Liquid-Liquid Separation in Gravitational Subsea Separators
Authors: Palermo Thierry , Lebrun Jean Philippe , Brocart Benjamin , Noik Christine , Pagnier Philippe 
Published: October 2011

Abstract
Subsea liquid-liquid separation is still a challenging issue. Technical problems have to be faced in terms of interface control, water quality, outlet water cut or solid removal.
In this paper, only water separation from water-in-oil emulsions is discussed. Water separation can be envisaged to manage hydrates without deployment of preservation phases during shut-down or if poor insulation of flowlines is considered. In this case, particularly in comparison with the quantity of oil remaining in re-injected water, a significant quantity of water remaining in oil is acceptable. However, a maximum water cut must be guaranteed to ensure transportability of hydrate slurry. Moreover, related to water depth, only compact separators with short residence times can be installed.
The key point is to evaluate, depending of oil properties, whether or not such a strategy based on subsea water separation can be considered as a good option. Thus, a 1D model has been developed to determine, for a given oil, the separation efficiency as a function of production conditions and separator size. Oil properties, regarding water-oil separation, are evaluated from monitored bottle tests and are accounted for in the model as "coalescence" parameters. Validation of the model has been achieved thanks to experiments carried out for real crude oils in a pilot loop including an instrumented gravitational separator (3m length, 0.7m in diameter).
A description of the pilot loop and the 1D model is given. The main physical concepts on which the model is based are presented: coalescence during settling and coalescence between the dense pack layer and water leg. Simulations allow us to discuss the possibility in reducing the size of separators for subsea applications.

Introduction
Subsea liquid-liquid separation is still a challenging issue. Technical problems have to be faced in terms of interface control, water quality, outlet water cut or solid removal.
In this paper, only water separation from water-in-oil emulsions will be discussed. Subsea water separation can be envisaged to manage hydrates without deployment of preservation phases during shut-down or if poor insulation of flowlines is considered (1). In this case, particularly in comparison with the quantity of oil remaining in re-injected water, a significant quantity of water remaining in oil is acceptable. However, a maximum water cut must be guaranteed to ensure transportability of hydrate slurry. Moreover, related to water depth, only compact separators with short residence times can be installed.
The key point is to evaluate, depending of oil properties, whether or not such a strategy based on subsea water separation can be considered as a good option. In order to improve our capability in predicting the performance of gravitational horizontal separators, a collaborative R&D program between TOTAL and IFPEN was launched some years ago. This program mainly dealt with the elaboration of a methodology including both experiments and modeling. This methodology is based on the determination of "coalescence" parameters from laboratory batch-settling experiments by matching a model, so-called batch model, with experimental data. Then, these parameters are used as input data in a 1D steady state model, so-called separator model, which enables separation efficiency to be calculated. Validation of the model has been achieved thanks to experiments carried out for real crude oils in a pilot loop including an instrumented gravitational separator (3m length, 0.7m in diameter).
A description of the pilot loop as well as the batch and separator models will be given. The main physical concepts on which the models are based will be presented: coalescence during settling and coalescence between the dense pack layer and water leg. Simulations will allow us to discuss the possibility in reducing the size of separators for subsea applications.