1. A bit is a drilling tools used to create cylindrical holes. A Bit is held by in a drill which rotate the bit to create cylindrical holes on materials. Polycrystalline Diamond Compact is a type of Bit which is an alloy of different metallic materials including Diamond. There are a number of selection parameters to consider in developing of drill bit material. These factors include material to be drilled, equipment to be used together with the bit, intensity of the drill and objective of drill intended. Technical requirements together with the kind of coolant to use will also be considered.
The hardness or softness of the materials to be drilled plays a key role in selecting material type for drill bit. If the material to be drilled is hard, then it is recommended to use a metal bonded diamond drill. If the drilling is for softer material, nickel bonded diamond drill will be adequate. This also has a cost effect with metal bonded diamond being more costly nickel bonded diamond. Material hardness is determined by the ability of the material to resist deformation - the higher the resistance, the harder the material. Also, the cost of the material to be drilled determines the drill bit composition and so are the material thickness, geometry and density Jayanta (2007, p. 505).
Equipment to be used with the drill bit determines what type of material make the bit. The condition of the equipment will determine the Rotation per Minute (RPM) as well as the feed time factor. Equipments which yield high rotation will normally go hand in hand with drill bit for softer materials and the vice versa.
The intensity of drilling will also determine selection of the bit material. If the drill is targeting industrial applications, the intensity of drilling will be higher. According to de la Vergne (2003, p. 4 -12) research drilling also vary with intensity. It could be a long time venture or short time one to collect sample materials. Metal bonded diamond bits will be economical in the long run if the drilling activity is prolonged.
2. Compressive strength is the maximum force which can be applied to a given rock without breaking it. The unit used to measure such strength is noted in Pound per square inch or Newton per square meter. Where 1 Newton is equivalent of 1 Kg-m/s2 and it gotten by multiplying the mass by the gravity force, 9.81m/s2. Bailey Perrot (1974, p. 117 – 128). Compressive strength is gotten by dividing the force over the area it is acting upon and denoted with ‘s’. The stress formula is given by
s = P/A,
Where P equals Force
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For example, granite has compressive strength of about 200×106N/m2. If the force applied to a given volume of granite is below this strength, then this force can not break it. Force which is applied on to a rock must be such that it’s higher than compressive strength of the rock so as to penetrate it. The bit should be able to withstand the weight exerted upon it during penetration. This will determine the drillability of the material to be drilled. The force to apply on each type of material is informed by its compressive strength and he bit type to support it. Hawk, Wilson (2001, p. 1331–1370). Also, the rotational rate of the drill and bit design will determine the rate of penetration (ROP).
List of components used in drill bit assembly and their mechanical properties
3. Drill bits comprises of both electrical and mechanical components. The Mechanical subsystem is made up of components whose goal is to translate, transfer or apply forces. The mechanical subsystem components includes, drill chuck, gear, and (ball) bearings. Hartman (1992, p. 3).
Drill chuck is a three jaw attachment that applies radial holding grip to different size drill bit and shafts. The maximum number of drill bit and shafts which can be hold for a particular drill depends on the drill chuck size. Drill chuck is attached to the drill shaft with a left-handed thread fastener. The fastener is turned in anti-clockwise direction during installation. The chuck turns in a clockwise manner to give the fastener a continuous tightening during use. Stuart (1992, p. 65)
The three-jaw configuration is designed to give axial alignment between the chuck rotational axis and centre of the bit. The continuous tightening of the attachment fastener and drill bit are important features in ensuring the bit does not loosen out during operation. The other mechanical components are gears. According to Stuart (1992, p. 65) this is the intermediary transmission between motor and the chuck. The transmission consists of gear pairs. It transfers the mechanical motion and energy from the motor to the chuck. Gears are mounted in the shaft and are categorized according to tooth forms, the arrangement of the shaft and quality. The arrangement of the shaft determine the direction of the input and the output motion
Rolling bearings are the other type of mechanical components found in drill bit assembly. Translational or rotational motion of one component relative to another, involve a resistance which range according to the type of material in contact, the mating surface and the character of the movement taking place. Richard (1983, p. 33)
To overcome the friction, energy is required. The more the friction the more heat is produced and more energy expended. Rolling bearings can either be ball or roller bearing or consist of two rings with a set rolling elements running in their truck. Richard (1983, p. 33) notes that the bearing is able to mitigate the effect of friction and minimal power use and for optimal performance.
Various forces and effects acting in bits during drilling operations with free body diagram
4. Axial force is a force which exists parallel to the drill centerline. It is the force due on major cutting edge. Axial force is higher at the drill center as opposed to the peripheral parts of the cutting edge. Bailey, Perrot, (2000, p. 89). Torque force is the amount of force exerted to rotate a drill and cut a hole in materials. This force need to be enables the drill bit to rotate and cut a hole through the torsion strength. The torsion strength being the force to rotate a drill to handle the resistance of the material drilled. One way to ensure a drill has a high torque force is to equip it with large motors. This increases the torsion strength of the drill. Bailey, Perrot, (2000, p. 89)
Drilling specific energy which can be defined as the amount of energy required to remove a unit of rock. This equation derived from consideration mechanical energy used by the bit during operation. The equation for specific energy is;
SE= (20WN/DF) t
SE=Drilling specific energy (MJ/m3)
W= weight on bit (kg)
N= Rotating speed (rpm)
D= the hole diameter (mm)
F = Footage (ft)
t = Rotating time (min)
Bailey, Perrot, (2000, p. 116) observes that, being the amount of energy that enables a bit to drill on a rock, it’s a key measure of efficiency and condition of a bit condition. Unlike penetration rate, drilling specific energy is not sensitive to change in weight and rotary speed. This attributes make it practical in bit selection (Bailey, Perro, 2000, p. 116).
Explain how the speed of the drill bit related with rate of penetration. Explain optimum conditions followed in oil rig operations
5. Rate of penetration (ROP) is the rate at which the drill bit breaks the rock being drill to progressively deepen the hole. It is normally measured in feet per minute or meters per hour. Drill bit speed is the number of rotation the bit makes per a given time, mostly measured in minute Todd, Robert ,Allen, Dell, Alting (1994, p. 43 – 48)
Feed would normally be proportional to the ROP, but this is normally not the case. The drill bit speed depends on a number of factors. This include; type of material been drilled and its hardness, the size of the hole, the type or how hard the bit is as well as its sharpness among others Todd, Robert ,Allen, Dell, Alting (1994, p. 43 – 48). Thus, hard rock will necessitate lower speed rate than soft rocks. When high speed is employed, the rate of tear of the bit also goes high especially if no coolant is applied. Though high speed dictates ROP, the bit will wear out faster.
The optimum conditions in an oil rig operation will first be set by the type of the drill bit used in respect with the types of rock to be drilled. The Crystalline Diamond Compact bit will be the best choice for the bit. This is because diamond drills are designed with a diamond tip that grinds in to hard materials. The life of the diamond bit will be determined by the hardness, abrasiveness and compressive strength of the material to be drilled. It will also depend on the drilling technique used. This include, speed, pressure and lubrication Todd, Robert ,Allen, Dell, Alting (1994, p. 43 – 48). When the type of rock found in the drill rig is determined, the best speed will be identified from a conventional table of materials and corresponding speed. The initial speed however should be 75% of the recommended speed for the particular rock. This can be gradually be increased if the bit shows no sign of wear up to the recommended speed. For a longer life of the bit, adequate coolant should be applied to prevent the bit wear. Different materials have difference hardness and abrasiveness. Hardness of material is measured using Knoop Hardness Scale (kg/mm2). Diamond is the hardest material known with a Knoop measurement of 7,000kg/mm2. Tungsten Carbide is the next hardest natural material next to Diamond with 2,000kg/mm2. This makes Diamond the ideal material for drill bit to use on hard rock and materials Todd, Robert ,Allen, Dell, Alting (1994, p. 43 – 48)
Diamond speeds differ depending on manufacturer and the drill type diamond. For instance Glastar Blunt Nose Diamond Drills can be used at a higher speed. Diamond Sure Diamond Core Drill Bits designed to drill glass, stone and tiles are used at slow speed. The speed is even slower when the hardness and abrasiveness of the material increases. The circumference of the bit increases as the diameter of the bit become larger. The drill RPM speed should be decreased to mitigate the high speed at which the outside edge is rotating for optimum results. Thus, if the circumference of the drill bit is large, the speed should be lower and vice versa Todd, Robert ,Allen, Dell, Alting (1994, p. 43 – 48)
Drill pressure is another factor to consideration in determining for optimal drilling operation oil rig. Speed and increased pressure increases the cutting time but increases friction and reduces the life of the drill bit. The pressure should be calibrated in relation to material hardness and the speed of drilling. If Diamond drill bit turns brown, yellow or black color, it is an indication of high speed and pressure on the bit. When drilling hard materials such as ones found in oil rigs, the pressure applied should little to moderate to avoid heat build up and bit wear and tear. High pressure and speed does not only cause overheating of the bit. It also can cause heat up of the surrounding surface causing heat fractures and breakage to occur. Todd, Robert ,Allen, Dell, Alting (1994, p. 43 – 48)
Lubricant is also a key consideration in achieving optimum drill optimization. Water or coolant is used as both as lubricant and cool the drill bit. The lubrication reduces the heat build up and helps in prolonging the life of the bit. Water is the most effective and cheap coolant. Oil lubricants are both expensive and reduce life span of Diamond bit. There are various method and techniques of apply lubricant in a drill bit. Each method differ rent from the other considerably. The best method however, is the one which will ensure the bit get enough lubrication for a long life of the bit. Thus, in oil rig, the best lubricant and coolant to use is water for optimal results
Todd, Robert ,Allen, Dell, Alting (1994, p. 43 – 48)
For optimum operations on oil rig the above factors have to be at play. Reduced drill speed to match the hardness of the rock. The drill pressure should be reduced since most of the oil rig fields have most of the hard rocks. Also to be taken care of increase use of water lubrication. When these considerations are factored in drill operation, the drill bit life will be extended considerably. The end cost factor will also be checked to moderate and the optimum operation will have been achieved.
6. How stress concentration factor is significant in design of drill bits. Also explain whether geometry of bit plays any role in ROP Todd, Robert ,Allen, Dell, Alting (1994, p. 43 – 48)
Stress concentration is the location in an object where stress is concentrated. An object is strongest when the whole body of the object is intact making the force acting on it to be evenly distributed. When cracks or holes develop in an object, the area of the object is reduced making resulting in such a force to increase the stress in the reduced area Gillespie, Laroux (2008, p. 78–79).
This can result in material falling as a results stress concentration exceeding cohesive strength. Stress concentration can be increased by cracks, holes or contaminants (foreign particles). It is also caused by geometric discontinuities like sharp corners and cross sectional area of the object. The stress factor is highly concentrated in a crack. Drilling a hole in the crack will reduce the SCF by making it brunt. This is a solution to the SCF Gillespie, Laroux (2008, p. 78–79).
In designing of drill bit, the consideration of SCF is taken into account. As mention earlier, most rock have natural crack, sharp bends and more important, impurities which helps in increase in SCF in a given rock mass area. When the SCF is high, the likelihood of the rock breaking is high. This is because the drill bit will tend to deviate towards the area of the rock where the SCF is highly concentrated during drilling. This will cause bending of the bit in due course. Such bending will ultimately affect operations and the bit will be damaged Todd, Allen, Alting, (1994, p. 43 - 48).
To help in mitigating this foreseen challenge, the length of the bit should be determined by its diameter. A long bit with a little circumference will not last long. When designing drill bit, the bit has to be designed in such a way as to give more stress concentration around the tip. This can be achieved by designing the bit with glove cuts at the tip Todd, Allen, Alting, (1994, p. 43 - 48).
Bit geometry and penetration force
According to new studies, penetration forces increases with the increasing size of bit tips. It also show cutting force decreases with decrease in bit tip size. Important characteristics of bit design are to consume less energy during penetration and to maximize cutting force. Drill bit with large size bit tip both have a higher penetration rate and cutting force Gillespie (2008, p. 78 – 79). Alternatively, bit with small bit tip have a higher resultant force. This is as a result of dissipating energy in grinding materials. Thus, small bit tip tends to consume more energy than large bit tips. The cutting process forces required by a bit can also be reduced by cutting tip gloves. This is because higher stress concentration develops on the rock by these ridges. Consumption of specific energy is higher in small bit tip than large ones Gillespie (2008, p. 78 – 79). Thus two major parameters affecting performance of cutting tools are streamlining the bit tip and the bit size. The streamlining of the bit tip not only requires less cutting force, but limit cutting and abrasion of the rock by the bit. The sizes of the bit tip transmit higher fracture intensity on rock underneath the bit, hence higher ROP with less required force and less specific energy Gillespie (2008, p. 78 – 79).
Rate of Penetration (ROP) is the rate at which the drill bit have to breaks the rock that is under it in order to deepen the drill hole. When drilling in hard rock area, the greatest challenges with the drill bits are impact damage, damage by heat and wear resulting from abrasive impact. All this can have negative impact on the bit performance. It’s the aim of every bit manufacturer to develop bits which can be able to counter the above challenge and give optimum results. This also has to do with cost. Drill bit geometry should be such that optimum results are achieved with least cost. Thus, as it has been noted, higher ROP can be achieved through the right bit design Gillespie (2008, p. 78 – 79).
Explain the power transmission mechanism used for rotating the drill bits. Also explain the limitations industry face on power transmission to high depth conditions
7. The principal drilling methods used to do mining are mechanical one which a drill drives cutting tools in to rocks by using static or dynamic force. For small scale drilling undertaking, percussion rock drillings are the most used equipments. Rotary crushing drills or down the hole drills (DHDs), are the mostly employed in large scale surface mine. When drilling in rock, the drill bit is driven to the rock by means rotation. The essential components of a drilling system are the rock drill, the feed equipment, drilling rods, bit, support against drilling reactions, power source and cuttings disposal equipment Hawk, Wilson (2001, p. 1331–1370)
Various drilling model set ups have varying components though the power or the rotating system is found in all. In rotary rig for instance, the engines are the main source of powering the rotary table and raise and lower equipment in the well. The drilling lines wound around a reel in a draw works. The engines are also connected to the draw work and let the drilling line to freely run in or out. The drilling line goes over a pulley at the top of the derrick/mast down to another pulley called the traveling block. The rotating system is used to cut the hole. Suspended on the hook below the traveling block is the swivel. The swivel allows the drilling line to rotate while connected to the traveling block. The end result is the rotation of the cones. This in turn rotates the bit Hawk, Wilson (2001, p. 1331–1370)
Explain various failure modes in drill bits during operations. List out major failure occur to bit during drilling in Middle East environment.
8Failure mode is a result of failure mechanism. In drilling, it’s important to identify all potential failure modes for the associated function, components, or system. The information obtained is used in system failure effect analysis. In many instance, a component or system has more than one failure modes Hawk, Wilson (2001, p. 1331–1370)
A drill bit can fail in different situations. The include chisel edge wear, outer corner wear, flank wear, margin wear, breakage, crater wear, chipping at lip, drill asymmetry and lip height differences. Each mode produces different signature modes Hawk, Wilson (2001, p. 1331–1370)
Chisel edge wear compare to flawless bit, a bit with a chisel edge wear increase the average thrust and literal vibration at the entrance. The thrust to torque ratio for a drill bit with chisel edge wear is larger than a normal new drill bit Todd, Allen, Alting, (1994, p. 43 - 48). Flank wear increases the thrust and moderate increase in torque. Flank wear does not cause thrust variation and literal vibration Todd, Allen, Alting, (1994, p. 43 - 48).
Crater wear increase torque but not thrust. Crater wear cause thrust to torque to be smaller than for new drill bit Todd, Allen, Alting, (1994, p. 43 - 48). Corner wear. A drill bit with corner wear increases both thrust and torque. It raises torque more than the thrust as functions on the outside radius making thrust to torque ration to be higher than in a new drill bit Todd, Allen, Alting, (1994, p. 43 - 48). Margin wear have no much effect on thrust and torque. The margin edge of the drill bit come into contact with drill hole surface. Margin wear has a bigger impact on the hole quality.
One of the signs of marginal wear is vibration which occurs at the final stages of the drilling process Todd, Allen, Alting, (1994, p. 43 - 48).
Chipping at lips increases torque and thrust moderately depending on chip size, shape, and location. The increase thrust fluctuation results from unbalanced cut Todd, Allen, Alting, (1994, p. 43 - 48). Drill asymmetry is as result of unbalance wear or bend axis of the drill bit. There is increase in vibration in the axial direction and literal vibration through all stage of the drilling process Todd, Allen, Alting, (1994, p. 43 - 48).
Breakage: drill bit breakage causes changes the drill cutting geometry and causes disruptive sensor signals changing the torque, literal vibration, and thrust average fluctuation Todd, Allen, Alting, (1994, p. 43 - 48).
Lip height difference: this is caused by poor grinding. The signature for lip height difference is the thrust fluctuation Todd, Allen, Alting, (1994, p. 43 - 48).
Major modes of drill bit failure in the Middle East include breakage, chisel edge wear, and flank wear. The greatest cause of failure is due to rock formations in the region. The oil rig rock formation has some of the hardest rocks known. de la Vergne (2003, p. 54).
9. Collect few post failure pictures of drill bits and highlight the failure point. Explain the causes of failure on the collected pictures.
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