Experimental Study On Dynamic Response Of Smooth And Straked Risers In Tandem Arrangement

Riser systems,known as the drill conductor systems linking surface floating bodies to seabed wellheads,are the basic installations for a floating production system to deliver liquids to ships.For these systems,vortex-induced vibration(VIV)constitutes one of the main drivers for their fatigue damages.Installing a helical strake on the surface of the riser has proved to be an effective way of passive suppression.Neverthess,most sutides on VIV of riser have relied on an isolated cylinder or isolated flexible riser model,when in real practice,one platform is usually designed for holding more than ten risers which appear as riser groups during service.When external flow passes by them,the risers will be subjected to an "interference effect" caused by the wake current from other risers and the forces from the adjacent risers.This effect can make a great difference to the VIV pattern of the risers,making it very diferent from that of a single riser.In order to examine how interference effect impacts the vortex-induced vibration of deep water risers,an experiment was conducted in a combined wave-current water flume on two tandem risers subjected to uniform flow.The riser model has an effective length of 2.0m,an outer diameter of 18.0mm,the thickness is 1mm,and the aspect ratio is 111.11.Two groups experimental researches on interference of two risers were conducted.One is the upstream riser and the downstream were smooth risers,another is the upstream riser was straked riser fitted with three-strand helical strakes with pitch 17.5D and height 0.25D,and the downstream one is a smooth riser.By varying the external flow velocities and spacing,and through comparisons with the dynamic response of isolated smooth and isolated straked riser,the paper observed how interference effect impacts the dynamic characteristics and dynamic response of two risers in tandem arrangement,revealed how the suppression efficiency of the three-strand helical strakes responds to spacing ratio and external flow velocity,and explored the wake excitation effect of inter-riser fluids on the downstream riser and their dynamic feedback to the upstream riser.The experimental results indicate that for two smooth risers arranged in series,mutual interference occurs between the serial risers;upstream smooth risers are less affected,and the dominant frequencies of cross-flow(CF)and in-line(IL)are slightly lower than isolated smooth riser,and the differences between the spacing ratios are small.The upstream smooth riser is less sensitive to the spacing ratio.When Ur>6.73;the CF dominant frequency of the downstream smooth riser with different pitch ratios is discretized.In the IL direction,the smaller the pitch ratio,the greater the interference frequency of the dominant frequency,and the greater the difference between the dominant frequency of the isolated smooth riser.The feedback effect of the downstream smooth riser enhances the CF displacement of the upstream smooth riser.The displacement of the upstream smooth riser at four spacing ratios exceeds that of the isolated smooth riser.There is a difference in the displacement of the upstream smooth riser with different spacing ratios.The downstream smooth riser is shielded by the upstream smooth riser and the tail vortex is excited.The displacement of the downstream smooth riser is higher than that of the isolated smooth riser.The dominant frequency of the downstream smooth riser is different from that of the isolated smooth riser.For straked and smooth risers in tandem arrangement,the dominant frequency of downstream smooth riser is different from that of isolated smooth riser.The first-order frequency lock-in region widens in the CF direction,and the process of converting the first-order to second-order dominant frequencies in the IL direction occurs,and the sensitivity to the change of the spacing ratio is low;as the reduced velocity increases,the effect of shielding effect of the upstream straked riser gradually weakens,the wake excitation gradually increases,and the CF displacement response of the downstream smooth riser gradually exceeds the isolated smooth riser;the IL displacement is lower than the isolated smooth riser.The downstream smooth riser has a disturbing effect on the wake area of the upstream straked riser.When the reduced velocity Ur<6.73,the vibration suppression effect of the three-strand helical strakes is enhanced,and the vibration suppression efficiency is highest when the spacing ratio is 8,up to 99%;When Ur>6.73,the vibration suppression effect of the three-strand helical strakes is weakened,and the vibration suppression efficiency is only 63%when the spacing ratio is 5.