An Innovative Technique for Transient Liquid Management of Hilly Gas Pipelines

It is known that for low liquid loading flows there is a critical velocity, below which liquid loading occurs. Pipelines are designed and operated to avoid liquid loading conditions, considering the constraints of these conditions. However, the characteristics of low liquid loading flows can be used to the operator's advantage. The average steady-state holdup increases drastically by entering the liquid loading region. Therefore, when ramping down to below the critical gas velocity, the pipeline starts accumulating liquid. When ramping up to above the critical gas velocity, the pipeline progressively flushes out large portions of its liquid content. However, the pipe deliquification process is much faster than the pipe filling, since the filling process is limited by low liquid flowrates. Thus, if the pipeline flowrate oscillates above and below the critical gas velocity, its liquid content remains close to the low liquid content equilibrium. This work studies an undulated 120-mile long and 32-inch ID gas pipeline operated at a relatively low flowrate, corresponding to gas velocities below the critical velocity. A transient multiphase flow simulator is used to model the oscillations in total flowrate, and therefore, packing and unpacking of the gas. A choke is used to control the rate and pressure at outlet, with simulation time of 24 hours. During packing, the total gas mass within the pipeline increases, the velocities are low, and liquid loading is observed. After reaching the maximum packing pressure, the pipeline is blown down, achieving gas velocities higher than the critical velocity. Simulations are conducted for varying liquid rates and packing-unpacking ratios. The combined production over the simulation time is kept constant for both steady-state flow and transient packing-unpacking. However, depending on the undulations and liquid flowrate, the transient packing-unpacking of the pipeline can help reduce the total liquid content of the pipeline significantly. The results of this study prove the importance of transient flow analysis in better understanding flow dynamics in pipelines. The proposed packing-unpacking technique can provide an operational guideline to minimize loading in natural gas pipes.