| Marin riser is the link connected between the oil well on seabed and the floating platform that generally used to oil drilling and production. Top tension riser (TTR) is supported by a top end tension provided by floating platforms and hinged at the bottom with the wellhead in the deep water. There is high pressure internal fluid (oil or gas) transporting in the riser and combination of external waves and currents is exerted on the riser. Vortices that are shed by constant current may cause risers Vortex-Induced Vibration (VIV). When vortex shedding frequency approaches the natural frequency, phenomenon of''lock-in''occurs and the oscillation amplitude will increase distinctly which may result in great fatigue damage to the riser. Vortex-Induced Vibration (VIV) is a complex phenomenon of fluid and structure coupling vibration that has been investigated extensively both at home and abroad.However, due to many factors (e.g., Reynolds number, internal flow, top tension and characteristics of riser) especially considering with complex ocean environmental loads, dynamic response analysis of VIV has more difficulty. This thesis present the investigation of VIV by tests that carried out in a large wave-current flume. And following works are done.(1) Based on the continuous wavelet transform method of Morlet wavelet, time-frequency analysis method which describes how the frequency content of a signal varies with time was gained. Detailed interpreted the physical phenomena of vortex-induced vibration. Denoised signal was gained by wavelet transform denoising technology. Amplitudes of oscillation were obtained use the modal decomposition method. Risers fatigue life is calculated based on the S-N fatigue approach under the assumption of linear cumulative damage (Palmgren-Miner rule).(2) In order to study the VIV characteristics of top tension riser (TTR) and flexible riser subjected to marine current, modal tests were conducted in a flume. The results show obvious"lock-in"region of TTR and severe vibration and amplification of amplitude. The in-line vibration amplitudes is generally less than cross-flow direction but can not be ignored on fatigue analysis. There is a strong correlation between the motions of in-line and cross-flow, the figure-of-eight trajectory shapes of VIV are observed. Natural frequency and VIV amplitudes are investigated by change factors of TTR, such as internal flow, top tension and mass ratio. Multi-modal interaction and the mode jumping phenomenon are found in flexible riser VIV tests.(3) Thesis presents experimental investigation on vortex-induced vibration of riser's interference both Parallel form and Tandem form. Results indicate that when the riser is in the wake of another, the oscillations induced by the vortex shedding are considerably modified by and strongly depend on the distance of the two cylinders. Compare with the alone riser, dynamic characteristics, dynamic response and the form of vortex shedding are considerably modified.(4) Displacements time history curves are gained by limited measure points on riser. Assuming fluid force at small period of time is linear, hydrodynamic loads can be identifid on each measure point of riser by method of precise integration on time domain. A wake oscillator model is use to simulate the riser near wake dynamics, differential equations are derived based on work-energy principles. The experimental results verify provide numerical simulation on fluid-structure interactions.(5) A new suppression device of crescent-shaped flow spoiler was designed and seven different layout schemes were arranged. VIV model tests with six level currents were conducted in a large wind-wave-current flume. Vibration frequency evolution with time of the riser installed with suppression devices was analyzed by Morlet wavelet transform. The principle of VIV suppression was interpreted through vibration characteristic aspects. Fatigue life of the riser shows that vibration amplitudes were reduced and the riser fatigue life was improved.
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