Oil and gas production handbook - Process Control and ...

efore 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.
Finally the water is collected in the water de-gassing drum. Dispersed gas will
slowly rise to the surface and pull remaining oil droplets to the surface by flotation.
The surface oil film is drained, and the produced water can be discharged to sea.
Recovered oil in the water treatment system is typically recycled to the third stage
separator.
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