Experimental Investigation Of Vortex-Induced Vibration And Suppression Devices Of Marine Riser

Marine risers are the main connecter between oil platform and the well at the seabed, suitable for fixed/floating platforms, drilling ships and production vessels. As an integral component in the offshore drilling and production activities, risers generally contain flowing mud or high-pressure/high-temperature oil and gas, while simultaneously bearing the various external actions due to wave, current, ice, and vessel motions. When the current flows across the riser at a certain velocity, vortex will be formed behind the riser and the shedding of these vortexes will cause the riser to vibrate. If the riser's natural frequency is close to that of the vortex shedding, a frequency'lock-in'phenomenon will happen, i.e. the vortex shedding frequency is dominated by, and fixed nearby to, the riser's natural frequency. This will lead to strong vortex-induced vibration (VIV) of the riser and aggravate its fatigue damage.When internal fluid travels along the curved path inside the deflected riser, it will be accelerated due to the riser's curvature and its lateral motion. This acceleration will reversely act against the riser and cause additional vibration. The tension applied at the riser top affects the riser's stiffness, and hence changes its vibration characteristics. Therefore, study about internal flow and the top tension will contribute to a better understanding on this regard. Besides, finding an effective VIV suppression method is important from an engineering point of perspective, as this will greatly enhance the riser's fatigue performance.In this thesis, experiments are conducted at a wind-wave-current tunnel on both bare risers made of plexiglass, and risers with VIV suppression devices attached. The experiments on bare risers take account of changes of internal flow, top tension and external current velocity, while other experiments are conducted on risers attached with 5 different kinds of suppression devices. Analyses on the experimental results show that: with the increase of internal flow velocity, the riser vibrates with larger amplitude and at a lower frequency, but the effect of internal flow becomes insignificant when the current velocity is relatively large; increase of top tension increases the riser's vibrating frequency but with a decreased amplitude; with respect to the VIV suppression effects of different devices, it is found that all the single spiral, double helix and triple helix crescent-shape suppression devices can reduce considerably the riser's vibration amplitude; the crescent-八shape suppression device also decreases the riser's VIV well, making virtually no change to the riser's upstream face. Combining the experimental results with existing knowledge, the thesis also makes some recommendations on design specifications of marine risers.