Page 131 - Drilling Technology in Nontechnical Language

122 Drilling Technology in Nontechnical Language Second Edition

A conventional platform could be installed with the capacity to drill
quite a lot of wells. A large platform might have the capability to drill more
than 30 wells by piledriving conductors into the seabed through guides
set into the platform substructure. The rig package sits on top of large
steel beams, where it is moved around the available well “slots” by using
hydraulic cylinders to skid the rig around on the beams. Once the wells are
all drilled, the rig package may be removed to reduce platform weight and
increase space.

In deep or very deep water, a platform may be designed to float but is
tethered to the seabed with steel pipes. This is called a tension leg platform
(TLP). Advantages of a TLP include the following:
▪ The platform is completed in an onshore yard, while the wells

may be drilled with a floating rig through a template. Once the
platform is ready, it is floated to the location, where the legs are

attached and the wells connected back to the platform. The time
from the decision to develop the field to producing oil can be

relatively short, as platform construction and drilling happens at
the same time.

▪ After the field is abandoned, the platform can be untethered,
removed, refurbished, and redeployed to another location or to
a yard for disposal. The future cost to the industry of removing

fixed platforms worldwide is likely to be huge, even if some

of them can be toppled on site and left as artificial reefs to

encourage fish stocks. The cost of removing a TLP will be much

smaller than for a conventional fixed platform.
▪ A TLP platform could be installed in water as deep as 10,000 ft.

This is far deeper than any fixed structure that could be used.

Submersible

Two types of submersible rig are used:
▪ A barge-type structure with a flat bottom, which is towed to a

location in shallow, static water and sunk to sit on the bottom
in up to about 20 ft of water. These rigs are also called swamp
barges. The derrick can be adjusted to account for slight tilts

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