Dynamic Response Of A Pipeline-Riser System Under Severe Slug Flow:Theoretical Modeling And Numerical Simulation

Severe slugging is an unsteady and transient flow phenomenon that occurs in a flowline-riser pipe system when a low flowing oil blocks the passage of the commingled gas caused by the downward declination of the flowline due to unevenness of the seabed terrain.This flow behaviour causes pressure fluctuations at the riser base due to the accumulation of liquid slugs whose length is longer than the riser height.The production of these slugs is followed by a chaotic blowout of the already compressed upstream gas owing to its blockade and the whole cycle is repeated all over and over again.For deepwater riser system,this dynamic and cyclic flow behaviour worsens the dynamic response of the system and is one of the main factors for the fatigue failure of the riser pipe.It is very difficult to study the dynamic coupling effect of severe slugs and the response of riser system because of the fluctuations of the fluid?s instantaneous pressure and the complexity of its physics which involves slug generation,slug production,gas blowout and liquid fallback.This doctoral thesis presents a theoretical and numerical simulation study done on this subject and the core progress made is embodied herein as follows:(1)Through a thorough study of the physics of severe slugging and the mechanism of its interaction with the riser structure,the dynamic model and the corresponding differential equation of the riser system reflecting the dynamic coupling of severe slugs and deepwater riser is established,which lays a theoretical foundation for the design and safe operation of the riser system under the condition of severe slug flow.(2)An ANHE-FD hybrid algorithm is proposed which combines an adaptive node-based height evaluator(ANHE)algorithm and finite difference(FD)approximations to obtain the numerical solution of the motion equation for the transverse vibration of the riser structure during liquid slug buildup.This provides a computational technique for the analysis of fluid-structure interaction(FSI)in severe slug flow.(3)By combining the multiphase flow simulator OLGA 7 and the structural analysis software ABAQUS,a numerical method is proposed to solve the dynamic coupling between the severe slug flow and the response of deepwater riser,and a possible solution is proposed for obtaining the whole dynamic coupling of the riser response and the slug generation by using the CFD-FEM co-execution simulation module in ABAQUS.(4)By using the proposed computational method,a large number of parameters are analyzed for the coupled dynamic response of deepwater riser system and severe slug flow.The relationship between slugging characteristics and dynamic behaviour of riser structure is obtained,and these relations and the corresponding laws are of great significance to the design of deepwater risers and the effect for reducing the unsteady flow nature of severe slugging in production flowlines.The research work in this thesis provides a scientific basis for the optimization of the design of deepwater riser system and the effect of preventing the pipe system from slugging and obtains a quite ideal result that will limit this problem that plagues the offshore oil and gas industry.