Research On Fluid-solid Coupling Dynamics Behavior Of Multi-wellhead Marine Riser Group

The multi-wellhead marine riser group is an important part of offshore oil exploration and transportation,which plays a key role in the production and transportation of offshore oil and natural gas.However,because of its complexity and variability of the working environment,the vibration phenomenon of marine riser group can not be avoid,and this will lead to the riser fatigue failure occurred,affecting the extraction of oil and gas,even it will result an accident of marine oil spill,seriously impact on the ecological environment.Therefore,the vibration and prevention and control problems of the multi-wellhead marine riser should be paid more attention.The flow induced vibration of riser is a main reason for the failure.For flow induced vibration of marine riser group.There is a strong interaction between the marine riser group and the fluid and between the riser and the riser,more precise physical models and theoretical models need to be constructed to analyze the interaction between structures and fluids and the complex dynamic response characteristics of the marine riser groups.In this paper,the structure of marine riser group in single phase uniform flow is studied.A numerical model is established to analyze the flow induced vibration of a multi-wellhead marine riser group based on two-way fluid-structure interaction method,and the interaction between the structure and the fluid is analyzed.The dynamic characteristics of Marine riser group is studied under different factors(number of different riser,different lay ratio and different way of arrangement).In addition,the fluid elastic instability of the marine riser group is discussed,and based on that analysis,the vibration prevention of the Marine riser group is carried out.Specific research findings:(1)The main vibration direction of each riser is in the direction of downstream direction and the vibration amplitude is basically the same when the velocity of external fluid is low and the number of riser is small.When the pipeline in the downstream direction increases,the downstream riser will vibrate significantly increases under the action of the trailing vortex,and the main vibration direction will become transverse vibration.(2)The study on different lay ratio of riser group found that:Whether in the direction of flow or in the direction of the flow,because of the interaction between the riser and the fluid elastic force,the vibration effect is obvious When the lay ratio is smaller.The interaction between the riser was gradually reduced when the lay ratio of the riser was greater than 3.5,and the critical section lay ratio of the interaction between the riser is between 3.5 and 4.(3)The study on the arrangement of different positions of marine riser groups found that:In the case of the pitch ratio fixed in various configurations,the vibration in the downstream direction are basically the same,which are the simple harmonic oscillations with the same frequency and the same amplitude.The vibration modes of each marine riser in the transverse direction are different,which indicates that the influence of the factors the position of the risers is mainly reflected in the transverse vibration of the riser.(4)Study on the fluid elastic instability of Marine riser group:most of the instability occurs in upstream risers or downstream risers.The critical velocity is basically the same regardless of the arrangement for a certain lay ratio marine riser group system.(5)Research on the vibration prevention of marine riser group:the two kinds of vibration prevention are partition plate and fairing,and the optimum partition length of partition plate is ID,and it is best installed in the upstream riser of the marine riser group system;The optimum height of the fairing is 0.5D,which is suitable for installation in the downstream riser.