DRILLING BITS (EN: drilling bit, bore bit; DE: Bohrmeihßel; FR: trépan, outil de forage; ES: broca, trepano; RU: буровые долота) are the major element of the drilling tool for the mechanical destruction of the rock during the process of the drilling of the borehole. The term "drilling bit" has preserved itself since the early time of the development for the technique of the drilling, when the single method for the penetration of the borehole was the percussive drilling, during which the drilling bit was similar to the carpentry tool with the same name. According to the purpose, they distinguish 3 classes of the drilling bits: for the continuous drilling (the destruction of the rock throughout the entire bottomhole within the borehole), for the coring drilling (the destruction of the rock throughout the annulus of the bottomhole within the borehole, with the leaving of the core within the central part of the borehole), and for the special purposes (the cutting bits, the reamers, the milling cutters, and others). According to the character of the impact onto the rocks, the drilling bits are sub-divided into 4 classes: crushing, crushing-shearing, wearing-cutting, and cutting-shearing. According to the shape of the working part (which is destructing the rocks), they distinguish the roller-cone and bladed drilling bits.
There is performed by the roller-cone drilling bits the greater part of the total quantity of the drilling for the petroleum, gas, and blasting boreholes. The roller-cone drilling bit (or the drilling bit for the coring drilling) comprises (one, two, three, four, or six conical) spherical or cylindrical roller-cones, which are mounted onto the rolling or sliding bearings (or onto their combination) on the trunnions of the sections of the drilling bit. The major variety of the roller-cone bits for the continuous drilling is the drilling bit with the triple roller-cone (see the Figure # 1, a), while during the drilling of the deep boreholes, there has also got the distribution the drilling bit with the single roller-cone (see the Figure # 1, b).
Depending on the structure of the casing, they sub-divide the roller-cone drilling bits into the sectional and whole-casing types. Within the sectional drilling bits, the casing is welded together of the individual (two, three, or four) sections (legs), on the trunnions of which there are mounted the roller-cones; within the whole-casing roller-cones, the casing is made of the cast metal, and there are welded to the casing the legs with the roller-cones, which are mounted onto the trunnions of the legs. For the attaching of the drilling bits to the drilling string, there is provided the external conical screw thread (nipple) for the sectional roller-cone bits, while there is provided the internal conical screw thread (coupling) for the whole-casing drilling bits. Within the USSR, there are issued 13 types of the roller-cone drilling bits for the continuous drilling, with the diameters of 46-508 millimetres (according to the GOST 20692-75 governmental standard).
According to the principle of the impact onto the rocks, the roller-cone drilling bits are sub-divided into the crushing and crushing-shearing types. The drilling bits of the crushing action are characterized by the minimal sliding of the teeth during the rolling of the roller-cones around the bottomhole, and by the absence of the milling action onto the wall of the borehole by the peripheral teeth; they distinguish the following types of them: T - for the drilling of the hard rocks, TZ - of the hard abrasive rocks, TK - of the hard rocks with the streaks of the strong rocks, TKZ - of the hard strong abrasive rocks, K - of the strong rocks, OK - of the very strong rocks. The roller-cone drilling bits of the crushing-shearing action are characterized by the increase of the sliding of the teeth during the rolling of the roller-cones around the bottomhole and the wall of the borehole. The types of the drilling bits with the crushing-shearing action: M - for the drilling of the soft rocks, MZ - of the soft abrasive rocks, MS - of the soft rocks with the streaks of the rocks with the medium hardness, MSZ - of the soft abrasive rocks with the streaks of the rocks with the medium hardness, S - of the rocks with the medium hardness, SZ - of the abrasive rocks with the medium hardness, ST - of the rocks with the medium hardness with the streaks of the hard rocks. There serve as the rock-destructing element (armament) for the roller-cone drilling bits the milled teeth, or the hard-alloy toothlets, which are pressed into the drilling bit, and the combinations of the teeth with the toothlets on the surface of the roller-cones. In order to improve the wear resistance of the milled teeth of the roller-cones against the abrasive wear, they weld onto these teeth the hard alloy, which is composed of the grains of the tungsten carbides. For the decrease of the wear of the drilling bit along the diameter, the peripheral crowns of the drilling bits of the S, ST, and T types have the L-like or T-like shape. The geometrical shape and the parameters of the teeth (height, length, pitch, and also the offset of the axes of the roller-cones) are different (they decrease from the M type to the T type) and depend on the physical properties of the rocks, which are drilled through. The modern armament of the roller-cones for the drilling bits is made of the hard alloy toothlets, which are inserted into the roller-cone, with the prismatic (the MZ, SZ, MSZ, and TZ types) and spherical (the TK type) working heads. The support of the roller-cone drilling bits, during the process of the rotation of the roller-cone, ensures the transmission of the axial load from the drilling string through the trunnions and rolling bodies to the armament of the roller-cone, which is located within the contact with the rocks of the bottomhole within the borehole. Within the supports of the drilling bits, there are used as the radial bearings, the roller, ball, and sliding bearings, there are used as the angular-thrusting bearings, the ball bearings, and there are used as the thrusting bearings, the sliding bearings. There are shown on the Figure # 2 the most known schemes for the supports, which they use within the roller-cone drilling bits. There exist within each support the locking ball bearing, which is retaining the roller-cone on the trunnion, and which is absorbing the axial component of the load onto the drilling bit.
The quantity of the rollers and balls within the support of the roller-cones, and their dimensions, depend on the size of the drilling bit, while the scheme of the support depends on the mode of the drilling. The drilling bits, which are used for the drilling with the high frequency of revolutions (more than 250 revolutions per minute), have the support with the rolling bodies without the sealing (the 1AV series); the drilling bits for the drilling with the medium frequency of revolutions (up to 250 revolutions per minute) have the support according to the scheme with the roller-ball - sliding - thrusting heel without the sealing (the 1AN series) or with the sealing with the help of the terminal lip (the 2AN series). The drilling bits for the drilling with the low frequency of revolutions (up to 60 revolutions per minute) have the sealed oil-filled support according to the scheme with the sliding - ball - sliding - thrusting heel, with the radial sealing lip. Within the drilling bits with the sealed oil-filled support, there exists within the thickened part of the leg the special tank with the grease, within which there is mounted the elastic bag, which is changing the shape during the increase of the pressure during the descent of the drilling bit into the borehole, and which is helping the displacement of the grease along the lubricating channels to the elements of the support, which are rubbing one against another. At the same time, the sealing lip must ensure the sealing of the supports at the side of the end of the roller-cone. This is achieved by the rigidity of the terminal lip, and by the tight fit of this lip to the end of the roller-cone.
For the supply of the washing liquid through the drilling bit to the bottomhole within the borehole, there exist within the roller-cone drilling bits the special washing or air blowing devices. Depending on the constructive realization, they distinguish the roller-cone drilling bits with the central or lateral washing, and also with the blowing by the air. The drilling bits with the central washing have one hole within the centre of the drilling bit, or 3 holes or slits within the casing (washing plate), through which holes the washing liquid is directed onto the roller-cones within the central part of the borehole. Within the drilling bits with the lateral washing (the hydraulic jet drilling bits, see the Figure # 3, a), the washing liquid is directed through the nozzles between the roller-cones into the peripheral zone of the bottomhole within the borehole.
Within the drilling bits with the blowing by the air (see the Figure # 3, b), gas or air-water mixture, one part of the flow, through the central hole within the casing, is supplied to the roller-cones, while the other part, through the special channels within the legs and their trunnions, arrives into the cavity within the supports of the roller-cones, for their cooling and cleaning of the drilling sludge. During the drilling of the blasting boreholes, they use within the drilling bits with the blowing by the air the reverse valves, which ensure the immediate closure of the central blowing channel within the drilling bit after the termination of the supply of the air, and thus prevent the suction of the rock particles into the cavity within the casing of the drilling bit above the entrance into the blowing channels of the legs.
The bladed drilling bits are purposed for the drilling by the rotary method of the soft rocks, and of the rocks with the medium hardness.
The bladed drilling bits (see the Figure # 4, a, b, v) comprise the forged casing with the connecting screw thread, to which there are welded 3 or more blades. For the two-bladed drilling bit, the casing and the blades are stamped as the single whole product. For the improvement of the wear resistance of the drilling bits, the blades are reinforced with the hard alloy. The plates of the hard alloy are melted onto the front face of the blades into the specially milled slots. The lateral faces of the blade (which are calibrating the wall of the borehole) are reinforced with the cylindrical toothlets (the VK8-V alloy), which are pressed into the drilled holes. The gaps between the toothlets are welded onto with the hard alloy. Within the USSR, they manufacture the bladed drilling bits (the GOST 26-02-1282-75 governmental standard) with the washing together with the cylindrical holes within the casing (the 2L type, the diameters are 76-165.1 millimetres, the speed for the movement of the washing liquid is up to 50 metres per second), and with the replaceable hydraulic jet nozzles within the casing (the 3L type, the diameters are 120.6-469.9 millimetres, the speed of the washing liquid is at least 90 metres per second). The wearing-cutting drilling bits (the ZIR type) have the diameters of 190.5-269.9 millimetres. They manufacture the pike-shaped drilling bits (the P type, the diameters are 98.4-444.5 millimetres) as the two varieties: C - for the drilling through the cement plugs, and through the metallic parts at the bottom of the casing strings; R - for the expansion of the shaft within the borehole. There also belong to the bladed drilling bits the drilling bits for the cable-churn drilling. For the drilling without the washing of the borehole, they use the auger drilling bits (see the Figure # 4, g).
For the auxiliary works (the drilling through the cement plugs, and through the metal within the borehole) there are issued the milling drilling bits: the FR type in the form of the flat-bottom milling bits, the lower working surface of which is equipped with the hard-alloy toothlets or plates, which are protruding above the casing of the drilling bits; the DFTS type with the situation of the hard-alloy toothlets along the 3 spirals, which are having the smooth transition from the central channel of the drilling bit onto the working sphere.
For the drilling of the boreholes with the retrieval of the core, they use the roller-cone and bladed drilling heads, which they manufacture for the special core-receiving devices with the removable and non-removable core-receivers. The coring drilling bits with the removable core-receiver permit to retrieve the core from the bottomhole within the borehole without the lifting of the drilling string.
They retrieve the core receiver with the core from the borehole by the slip, which is lowered into the drilling pipes by the special winch, and they lift the drilling head only after its wear together with the drilling string. In case of the work by the coring drilling bits with the non-removable core receiver, it is necessary to lift the entire drilling string for the retrieval of the core from the borehole; in this case, the heads are often not worn. The core-forming elements of the drilling bits transmit onto the core the minimal transverse forces, which fact decreases the probability for the destruction of the core; the washing channels within the drilling heads are situated so, that the jet of the washing fluid passes besides the core receiver.
They manufacture the drilling bits and the drilling heads of the sturdy and wear-resistant materials, because during the process of the drilling there act onto the drilling bit the axial and percussive loads, torque, and also the pressure and chemical activity of the washing liquid. For the sections (legs) and roller-cones of the drilling bits, they use the chromium-nickel-molybdenum, chromium-nickel, and nickel-molybdenum steels. There are issued the drilling bits and the drilling heads, which are equipped with the natural or synthetic diamonds (see the "Diamond drilling" article). They manufacture the certain types of the drilling bits of the steels, which are obtained by the electroslag and vacuum arc remelting processes.
The improvement of the drilling bits is performed in the direction to the improvement of their designs: the creation of the new schemes for the supports with the sealed oil-filled supports for the slow-rotating and fast-rotating drilling; the usage of the new shapes for the hard-alloy toothlets; the searching for the more wear-resistant materials; the improvement for the precision for the manufacturing of the parts, and for the assembly of the drilling bits, and also the usage of the more perfect schemes for the supply of the washing liquid into the bottomhole within the borehole. See also the "Drilling crown" article.
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