Study On Gas-liquid Slug Flow Characteristics And Flow-induced Vibration Of Marine Riser

As significant hubs in marine oil and gas industry,marine risers are known as the lifeline of offshore oil.Risers are affected by external ocean currents as well as internal oil and gas flow,thus making their vibration complicated.The vibration induced by external currents(Vortex induced vibration)has been done lots of research,relevant theory is mature.At the same time,slug flow is the most common flow pattern in risers,the slug induced vibration cannot be ignored.At present,there are few studies of two phase flow and flow induced vibration on flexible risers.Therefore,the target of this work is to investigate the slug flow and slug induced vibration of a free hanging flexible riser by comprehensive utilization of experimental study and numerical simulation.Firstly,according to the similarity principle,a generalized model of experiment free hanging flexible riser is designed.One high speed camera was utilized to capture the displacement of riser and flow details.Two pressure sensor arranged on the top and bottom of riser.In order to capture the flow characteristics,water was stained firstly.The flow characteristics and vibration response of 30 varied gas and liquid velocities were tested.It is found that three flow pattern appeared in total 30 case,they were quasi-severe slug flow,hydrodynamic slug flow and transition flow.The vibration of riser showed a certain periodicity,with the increase of liquid velocity,the displacement of riser showed a tendency to decrease gradually.The root mean square(RMS)amplitude curves emerged one peak in x direction while two peak in z direction.The trajectory presented 1-figured pattern in the bottom of riser,spindle shape in the middle,and long stripes in the top respectively.The greater the liquid velocity,the grater the average pressure in the bottom of riser,and the smaller the pressure fluctuation range.The distribution of fluid in varied groups was significantly different,the higher the liquid velocity,the shorter the length of the liquid slug,the greater the velocity of the slug.The four cycles of the quasi-severe slug flow can be clearly observed in specific groups.Stable riser would cause the average pressure changing and the fluctuation amplitude getting smaller.Long liquid slugs were divided into numbers of short slugs by the gas,which was the direct cause of the pressure changing.A two-way fluid-structure interaction numerical method of 3D two phase flow and slug induced vibration was developed based on computational fluid dynamics software.The numerical results were in good agreement with the experimental results with a reasonable range of error of 30%.The numerical simulation could show the equivalent stress distribution of the riser surface during the vibration process,which indicated that the fixed point of the upper and lower ends of the riser is the most stress position.The numerical simulation results showed a well explain and supplement on the experimental results.