Fluid Circulation
The fluid circulation across the bit face must be designed to remove the cuttings efficiently and also to cool the bit face. These requirements may be satisfied by increasing the fluid flowrate and/or the design of the water courses that run across the face of the bit. This increased fluid flow may however cause excessive erosion of the face and premature bit failure. More than three jets are generally used on a PDC bit.
It can be appreciated from the above discussion that there are many variations in the design of drillbits. The IADC has therefore developed a system of comparison charts for classifying drillbits according to their design characteristics and therefore their application. Two systems have been developed: one for roller cone bits; and one for Fixed Cutter bits.
The IADC bit comparison charts (Table 1) are often used to select the best bit for a particular application. These charts contain the bits available from the four leading manufacturers of bits. The bits are classified according to the International Association of Drilling Contractors (IADC) code. The position of each bit in the chart is defined by three numbers and one character. The sequence of numeric characters defines the “Series, Type and Features” of the bit. The additional character defines additional design features.
Column 1 — Series
The series classification is split into two broad categories: milled tooth bits (series 13); and insert bits (series 4-8). The characters 1- 8 represent a particular formation drillability.
Series 1-3 bits are therefore milled tooth bits which are suitable for soft, medium or hard formations.
Series 4-8 bits are insert bits and are suitable for soft, medium, hard and extra hard formations.
Column 2 — Type
Each series category is subdivided into 4 types according to the drillability of the formation (i. e. a type 3 is suitable for a harder formation than a type 2 bit within the same series).
The classification of the bit according to series and type specification will be dependant primarily on the cutter size and spacing and bearing and cone structure discussed in the previous sections.
Row 1 — Features
The design features of the bit are defined on the horizontal axis of the system. There are slightvariations in thefeatures describedonthecomparisoncharts, depending on the comparison chart being used in the chart shown in Table 1 the numerical characters define the following features:
1 Means a standard roller bearing
2 Means air cooled roller bearings
3 Means a roller bearing bit with gauge protection
4 Means sealed roller bearings are included
5 Means both sealed roller bearings and gauge protection included
6 Means sealed friction bearings included (for both milled tooth and insert
bits)