| The submerged floating pipeline is one of the new concepts to transfer fluid and gas in offshore engineering.It is different from the traditional subsea pipeline for that it is floated in the mid-deep ocean with the balance between its gravity,buoyance and traction from other devices.Because of its unique configuration,the submerged floating pipeline has many advantages in deep sea offshore engineering.Therefore it proves to be a part of the future solutions for deep water gas and fluid transportation.Ocean current is the main concern in the design of the submerged floating pipeline.When the current flows through the pipeline,vortex-induced vibration(VIV)of the pipeline may occur.For the submerged floating pipeline,the VIV is the main part of its dynamic responses,and one of the main causes for the fatigue damage.As a result,to make a research on the VIV of the submerged floating pipeline is helpful for its design and safety.Meanwhile,nowadays the submerged floating pipeline is also designed to be connected with floating offshore platforms for oil,gas and fluid transportation.When the floating structures have motions on the ocean waves,these motions will result in the dynamic responses of the pipeline,causing the tension force of the pipeline to change with time.This may excite the parametric resonance of the pipeline.If the parametric resonance occurs,the pipeline will have very large vibration,which will make its structure damage.Thus the prediction of the parametric resonance is necessary for the design of the submerged floating pipeline.In this thesis,three different forms of the submerged floating pipeline is studied.The theoretical models for the pipeline's dynamic analysis are established.Through the numerical computation,the VIV,parametric resonance and coupled vibration with both excitations of the submerged floating pipeline are researched.The main works are as follows:(1)Analysis of the VIV of the submerged floating pipelineBy the wake oscillator model,the VIV equations of the pipeline is derived.With infinite differential method,simulations and analysis of pipeline's VIV are made.The result shows that within different current speed,the lock-in areas are continues for the VIV of the submerged floating pipeline.The increase of hydrodynamic damping coefficient will decrease the vibration response,which means adding bumps and helical strakes on the surface of the pipeline can effectively reduce the VIV response.(2)Mathieu parametric resonance prediction and coupled vibration analysis of the submerged floating pipelineWith the analysis of the motions of the floating structures on irregular waves,the parametric excitation model of the pipeline is established.Then Mathieu equation is derived.Mathieu instability diagram is used for the prediction of the parametric resonance of the submerged floating pipeline.The analysis shows that if damping coefficient increases,the stable zone will become bigger,which means that the larger damping can make the pipeline avoid the parametric resonance.With consideration of both vortex and parametric excitations,the coupled vibration analysis model is established.Mode jump phenomenon is observed in the simulation,and the parametric excitation is considered as one of the possible reasons for this phenomenon.(3)Hill parametric resonance prediction and coupled vibration analysis of the submerged floating pipelineHill equation for the submerged floating pipeline's parametric vibration is derived through irregular wave theory and principle of linear superposition.The parametric resonance is predicted by Hill instability diagram,and the influence of the parametric instability under different damping and ratios of the motion amplitude of the floaters is studied.Finally,the simulation of coupled vibration of the submerged floating pipeline is made.The comparison with the single-frequency results is also conducted.The study shows that it is necessary for designers to take all possible motions of the floating structures into consideration. |
PDF Full Text Request