Computational Fluid Dynamics Analysis On Vortex-induced Vibrations Of Offshore Tubular Structures

Vortex-induced vibrations(VIV), which always induces resonance damage or short-term fatigue failure for structures, is a key factor considered in design and safety running of offshore structures, especially for riser and subsea pipe. With the development of offshore engineer extending to deepwater and the research furthering, mechanism of VIV process and fluid-structure coupling analysis become the promising direction for VIV research development. In this thesis, VIV of offshore structures is investigated based on CFD technology, which is supported by"863"project: deepwater riser and wellhead technology, and another project researching on safety running and risk management of Hangzhou-Bay subsea pipeline, sponsored by Sinopec. Considering shortage of common VIV analyzing model in which the mechanism of vortex shedding process is always ignored and empirical formula or experimental data is used, the paper proposes a modification of conventional VIV analyzing method based CFD, which provides a reference to the design of offshore structures.In this paper, CFD software is used to simulate the process of vortex shedding, according to which, VIV mechanism and the law of vortex formation are explained, the related fluid field parameter, lift and drag force corresponding to different Re number are described, and also, the difference between 2D potential hypothesis and 3D large eddy simulation is described. Based on mass-spring model considering fluid-structure coupling, the changing rule between lift coefficient and transverse displacement was discussed: with current velocity increasing, lift coefficient and displacement increase to the maximum and then decrease slightly; Using forced vibration theory, the relationship of lift or drag coefficient amplitude with the vibration amplitude of structure, and the lift coefficient frequency with the structure frequency are investigated, meanwhile, the phenomenon of frequency"lock-in"is observed. Considering the interaction of adjacent flow field, changing law of fluid field parameters for two tubular structures arranged in tandem or side-by-side is disclosed: in tandem pipes, the force on downstream pipe is more serious than upstream pipe; in side-side pipes, forces on both pipes are strengthened due to the interaction of wakes. The method of avoiding or impairing VIV effect is suggested. Through simulating the fluid field, the mechanism of kinds of VIV restrain apparatus is explained, furthermore, the optimal fairing structure size and installation requirement are described. The effect of a new apparatus, which is called spoiler, installed on subsea pipe, on flow field around subsea pipe is analyzed, consequently, the mechanism of self-bury is explained; According to the significant asymmetry of lift coefficient caused by spoiler, two new concepts, lift coefficient amplitude and lift coefficient additional value are proposed for the first time, and a simplified model calculating lift force is presented. On the basis of the mature strip theory used in analysis on ship and wave loads, supposing that in uniform current the fluid force along the pipe is equal, therefore a new model for VIV analysis with consideration of fluid-structure coupling is proposed and the iterative process is illustrated through a practical example.