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Figure 4.1 – Drilling ships
Drilling equipment is passed through the vessel's moon pool and
connected to the well equipment below via riser pipe, a somewhat flexible
pipe that extends from the top of the subsea well to the bottom of the
drillship. Drillships are differentiated from other offshore drilling units by
their easy mobility. While semisubmersible rigs can also drill in deep
waters, drillships are able to propel themselves from well to well and
location to location, unlike semisubs, which must rely on an outside
transport vessel to transfer them from place to place.
While capable of drilling in deep and ultra-deep waters, a
disadvantage to using a drillship is its susceptibility to being agitated by
waves, wind and currents. This is especially troublesome when the vessel
is actually drilling, because the drillship is connected to equipment
thousands of feet under the sea.
A proper mooring system on a drillship is integral to drilling
successful wells. In shallower waters, drillships are moored to the seafloor
with anywhere from 6 to twelve anchors. Once the water depth becomes
too deep, drillships depend on dynamic positioning systems (DPS) to keep
the vessel in place while drilling. DPS relies on several thrusters located

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