The cones of a roller cone bit are mounted on journals as shown in Figure 6. There are three types of bearings used in these bits:
1. Roller bearings, which form the outer assembly and help to support the radial loading (or WOB)
2. Ball bearings, which resist longitudinal or thrust loads and also help to secure the cones on the journals
3. A friction bearing, in the nose assembly which helps to support the radial loading. The friction bearing consists of a special bushing pressed into the nose of the cone. This combines with the pilot pin on the journal to produce a low coefficient of friction to resist seizure and wear.
All bearing materials must be made of toughened steel which has a high resistance to chipping and breaking under the severe loading they must support. As with all rock bit components, heat treatment is used to strengthen the steel.
The most important factor in the design of the bearing assembly is the space availability. Ideally the bearings should be large enough to support the applied loading, but this must be balanced against the strength of the journal and cone shell which will be a function of the journal diameter and cone shell thickness. The final design is a compromise which ensures that, ideally, the bearings will not wear out before the cutting structure (i.e. all bit components should wear out evenly). However, the cyclic loading imposed on the bearings will, in all cases, eventually initiate a failure. When this occurs the balance and alignment of the assembly is destroyed and the cones lock onto the journals.
There have been a number of developments in bearing technology used in rock bits :
The bearing assemblies of the first roller cone bits were open to the drilling fluid. Sealed bearing bits were introduced in the late 1950s, to extend the bearing life of insert bits. The sealing mechanism prevents abrasive solids in the mud from entering and causing excess frictional resistance in the bearings. The bearings are lubricated by grease which is fed in from a reservoir as required. Some manufacturers claim a 25% increase in bearing life by using this arrangement (Figure 7).
Journal bearing bits do not have roller bearings. The cones are mounted directly onto the journal (Figure 8). This offers the advantage of a larger contact area over which the load is transmitted from the cone to the journal. The contact area is specially treated and inlaid with alloys to increase wear resistance. Only a small amount of lubrication is required as part of the sealing system. Ball bearings are still used to retain the cones on the journal.
Institute of Petroleum Engineering, Heriot-Watt University
A further development of the PDC bit concept was the introduction in the later 1980’s of Thermally Stable Polycrystalline (TSP) diamond bits. These bits are manufactured in a similar fashion to PDC bits but are tolerant of much higher temperatures than PDC bits.
Thermally Stable Polycrystralline – TSP – Diamond bits were introduced when it was found, soon after their introduction, that PDC bit cutters were sometimes chipped during drilling. It was found that this failure was due to internal stresses caused by the differential expansion of the diamond and binder material. Cobalt is the most widely used binder in sintered PCD products. This material has a thermal coefficient of expansion of 1.2 x 10-5 deg. C compared to 2.7 x 10-6 for diamond. Therefore cobalt expands faster than diamond. As the bulk temperature of the cutter rises above 7300 C internal stresses caused by the different rates of expansion leads to severe intergranular cracking, macro chipping and rapid failure of the cutter.
These temperatures are much higher than the temperatures to be found at the bottom of the borehole (typically 1000 C at 8000 ft). They, in fact, arise from the friction generated by the shearing action by which these bits cut the rock.
This temperature barrier of 7300 C presented serious barriers to improved performance of PCD cutter bits. Manufacturers experimented with improving the thermal stability of the cutters and Thermally Stable Polycrystralline Diamond Bits were developed. These bits are more stable at higher temperatures because the cobalt binder has been removed and this eliminates internal stresses caused by differential expansion. Since most of the binder is interconnected, extended treatment with acids can leach most of it out. The bonds between adjacent diamond particles are unaffected, retaining 50-80% of the compacts’ strength. Leached PCD is thermally stable in inert or reducing atmospheres to 12000 C but will degrade at 8750 C in the presence of oxygen. Due to the nature of the manufacturing process the thermally stable polycrystalline (TSP) diamond cannot be integrally bonded to a WC substrate. Therefore, not only is the PCD itself weaker, but the excellent strength of an integrally bonded Tungsten Carbide (WC) substrate is sacrificed.Without the WC substrate, the TSP diamond is restricted to small sizes (Figure 20) and must be set into a matrix similar to natural diamonds.
A new generation of diamond bits known as polycrystalline diamond compact (PDC) bits were introduced in the 1980’s (Figure 5). These bits have the same advantages and disadvantages as natural diamond bits but use small discs of synthetic diamond to provide the scraping cutting surface. The small discs may be manufactured in any size and shape and are not sensitive to failure along cleavage planes as with natural diamond. PDC bits have been run very successfully in many areas around the world. They have been particularly successful (long bit runs and high ROP) when run in combination with turbodrills and oil based mud.
PCD (Polycrystalline Diamond) is formed in a two stage high temperature, high pressure process. The first stage in the process is to manufacture the artificial diamond crystals by exposing graphite, in the presence of a Cobalt, nickel and iron or manganese catalyst/solution, to a pressure in excess of 600,000 psi. At these conditions diamond crystals rapidly form. However, during the process of converting the graphite to diamond there is volume shrinkage, which causes the catalyst/solvent to flow between the forming crystals, preventing intercrystalline bonding and therefore only a diamond crystal powder is produced from this part of the process.
In the second stage of the process, the PCD blank or ‘cutter’ is formed by a liquid-phase sintering operation. The diamond powder formed in the first stage of the process is thoroughly mixed with catalyst/binder and exposed to temperatures in excess of 14000 C and pressures of 750,000 psi. The principal mechanism for sintering is to dissolve the diamond crystals at their edges, corners and points of high pressure caused by point or edge contacts. This is followed by epitaxial growth of diamond on faces and at sites of low contact angle between the crystals. This regrowth process forms true diamond-to-diamond bonds excluding the liquid binder from the bond zone. The binder forms a more or less continuous network of pores, co-existing with a continuous network of diamond. Typical diamond concentrations in the PCD is 90-97 vol.%.
If one requires a composite compact in which PCD is bonded chemically to a tungsten carbide substrate (Figure 18), some or all of the binder for the PCD may be obtained from the adjacent tungsten carbide substrate by melting and extruding the cobalt binder from the tungsten carbide. The cutters can be manufactured as disc shaped cutters or as stud cutters, as shown in Figure 19.
Institute of Petroleum Engineering, Heriot-Watt University
The hardness and wear resistance of diamond made it an obvious material to be used for a drilling bit. The diamond bit is really a type of drag bit since it has no moving cones and operates as a single unit. Industrial diamonds have been used for many years in drill bits and in core heads (Figure 1).
The cutting action of a diamond bit is achieved by scraping away the rock. The diamonds are set in a specially designed pattern and bonded into a matrix material set on a steel body. Despite its high wear resistance diamond is sensitive to shock and vibration and therefore great care must be taken when running a diamond bit. Effective fluid circulation across the face of the bit is also very important to prevent overheating of the diamonds and matrix material and to prevent the face of the bit becoming smeared with the rock cuttings (bit balling).
The major disadvantage of diamond bits is their cost (sometimes 10 times more expensive than a similar sized rock bit). There is also no guarantee that these bits will achieve a higher ROP than a correctly selected roller cone bit in the same formation. They are however cost effective when drilling formations where long rotating hours (200-300 hours per bit) are required. Since diamond bits have no moving parts they tend to last longer than roller cone bits and can be used for extremely long bit runs. This results in a reduction in the number of round trips and offsets the capital cost of the bit. This is especially important in areas where operating costs are high (e.g. offshore drilling). In addition, the diamonds of a diamond bit can be extracted, so that a used bit does have some salvage value.
Diamond has been used as a material for cutting rock for many years. Since it was first used however, the type of diamond and the way in which it is set in the drill bit have changed.
Roller cone bits (or rock bits) are still the most common type of bit used world wide. The cutting action is provided by cones which have either steel teeth or tungsten carbide inserts. These cones rotate on the bottom of the hole and drill hole predominantly with a grinding and chipping action. Rock bits are classified as milled tooth bits or insert bits depending on the cutting surface on the cones (Figure 2 and 3).
The first successful roller cone bit was designed by Hughes in 1909. This was a major innovation, since it allowed rotary drilling to be extended to hard formations. The first design was a 2 cone bit which frequently balled up since the teeth on the cones did not mesh. This led to the introduction of a superior design in the 1930s which had 3 cones with meshing teeth. The same basic design is still in use today although there have been many improvements over the years.
The cones of the 3 cone bit are mounted on bearing pins, or arm journals, which extend from the bit body. The bearings allow each cone to turn about its own axis as the bit is rotated. The use of 3 cones allows an even distribution of weight, a balanced cutting structure and drills a better gauge hole than the 2 cone design. The major advances in rock bit design since the introduction of the Hughes rock bit include:
1. Improved cleaning action by using jet nozzles
2. Using tungsten carbide for hardfacing and gauge protection
3. Introduction of sealed bearings to prevent the mud causing premature failure due to abrasion and corrosion of the bearings.
Drag bits were the first bits used in rotary drilling, but are no longer in common use. A drag bit consists of rigid steel blades shaped like a fish-tail which rotate as a single unit. These simple designs were used up to 1900 to successfully drill through soft formations. The introduction of hardfacing to the surface of the blades and the design of fluid passageways greatly improved its performance. Due to the dragging/scraping action of this type of bit, high RPM and low WOB are applied.
The decline in the use of drag bits was due to:
– The introduction of roller cone bits, which could drill soft formations more efficiently
– If too much WOB was applied, excessive torque led to bit failure or drill pipe failure
– Drag bits tend to drill crooked hole, therefore some means of controlling deviation was required
– Drag bits were limited to drilling through uniformly, soft, unconsolidated formations where there were no hard abrasive layers.