Field data are presented for two parallel risers connected to the same inlet. The data show that the multiphase flow is split equally over the two risers at high flow rates; however, below a critical flow rate the system diverges: the flow passes preferentially through one riser, and the other riser accumulates liquid. This unequal split is unwanted from the operational point of view, as it leads to a larger pressure drop over the risers and liquid accumulation in one riser. The instability is also studied in a design study of a gas-condensate field with 1D simulations. It is shown that the 1D simulations reproduce the maldistribution of the liquid content of two parallel flowlines. Based on the simulations, an explanation is proposed for the occurrence of the unequal split.
The sizing of a gas-condensate flowline is mainly driven by two constraints. The first constraint is that the pressure drop over the flowline at the highest or "nominal" flow rate must be as small as possible, in order to have the smallest possible backpressure on the wells and therefore the largest production. This constraint imposes to choose a flowline diameter as large as possible to reduce the gas velocity and the wall friction. The second constraint concerns low flow rates: at small gas velocities, the gas exerts a small drag on the liquid layer, and because of gravity the liquid content becomes large in upward sections of the flowline. A large liquid content promotes in turn large liquid surges due to flow instabilities or ramp-up and pigging operations, and therefore the need of large and expensive receiving facilities at the outlet of the flowline. Additionally, a large liquid content increases the pressure drop over the flowline due to the static head. Therefore, the second constraint imposes to choose a flowline diameter as small as possible to have sufficiently large gas velocities in the flowline. The two design constraints are contradictory: a large diameter will give a smaller pressure drop, i.e. a larger production at the nominal flow rate, but it will require larger receiving facilities at low flow rates or a higher production cut-off. Thus, the sizing of a gas-condensate flowline requires determining the optimal diameter of the flowline and the optimal size of the receiving facilities with respect to the operational flexibility and the economics over the life of the field.