Oilfield Catalog - Gates Corporation

Rig Technology
Semi-Submersible
Popular because of its stability, the floating
semi-submersible is a common sight in the
North Sea.
In contrast to a submersible, which can be
completely flooded to sit on the seafloor, a
semi-submersible floats just beneath the
surface on submerged pontoons that use
water for ballast.
Anchors and computer-controlled propellers
keep the vessel on the well site.
Semis are one of the two most frequently
used floating rigs. The other is the drill ship.
Drill Ship
Drill ships are useful in deep and remote
waters, because they can generally handle
more weight than other types of offshore
rigs. Drill ships can better handle the weight
of a long drill string, and they can carry
larger amounts of supplies, so resupplying is
less frequent.
On drill ships, the rig itself is usually located
amidship. A sealed hole called the moon
pool allows the drill string to pass through
the hull.
As with semi-submersibles, the drill ship is
kept in position with computer controlled
thrusters, an anchor system, or both.
Components of an
Offshore Rig
In many ways, a rotary drilling rig uses the
same basic principles, whether it is on land
or on a unit above water. That is, the bit
turns on the bottom weighted by drill collars
and rotated by pipe, which is turned by a
rotary table and suspended in the hole by a
block-and-tackle system.
But the fact that the drill string must be in
water has led to some highly specialized
offshore technology. This technology is different
for bottom-supported rigs and for rigs
that float.
Casing
On a bottom-supported offshore rig, casing
isn’t just a pipe liner inside the borehole.
It also extends from the borehole on the
seafloor to a point above the water surface,
just below the rig floor. This casing is called
conductor casing. The casing is necessary
to guide the drill string and also to return
the drilling fluid and cuttings to the surface.
Marine Riser System
On floating rigs, a marine riser system is
used instead of conductor casing. This is
because floating rigs must adjust to the
relative movement between ocean surface
and the fixed wellhead on the ocean floor.
The marine riser isn’t the only system that
differs on a floating rig. Blowout prevention
is another.
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