| Nowadays, with the great progress of the offshore oil exploration andexploitation, risers, considered as key components connecting drilling wells on theseabed and platforms, are attracting more attention. Flexible risers have alreadyshown their apparent merits, as almost all the top oil producers start to discoverdeepwater fields. In the deepwater environment, flexible risers would perform a seriesof special dynamic response phenomenon, affected by the complex marine conditions(such as currents, waves etc.). When confront with currents, the vortex sheddingwould be detected, that is the phenomenon of Vortex Induced Vibration (VIV). TheVIV phenomenon would directly induce to periodic oscillations in In-line directionand Cross direction. When the frequency of vortex shedding gradually comes near thenatural frequency of flexible risers, the resonance phenomenon would emerge, whichwould make amplitudes of both sides soar. When subject to VIV dynamic response fora long time, flexible risers would incline to subjecting to fatigue failure. Additionally,the step to the deepwater field is limited.During the practical work process, flexible risers, subjecting to the currentimpact, inter tension, offset and low drift of platforms, would display dynamicbehaviors. The primary part of this thesis is supported by VIV Model Test of a newtype flexible riser, the key parameters and work conditions of which are provided by arenowned foreign riser manufacturer. Scale model test would be adopted as a primarymethod. And following works are done.1) At first, using hydrodynamics similar principles and reducing the proportion,the flexible riser model can well meet the experiment demands. Then, with hingedends and diverse top tension, the flexible riser model is tested at a special range ofcurrent speed. Meanwhile the strain data and some other data are collected all the time.After data processing, the analysis of dynamic response characteristic of the flexibleriser is conducted comprehensively. It will greatly reveal the Vortex-Induced Vibration mechanism and analyze the resonance region of target flexible riser. Thecore factors of resonance would be verified (current speeds and some other factors).2) In the next stage, based on the model test proposal, the analysis ofcorresponding boundary conditions are considered, and some transformation andsimplification of boundary conditions are performed. After the manufacture andassembling of the device of applying displacement, the phase two of the flexible risermodel is tested as the same conditions. The results would be used to analyze theimpact of applying displacements, as well as form a contrast. Besides, the conclusioncan be provided as helpful references to practical works. Consequently, it would hasvery high application value.3) When coping with the strain data, a series of data processing software, such asMATLAB would be use to make time-domain analysis of amplitudes. Besides, viaFast Fourier Transform, the corresponding frequency domain analysis would beconducted, to achieve the vibration frequency at target site. The amplitudes andfrequencies, as key analysis factors, can have a precise description of the real-timedynamic characteristics of the flexible riser model. After that, some general dynamicresponse regulations can be got. During the experiment period, the specialPhotographic devices, comprising underwater DVs and overwater DVs, are used tomake the VIV process better captured. By means of tracking substance, the vortexshedding form in the wake zone can be well captured; the main role of underwaterDVs is to monitor the real-time gesture of the flexible risers underwater. Part of thevideo data can be served as direct proof of resonance phenomenon. |
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