Integration of 1D and 3D Dynamic Flow Simulation Helps Meet Deepwater Flow Assurance Challenges


Gas-lift is another 3D phenomenon that cannot be modelled with 1D codes. Gas lift is sometimes used to address slugging by pumping pressurised gas through the pipe to push the water forwards and prevent slugging.


Increasing Need for 3D Simulation


Three-dimensional simulation (CFD) can address these and virtually all other deepwater fluid flow problems because of its ability to simulate multiphase flow in a 3D environment without the need for simplifying assumptions that require empirical data. CFD solves the fundamental nonlinear differential equations that describe fluid flow (the Navier–Stokes and allied equations) for predefined geometries and a set of initial boundary conditions, process flow physics and chemistry.


A CFD simulation provides fluid velocity, pressure, temperature, gas/liquid composition and other variables throughout the solution domain for problems with complex geometries and boundary conditions. As part of the analysis, an engineer may change the geometry of the system or the boundary conditions (re-design ideas) and observe the effect of the changes on fluid flow patterns or distributions of other variables, such as gas composition.


CFD is becoming increasingly important in the deepwater environment because engineers often have no empirical data or practical experiments to guide them. Several obstacles have prevented engineers from taking greater advantage of CFD codes in the past.


The first is that CFD simulations for the complex geometries found in subsea drilling and production used to take an unacceptably long period of computer time to solve. Improvements in solution algorithms and the move towards massively parallel high-performance computing configurations (clusters) have, however, overcome this obstacle.


Another obstacle is that engineers only want to use CFD for the complex geometries involved in subsea equipment. They want to continue to use 1D calculations for straighter pipeline runs and other simple geometries because of 1D’s greater speed and simplicity in handling these types of geometries. With the 1D and 3D codes each simulating specific sections of the fluid flow, each code ideally becomes dependent upon the other for information, such as flow velocities, flow rates and pressures at the boundaries between 1D and 3D areas. In the past this required a cumbersome process whereby the analyst manually moved data files back and forth between 1D and 3D simulations.


Consider the following example using separate 1D and 3D codes. The analyst might first simulate the entire system using a 1D code with the realisation that accuracy will suffer in those areas where the geometry is complex. The analyst would then extract flow conditions at the boundaries of the geometrically complex areas and use them as the boundary conditions for the 3D simulation. The 3D simulation would provide much greater clarity into what is happening in the complex areas. These changes would of course affect the 1D simulation, so the 3D simulation results would need to be entered into the 1D simulation so that it could be re-run. Depending on the stage in the design process and the level of accuracy that is needed, several more iterations of manually exchanging boundary conditions


EXPLORATION & PRODUCTION – VOLUME 8 ISSUE 2


between the 1D and 3D simulation might be needed. All of these iterations would potentially have to be repeated for every design change and every time that it is necessary to investigate a new set of operating conditions.


Seamless Integration between 1D and 3D Simulation The recent automation of information exchange between simultaneously running 1D and 3D codes greatly reduces the time required to perform this type of analysis. At the same time, it improves the accuracy of the analysis by providing a seamless flow of information between the two codes. This, in turn, makes the results of each more accurate.


The most prominent example of this development is the integration of STAR-CCM+, one of the most popular 3D codes in the energy industry, with OLGA, the leading 1D piping system dynamics code. The STAR-CCM+ user can run OLGA as a slave process, which causes the two codes to exchange data at each time step, much more frequently than is possible using manual methods. STAR-CCM+ boundaries are associated with OLGA zones and STAR-CCM+ phases are associated with OLGA species to automatically pass information between them at the physical interfaces.


In a typical example, a long pipeline was modelled using OLGA, while STAR-CCM+ was used to simulate the multiphase transient characteristics of a large, geometrically complex slug catcher at


As the oil and gas industry moves to increasing depths, physical experiments performed at shallower depths become decreasingly relevant.


different flow rates and gas–liquid ratios. The analysis was used to study the adequacy of the slug catcher design to handle a wide range of flow conditions. This simulation made it possible to validate the slug catcher for use throughout the life of the asset, greatly reducing the possibility of having to replace the slug catcher at a potential cost of tens of millions of dollars.


Conclusions


A new generation of integrated advanced functionality seamlessly integrates OLGA multiphase 1D flow simulation software that predicts the dynamic behaviour of fluid flow in extensive piping systems with full 3D CFD software (STAR-CCM+) that accurately predicts flow in complex geometries. The new STAR-CCM+-based STAR-OLGA Link automatically launches OLGA and STAR-CCM+ co-simulations and passes the data back and forth at each time step.


This combination of 1D and 3D software makes it possible to simulate complex systems of piping and equipment at a much higher level of accuracy and in much less time than was possible in the past. This approach helps meet the challenges of flow assurance, preventing potential fluid-related problems from impacting oil and gas production throughout the life of the asset. n


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