Flow Characteristics And Vibration Analysis In Flexible Riser Considering Hydrate Phase Transition

Natural Gas Hydrate(commonly known as "flammable ice")is an ideal energy source for future low-carbon society.With the gradual depletion of onshore oil and gas resources,the exploitation and transportation of natural gas hydrate is imperative.According to the characteristics of weak cementation and non-diagenesis of hydrate reservoir in South China Sea,Academician Zhou Shouwei proposed the solid-state fluidized mining method,which breaks the hydrate into particles at the seafloor and transports them to the sea surface through riser.During the process of hydrate particles rising from the seabed to the sea surface,due to the decrease of pressure,the particles decompose continuously,and the solid-liquid two-phase flow becomes solid-liquid-gas three-phase flow.At present,the research on hydrates by scholars in China and abroad mostly focuses on the static decomposition process or the flow process under the constant decomposition rate.However,pressure change has a great influence on the decomposition rate of hydrate.By simulating the decomposition process of hydrate particles under different pressures,the following studies are made on the hydrate flow characteristics in the flexible riser and the coupled vibration characteristics of the riser under the combined action of hydrate slurry and outward flow.(1)By analyzing the temperature and pressure,which are the two main factors affecting the decomposition rate of hydrate particles,it is found that the temperature change during the whole process is very small.The decomposition rate model of hydrate particles on pressure is established.According to the decomposition rate,the decomposition process of hydrate particles is simulated by adding source terms through UDF(User-Defined Function).(2)Analyse all factors affecting the flow characteristics of hydrate,determine the range of variation of each factor,set up experimental scheme by controlling variable method,and finally design 23 groups of simulation schemes according to five factors: pipe diameter,hanging angle,velocity,particle diameter and particle volume fraction.(3)Assuming that the riser is fixed,an internal fluid domain model is established.According to the simulation scheme,the influence of the above five factors on the flow characteristics of hydrate is analyzed.The results show that each factor has varying degrees of influence on gas holdup(cross-section gas holdup)and particle distribution in the riser.Among them,velocity and particle volume fraction have the greatest influence on gas holdup,while particle diameter and particle volume fraction have the greatest influence on particle distribution.Other factors besides particle diameter will influence the transportation speed of particles,especially the volume fraction of particles.The lift and energy consumption of lifting hydrate decrease with the increase of pipe diameter and hanging angle,and increase with the increase of velocity and particle diameter.With the increase of particle volume fraction,it first decreases and then increases,and the volume fraction is The minimum value is reached at 25%(4)The overall model of external flow domain and riser is established,and the coupling vibration characteristics of riser under the combined action of internal and external flow are analyzed.The influence of the internal flow field is simulated by loading the stabilized pressure of the internal flow field on the inner wall of the riser,and the external fluid domain is divided into dynamic meshes to analyze the coupled vibration response characteristics of the riser under different external flow velocities and riser hanging angles.The results show that the decomposition of hydrate can effectively increase the transportation speed of hydrate particles and reduce the energy consumption of transportation.The increase of external fluid velocity will increase the vibration amplitude and frequency of riser and increase the upper end stress.There is no fixed relationship between the hanging angle and the vibration amplitude and frequency of the riser.Changing the hanging angle can reduce the stress of the riser to a certain extent.