Modelling and optimization of compact subsea liquid-liquid separation system

Subsea separation systems require compactly sized separators because they are easier to install on the sea bed and require less material of construction to withstand high pressures subsea. The compactness of the separators is brought about by using cyclonic forces, which are many times stronger than gravitational force to drive separation. Existing models are typically intended for design purposes, but they are computationally intensive and are not suitable to be used in numerical optimization methods. Hence, in this work, we developed a simple, yet reasonably accurate model for a subsea separation system, in which the oil-water emulsion feed undergoes a preliminary bulk separation in a gravity separator into two crudely separated streams. A further purification is carried out for the oil-rich stream in a dewaterer and the water-rich stream in a deoiler. Our models calculate the oil cut in the outgoing streams based on separator design and operational parameters, such as flow split, feed flow rate, and oil cut of the incoming stream. The deoiler model was calibrated using data from literature and the system was used for optimization to maximize the oil cut in the oil-rich product using the flow splits of the individual separators as degrees of freedom. The model was used to study the optimal flow splits corresponding to different feed rates and inlet oil cuts.