Page 132 - Drilling Technology in Nontechnical Language

Chapter 5 – RIG SELECTION AND RIG EQUIPMENT 123

caused by the bottom being at a slight gradient. These rig types
are not common, with only about 12 operating worldwide.
▪ A structure similar to a lightly built semisubmersible, with
columns and pontoons. The rig is floated to the location, where

it takes on ballast to sink to the bottom in up to about 130 ft of
water. The bottom has to be fairly level. Deck load capacity while
moving is low, but once on bottom, this type of submersible can
take on board a large weight of supplies. In the water depths that
these rigs would operate in, a jackup type rig would also be able
to operate well.

Rig Systems and Equipment

Dynamic positioning

With a drillship or semisubmersible rig, it is possible to station the rig
above the well by dynamic positioning.
Underneath the rig are attached small propellers mounted on housings
that swivel. On the sea bed, transponders are placed at known locations.
Transponders are electronic devices that wait for a coded signal from
an interrogating transmitter. When that signal is received, a reply is
transmitted. The interrogating transmitter measures the time for the signal
reply and, knowing the speed of the transmission wave, can calculate the
distance from itself to the transponder. If four transponders are placed on
the seabed a reasonable distance apart, the 3-D position of the interrogator
in relation to the transponders can be calculated. The rig positioning system
uses these transponders to keep a constantly updated rig position relative
to the wellhead. If the rig starts to move away from above the wellhead,
the positioning system computers control the thrusters to push the rig back
to its defined position.

As described in chapter 4, a floating rig uses a blowout preventer on
the seabed, and the rig connects to the top of the BOP with a riser pipe.
Between the top of the BOP and the riser is a special joint that allows some
angular movement between the riser and BOP. However, if this angle is
exceeded (usually 5°), something will be damaged. If the rig moves outside a
certain distance from vertically above the wellhead, a disconnect sequence
has to be initiated. This sequence closes the BOP, cuts the drillpipe, and

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