Roller Tri-Cone Bit

Early bits possessed two cones that had no interaction or meshing, these were prone to balling (where drilled cuttings collect and consolidate around the bit) in soft formations. These were superseded by the tri-cone bit, the most common type used in modern drilling (Figure 20). These possess 3 cones, which are intermeshing and therefore self-cleaning, with rows of cutters on each cone.

Cutters are of two principle types: milled teeth (Figure 20) or tungsten carbide inserts (TCI), and can be of varying size and hardness according to the lithology expected. A lot of heat is generated by friction during drilling and this heat must be dissipated. Cooling, together with lubrication, is an important function of the drilling fluid. This exits the drillstring through ports in the bit that are called jets or nozzles; one jet is positioned above each cone.

Jets are replaceable and can be of varying size, the smaller the jet, the greater the velocity and force of the mud exiting the bit. Jet sizes are expressed in millimeters or in 32nds of an inch. If no jet is set into the port, it is known as an open jet (the size is one inch, that is, thirty-two 32nds).

Figure 20: Tri-cone Milled Tooth Bit

Roller bits are classified by a system developed by the International Association of Drilling Contractors (IADC), where most have a 3 digit IADC code to describe the series, type and design (Table 1). The following are examples:

 Hughes ATM22: IADC code 517—Soft chisel type TCI bit, softest in the range, with friction-sealed journal bearings and gauge-protected.

 Reed MHP13G: IADC code 137—Soft milled tooth bit, moderately hard in the range, with friction-sealed journal bearings and gauge-protected.

Some bits have a fourth category to describe additional features about the bit. Examples include air application (A) bits, centre jets (C), deviation control (D), extra gauge (E), horizontal steering (H), standard steel tooth bit (S), chisel shaped inserts (X), conical shaped inserts (Y).

2.4.2.1 Bit Terminology

Figure 21 illustrates the naming convention for the various parts of tri-cone bits.

Figure 21: Tri-Cone Bit—Terminology 2.4.2.2 IADC Bit Classification

Table 1: IADC Bit Classification

Series

Type of cutting structure

1 Soft Milled Tooth

2 Medium 3 Hard

4 Very soft Chisel Tungsten Carbide Insert 5 Soft

6 Medium Conical

7 Hard 8 Very hard

Type

5 Gauge protected and sealed bearing 6 Friction, sealed journal bearing

7 Friction, sealed journal bearing, gauge protected 8 Directional

9 Other

2.4.2.3 Cone Action

As cones roll on the bottom of the hole, a sliding action gouges and scrapes the formation. Cones have more than one rolling center because of the number and alignment of cutter rows, but this is restrained by the weight of the drill collars acting on the bit.

Rotation is around the bit center-line so that the teeth must slide and scrape as they roll. This action is minimized in the design of the hard bits (by having no cone offset) to reduce wear, but action is still not pure rolling.

The sliding action produces a controlled tearing, gouging, and scraping action on the formation, leading to fast and efficient chip removal. For soft formations, the scraping action is enhanced by offsetting the cones. This leads to faster drilling and the amount of scraping action depends on the degree of offset. Soft formation bits may have an offset of 1/4 or 1/8 inch in medium bits, and no offset for hard bits.

2.4.2.4 Bearing Types

 Unsealed—These are grease-filled and exposed. Their life is short because they are exposed to metal fatigue and abrasion from solids.

 Sealed and self-lubricating—Metal fatigue still exists, but abrasion from solids is eliminated as long as there is a seal.

 Sealed journal bearings—These have a longer life, but wear can come from seizure of the sliding metal-to-metal surfaces on the bottom side of bearings. If the seal fails, drilling mud leaks into the bearing, displacing the grease. Overheating causes rapid failure of the bearing. The bearing has a pressure compensation system that minimizes the pressure differential between the bearing and the mud column pressure.

2.4.2.5 Teeth

The size, shape and separation of teeth affect the efficiency of the bit in a formation of varying hardness.

The tooth design also determines the size and form of the drilled cuttings produced and used for formation evaluation.

For soft formations, the teeth are typically long, slender, and widely spaced. The longer teeth allow deeper penetration into the soft formation. This deeper penetration is maintained as the teeth become worn by making the teeth as slender as possible. The wide spacing prevents the soft formation from balling or packing between the teeth. The cutting action is one of gouging and scraping and the cuttings typically produced are large and freshly broken.

Bearing size and strength are restricted in soft formation bits depending on the size of the teeth. This normally does not produce a problem because only low weights or force must be applied to the bit to achieve formation failure and penetration.

For formations of medium hardness, shorter and broader teeth are used. Deep penetration is limited by the formation hardness so that longer teeth are unnecessary. The length is such that as much penetration as possible is achieved. At the same time, wear caused by the firmer formation is kept to a minimum.

Wide spacing allows for efficient cleaning even though balling is not as important as in a soft formation.

For hard formations, short and broad teeth produce a crushing and chipping action rather than scraping and gouging. The drilled cuttings are smaller, more rounded, crushed and ground. Tooth spacing is not required for cleaning because cuttings are smaller with a lower concentration or volume, resulting from lower penetration rates.

Increased life in hard and abrasive formations can be produced by hard-facing the milled steel teeth or by using tungsten carbide inserts (TCIs). For harder formations, fewer and smaller teeth facilitate larger and stronger bearings that can withstand the higher forces that cause failure.

2.4.2.6 Operating Requirements

Hard and abrasive formations require a higher force or weight on bit (WOB) to be applied to the bit. The greater weight impacts the bearings so a corresponding lower RPM is applied to minimize bearing wear.

To prevent impact failure or cracking of insert cutters, the WOB required is slightly lower for an equivalent TCI bit.

Softer formations require lower weight on bit to achieve penetration, therefore higher RPM can be applied.

Similar parameters are required for tooth and TCI bits. Too much weight being applied could break the longer teeth or inserts.

Generally, rate of penetration (ROP) is faster with more weight applied to the bit and/or higher RPM, but too much weight can have detrimental effects such as bit balling in softer formations, failure of roller bearings, seizure of journal bearings, and breakage of teeth or inserts.