Page 471 - Offshore Electrical Engineering Manual

458 APPENDIX A: Guide to Offshore Installations

contain various additives for better lubrication of the drill bit in abrasive rock forma-
tions or better sealing in highly permeable rock formations.

SHALE SHAKER
The mud is circulated by mud pumps using yet another DC variable-speed drive
controlled from the driller’s console. The drill cuttings conveyed to the surface are
then sieved out by a vibrating screen called a shale shaker before returning to a pit
for recirculation.
The moving column of mud in the hole is vital to the safety of the drilling opera-
tion, as its weight helps prevent a ‘blowout’ if oil or gas at very high pressures is
encountered.
The petroleum engineer may increase the density of the mud at critical stages of
drilling where high-pressure gas or oil pockets are likely. This increases the weight
of the column of mud in the hole and reduces the blowout risk.

BLOWOUT PREVENTER
If a pocket of high-pressure oil or gas is unexpectedly pierced by the drill, the hydro-
static pressure and viscosity of the mud in the drillhole will not be sufficient to pre-
vent the oil and gas from blowing the mud back and erupting, often in a fierce jet
from the well. If this very hazardous condition, known as a blowout, is not quickly
controlled, it may lead to serious fires and explosions, and possibly the loss of the
entire installation. To guard against blowouts, up to six heavy-duty valves collec-
tively known as the blowout preventer are fixed beneath the raised floor of the derrick
or on the seabed. If there is a risk of a blowout occurring, the well can be closed by
these valves, even whilst drilling is in progress.

WIRELINE LOGGING
One of the temporary facilities the electrical supply of which is often problematical for
electrical engineers is the wireline cabin. This is because they are almost invariably in
the hazardous area created by the wellheads. They are necessary because a continuous
log often needs to be made of the electrical and other properties of the rock formations
drilled through. Different rock types, their fluid content and physical properties can be
distinguished, and with information obtained by analysis of drill cuttings and specially
drilled core samples, a map of the oil or gas field geological structure may be built up.

WELL COMPLETION
Once oil or gas is found in sufficient quantities to be economically viable, production
wells may be drilled. In this case the hole is periodically lined with steel casing of dif-
ferent diameters. The production casing is the longest and smallest diameter, typically
9.5/8 in., followed by intermediate casings and ending up with 30-in. surface casing.