Page 67 - Hydrocarbon

54 Drilling Systems and Equipment

Figure 4.7 Roller cone bit (left) and PDC bit.

On a rock bit, the three cones are rotated and the attached teeth break or crush
the rock underneath into small chips (cuttings). The cutting action is supported by
powerful jets of drilling fluid which are discharged under high pressure through
nozzles located at the side of the bit. After some hours of drilling (between 5 and
25 h depending on the formation and bit type), the teeth will become dull and the
bearings wear out. Later on we will see how a new bit can be fitted to the drill
string. The PDC bit is fitted with industrial diamond cutters instead of hardened
metal teeth. This type of bit is popular because of its much better rate of penetration
(ROP), longer lifetime and suitability for drilling with high revolutions per minute
(rpm), which makes it the preferred choice for turbine drilling. The selection of bit
type depends on the composition and hardness of the formation to be drilled and
the planned drilling parameters.
Between the bit and the surface, where the torque is generated, we find the drill
string (Figure 4.8). Whilst primarily being a means for power transmission, the drill
string fulfils several other functions, and if we move up from the bit we can see what
those are.
The drill collars (DCs) are thick-walled, heavy lengths of pipe. They keep the
drill string in tension (avoiding buckling) and provide weight onto the bit. Stabilisers
are added to the drill string at intervals to hold, increase or decrease the hole angle.
The function of stabilisers will be explained in more detail in Section 4.5. The bottom
hole assembly (BHA) described so far is suspended from the drill pipe, made up of
30 ft long sections of steel pipe (joints) screwed together. The drill string is
connected to the kelly saver sub. A saver sub is basically a short piece of connecting
pipe with threads on both ends. In cases where connections have to be made up and
broken frequently, the sub ‘saves’ the threads of the more expensive equipment. The
kelly is a six-sided piece of pipe that fits tightly into the kelly bushing which is fitted
into the rotary table. By turning the latter, torque is transmitted from the kelly down