Based on the success of foam in subsurface applications it is of interest to investigate whether foam can also help overcome liquid management problems in surface flowlineriser systems. Therefore, flow experiments were carried out in the flow loop at the Shell Technology Centre Amsterdam. The facility consists of a 100 m long horizontal flowline (with 50.8 mm diameter) followed by a 16.8-m vertical riser (with 44 mm diameter). Air and water are the working fluids, and operation is at atmospheric outlet pressure. Foam is created by adding "DreftTM" (a dishwashing detergent) in various concentrations to the water/air flow. Experiments were taken both without and with foam. Various measurement techniques were used: differential pressure sensors, flow visualization, and Distributed Acoustic Sensors (DAS) (fibre optics). The focus is on (growing) slugs in the horizontal flowline, and severe slugging in the flowline-riser. It can be concluded from the small-scale lab experiments that adding a surfactant mitigates (growing) slugs in (nearly) horizontal flowlines, whereas the severe slugging cycle in a flowline-riser configuration cannot be broken.

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A possible new development concept for Floating LNG may include a single flowline along the sea floor that splits into dual or more flexible risers. Since the gas co-produces condensate and water, design rules are needed for the splitting of the phases at the riser base manifold. Multiphase flow splitting is much more complex than single-phase flow splitting. The latter is fully determined by the back pressure on each riser, but for multiphase flow the phase split (Liquid-Gas Ratio) into each of the risers may also depend on other factors, such as the flow regime in the riser/flowline, the precise geometric details of the splitting configuration and other parameters (e.g. the momentum flux ratio in the flowline). Ideally the phase volume ratio should be fully equal over the two risers and remain the same as in the flow line. To find the proper design rules for multiphase flow splitting, which ensures equal phase volume split, we have set up a research programme that includes lab experiments and simulations using Computational Fluid Dynamics. The hypothesis is that phase maldistribution can occur if the gas flow rate in the risers is so low that it gives churn flow or hydrodynamic slug flow in the risers, whereas an equal phase volume split is expected if the gas flow rate is sufficiently high to produce annular flow in both risers. The flow facility at the Shell Technology Centre in Amsterdam transports air and water through a 2′, 100 m long flowline, splitting into dual, about 15 m high, risers, having a diameter of 2′. The pressure is atmospheric at the riser top. For the splitting configuration, we used a symmetric lay-out, which is a so-called Impacting Tee. We created the splitting curve by systematically changing the opening of the chokes at the top of the risers. At low gas flows a non-symmetric flow split was found, with flip-flopping and hysteresis in the risers. For example, a stagnant liquid flow could develop in one riser, and churn flow in the other riser, with a sudden swap of flow behaviour between the two risers. This maldistribution gradually disappeared as the gas flow rate was increased. CFD simulations were carried out with the Fluent package using a Volume of Fluid approach for the multiphase flow through the symmetric splitter. As in the experiments, the CFD simulations also give the preference of all flow to be produced through a single riser, with a stagnant liquid column in the other riser. In the experiments this maldistribution disappeared for a sufficiently high gas throughput. For the first flow rate where an equal split is found in the experiments, however, the CFD model still predicts that all of the flow exits through only one of the two risers.@en