Page 475 - Offshore Electrical Engineering Manual

462 APPENDIX A: Guide to Offshore Installations

Some liquids have the property of absorbing water vapour from gases and are
used to prevent or restrict formation of hydrates.
Ethylene glycol or similar agents are used for this purpose offshore. Gas is passed
through a pressure vessel into which a fine spray of the agent is introduced.

WATER INJECTION
Seawater is drawn up by lift pumps and, along with treated produced water, is
injected into the oil bearing formations via a separate injection well. The purpose
of this is to artificially maintain well pressure by replacing the oil, gas and water
removed via the producing well. The injection pumps required are usually very
large, in the order of several megawatts. The reinjection of produced water has
the added advantage of disposing of a potential environmental pollutant, although
with high oil prices it is economically attractive to centrifuge out quite low levels
of entrained oil.

GAS REINJECTION
In the early days of oil production, it was accepted that the gas removed from the
separators could be flared, should it be found uneconomical to pipe the gas ashore.
Nowadays, with established North Sea gas pipe networks such as ‘FLAGS’ it is nor-
mally possible for the producing company to tie in to an established gas line and
supply the gas via its onshore terminal to the particular national gas utility. In any
case, the flaring of large quantities of gas except in abnormal circumstances is strictly
controlled by national regulations.
Where the gas export facilities of a platform are nonexistent or restricted to the
extent that oil production would be affected, it is necessary to reinject the gas as an
acceptable alternative to flaring. It also has the advantage of helping to maintain well
pressure and hopefully may be largely recoverable at a later date.
A large compressor train of two or three stages is normally required to obtain the
gas flow rates and pressures necessary for reinjection. A typical scheme would be of
two stages utilising axial compressors driven by large induction motors, followed by
a third-stage reciprocating compressor driven by a synchronous motor to ease volt-
age and frequency control problems for the platform generation. The problems of
large process loads such as these, when powered from a small number of generators,
isolated from a national grid network, are discussed in PART 1 Chapter 2.

OIL STORAGE AND EXPORT
Oil that has been through the platform processes may be exported immediately via
large main oil line pumps or, where no export pipeline exists, may be stored in the
subsea cells of the platform structure before offloading by a tanker. As the oil from the
particular field may be above the average value of the oil from a shared export pipe net-
work, it may be economically advantageous to export by tanker as well as by pipeline.