Page 65 - Hydrocarbon

52 Drilling Systems and Equipment

Figure 4.5 Tender-assisted drilling.

spacious barge anchored alongside (Figure 4.5). It is thus possible to service a whole
field or even several fields using only one or two tender-assisted derrick sets. In
rough weather, barge type tenders quickly become inoperable and unsafe since the
platform is fixed whereas the barge moves up and down with the waves. In these
cases and in the hostile environment of the North Sea, a modified semi-submersible
vessel may serve as a tender. Currently, purpose-built semi-submersible tenders are
being introduced for some future field developments.

4.3. Drilling Systems and Equipment

Whether onshore or offshore drilling is carried out, the basic drilling system
employed in both the cases will be the rotary rig (Figure 4.6). The parts of such a unit
and the three basic functions carried out during rotary drilling operations are as follows:

Torque is transmitted from a power source at the surface through a drill string to
the drill bit.
A drilling fluid is pumped from a storage unit down the drill string and up
through the annulus. This fluid will bring the cuttings created by the bit action to
the surface, hence clean the hole, cool the bit and lubricate the drill string.
The subsurface pressures above and within the hydrocarbon-bearing strata are
controlled by the weight of the drilling fluid and by large seal assemblies at the
surface (BOPs).