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 N m3/hr and a liquid (water) flow rate of 2 m3/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 N m3/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 N m3/hr), production through a single riser was measured in the experiments.
Gas-condensate fields produce gas and a limited amount of liquid, which consists of condensate and water. Floating LNG (FLNG) is a new way of producing resources from remote fields. A possible concept is using a single flowline that splits into two or more flexible risers. Thus there is vertical upflow in all of the pipe branches downstream of the splitting junction. Due to the limited diameter of flexible risers and limits on the maximum allowed velocity, more than one riser will be needed to transport the produced fluid from the seafloor up to the FLNG vessel. The FLNG vessel is expected to receive fluid produced from multiple gas fields, some of which may be located a significant distance away. Thus having one flowline for each riser can be cost-prohibitive. Being able to achieve stable operation while splitting the flow from a single flowline into multiple risers at the riser base can thus make producing from these distant fields economically viable.