CFD for the multiphase flow splitting from a single flowline into a dual riser

Three-dimensional computational fluid dynamics (CFD) simulations were carried out with ANSYS Fluent 15.0 for the splitting of two-phase, gas-liquid flow from a horizontal flowline to two vertical risers. This piping configuration is representative of what is being envisioned for transport of gas condensate from a subsea production manifold to a floating LNG vessel. The flow conditions that were recently tested in air-water experiments at Shell Technology Centre Amsterdam were simulated. The flow was split using a horizontal impacting tee. The pipe diameter was about 2" throughout. Simulations were done for a gas (air) flow rate of 60 Nm ³/hr and a liquid (water) flow rate of 2 m'/hr. In the simulations, the flow split was varied by increasing the pressure at one of the two outlets, leaving the other outlet pressure constant. The CFD simulations show that if the two outlet pressures are held constant, an unstable flow condition exists that gives transients that finally end up in a state where all flow goes through one riser, whereas a stagnant liquid column is found in the other riser. It was also found that an initially imposed symmetric flow split (i.e., outlet pressures of the two risers being equal) lost its symmetry as the simulation evolved over time, because of the inherent instability and fluctuations in the flow introduced at the impacting tee. This indicates that only two states can exist in this configuration: one in which all flow goes through the first riser and the other in which all flow goes through the second riser. At the considered flow rate of 60 Nm³/hr this behaviour in the CFD simulations is different from the experiments where some finite production through both risers is found. However, for a lower gas throughput (i.e., at 40 and 20 Nm³/hr), production through a single riser was measured in the experiments.