| With the increasing depletion of oil-gas resources in the land and offshore,oil-gas development in deep-sea area has been paid more attention in recent years.As a deep-sea oil-gas production and storage platform,the offshore floating platform has become the key to oil-gas development.When the sea current flows through the platform cylinder,it generates the vortex-induced motion(VIM).The intensive oscillation generated by VIM will seriously affect the production activities of the platform.Therefore,how to reduce the VIM of the cylindrical platform has become a key issue in the deep-sea research field.At present,due to the limitations of experimental condition,the model tests of the VIM mostly focus on low Reynolds number.There are few studies on the VIM with multi-degrees of freedom at high Reynolds number.However,for the actual platform in deep sea,the Reynolds number can reach more than 10~5,and the VIM characteristics at low Reynolds number are significantly different from which of the actual platform.Therefore,in this thesis,the multi-degrees of freedom VIM of floating cylinder at high Reynolds number is studied and analyzed by combining experimental and numerical simulation.The VIM is systematically studied from the following aspects:In this thesis,the characteristics of the VIM are studied firstly by a simple basic experiment.The VIM of a large diameter cylinder was studied with two-degrees of freedom,sway and surge motion.The response amplitude,motion frequency and trajectory are studied and analyzed.The influence of different aspect ratio and center of gravity on the VIM was researched.The results show that the VIM of cylinder at high Reynolds number has the general characteristics of the VIM,and there is an obvious vortex-induced resonance phenomenon.The amplitudes of sway and surge increase first and then decrease,and the motion trajectory is"8".With the increase of model’s aspect ratio and the increase of the center of gravity,the VIM response amplitude of the cylinder will increase slightly.In order to verify the accuracy of the model test and to supplement the vortex shedding state of the VIM which cannot be obtained by experiment,the numerical simulation about the VIM with the same conditions as the experiment was carried out.Based on the STAR-CCM+solver,two-degrees of freedom VIM of the floating cylinder was predicted by using the Overset Mesh method and the dynamic fluid-structure coupling model.The data of numerical simulation was compared with the experimental data.The results show that the numerical results of sway and surge are well fitted with experimental results in the trend,but the numerical results are slightly larger than the experimental results,especially in the lock-in area.In the numerical simulation,the vortex shedding state of VIM is symmetrical.Since the actual platform is not a simple cylinder,and the two-degrees of freedom study cannot comprehensively reflect the coupling characteristics of VIM.Therefore,based on the two-degrees of freedom experiment of the first cylinder model,the multi-degree of freedom VIM test of the new S-Spar platform is further carried out.In this experiment,the four-degrees of freedom of VIM of S-Spar model were studied respectively,the response frequency,amplitude and trajectory of VIM were analyzed,and the coupling effect between different degrees of freedom was discussed.The effects of different conditions on VIM were studied by changing the center of gravity.The results show that the different degrees of freedom have their own unique response characteristics,in which sway dominates the movement,and there are coupling effects between the different degrees of freedom.There is frequency interference between different degrees of freedom.The center of gravity has a significant effect on the maximum response amplitude and the range of lock-in area.The stability of S-Spar is better than that of traditional cylinder,which can greatly reduce the response amplitude of the VIM. |
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