| With the development of modern marine engineering technology,flexible pipes have been widely used in the deep-water mining industry.Such deep-water flexible pipes,which transport submerged natural resources,are susceptible to multi-mode vortex induced vibration(VIV)in complex current environments,causing fatigue damage and structural damage to them.Therefore,it is extremely important to predict and mitigate the risk of marine pipelines,and it is of great scientific and practical importance to study the vibration response mechanism of flexible pipes under the coupling effect of internal flow and external current and the suppression measures of VIV.In this thesis,the multi-physics field numerical calculation software STAR-CCM+ is used to achieve the numerical prediction of the multi-mode VIV response of a slender flexible pipeline model at high Reynolds number based on the viscous Navier-Stokes flow field control equations and the structural dynamics control equations.Combining numerical simulations and vibration signal analysis,the effectiveness of the temporal-spatial multi-mode interactions and VIV suppression methods for flexible pipes under the coupling effect of internal flow and external current is investigated.Firstly,the grid dependence study and the fluid-structure interaction(FSI)reliability verification of the adopted numerical methods are carried out.The 3D PFE(position-frequency-energy)combined with TFE-HS(time-frequency-energy Hilbert spectrum)diagrams are applied for correlation analysis to visualise the vibration modes and corresponding vibration energies involved at various locations along the pipe span,revealing the temporal-spatial multi-mode interaction of the flexible pipe and the resulting vibration energy.The mechanism of temporal-spatial multi-mode interaction and the resulting vibration energy transfer in flexible pipes is revealed.A local transient upper envelope(UE)method is used to identify spreading standing waves,travelling waves and mixed standing-travelling waves,and to characterise the mode competition in the temporal-spatial axis.Details of the full thesis are as follows:Firstly,a numerical study of the multi-mode coupled vibration response of a flexible pipe considering different external shear current velocities,internal flow velocities and internal fluid densities is carried out.It is shown that increases in both internal flow velocity and density amplify the in line(IL)mean static deflection of the flexible pipe,but that the involvement of internal flow is not usually effective in changing its principal vibration mode.In the TFE spectrum,alternating amplitude peaks and troughs are found due to mode switching.Usually the vibration response at the amplitude trough position has a broad frequency characteristic,while the vibration at the amplitude peak is often dominated by a single frequency or narrow-band response.Based on the span waveform analysis,it is found that standing and travelling waves usually correspond to the relative equilibrium and instability of energy transfer between fluid-structure and mode-mode in the local zone,respectively.In addition,the amplitude peak is usually a high-order dominant frequency response with a distinct “8” shaped trajectory,while the trough is a low-order sub-dominant frequency response with significant multi-mode competition and a relatively chaotic trajectory.Finally,through the characterisation of the lift and vibration response,it is found that due to the limited pipe length and limited vibration amplitude,the vibration process is synchronous with the lift,i.e.the hydrodynamic force and structural deformation are in phase.In addition,a comparative study of the VIV response characteristics of airfoil,textured and smooth pipes was carried out by applying the two-way FSI numerical method to investigate the effectiveness of shaped surface structures such as spoilers and surface textures on the passive suppression of VIV.The results of the calculations show that the surface texturing of the flexible pipe and the surface structure characteristics of the spoiler can effectively suppress the VIV response of the pipe,especially when the velocity is relatively large,which can significantly reduce the vibration response amplitude of the pipe and play a role in weakening the fatigue damage of the pipe.This study provides some theoretical reference for the design of marine pipes with shaped structures such as airfoil and textured pipes using passive vibration suppression. |
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