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Types of Offshore Drilling Rigs
There have been many types of drilling rigs used since the
industry took its first steps into the offshore areas. Figure 4.7
shows some of these variations. Today, however, there are three
important types of mobile offshore drilling rigs that operate in the
open marine areas. These are the jackup, the semi-submersible and
the drillship (Figure 4.7).
Jackup rigs are usually towed to the drilling location. At
the site, the legs are lowered to the seabed and the platform is
jacked up to a safe level above the sea. When drilling operations
are completed, the legs are "jacked up" high enough so that the rig
can float freely and be towed to the next location. The maximum
water depth in which jackup rigs are used is about 90 metres.

Figure 4.7 - Drilling platform

The semi-submersible rig is comprised of a drilling
platform mounted on columns above two large pontoons. During
rig moves, only the pontoons are submerged. At the drillsite, the
pontoons are partially flooded so that the rig rides deep in the
water providing a remarkably stable drilling platform. Most semi-
submersibles in existence today are moored by anchors, but an
increasing number are adopting dynamic positioning systems.

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