4.2.5 Coalescer
After the third stage separator, the oil
can go to a coalescer for final removal
of water. In this unit the water content
can be reduced to below 0.1%. The
coalescer is completely filled with
liquid: water at the bottom and oil on
top. Inside electrodes form an electric
field to break surface bonds between
conductive water and isolating oil in an
oil water emulsion. The coalescer field plates are generally steel, sometimes covered
with dielectric material to prevent short circuits. The critical field strength in oil is in
the range 0.2 to 2 kV/cm. Field intensity and frequency as well as the coalescer grid
layout is different for different manufacturers and oil types.
4.2.6 Electrostatic Desalter
If the separated oil
contains unacceptable
amounts of salts, it can be
removed in an
electrostatic desalter (Not
used in the Njord
example) The salts, which
may be Sodium, Calcium
or Magnesium chlorides
comes from the reservoir water and is also dissolved in the oil. The desalters will be
placed after the first or second stage separator depending on Gas Oil Ratio (GOR)
and Water cut. Photo: Burgess Manning Europe PLC
4.2.7 Water treatment
On an installation such as this, when the water cut is high, there will be a huge
amount of produced water. In our example, a water cut of 40% gives a water
production of about 4000 cubic meters per day (4 million liters) that must be cleaned