Scour Mechanism And Characteristics Of Vortex-Induced Vibrations Of A Submarine Free Spanning Pipeline

Submarine pipeline is the lifeline of transportation of marine oil and gas.Scouring near pipeline always takes place under the effect of ocean current in severe marine environment,which may cause pipe to be suspended.Vortex-induced vibration(VIV)of suspended pipeline would be induced by incoming ocean current.Structural fatigue damage would be caused by VIV,which would result in structure failure in severe cases.Scouring mechanism and VIV characteristics of a submarine free spanning pipeline were investigated respectively,which had practical engineering guide significance.Meanwhile,the research made up the deficiency of research field of scouring and VIV of submarine pipeline,which had important theoretical value.Pipe scouring was simulated using FLOW3 D.Sediment scour and k-? turbulent model were combined to simulate pipe scouring process under uniform flow with 0.5 diameter gap between pipe and seabed,getting the following conclusion: Regular vortex shedding exists behind the pipe when pipe scouring achieving stability.There is no apparent vortex shedding behind the pipe when scouring is obvious;The shape of scour hole is the result of combination of scouring and deposition;Initial scouring occurs in the upstream of the pipe;Maximum depth of the scour hole gradually moves downstream as time goes on,eventually stays at downstream of the pipe after scouring reaches stability.A towing tank investigation on vortex-induced vibrations of a simply-supported pipe model was conducted.The aspect ratio(length/diameter)of pipe model was 195.5 and the mass ratio was 1.82.The structural vibration was measured by means of strain gages and a modal method was used to transform the experimental signals into displacement.Both CF-VIV and IL-VIV were investigated,such as dominate mode number,dominate frequency,Max RMS displacement,Max displacement,Max RMS Strain.The coupling effect between IL and CF VIV was analyzed emphatically.It can be found that the coupling phenomenon of IL-VIV and CF-VIV is very obvious.The trajectory at different positions of the pipe model appears many complex forms,such as “8-shape”,“inverted teardrop” and “mouth-shape”.Soil supporting device was designed and another towing tank investigation was conducted to investigate VIV considering fluid-pipe-seabed interaction.The pipe model had a free span in length of 4.6m,and the span shoulder on both ends were 0.5m.The pipe model was laid 5 diameter of itself under soil surface.Sandy soil specimen was collected from Bohai sea.Free decay experiments were conducted in still water to investigate natural frequency of the pipe model.The paper had a preliminary analysis on vibration frequency and strain signals.Results show that the suspended span pipeline vortex-induced vibration remains harmonic characteristic.