Experimental Study On Active Vibration Suppression Technology Of Vortex-induced Vibration Of Deep Sea Riser

In the offshore oil and gas development system,the deep sea riser is used to connect offshore platforms and subsea wellheads.It is a pipeline necessary for all types of platform structures in the marine oil and gas development system to transmit oil and gas.There is generally high-pressure oil or air flow through the inside,and the outside needs to withstand the effects of various marine environmental loads such as waves,currents,ice and earthquakes.Under these complex environmental loads,the riser generates forced excitation vibration in the in-line direction;At the same time,due to the effect of the ocean currents,vortices can be formed alternately on both sides of the riser.The vortex shedding generates a periodic variable force,which causes the riser to generate vortex-induced vibration(VIV)in the direction of lift perpendicular to the in-line direction.VIV is the main factor leading to fatigue damage,and once the riser is damaged,it will cause huge economic losses and environmental disasters,and cause irrecoverable damage to the original fragile marine ecosystem.Due to the limitation of the direction of waves and incoming flow and the attachment of marine plankton,the use of existing vibration suppression devices has limitations,which affects the service life requirements of the riser,and with the development of marine oil and gas development in deep and ultra-deepwater,active suppression shows its superior characteristics,and research on active suppression of riser VIV is of great practical significance for reducing the risk of damage to the riser during service.Simultaneously,improving the active vibration suppression mechanism of the riser VIV has important theoretical significance and engineering application value.In this paper,two new types of active vibration suppression devices,water jetting type,and air jetting type are designed.By changing the multi-stage external flow velocities and changing the parameters such as excitation spacing of the active vibration suppression device,the comparative test of the riser VIV before and after active suppression was carried out in the wave-current combined flume.The riser model uses a plexiglass tube with a length of 2.0m,a diameter of 18.0mm,and a wall thickness of 1.0mm.Six measurement points are arranged along the axis of the riser model.Each measurement point is arranged at 90° with 4 fiber grating sensors to measure the vibration response in the in-line(IL)and cross-flow(CF)direction.For the water jetting active vibration suppression test,the water jetting active vibration suppression device is placed behind the riser,the water jet hole is parallel to the riser axis and is facing the direction of the incoming flow,and the design is 2.5D,5.0D,7.5D,10.0D,12.5D,15.0D,a total of 6 excitation spacings;for the air jetting active vibration suppression test,the air jetting active vibration suppression device is placed in front of the riser,and the air jet holes are all facing upward,and 10.0D,15.0D,30.0D,a total of 3 excitation spacings.Based on methods such as modal decomposition and wavelet transform,the dimensionless displacement of the riser before and after vibration suppression,the dominant frequency,the dominant mode,the Strouhal number,the displacement trajectory evolution and other parameters are analyzed to explore the dynamic response law of riser before and after active vibration suppression under uniform flow.The influence factors and suppression mechanism of suppression effect and performance of water jetting and air jetting active vibration suppression devices under different excitation spacings are studied to provide a theoretical basis and data reference for the design and manufacture of deep sea riser active vibration suppression devices.The results show that the water and the air jetting active vibration suppression devices have obvious suppression effects on the VIV of the riser at different excitation spacings,and the displacement amplitude and vibration frequency of the riser have different degrees within different excitation spacings.The coupling relationship between the excitation spacing and the reduced velocity is the key factor for the two active vibration suppression devices to achieve the optimal vibration suppression effect.For the water jetting active vibration suppression device,compared with the riser before vibration suppression,the displacement amplitude of the riser after vibration suppression is significantly reduced.The optimal vibration suppression efficiency of the water jetting active vibration suppression device can reach 92%,and it is not that the smaller the excitation spacing is,the better the vibration suppression effect of the active vibration suppression device is,but there is a sensitive spacing.As the reduced velocity increases,when the excitation spacing of the water jetting active vibration suppression device changes from 7.5D to 10.0D,the vibration suppression efficiency is polarized,and a clear "transition area" appears.Because the water jetting active vibration suppression device effectively destroys the strong nonlinear coupling relationship between the CF and IL directions of the riser,the displacement trajectory of the riser changes from "8" to "O".For the air jetting active vibration suppression device,after the horizontally rising air curtain flow enters the strong turbulence region,the two-way circulation formed by it has a significant effect on the dominant frequency of the riser when the velocity is reduced,and the VIV energy is attenuated by orders of magnitude.When the position of the air curtain is in the weak disturbance region,the riser resumes regular VIV,the bubble injects momentum into the shear layer and the boundary layer,lengthens the formation length of the vortex,and the dominant frequency decreases slightly.When the air curtain is located in the strong turbulence region,the reverse circulation formed by it hinders the formation and development of the boundary layer.At the same time,the rising air curtain interrupts the riser wake,which effectively reduces the riser amplitude and keeps the amplitude stable at the same amplitude.In addition,the stable amplitude when the air curtain is located in the strong turbulence region is weakly affected by the reduced velocity.At high reduced velocity,the air jetting active vibration suppression device has more advantages.The air curtain generated by the air jetting active vibration suppression device at different excitation spacings enters the strong turbulence region at different velocities and inclination angles,and the peaks of vibration suppression efficiency appearing in the spacing are also slightly different.