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278 Oil and Gas Processing
oil skimmer
water outlet
inlet
water weir
CPI
Figure 11.12 Corrugated (or tilted) plate interceptor.
In a gas flotation unit, air is bubbled through oily water to capture oil particles
which then rise with the bubble to form a scum at the surface of the flotation unit.
The scum can be removed by rotating paddles. Chemicals are often added to
destabilise the inlet stream and enhance performance.
Hydrocyclones (Figure 11.13) have become common on offshore facilities and
rely on centrifugal force to separate light oil particles from the heavier water phase.
As the inlet stream is centrifuged, the heavier water phase is ‘spun’ to the outside of
the cyclone whilst oil particles move to the centre of the cyclone, coalesce and are
drawn off upwards. The heavier water is taken out at the bottom.
To ensure disposal water quality is in line with regulatory requirements (usually
40 ppm), the oil-in-water content is monitored by solvent extraction and infrared
spectroscopy. The specification of 40 ppm refers to an oil-in-water content typically
averaged over a 1 month period. Oil-in-water standards are generally tightening and
whilst 40 ppm remains acceptable in some areas, 10 ppm or less is becoming more
common. In closed marine environments such as the Caspian Sea, partially closed
environments like the Arabian Gulf and elsewhere, many companies are re-injecting
produced water back into reservoirs to meet prevailing or self-imposed corporate
environmental standards. This alternative is known as produced water re-injection (PWRI).
11.1.2.7. Multiphase pumps
Since the 1990s, multiphase pumps, capable of simultaneously pumping oil, water
and gas, have gained increased acceptance in oil field production. In some cases,
they have replaced conventional processing equipment at the wellhead or platform,
effectively exporting processing requirements downstream to central gathering
facilities. Eliminating in-field equipment such as separators, compressors, individual
pumping equipment, heaters, gas flares and separate flowlines can also significantly
reduce processing costs.
Additional benefits include reduced environmental impact for onshore
installations as multiphase pumps require only a fraction of the space conventional