical Experiment on Interaction between Large-scale Structures and Special Environmental Forces"supported by National Natural Science Foundation of China under grant number 50538050. The purpose of this research is to build a numerical tool for predicting VIV response on flexible riser directly. Corresponding experiments were done for providing reference data to numerical model.The main contents including:1. A device was developed to enforce excitation on one of riser's ends for simulating the motions of platform due to wind or wave. Four kinds of cases were set: (a) Decay tests were performed in both air and still water to measure structural characteristics of the riser model, such as natural frequencies and modal damping ratios in corresponding fluid medium. (b) By varying uniform currents without applying end excitation,"lock-in"phenomena and continuous appearance of three-branch-response were observed. (c) By varying amplitudes and frequencies of harmonic excitation at one end, response data in air and water were collected, they were reference of latter analysis. (d) By combining all cases in (b) and (c) and by comparing with the corresponding results, the effect of end oscillation on flexible VIV was studied.2. Overlapping grid was applied successfully to deal with moving boundary.Finite volume method (FVM) and Arbitrary Lagrangian Eulerian Method (ALE) were used with overlapping grid to couple the fluid field, structural field and mesh field, and then an effective and efficient VIV model of 2D cylinder was built. A numerical tool VIV2D was made through programming languages C++ and MATLAB. Two of the outstanding merits of the model are: (a) no limit to moving velocity and moving range; (b) no necessary to remesh the computational domain but to find new donors for fringe nodes in the progress of motion. It is very continent to simulate one degree of freedom (DOF) VIV or two DOF VIV, self-excited (VIV) or forced oscillations. Parameter analysis was also made about three key factors on dimensionless amplitudes.3. The 2D VIV model, with combination of strip theory and FEM, was expanded to simulate flexible riser VIV in a quasi-3D way and then a numerical tool RISER was built. That was to treat each finite element along riser's axis like a 2D cylinder, but they were coupled through FEM to obtain the whole response of the flexible riser. Two types of cases were considered by referring to that in experiment: (a) No end excitation was enforced in uniform current. Correctness and validity of the numerical model was evaluated through comparison between experimental and numerical results. (b) Effect of end excitation on VIV of flexible riser was also investigated numerically.The research in this dissertation involved many aspects of VIV. It improved the understanding of VIV especially that of flexible riser through present work. More realistic boundary conditions of a flexible riser were considered in numerical and experimental studies, and then it is a valuable test for building more accurate prediction model of flexible riser VIV. Instructive references about experiment and numerical studies of flexible riser VIV were also provided.
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