Page 51 - 4822

The third type of mobile rig, the drillship (Figure 9.5), is
comprised of a drilling platform mounted on a conventional ship's
hull. These drilling units have also been moored by anchors in the
past, but today most are using DPS technolo gy. The drillship
experiences greater heaving and rolling motion than the
semisubmersible and, therefore, is more limited in the severity of
seas in which it can operate. On the other hand, the greater
mobility of the drillship gives it advantages in areas where it may
have to move off location under short notice, such as in areas with
a high iceberg population.
Drillships and semi-submersibles have not yet reached their
limits with regard to the water depths in which they can operate.
Using dynamic positioning systems, they have been able to drill in
water depths exceeding 2000 metres. Both semi-submersibles and
drillships have been used on the Grand Banks. The jackup has not
been used in the Newfoundland offshore, but has been routinely
used off Nova Scotia.
Drilling offshore is much more expensive than drilling
onshore. The cost of drilling is very much influenced by
environmental conditions. Factors such as distance from land,
water depth, well depth, weather conditions, the presence of pack
ice and icebergs all affect the cost of an offshore well. For
example, on the Grand Banks of Newfoundland, in 1991, the
average well takes from 60 to 70 days to drill and costs
approximately $16 million. These well costs can only be justified
in areas where there is a good chance of finding very large fields.
Offshore Newfoundland and Labrador is one such area. Figure 9.6
shows other areas in the world where offshore drilling has
occurred [18].
Supply Vessels
An offshore drilling rig located hundreds of kilometres
from land with a crew of about 100 people, needs a lot of supplies.
These include food for the crew, fuel for the engines and
generators, sacks of cement and sacks of bentonite and barite
which are mixed with water to make the drilling mud. It needs
hundreds of tons of drill pipe, casing, and, of course, the huge and
massive sections of pipe that are put together to form the riser. It is

51