| The marine riser is the key equipment connecting the floating platform and the seabed wellhead,and VIV is the main reason of its fatigue damage,which contains complex and abundant dynamic content.In practical engineering,the marine riser is basically operated for a long time.As time goes by,Marine organisms will absorb on its surface,thus affecting its surface roughness.The surface roughness makes the dynamic characteristics of the riser VIV more abundant and complex,including jumping,multi-frequency vibration,broadband vibration,resonance and other dynamic contents.In this paper,based on the bidirectional fluid–structure coupling method of CFD(Computational Fluid Dynamics)and CSD(Computational Structure Dynamics),a numerical simulation program for VIV of a smooth cylinder and a smooth riser is developed.On this basis,the modified model of rough wall velocity gradient is introduced,and the calculation program of a rough cylinder and a rough riser wall velocity gradient is compiled and embedded into the numerical calculation program of VIV of a smooth cylinder and a smooth riser,so as to construct the numerical calculation program of VIV of a rough cylinder and a rough riser.The program is used to study the effects of various key parameters(different roughness,different inflow velocities,etc.)on the mode of vibration,vibration response characteristics,dynamic characteristics,wake vortex shedding pattern and vibration trajectories of cylinders and risers.The differences of VIV characteristics between smooth cylinder and rough cylinder,between smooth riser and rough riser are systematically studied to reveal the nonlinear dynamic behavior of VIV of a smooth cylinder,a rough cylinder,a smooth riser and a rough riser,such as jumping,multi-frequency vibration,broadband vibration,resonance,etc.The influence mechanism of surface roughness on VIV of a cylinder and a riser is explored to provide scientific theoretical basis and practical engineering method for vibration control and engineering design of rough marine riser.In general,this paper focuses on the VIV of marine riser considering the surface roughness.The object of study goes from a smooth cylinder to a rough cylinder,to a smooth riser,and finally to a rough riser.The VIV characteristics of a Marine riser considering surface roughness are systematically studied from a cylinder(2D)to a riser(3D)and from a smooth surface to a rough surface.The main research work and achievements of this paper are summarized as follows:(1)It is proposed that the initial conditions of the numerical calculation starting from the static cylinder structure and the exact setting of the viscosity coefficient matched with the physical experiment are the keys to successfully capture the super upper branch of the cylindrical cylinder VIV and 2T wake vortex shedding pattern in the numerical calculation.The laws of fluid force,vibration response characteristics,vibration trajectory,wake vortex shedding pattern and other aspects of a smooth cylindrical cylinder VIV are systematically analyzed and summarized by using the numerical calculation program of a smooth cylindrical VIV.Through a systematic comparison and analysis with the numerical simulation results reported by other scholars and the classical physical experiment results,the numerical simulation results in this paper more accurately simulate the vibration response branches and wake vortex shedding pattern of the smooth cylinder,especially the successful capture of the2T wake vortex shedding pattern and the super upper branch.(2)The modified model of velocity gradient on the rough wall surface was introduced to construct a numerical calculation program for the VIV of a rough cylinder,and the characteristics of the VIV of a rough cylinder with roughness K_s/D=0.005,0.01 and 0.02 were systematically studied.The results show that,compared with the smooth cylinder,the larger the surface roughness is,the shorter the super upper branch of the VIV of a rough cylinder is and the smaller the maximum vibration amplitude can be.When the surface roughness is small,the lock-in region of the rough cylinder VIV does not change obviously.When the surface roughness is large,the lock-in region of the rough cylinder VIV decreases obviously.When the surface roughness is large,the maximum value of the lower branch of the VIV of a rough cylinder increases obviously.The surface roughness of a cylinder can inhibit the chaotic motion of a cylinder at a lower reduced velocity.The surface roughness of the cylinder can promote the chaotic motion of a cylinder at a high reduced velocity.With the increase of the surface roughness,the surface roughness of a cylinder will inhibit the 2T wake vortex shedding pattern.(3)A new model which can take into account the variation of axial tension is proposed,based on the Newmark-?method,the VIV program of a smooth riser was developed,and the VIV characteristics of a smooth riser were studied.The results show that the numerical simulation results in this paper accurately capture the bimodal characteristics of the vibration amplitude ratio curve of the smooth riser,and the vibration frequency of the smooth riser is consistent with the experimental results.With the increase of the inflow velocity,the streamwise and transverse vibration frequency increases gradually,and the vibration modes of the streamwise and transverse rise step by step.Compared with transverse vibration,the streamwise vibration enters the second vibration earlier.When the inflow velocity is large,the streamwise direction of the riser will have multi-frequency vibration,that is,in addition to the main frequency,there is a secondary frequency twice the main frequency;the transverse direction of the riser will also have multi-frequency vibration,that is,in addition to the main frequency,there are secondary frequencies two and three times the main frequency.(4)The modified model of velocity gradient on the rough wall surface was introduced to construct a numerical calculation program for the VIV of a 3D rough riser,and the difference of the VIV between the smooth and rough riser was compared.The results show that the numerical simulation results are in good agreement with the experimental results.With the increase of surface roughness,the streamwise vibration response amplitude of the riser decreases.With the increase of the surface roughness,the transverse vibration response amplitude of a riser increases slightly at a lower reduced speed.With the increase of surface roughness,the transverse vibration response amplitude of the riser decreases at a higher reduced speed.Compared with the smooth riser,the vibration frequency of the riser with larger surface roughness is increased.With the increase of surface roughness,the transverse vibration frequency of the riser increases obviously at a higher reduced velocity.Larger surface roughness will enhance the multi-frequency and broadband vibration characteristics in the streamwise direction of the riser,but smaller surface roughness will inhibit the multi-frequency and broadband vibration characteristics in the streamwise direction of the riser.The transverse vibration of the riser shows the multi-frequency vibration characteristics,that is,in addition to the main frequency,there is the secondary frequency twice or thrice the main frequency.With the increase of the surface roughness,the secondary frequencies twice or thrice the main frequency of the riser gradually weaken,the multi-frequency vibration characteristics gradually decrease,but the broadband vibration characteristics gradually increase.When the inflow velocity is high,even the small surface roughness will have obvious influence on the wake vortex shedding pattern.The surface roughness will inhibit the 2T and 2P wake vortex shedding pattern,and enhance the 2C wake vortex shedding pattern. |
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