Experimental Study On Vortex-induced Vibration And Interference Of Deepwater Marine Risers Transporting Fluid

Marine risers are the main connecter between a platform and the mouth of awell in the seabed and can be applied to floating platform, fixed platform and drillingshipping systems. They can be used for drilling, production and intervention.Marine risers are in very harsh environment in the deep ocean and they aresubjected to many kinds of loads including waves, currents, floating platform motionat the upper end, ice, etc. When current or wave flows across the risers at certainspeed, there will appear vortex shedding, and if the frequency of the risers is close tothat of the vortex shedding, oscillation will make the vortex shedding frequency fixednearby the structural natural frequency and cause large amplitude vibration and thefatigue failure of the riser, which is called ‘lock-in’ state. Vortex-Induced Vibration(VIV) is a complex phenomenon of fluid and structure coupling vibration. For VIVphenomenon, many influencing factors, for example, Reynolds number, top tension,current velocity, internal velocity, and mode, especially considering with complexocean environmental loads, which will accelerate the fatigue failure of the riser. Thefailures of the riser will not only incur huge loss for the project itself but also lead tosecondary haphazard. So further researches on the vortex-induced vibrations(VIV)and dynamic behavior of the riser under the excitation of waves and current are verynecessary.In order to study the VIV characteristics of large length-to-diameter-ratio riserwith internal flow subjected to marine current, tests of VIV were conducted in a largewave-current pool. Model materials were made of copper, with external diameters of20mm and a total length of6.2m. The model riser was attached vertically to thesupporting structure and different top tension, different external current velocities,different internal velocities, which were acted on the model. The amplitude, frequency and mode were measured respectively. The cylinder model was equipped with twelvemeasured stations which are distributed averagely along the riser and each wasformed by four strain gauges to measure both in-line direction and cross-flowdirection oscillation responses. In order to research the dynamic response of a largelength-to-diameter-ratio riser in a stepped current, the experimental data werecontrastive studied by dynamic response of in-line direction and cross-flow direction.The effect of varying top tensions on the natural frequency, the effect of varyinginternal velocities on the amplitude with different top tensions and the effect ofvarying internal velocities on the top tensions were studied. The experimental resultsshowed that there existed a strong correlation between the motions of in-line andcross-flow with a frequency doubling phenomenon and the in-line vibrationamplitudes is generally less than cross-flow direction. During the region of theexternal current velocities, the highest mode number is four. The following results canbe drawn. that the internal flow and top tension have some influence on thefundamental frequencies, dynamic response amplitude and mode numbers, especiallythe internal flow has important effect on the top tension.In order to study the effect of suppression device on VIV, a helical strakesdevice with trapezoid cross section was designed. Modal tests were conducted in alarge wave-current pool. According to change pitch, number of strakes anddistribution rate, the effect of the device was detected. The experimental resultsindicate that double helix and triple helix have a better suppression effect than singlehelix and different pitch also has an important effect on suppression effect. For theexperiment of riser with different distribution rate, the distribution is90%,70%,50%,30%respectively. The experimental results indicate that suppression effect of30%distribution rate is less than other three distribution rate, the Decreasing Value is about60%of the amplitude, the Decreasing Value of another three are more than90%.The risers have several arrangements, such as tandem, side-by-side and stagedconfigurations. When a riser is in the wake of another, the oscillations induced by thevortex shedding are considerable modified and strongly depended on the distance ofthe two risers. Compared with the single riser, dynamic characteristics, dynamic response and the form of vortex shedding are considerably modified. In order to studythe vortex-induced vibration of marine risers with interference, two groupsexperimental research on interference of two risers was conducted. One is two barerisers, which arrangements involve tandem and side-by-side, another involve one bareriser and one riser with triple strakes, which is side-by-side. Different external currentvelocities, different distance of the two cylinders were designed and compared with asingle bare riser. The experimental results indicate that when the riser is in the wakeof another, the oscillations induced by the vortex shedding are considerably modifiedand strongly depend on the distance of the two cylinders. Compared with the singleriser, dynamic characteristics, dynamic response and the form of vortex shedding areconsiderably modified.