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Figure 6.2 - Photo: Burgess Manning Europe PLC

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 before
discharge to sea. Often this water contains sand particles bound to
the oil/water emulsion.
The environmental regulations in most countries are quite
strict, as an example, in the North-East Atlantic the OSPAR
convention limits oil in water discharged to sea to 40 mg/liter
(ppm). It also places limits other forms of contaminants. This still
means up to one barrel of oil per day for the above production, but
in this form, the microscopic oil drops are broken down fast by
natural bacteria.
Various equipment is used; the illustration shows a typical
water treatment system. Water from the separators and coalescers
first goes to a sand cyclone, which removes most of the sand. The
sand is further washed before it is discharged.
The water then goes to a hydrocyclone, a centrifugal
separator that will remove oil drops. The hydrocyclone creates a
standing vortex where oil collects in the middle and water is forced
to the side.

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