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

Marine riser is an important base structure in the ocean oil-gas development andbears complex ocean environmental load due to wind wave and current. When acurrent flows through the marine riser at a certain velocity, there will be vortexshedding periodically behind the riser which causes the riser to vibrate. If thefrequency of vortex shedding is close to natural frequency of the riser, a frequency‘lock-in’ phenomenon will happen. This will lead to strong vortex-induced vibration(VIV) of the riser and fatigue life of the riser decrease.Installation of vibration suppression devices is an effective way to suppressvortex-induced vibration and improve the fatigue life of riser. One type of three-starthelical strakes of trapezoidal cross section and two kinds of Crescent-shaped spoilerwas designed. In order to investigate characteristics of vortex-induced vibration andthe effect of suppression devices, the VIV experiment was conducted in the largeflume of the laboratory.Firstly, the characteristics of vortex-induced vibration were studied. This studyshows that the vibration of the riser is strongest when the velocity of current flows inthe ‘lock-in’ region. The vibration of the riser weaken obviously when the velocity outof ‘lock-in’ region. And the microstrain amplitude of transverse vibration is an orderof magnitude larger than that of the in-line.Secondly, three-start helical strake of trapezoidal cross section was studied inlaboratory. By means of changing different strake coverage length and the form ofcoverage of three-start helical strake, the effect of suppressing VIV was studied. Bothin-line and cross-flow data of dynamic responses were obtained by resistance straingage instrumentation from the experiment and the fatigue life of riser is analyzed byrain-flow counting method. The experimental results indicate that with the samecurrent speed, each condition of coverage length can suppress VIV, and as the rate ofcoverage increase, the vibration of marine riser decrease and the fatigue life increase. The in-line vibration frequency of marine riser covered with helical strakes decreases.The cross-flow vibration frequency decreases when marine riser covered with helicalstrakes of high coverage rate. When marine riser covered with helical strakes of lowcoverage rate, the cross-flow vibration frequency is almost the same as that of a bareriser.Lastly, two sizes of crescent spoiler are designed in this thesis. One is the bigcrescent spoiler; the other is small crescent spoiler. The riser is covered with crescentspoiler as erected ‘八’ and inverted ‘八’. The spaces between ‘八’ or inverted ‘八’are3Dand5D. The vibration suppression performance is researched through by means ofexperiments. The result proves that crescent spoiler can suppress transverse vibrationof riser effectively. When covered with big crescent spoiler as inverted ‘八’,transverse vibration of riser can be reduced as much as95%if the space betweeninverted ‘八’ is3D. The In-line vibration can not be reduced by crescent spoiler. If thespace between ‘八’ or inverted ‘八’ is too wide, crescent spoiler will increase in-linevibration. The space between ‘八’ or inverted ‘八’ decide vibration suppression effectof crescent spoiler. And the rib height of crescent spoiler is very important.