Page 245 - Petroleum and Gas Field Processing

of the treated water is recycled back to the unit using a pump. The
recycled water flows through a venturi and, due to the reduction in
pressure, sucks gas from the vapor space at the top of the unit. The gas is
released through a nozzle near the bottom in the form of small bubbles
that carry the oil droplets to the surface as they rise. Finally, the oil is
skimmed and collected in a chamber for removal out of the unit.
Normally, a dispersed gas flotation unit consists of three or four of
the cells described. The water to be treated moves from one cell to the next
for further removal of oil. Typically, the oil removal efficiency of one cell
is about 50%. Therefore, a three-cell unit will have an overall efficiency of
87%, whereas a four-cell unit will have an overall efficiency of 94%.
Flotation unit manufacturers have patented design and produce standard
units that are typically designed to handle produced water flow rate
of about 5000 BPD. For higher flow rates, additional units are added
in parallel. Flotation units are capable of removing oil droplets smaller
than 30 mm.

9.5 OFFSHORE WATER DISPOSAL EQUIPMENT

Produced water in offshore operations should not be dumped directly into
the sea after treatment. In addition to the treated produced water,
rainwater, and equipment-washdown water represent another source of
oil-contaminated water that needs to be disposed of properly. For this
purpose, offshore production platforms should be equipped with some
form of a disposal device that disposes of the water deep enough below the
surface of the sea and away from the wave action to prevent sheens from
occurring. The most common of these disposal devices are the disposal
piles, skim piles, and SP piles; these are described in the following
subsections.

Disposal Piles
Disposal piles are the simplest form of offshore water disposal devices.
The disposal pile is simply a large diameter open-ended pipe that is
attached to the platform and extends to a specific minimum depth below
the surface of the sea. The diameter of the pile is determined based on the
total flow of water to be disposed of and the water and oil gravities. In
shallow water, the disposal pile should extend down to near the seafloor.
In deep water, however, the depth of the pile below the normal water level
is determined such that a high level in the pile will be sensed and the

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