Page 90 - Oil and Gas Production Handbook An Introduction to Oil and Gas Production

though distillation or reverse filtering. Onshore potable water is provided by
purification of water from above- or underground reservoirs.

Reverse filtering or osmosis requires a membrane driving pressure of about
7000 kPa/1 PSI of pressure per 100 ppm of solids dissolved in the water. For
seawater with 3.5% salt, 2.5 MPa, 350 PSI is required.

6.7.2 Seawater
Seawater is used extensively for cooling purposes. Cold water is provided to
air compressor coolers, gas coolers, main generators and HVAC. In addition,
seawater is used for the production of hypochlorite (see chemicals) and for
fire water. Seawater is treated with hypochlorite to prevent microbiological
growth in process equipment and piping.

Seawater is sometimes used for reservoir water injection. In this case a
deaerator is used to reduce oxygen in the water before injection. Oxygen
can cause microbiological growth in the reservoir. The deaerator is designed
to use strip gas and vacuum.

6.7.3 Ballast water
Ballast systems are found on drilling rigs, floating production ships, rigs and
TLP (tension leg platforms). The object is to keep the platform level and at a
certain depth under varying conditions, such as mode of operation
(stationary drilling, movement), climatic conditions (elevation of rig during
storms), amount of produce in storage tanks, and to adjust loading on TLP
tension members.

Ballasting is accomplished by means of ballast tanks, pumps and valves,
which are used in combination with position measuring instruments and
tension force meters (TLP) to achieve the desired ballasting.

If fresh water is produced, it can be used as ballast to avoid salt water.
Additionally, if ballast water has become contaminated from oil tanks, it must
be cleaned before discharged out to sea.

6.8 Chemicals and additives

A wide range of chemical additives are used in the main process. Some of
these are marked in the process diagram. The cost of process chemical
additives is considerable. A typical example is antifoam where a
concentration of about 150 ppm is used. With a production of 40,000 bpd,

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