Page 106 - Drilling Technology in Nontechnical Language

Chapter 4 – PLANNING AND DRILLING A DEVELOPMENT WELL OFFSHORE 97

The riser is supported by tensioner wires attached to the bottom part
of the telescopic joint. These tensioner wires are attached to pulleys on
the ends of hydraulic rams, which are powered by pressurized nitrogen
cylinders. In effect, the tensioner acts like a very powerful spring, the
strength of which is set by adjusting the nitrogen cylinder pressure. In
very deep water, the tensioner force required to support the riser weight is

quite high. The riser pipes have shaped “floats” attached to their outside,
which reduce the amount of tensioner force needed. As the rig moves up
and down (heaves), the tensioners allow the wire to respond and maintain
the correct force to support the riser.

In summary, the rig has drilled a hole, cemented a conductor in place,
drilled surface hole, and cemented surface casing in place. The BOP
was tested on the rig, and then it was run on riser pipe and latched to the
wellhead housing on top of the surface casing. The BOP sits on the seabed,
controlled by hydraulic hoses from the rig.

Heave compensator

With pipe in the hole, either for drilling or for other operations such as
running casing, compensation must be made for rig heave. This ensures
that the drill bit stays on bottom with the correct weight on bit, or that the
casing can be landed in a controlled manner, while the rig floats up and

down on the waves (fig. 4–12).
The traveling block (used to suspend and control the drillstring) can
be compensated for in a manner similar to the way that the riser tensioners
work, as described above. The compensator cylinder hydraulic fluid

is energized by compressed nitrogen. The upward force exerted by the
compensator is adjusted by changing the nitrogen pressure. If the upward
force just suspends the drillstring, then reducing the nitrogen pressure to
“lose” the desired weight on bit will allow the bit to sit on bottom with the
desired downward force. The amount of heave that can be compensated
for is governed by the maximum stroke (movement) of the compensating
cylinder rod.

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