Experimental Investigation On Vortex-induced Vibration Of Deepsea Risers Under Different Excitation Water Depths

The deep-sea vertical pipeline structure(referred to as "riser" for short)is used to connect offshore platforms and seabed wellheads.It is the basic device for transferring oil and gas from the seabed wellhead to the platform,and the most complex structure in the deep-sea oil and gas development system.Inside the riser,there is generally high-pressure oil or air flow during the operation of the deep sea oil and gas development system,and the outside needs to withstand the load of complex marine environments such as waves,currents,ice and earthquakes.At the same time,there is a platform connected to the top,and the bottom interacts with the seabed,which makes the riser have the characteristics of harsh service environment,extremely complicated stress,and huge construction costs.Vortex-induced vibration(VIV)is an important factor that causes fatigue damage of the riser during service,especially the occurrence of "lock-in" phenomenon,which will greatly aggravate the fatigue damage of the riser.In actual sea conditions,the fluid form is intricate,and the velocity profile of the ocean has a certain velocity gradient with the height,and even if it is located in the same plane,there will be a velocity gradient with different positions.In the deep sea,the surface current velocity in the surface layer is often higher than that in the bottom layer,and it decreases linearly with the increase of the depth of the ocean,then the cross section shows an "inverted triangle" mode.To better simulate the "inverted triangle" ocean velocity profile model in real sea conditions,the VIV test of deep-sea risers under different excitation water depths is carried out in this paper in the wave-current combined flume.The riser model used a transparent flexible plexiglass tube with a length of 2.0m,an outer diameter of 18.0mm,and an aspect ratio of 111.11.Six measurement points were arranged along the axial direction of the riser model,and four fiber grating sensors were respectively arranged for each measurement point to measure the vibration response in the CF and IL directions.In the test,the riser was fixed on the support device in a standing posture,and a constant top tension was applied at the upper end.By dimensional changing the height of the excitation water depth and the hydrodynamic parameters such as the outflow velocity of all levels of water depth,the CF and IL amplitudes,frequencies,displacements and modes of the risers under different excitation water depths were measured to explore the law of dynamic response parameters such as dominant frequency,dominant mode,dimensionless displacement and vibration trajectory evolution process of the riser under different excitation water depths by conducting experimental studies on the riser VIV under different excitation water depths,further using the time-frequency scale diagram obtained by the wavelet transform method to analyze the characteristic frequency variation law of the riser under different excitation water depths and the frequency component characteristics in the time domain,thus revealing the sensitive characteristics of the height of the excitation water depth to the VIV of the riser.The results show that the law of vortex-induced vibration of the riser under different excitation water depths is quite different in different outflow velocity ranges,and the vibration firequency and displacement amplitude of the riser have different degrees of influence in the range of different excitation water depths.Changes in the height of the excitation water depth will change the additional mass and fluid damping of the riser and other parameters,which will affect the spatial correlation and stability of the vortex shedding behind the riser The time-frequency scale diagram of riser vibration under different excitation water depths obtained by wavelet transform can clearly reflect the intensity change and frequency components of the frequency in the time domain.It becomes very narrow and centered on the locked frequency.When the riser is in the"lock-in" region,the distribution range of the characteristic frequency becomes very narrow and centered on the "lock-in" frequency.The increase in the height of the excitation water depth gradually advances the starting point of the "lock-in" region of the riser,which further promotes the excitation of the higher-order vibration frequency of the riser structure.As the height of the excitation water depth increases,the stability of the accelerated movement of the surrounding fluid micelles caused by the additional mass is enhanced,and the viscous shear and fluid separation on the surface of the structure caused by the fluid damping are strengthened,making regularity more obvious at low flow velocities.Within the height of the dimensional excitation water depth,the dominant frequency,dominant mode and dimensionless displacement are highly insensitive to the excitation water depth at high flow velocity.The change of the excitation water depth will interfere with the correlation of the nonlinear coupling of the riser,so that the trajectory of the riser will show "O-shaped","X-shaped" and"crescent-shaped".