Page 473 - Offshore Electrical Engineering Manual

460 APPENDIX A: Guide to Offshore Installations

spacing at the reservoir to allow the gas to be removed in a more uniform manner.
The method may also be used where satellite wells are completed underwater.
In some cases where the field is shallow, such as the Morecambe Bay gas field,
wells may be drilled at an angle to the vertical using a special drilling rig with a
derrick which may be tilted to the correct angle by hydraulic jacks before drilling
commences.

A.3 PRODUCTION PROCESS ESSENTIALS

The processes outlined in the following are shown in the flow diagram in Fig. A3.1
and are typical of a large North Sea fixed production platform.

OIL, WATER AND GAS SEPARATION
Petroleum mixtures are usually complex and not easy to separate efficiently. Vapour
recovery and evaporation control must be engineered in a hostile environment with
minimum risk of fire or explosion.
Separators on offshore installations consist of large steel pressure vessels, either
horizontal or vertical, which by a combination of changing flow direction many
times and the action of gravity cause the heavier liquids to fall to the bottom. Special
(sometimes vertical) separator vessels are used on floating installations to allow the
separation process to occur in bad weather, with the ship rolling and pitching.
After the first stage of separation, the gas will still be ‘wet’ and require at least
one further stage of separation. Liquids which would revert to a gas at atmospheric
pressure are referred to as condensate and are normally re-entrained with the crude
oil before exporting from the platform by a subsea pipeline or tanker. On some instal-
lations, the gas is liquefied and also injected into the oil pipeline before exporting
ashore.
A large proportion of the flow from any North Sea well is water, and this must
be removed from the oil and gas simply because it has no economic value and it
would also cause corrosion in the export subsea pipeline. As the petroleum liquids
are less dense than water, they form a middle layer in the separators so that the ‘pro-
duced water’ can be drained off at the bottom. Level controllers ensure that the three
different outputs of each separator are maintained. Naturally occurring radioactive
materials (NORM) in the form of natural radioactive salts may collect in some sepa-
rators, and this will need to be removed and taken to Sellafield or a similar plant to
be disposed of safely.

HYDRATES

Mixtures of water vapour and natural gas at high pressure tend to form solid ice-
like crystals on cooling called hydrates. Hydrates can cause restriction or even com-
pletely prevent flow of gas.