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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.

Gas treatment and Compression
The gas train consist 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 to 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) allows the gas to recirculate. The components are described
below.

Heat exchangers
For the compressor operate in an efficient way, the
temperature of the gas should be low. The lower the temperature is
the less energy will be used to compress the gas for a given final
pressure and temperature. However both gas from separators and
compressed gas are relatively hot. When gas is compressed, it
must remain in thermodynamic balance, which means that the gas
pressure times volume over temperature (PV/T) must remain
constant. (PV = nkT). This ends up as a temperature increase.
Temperature exchangers of various forms are used to cool
the gas. Plate heat exchangers (upper picture) consist of a number
of plates where the gas and cooling medium pass between
alternating plates in opposing directions. Tube and shell
exchangers (next picture) place tubes inside a shell filled with of
cooling fluid. The cooling fluid is often pure water with corrosion
inhibitors.
When designing the process it is important to plan the
thermal energy balance. Heat should be conserved e.g. by using
the cooling fluid from the gas train to reheat oil in the oil train.

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