Numerical Simulations Of Flow Around Marine Risers And Experiments On Sensibility Of VIV Suppression

The pipeline structures applied in offshore engineering, such as marine risers,suffer the effects of flow and separation around them.When vortex-shedding occurs,periodic forces in cross-flow and in-line directions come into being, and they causethe vortex-induced vibration(VIV) of structures. If the frequency of risers is close tothat of the vortex shedding, the oscillation of structures will get large amplitudevibration and fatigue failure of the risers, which is called ‘lock-in’ phenomenon.With large amounts of pipeline structures applied,the interference of risers makesthe flow field around them very complicated. This article applied the method ofcomputational fluid dynamics(CFD), forms of flow field were simulated and drawnout by establishing the models of the flow around circular cylinders, and the rules ofvortex-shedding were also analyzed. Then choosing the usual helical strakes as thedevices for suppression of VIV, experiments were conducted under stepped current tostudy the influence of different arrangements on the results of suppression.Firstly,applying the software of CFD method and simplifying the marine risers ascircular and nonflexible cylinders, numerical simulation of2D static flow aroundthem were conducted to study the rules of drafting forms, frequency of vortexshedding, separation point of the shear layer and forces of structures because of thechange of Reynolds numbers and gap between cylinders.According with the analysisof the streamlines under the same Re number,the forming reasons of different wakestreams behind the circular cylinders were explained. Above all, the regions of G/Dcorresponding with different drafting forms and the rule of the regions changing withRe number were identified.According to the laws of the flow field and vortex shedding,the principle of VIVsuppression with helical strakes was explained by introducing the concept of thediversion angle and amount, they were used to guide the radial flow to separate andthe rules of vortex-shedding to change. Effects on the diversion angle and amount when screw pitch,number and coverage of helical strakes change were analyzed later.Considering the fact that tests of VIV suppression of marine risers were mostlystudied by transverse dragging and subjected to uniform current, copper pipes withslenderness ratio of310were chosen and trapezoid cross-section helical strakes withscrew height of0.25D were designed as the models to conduct the experiments onVIV of marine risers under stepped current. According VIV tests of single, double andtriple helical risers with different screw pitches and coverages, micro-strain timehistory curves in same point were collected and contrastive analysised in differentconditions. The effects and sensibilities of screw pitch,number and coverage of helicalstrakes on the amplitude and frequency in cross-flow direction were discussedespecially when the risers were near the third order lock-in range.