Second stage separator
The second stage separator is quite similar to the first stage HP separator. In
addition to output from the first stage, it also receives production from wells
connected to the low pressure manifold. The pressure is now around 1 MPa
(10 atmospheres) and temperature below 100ºC. The water content will be
reduced to below 2%.
An oil heater can be located between the first and second stage separator to
reheat the oil/water/gas mixture. This makes it easier to separate out water
when initial water cut is high and temperature is low. The heat exchanger is
normally a tube/shell type where oil passes though tubes in a heating
medium placed inside an outer shell.
4.2.4 Third stage separator
The final separator is a two-phase separator, also called a flash drum. The
pressure is now reduced to atmospheric pressure of around 100 kPa, so that
the last heavy gas components can boil out. In some processes where the
initial temperature is low,
it might be necessary to heat the liquid again (in a
heat exchanger) before the flash drum to achieve good separation of the
heavy components. There are level and pressure control loops.
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As an alternative, when production is mainly gas, and remaining liquid
droplets have to be separated out, the two-phase separator can be a knockout drum (K.O. drum).
4.2.5 Coalescer
After the third stage separator, the oil can go to a coalescer for final removal
of water. In this unit, water content can be reduced to below 0.1%. The
coalescer is completely filled with liquid: water at the bottom and oil on top.
Internal 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 of 0.2 to
2 kV/cm. Field intensity and frequency as well as the coalescer grid layout
are different for different manufacturers and oil types.
4.2.6 Electrostatic desalter
If the separated oil
contains unacceptable
amounts of salts, they
can be removed in an
electrostatic desalter
(not used in the Njord
example). The salts,
which may be sodium,
calcium or magnesium
chlorides, come from the reservoir water and are also dissolved in the oil.
The desalters will be placed after the first or second stage separator
depending on GOR and water cut. Photo: Burgess Manning Europe PLC
4.2.7
Water treatment
On an installation such as this, where the water cut is high, there will be a
huge amount of water produced. In our example, a water cut of 40% gives
water production of about 4,000 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.
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The environmental regulations in most countries are quite strict. For
example, in the Northeast Atlantic, the OSPAR convention limits oil in water
discharged to sea to 40 mg/liter (ppm).
Figure 6. Produced water treatment
It also places limits on other forms of contaminants. This still means that the
equivalent of up to one barrel of oil per day in contaminants from the above
production is discharged into the sea, but in this form, microscopic oil drops
are broken down quickly by natural bacteria.
Various pieces of equipment are used. This 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
removes 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
slowly rises and pulls 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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4.3 Gas treatment and compression
The gas train consists of several stages, each taking gas from a suitable
pressure level in the production separator's gas outlet, and from the previous
stage.
A typical stage is
shown on the right.
Incoming gas (on
the right) is first
cooled in a heat
exchanger. It then
passes through the
scrubber to remove
liquids and goes into
the compressor. The
anti-surge loop
(thin orange line)
and the surge valve
(UV0121 23) allow
the gas to
recirculate. The
components are described below.
