Transient Multiphase Simulation in Separator Vessel Internals Design in Saudi Aramco

The design of new multiphase separation plants or the replacement of vessels represents an opportunity to review and update the separation systems, to ensure the surface facilities lifetime with maturing production conditions. Potentially adverse operating scenarios involving slug flow could occur in the production header, leading to potentially damaging fluid forces acting on the high-pressure production trap internals. The internals design is based on the fluid forces occurring during a slug flow condition. This paper presents the application of 3-D Computational Fluid Dynamics (CFD) to simulate the realistic fluid forces acting on a novel separator inlet device, and to assess the structural integrity of the mechanical design. To generate slug conditions resulting from several adverse, but low probability, operating scenarios, OLGA, a 1-D transient multiphase flow hydraulic simulation application, was used to simulate the transient multiphase flow conditions in the production header entering the production separator using the 20-year production forecast. In a one-way coupling, the output from OLGA is used as input to the CFD model. The scenarios were carefully developed, thoroughly evaluated and agreed on by the different project stakeholders — technical services, operations and project management teams. The OLGA simulations produced the transient phase velocities and hold-ups that were applied as inlet boundary conditions to the CFD models of the separator. The transient CFD turbulent multiphase simulations used the Volume-of-Fluid model to track the gas-oil-water mixture in the inlet device and the inlet section of the separator. A discussion on the appropriate CFD multiphase model selection is presented. The hydrodynamic forces computed in the CFD model are integrated over the internal surfaces of the inlet device and mapped as a distributed pressure load for a Finite Element Analysis (FEA) structural model, to determine the mechanical reactions of the structure, a fluid-structure interaction (FSI) analysis. The methodology developed is intended for use in new internal designs for two- and three-phase separators, and slug catchers in gas/oil separation plants (GOSPs).