TCR BITS

Cutting Mechanism-Two types of drilling action take place at the bit.(a). CRUSHING ACTION takes place when weight applied to the bit forces inserts (or teeth) into the formation being drilled.(b). SKIDDING, GOUGING type of action results partly because the designed axis of cone rotation is slightly angled to the axis of bit rotation.

JOURNAL ANGLE is defined as an angle formed between axis of the journal and the horizontal.

It optimizes bit insert/tooth penetration into the formation being drilled; generally, bits with relatively small journal angles are best suited for drilling in softer formations, and those with larger angles perform best in harder formations.

FORMATION TYPE	JOURNAL ANGLE	REMARKS
SOFT	33°	Low angle allows a prominent cutter action and permits greater tooth depth.
MEDIUM	34° to 36°	Decreases cutter action
HARD	39°	Minimizes cutter action

The Journal Angle specifies the outside contour of the bit. The use of an Oversize angle increases the diameter of the cone and is most suitable for the soft formation bits. Although, this increases cone size, the gauge tip must be brought inwards to ensure the bit drills a gauge hole.

INTERFITTING TEETH

An important factor which affects the Journal Angle is the degree of meshing or interfit. The amount of interfit affects several aspects of bit design:

1.   It allows increased space for tooth depth, more space for bearings and greater cone thickness.
2.     Allows mechanical cleaning of bit, thus preventing bit-balling.
3.     It helps cutting action of the cones by increasing cone-slippage.

   

    In some formations, it is advantageous to design the cones and their configuration so that they do not rotate evenly but that they slip during rotation. This is called Cone slippage, it allows a rock bit to drill using a scraping action, as well as the normal grinding or crushing action.

Cone slippage can be designed into the bit in two ways. Since cones have two profiles: the inner and the outer cone profile, a cone removed from the bit and placed on a horizontal surface can take up two positions. It may either roll about the heel cone or the nose cone. When the cone is mounted on a journal it is forced to rotate around the center of the bit. This “unnatural” turning motion forces the inner cone to scrape and the outer cone to gouge. Gouging and scraping help to break up the rock in a soft formation but are not so effective in harder formations, where teeth wear is excessive.

Cone slippage can also be attained by offsetting the axes of the cones.

CONE OFFSET is defined as the horizontal distance between the axis of a bit and the vertical plane through the axis of its journal.

To increase the skidding-gouging action, bit designers generate additional working force by offsetting the center lines of the cones so that they do not intersect at a common point on the bit. 

Offset forces a cone to turn within the limits of the hole rather than on its own axis. Offset is established by moving the center line of a cone away from the center line of the bit in such a way that a vertical plane through the cone center line is parallel to the vertical center line of the bit.

The Skew Point is an arbitrary point along the journal center line and is the angle formed by the offset, the center line of the journal, and a line from the bit center to the skew point.

The skew direction is always positive, or in the direction of the rotation. This permits the tip of the teeth to ream the hole to full gauge. Negative skew would have the gauge face rubbing the hole wall, increasing gauge wear.

FORMATION TYPE	CONE OFFSET	REMARKS
SOFT	Maximum offset (3° Skew Angle)	Increases gouging, scraping action
MEDIUM	Limited offset (2° Skew Angle)	Limited cutter action
HARD	No offset (0° Skew Angle)	Minimized gouging, scraping action

Basic cone geometry is directly affected by increases or decreases in either journal or offset angles, and a change in one of the two requires a compensating change in the other. Skidding-gouging improves penetration in soft and medium formations at the expense of increased insert or tooth wear. In abrasive formations, offset can reduce cutting structure service life to an impractical level. Bit designers thus limit the use of offset so that results just meet requirements for formation penetration.

STEEL TOOTH CUTTING STRUCTURES
FORMATION	TOOTH PROPERTIES	TOOTH ANGLE
SOFT formation cutting structure	Few in number, widely spaced, and placed in a few broad rows. Slender and small tooth angles.	39° to 42°
MEDIUM formation cutting structure	Fairly numerous, with moderate spacing and depth. Teeth’s are strong, and are a compromise between hard and soft bits	43° to 46°
HARD formation cutting structure	Many teeth’s are present. They are closely spaced and are short and blunt. There are many narrow rows with large tooth angles	46° to 50°

TUNGSTEN CARBIDE CUTTING STRUCTURE

Earlier many people in the oil field thought that chisel shaped teeth significantly affected the drill rate in all formations. This was because early drilling practices used light bit weights but, today with the heavier bit weights, it tends to nullify the advantage of the chisel shape. Even the steel milled teeth break down under heavy weights. With this in mind, many “blunt” insert tooth designs were made, and seem to drill efficiently. Nowadays, most insert teeth have this blunt, conical shape.

CUTTING STRUCTURE W.R.T FORMATION

Soft formation bits require deep penetration into the rock so the teeth are long, thin and widely spaced to prevent bit balling.

Bit balling occurs when soft formations are drilled and the soft material accumulates on the surface of the bit preventing the teeth from penetrating the rock.

The long teeth take up space, so the bearing size must be reduced. This is acceptable since the loading should not be excessive in soft formations.

Moderately hard formation bits are required to withstand heavier loads so tooth height is decreased, and tooth width increased.

Such bits rely on scraping/gouging action with only limited penetration. The spacing of teeth must still be sufficient to allow good cleaning.

Hard formation bits rely on a chipping action and not on tooth penetration to drill, so the teeth are short and stubbier than those used for softer formations.

The teeth must be strong enough to withstand the crushing/chipping action and sufficient numbers of teeth should be used to reduce the unit load.

Spacing of teeth is less critical since ROP is reduced and the cuttings tend to be smaller.

The cutting structure for insert bits follows the same pattern as for milled tooth bits.

Long chisel shaped inserts are required for soft formations, while short ovide shaped inserts are used in hard formation bits. Tungsten carbide hardfacing is applied to the teeth of soft formation bits to increase resistance to the scraping and gouging action. Hard formation bits have little or no hardfacing on the teeth, but hardfacing is applied to the outer surface (gauge) of the bit.

GAUGE PROTECTION

Protection of the gauge surface is vital to the effectiveness of any bit. The gauge surfaces constantly ream the hole, and thus are subject to continuous abrasive wear.

Applying tungsten carbide in a steel matrix through a welding process, called Hardfacing, provides the best resistance to this type of wear. Gauge protection is improved as the amount of hardfaced surface area increases.

If the outer edge of the cutting structure is not protected by tungsten carbide hardfacing two problems may occur.

1. The outer surface of the bit will be eroded by the abrasive formation so that the hole diameter will decrease. This undergauge section of the hole will have to be reamed out by the next bit, thus wasting valuable drilling time.
2. If the gauge area is worn away it causes a redistribution of thrust forces throughout the bearing assembly, leading to possible bit failure and leaving junk in the hole (e.g. lost cones)

BEARING SYSTEMS

OPEN BEARING SYSTEMS

Non-sealed roller bearings, referred to as open bearing systems, are typically used in large-diameter (> 20 in.) bits. These bits are often used to drill from surface to relatively shallow depths with a simple drilling fluid system. This drilling application does not necessitate the use of seals in the bits. They rely on the drilling fluid for cooling, cleaning, and lubrication of the bearings.

SEALED ROLLER BEARING SYSTEMS have a sealed grease reservoir to lubricate the bearings. The bearing system is composed of:

1) A roller-ball-friction or roller-ball-roller bearings;

2) The seal, which retains the lubricant and prevents drilling fluid and abrasive cuttings from entering the bearing cavities;

3) The shirttail is designed and hardfaced to protect the seal;

4) A lubricant, an elasto-hydrodynamic type, is used to ensure minimum friction and wear;

5) The reservoir, which stores and supplies the lubricant to the bearings;

6) The vented breather plug, which transfers downhole fluid pressure against the lubricant-filled flexible diaphragm to equalize pressures surrounding the bearing seal.

Seals contribute significantly to the effectiveness of the lubrication and pressure compensation system by preventing drilling contaminants from entering the bearings.

Non-sealed bearing designs allow mud to enter the bearing to cool and lubricate, but suffer shorter bearing life than sealed bearings.

JOURNAL BEARING SYSTEMS

The primary cause of roller bearing failure is Journal Spalling, which causes destruction of the rollers and the locking of the cone.

To remedy this, the “Journal Bearing” is used. In this system cones are mounted directly on to the journal. This offers a distinct mechanical advantage over roller arrangements, in that it presents a larger contact area at the load bearing point. This distribution of the load eliminates the chief cause of roller bearing assembly failure i.e. spalling in the load portion of the bearing face.

Journal bearings consist of at least one rotating surface separated from the journal by a film of lubricant. The surfaces are specially designed so that the film of lubricant separates them; were they to touch, mating bearing components would gall or possibly fuse. As long as satisfactory lubrication is provided and loading remains within design limits, journal bearings are extremely efficient.

CUT AWAY OF JOURNAL BEARING

Journal bearing systems in the tungsten carbide insert bits features a metal bearing surface combined with a hardfaced journal and a lubricant. Specialized seals and reliable pressure equalization systems keeps the drilling fluid and formation contaminants out of bearings, and positively seals the graphite-based lubricant inside the bearing. Precision fit of the journal and cone distributes contact loading evenly throughout a near perfect arc. Bearing surfaces are finished to a carefully controlled surface texture to ensure optimum lubrication.

BEARING SEALS

Roller-cone bearing seals operate in an exceptionally harsh environment. Drilling mud and most cuttings are extremely abrasive, high operating temperatures can break down the elastomers from which seals are made and pressure pulses often occur in downhole drilling fluids that apply lateral loading on seals that must be resisted.

Bearing seals perform two functions:

1. On the interior side, the seal is excluding clean, functional lubricant from escaping the bit,
2. On the exterior side, the seal is excluding dirt and chemicals from penetrating the bit. 

The first and still most popular seal is the radial seal (used mainly on the sealed roller bearing bits). The radial seal is a circular steel spring encased in rubber, which seals against the face of the shank and the face of the cone.

The newer “O” ring seal is considered the most effective seal. The major problem confronting the “O” ring is tolerance, which must be precise in order to maintain an effective seal. An O-ring is installed in a seal gland to form a seal system. The gland holds the O-ring in place and is sized so that the O-ring is compressed between the gland and the bearing hub at which sealing is required.

GOOD PRACTICES

Adequate cleaning is even more important with sealed bearing bits. If drilled cuttings are allowed to build up around the shirttail, seal damage and premature bearing failure may result.

Gauge protection is also important to seal and bearing life, because seal damage can occur from shirttail wear caused by inadequate gauge protection.

Any time a sealed bearing bit is re-run, the seals and shirttail should be carefully checked for excessive wear or grooving.

LUBRICATION SYSTEM & LUBRICANTS Lubricants play a vital role in bearing performance. They provide Lubrication for both bearings and seals, A medium for heat transfer away from the bearings. To achieve these functions, lubricants are specially engineered and continually improved. Lubrication systems are engineered to provide reserve storage, positive delivery to the bearing system, capacity for thermal expansion, and pressure equalization with fluids on the bit exterior. The Lubrication System consists of a GREASE RESERVOIR large enough to ensure availability of lubricant for all lubrication functions throughout the life of the bit. Roller-cone bits typically contain one lubricant reservoir in each leg. Thus, for a three-cone bit, there are three reservoirs. A small positive pressure differential in the system ensures flow from reservoir to bearings. PRESSURE EQUALIZATION & PRESSURE RELIEF VALVE At installation, lubricant is at atmospheric pressure and cannot provide significant resistance to well-bottom pressures. Accordingly, internal lubrication system pressures equalize themselves with external bit pressures resulting, from a column of drilling fluids and cuttings contained in a well to prevent seal failure caused by differential pressure. Equalization is accomplished by a small relief valve installed in the lubricant reservoir system. It also releases any trapped pressure within the lubricating system due to sudden downhole pressure and temperature changes, which might otherwise rupture the seals. MATERIAL REQUIREMENTS The rock bit must be stronger than the rock it is to drill. The measurement of hard steel is measured on the Rockwell hardness tester scale (Rc). The tester uses a diamond pyramid indenter with a load of 150 kilograms. The deeper the indentation in the steel, the softer it is. The degree of hardness that can be produced in steel is determined by its carbon content, the higher the percentage of carbon (up to 0.7%), the harder the steel. By heat treating properly, it can be made up to about 65 Rc. Alloying elements improve the hardening potential in thick sections and cause the steel to have a more uniform response to heat treating. The steel must also be ductile (resistance to crack propagation). This ductility or toughness of metals is inversely related to hardness (the harder a metal, the less ductile. Softer the steel, the more ductile). Alloying elements improve the ductility of steels and toughness, and resistance to failure from impact loads. HEAT TREATMENT The desired metallurgical properties and physical strengths are developed through heat treating. As mentioned above, the strength is improved by increasing the carbon content at the surface by CARBONIZING, commonly known as CASE HARDENING.  This is essential for the teeth on milled tooth bits, and necessary for strength and wear resistance on the bearing surfaces. Toughness (resistance to impact and crack propagation) is attained by leaving the inner part or the core steel unchanged. The overall physical properties that are needed (strength and toughness) are achieved by heating the parts to a high temperature, then quenching them in oil. The maximum surface hardness of the carbonized section gets about 60 - 64 Rc (the hardness of a file). The core hardness will be about 25- 40 Rc, remaining tough and ductile. (Carburizing, also referred to as Case Hardening, is a heat treatment process applied to low carbon steel, as well as high alloy steel bearings etc that produces a surface which is resistant to wear, while maintaining toughness and strength of the core. Carburizing increases strength and wear resistance by diffusing carbon into the surface of the steel creating a case while retaining a substantially lesser hardness in the core.) Mill Tooth Bit Teeth The teeth on a mill tooth bit are sometimes hard-faced using tungsten carbide. This hard-facing can be on the gauge teeth (for hard formations), the inner teeth (for soft formations), or on both rows. Hard-facing is applied in such a way so that, as the teeth dull, the hard-facing causes a self-sharpening of the tooth. IADC CLASSIFICATION OF TCR BITS

CUTTING STRUCTURE SERIES			FORMATION TYPE	BEARING/GAUGE DESCRIPTION		ADDITIONAL FEATURES (OPTIONAL)
STEEL TOOTH BITS	1	Soft formations with Low Compressive Strength and High Drillability	1 refers to the Softest Formation in the series and 4 refers to the Hardest Formation in the series.	1	Standard Open Roller Bearing	A – Air Application B – Special Bearing Seal C – Centre Jet D – Deviation Control E – Extended Nozzles G – Gauge/Body       Protection H – Horizontal Steering       Application J – Jet Deflection L – Lug Pads M – Motor Application S – Standard Steel Tooth T – Two Cone Bit W – Enhanced Cutting       Structure X – Predominantly       Chisel Y – Conical Tooth Insert Z – Other Shape Insert
	2	Medium to Medium Hard Formations with High Compressive Strengths		2	Standard Open Roller Bearing, Air Cooled
	3	Hard Semi-Abrasive and Abrasive Formations		3	Standard Open Roller Bearing, Gauge Protection
INSERT BITS	4	Soft formations with Low Compressive Strength and High Drillability		4	Sealed Roller Bearing
	5	Soft to Medium Formations with Low Compressive Strengths		5	Sealed Roller Bearing, Gauge protection
	6	Medium Hard Formations with High Compressive Strength		6	Sealed Friction Bearing
	7	Hard Semi-Abrasive and Abrasive Formations		7	Sealed Friction Bearing, Gauge Protection
	8	Extremely Hard and Abrasive Formations

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