Liquid / Liquid Separation
On Statfjord C, a Statoil operated platform in the North Sea, a CDS-GasunieTM Inlet Cyclone was tested in the test separator. The purpose of the test was to evaluate the liquid / liquid separation performance in order to check the feasibility of revamping the main production separator on the platform. In order to evaluate the liquid / liquid separation performance the Test Separator was modified to provide 17 extraction points for liquid samples at various distances from the cyclone and at various heights. The arrangement of the test separator is shown in figure 5.
The actual test conditions were very challenging especially considering phase inversion due to the water cut range of 42% to 76% and droplet shearing within the inlet piping due to the high momentum values of up to 65,000 kg/ms?.
It was found that the inlet cyclone arrangement operated very well with oil in water quality that in the majority of cases kept below 40 mg/l meaning that it could be disposed to sea with no further treatment. Additionally the oil in water quality in the majority of the cases was below 2%, which again exceeds the usual requirement of 5%. These results being maintained up to an inlet momentum of 65,000 kg/ms? whereas the normal separator design criteria is between 6,000 kg/ms? and 10,000 kg/ms?. The complete set of results is shown below along with the water in oil results that were achieved with a simple deflector plate inlet device.
Defoaming
High injection rates of defoaming chemicals had been required to operate the production and test separators on the Mars TLP, a Shell operated platform in the Gulf of Mexico. In an effort to reduce the chemical consumption, different means of mechanically breaking the foam were investigated in the test separator including gas outlet axial flow demisting cyclones and inlet cyclones supplied by CDS Engineering. For two different wells, the chemical rate was reduced by 20% and 80% respectively when the existing internals were replaced by those referenced above.
On the basis of the successful test separator results, the four main production separators were retrofitted with new internals. Chemical reduction has been on the order of 50%. Other operational problems due to foaming listed below were also reduced:
Poor level control that lead to platform shutdowns
Liquid carryover in the gas outlet that lead to flooding of downstream scrubbers and compressors
Gas carry-under in the liquid outlet that lead to increased compression requirements
The figure below indicates increasing total production rates (red line) while still reducing the defoamer consumption (blue line). The CDS internals were installed in June 1998.
