High Pressure Hydrocarbon Separation Performance of In-Line Piggable Liquid Removal Unit - Pseudo Dry Gas Systems

Pseudo Dry Gas (PDG) technology is proposed as an alternative concept for transporting multiphase fluids (gas, condensate, and water) for long deep-water subsea tieback developments while delivering significant reductions in Scope 1 and 2 CO₂ emissions by the mitigation of compression (Reference 1). Using PDG technology, subsea multiphase pipeline networks can be extended to an excess of 300 km total length and considerably reduce the backpressure to the wells. The basis of the PDG system is to remove the liquid from the main pipeline using Piggable Liquid Removal Units. With the removal of the majority of liquid, the gravitational pressure losses in the system are considerably reduced allowing the pipeline to operate like a "Pseudo" Dry Gas system. The original prototype of this design underwent low pressure flow loop testing (Reference 2). This paper focuses on the configuration of the equipment and the supporting reasoning for high pressure and elevated temperature conditions, along with a key observations the results obtained from the testing program. The current work involved the fabrication of a 180bar rated PDG unit, a subsea magnetic drive pump and the supporting subsea control system for flow loop testing undertaken at TÜV SÜD National Engineering Laboratories (hereinafter NEL), UK. This development program is supported by operators in conjunction with the Net Zero Technology Center. The testing program is developed to encompass the expected operational envelope that the PDG units would see in service aligned to a broad range of asset-based study work snd to consider critical non-dimensional flow assurance parameters to ensure scalability. The testing parameters considered are pressure (up to 125bar), temperature (up to 40C), and inclination (zero- and two-degrees). In addition, various liquids are considered e.g., two hydrocarbon fluids (representing oil and condensate), brine water, and hydrocarbon and water mixture. An extensive number of test points at different liquid and gas superficial velocities are included both with and without the PDG unit installed to allow for baseline comparisons. Besides pressure, temperature, level gauges and flow meters; an X-ray machine and an iso-kinetic sampling device are installed to provide liquid holdup measurements and to measure the liquid entrainment in the gas, respectively. The latter measurement data is to be used to evaluate PDG separation performance at various liquid entrainment rate. The results show acceptable PDG separation performance at various high system pressures for all cases indicating marginal impact of increasing pressures in relation to the required performance. The liquid entrainment rate has been measured for different surface tension fluids, upstream and downstream of the PDG unit and results have been included in the calculation of the separation performance of the unit.