Study On The Coupling Effect Of Nonlinear Added Mass Coefficient And Fluid Damping On Vortex-induced Vibration

The marine riser system is an important part of the offshore oil drilling equipment.The oil from the seabed is transmitted through the X-tree,BOP,and marine riser to the FLNG or shuttle cruise ship.The riser can also be used as a drilling tool in the early stage of the oil production.As the sea current flows through the riser,the vortex shedding on the both sides of the riser will produce the force in the directions of in-line and cross flow.These forces cause the riser to displace which will change the flow in turn,resulting in a fluid-solid coupling.This coupling is called vortex-induced vibration or VIV for short.Vortex-induced vibration may cause fatigue failure of riser;therefore,an accurate VIV prediction model will demonstrate certain engineering value for evaluating service life.In this paper,the research methods of VIV in the domestic and abroad in the frequency domain and time domain were investigated and it was learned that the scholars usually use fixed value to replace the VIV parameters: added mass coefficient and fluid damping.Aimed at this question,the experimental study and the wake matrix model derivation were carried out.The relationship between the average top tension and the reduced velocity are derived and compared to the classical experimental data.The Generalized Integral Transform Technique(GITT)is used to decouple the riser vibration equation and the wake oscillator equation,and take nonlinear added mass coefficient and fluid damping from experiment into the decoupled equation,then analyze the influence on the vortex-induce vibration model of the riser.A scale model of the marine riser was set up in a wave-flow sink,which was made of a PVC pipe with effective length of 1.2m and aspect ratio of 1000.Both ends were hinged.Two fiber grating sensor strings were uniformly attached on the surface of the riser model in the cross-flow(CF)and in line(IL)direction.The hydrodynamic force was measured with a high-precision bellows load cell and the top tension was measured with a S-type tension sensor and a spring.The collected vibration signals were processed in MATLAB by Chebyshev filter,modal analysis method,integration method and a series of treatments to get the experimental parameters.The effects of reduced vibration,top tension and mass ratio of the riser on the added mass coefficient and fluid damping were studied through the flume experiment.Then the characteristic parameters such as amplitude ratio,frequency ratio and modal of vortexinduced vibration were analyzed to determine it.The experimental results show that the added mass coefficient and the fluid damping are both nonlinear parameters;the additional mass coefficient is related to the reduced velocity,but also to the mass ratio and the top tension.The top tension will affect the zero point of the added mass curve.The fluid damping is related to the reduced speed and amplitude ratio,and the relationship between the mass ratio and the top tension is not obvious.In the finite element software ANSYS,the flow around circular cylinder is simulated.For the different fluid velocity and different riser parameters,the corresponding added mass and fluid damping from experiment result are applied,and the curves of lift coefficient and vortex shedding are obtained,then to compare with theoretical research.