Introduction and Historical Perspective

Drilling and excavation are widely applied for many purposes, including making a hole in a wall, deep drilling in the search for oil and exploration of the Earth's subsurface (Clark, 1987). Most of what we have learned about early climates of planet Earth (e.g., ice ages) was obtained from cores taken by drilling through ice sheets and glaciers (e.g., Dansgaard, 2004). Increasingly, developers of drills are addressing complex challenges in extreme environments, for example penetrating the surface of planetary bodies. Drilling on the Moon is an example of such a challenge, which was successfully accomplished for the first time in 1970 by the Soviet's robotic Luna 16 lander. This success was followed with the growing efforts to drill on Mars and penetrating the surfaces of other extraterrestrial bodies and increasingly enabling scientists to investigate the history of our Solar System and understand better our own planet Earth (Zacny et al., 2008). To address the challenges presented by the large variety of materials that need to be penetrated in drilling into our own planet's surface, scientists and engineers have developed many types of drills, with mechanical drills being the most common. Mechanical drills use a bit with a tip that interacts with the drilled medium and applies cutting or breaking forces over a small area to achieve large shear and/or impact stresses. These types of bits are widely used and can be purchased at local hardware stores.

Penetrating solid objects, such as the ground, rocks and wood, has been achieved by creatures and plants in Nature ever since they first existed on Earth millions of years ago. The earthworm, termites (Figure 1.1), rodents, the woodpecker (Figure 1.2) and many others are capable of making holes and tunnels for their habitat and search for food (Bar-Cohen, 2005). Also, roots of plants have amazing capabilities to penetrate rocks and hard soil.

Since ancient times, humans have been digging through the ground and solid objects. There are many reasons to dig and, over the years, as more effective tools became available, the capability to penetrate various media increased. Some of the

Figure 1.1 Termites (see inside the top section of the cavity) make holes in wood.

Pileated Woodpecker Holeshttp://www.paulnoll.com/ Oregon/Birds/habitat-Woodpecker.html."/>
Figure 1.2 The pileated woodpecker performs "percussive" drilling to bore holes in trees to make their nest and also seek food. Courtesy Paul and Bernice Noll http://www.paulnoll.com/ Oregon/Birds/habitat-Woodpecker.html.

1.1 Introduction and Historical Perspective j 3

applications that required penetration of the ground included mining for resources, digging water wells, burying objects, supporting columns and structures and searching for food (including plant bulbs and roots). Advances in penetration tools were made as a result of discovering more effective fabrication materials, developing methods of processing and machining, increasing the capability to leverage forces and driving tools with the aid of mechanical, electrical, pneumatic and hydraulic actuators.

The use of metallic tools for the penetration of objects probably started in the Bronze Age when tools were made in the shape of an arrow that consisted of two distinct cutting edges. The use of bow drills dates back to the ancient Egyptians (3150-31 BC). As far back as 2550-2315 BC, the Egyptians may have used diamond drilling tools for the construction of the pyramids, and between 600 and 260 BC, the Chinese drilled holes up to 35 cm (14 in) in diameter to depths exceeding 600 m (~2000ft). About 1000 years ago, in 1126 AD, Carthusian monks used a percussive technique to drill for water, reaching a depth of around 300 m (1000 ft) (De Villiers, 2001).

The development of pneumatic drills dates back to Samuel Ingersoll's invention in 1871 and it made a major impact on the ability to drill. The electric drills in the past century have revolutionized our ability to penetrate tough materials on demand. The invention of the steam engine in the eighteenth century had an enormous impact on the capability to drill on large scales and with it came a surge in demand for coal to fuel steam engines. One of the most extracted materials is coal, which is also the most abundant fossil fuel on Earth: its predominant use has been for producing energy in the form of heat. In the eighteenth and nineteenth centuries, it was the most important energy source that fueled the Industrial Revolution and was a major driver in the development of tools for large-scale applications. The capability to mine hard rocks and the reduction in the cost and time required for excavation were a direct result of using mechanical drills powered by compressed air, that is, the pneumatic hammers that are known today as "jackhammers".

A rotary steam-driven drill was invented by Richard Trevithick in England in 1813 [Britannica on-line] and the first patented pneumatic drill was invented in 1849 by Jonathan Couch in Philadelphia, PA, USA (Encyclopedia Britannica, 1986). This pneumatic drill was a hammer-type drill that impacted a metal rod into rocks. This particular drill and many others, invented by Joseph W. Fowle (1849-1851) and Cave in Paris (1851), were driven by steam. Although these drills found numerous applications on the surface, they were not suitable for underground drilling, where in fact most of the drilling for mineral excavation was taking place. In the early stages, the steam machinery had to be kept close to the boiler, since lengthy hoses were not available and the steam lost its heat and pressure when transported over long distances. In 1851, Fowle patented the first use of a flexible steam hose and thus made the location at which the drills are positioned less dependent on the location of the boiler. Thus, Fowle's patent provided the basis for the design and development of modern rock drills.

In 1852, the use of compressed air for rock drilling for the creation of the Mont Cenis tunnel in the Western Alps was proposed by the physicist Colladon. The idea of using compressed air to drive the drilling tools was also utilized by the Italian engineer Germain Sommeiller and others between 1852 and 1860 (Peele, 1920) to develop new types of drills. The use of compressed air offered three major advantages that included:

  1. Low transmission losses.
  2. No safety issues related to leakage (as opposed to the case of steam, which posed a burn hazard to the operators when leaks occurred in the pipes).
  3. The "used" compressed gas provided additional ventilation in the working area (e.g., a mine shaft or a tunnel).

A notable advancement of the pneumatic drill was made in 1890 by C. H. Shaw [Britannica on-line], a machinist from Denver, CO, USA, who invented the first hammering drill with air-leg feed (Mining Magazine, 2006). A significant improvement in pneumatic drilling was made in an 1896 invention by the entrepreneur J. George Leyner from Colorado, who introduced the hollow drill bit. In this arrangement, compressed air was blown down the center of the drill and out through the hollow drill bit. The air not only removed the cuttings from the hole, which improved the drilling efficiency, but also helped to keep the drill bit and the drilled formation cool. Drilling hazards in underground mines were very common. Drillers suffered respiratory illnesses due to breathing the released fine dust generated by drilling rocks. This problem of dust was substantially reduced by J. George Leyner, who suppressed dust formation by introducing water into the drilled holes. The application of diamonds as cutting elements in drill bits was first proposed in 1869. Although diamond bits were (and still are) much more expensive than other types of bits, they provided much greater durability in cutting hard materials and this made the drilling process more effective and efficient.

The ever-increasing demand for resources led to the essentially complete exploitation of easy-to-mine areas and left more difficult areas (such as deeper mines or very remote mines in the cold Arctic regions) for the future. The gold mining industry now has the deepest mines in the world. For example, the AngloGold Ashanti gold mine Tau Tona (a Setswana word for Great Lion) outside Carletonville, South Africa, is 3.6 km deep (Figure 1.3). The mountains of tailings in the foreground of this figure are processed (crushed to powder and chemically treated) gold-bearing rock recovered from the mine. A great deal of drilling and blasting was carried out to produce these large hills. Currently, there are plans to reach as deep as 3.9 km, where more gold-bearing rocks can be found. At these great depths, rock temperatures reach 55 ° C and extensive air conditioning is required for the miners in order to cool the air temperature to the bearable level of 28 °C. The challenges of drilling to great depth under harsher conditions together with tighter economic considerations that include mining faster are increasingly raising the required capabilities of cutting tools. Therefore, there is a continuing need to improve the performance of cutting materials used at the tips of the drill bits in order to penetrate harder formations more quickly and with much greater durability.

The development of metallurgical processes to heat-treat steels for greater deformation resistance of drill rods dates back to the 1890s. This initial development led to improved hammer drills that were faster, lighter and significantly more effective.

Gold Bearing Rock Formations
Figure 1.3 Tau Tona gold mine in South Africa, at 3.6 km deep, is the world deepest mine (the shaft is visible in the background). The hills in the foreground are tailings of recovered and processed gold-bearing rock.

Today, drill rods can slide more freely in a chuck and do not deform when the piston hits their top end. The invention of the replaceable bit in 1918 by A. L. Hawkesworth, a mechanical foreman for the Anaconda Company in Butte, MT, USA, made drilling even more efficient, because now only cutting edges were replaced and not entire drill sections. After World War II, hand-held jackhammers were developed that could be flexibly attached to an air cylinder; these drills allowed miners to drill in any desired direction without the need to mount the drill on a support structure. The speed of drilling has subsequently increased and, in addition, new bits made of much harder materials such as tungsten carbide were produced. In the 1970s, hydraulic technology was applied to drive drills directly instead of being used to create the compressed air that powered the drills. Advances in technology of hydraulic drills led to the level of today's capability, having up to 2000 blows min-1 with hammering action that is much quieter than previously, with noise levels as low as 85 dB at a distance of 10 m (e.g., Altras Copco SmartRig) (Mining Magazine, 2006).

The invention of the electric rotary drill is credited to the scientist James Arnot, who developed this type of drill in 1889 while an employee of the Union Electric Company in Melbourne, Australia. He designed it primarily to penetrate rocks and coal. Six years later, the invention of the portable drill was patented by Wilhelm Fein in Germany. Another major milestone occurred in 1917, when Black & Dekker invented the trigger switch that was mounted on a pistol-grip handle of the drill (Decker and Black, 1917). Modern oilfield rotary drills were first introduced in 1884 (Maurer, 1968); but the first oil well was drilled in 1745 in France (Lee et al., 1988).

Figure 1.4 Assortment of bits from the middle of the twentieth century displayed at the Deutsches Museum in Munich, Germany. This photograph was taken by Jack Craft, Honeybee Robotics Spacecraft Mechanisms Corporation, and it is published courtesy of him. The description of each of these drill bits was translated from German by Jack Craft and it is provided with editing in the main text.

Figure 1.4 Assortment of bits from the middle of the twentieth century displayed at the Deutsches Museum in Munich, Germany. This photograph was taken by Jack Craft, Honeybee Robotics Spacecraft Mechanisms Corporation, and it is published courtesy of him. The description of each of these drill bits was translated from German by Jack Craft and it is provided with editing in the main text.

In Figure 1.4, an example is shown of an assortment of bits from the middle of the twentieth century that is displayed at the Deutsches Museum in Munich, Germany. These coring bits were designed for cutting cores out of rock. They consist of a cylindrical steel body and a cutting edge having a shape that is specifically designed for the specific rock formation hardness of rotary drills. Nowadays, diamonds are almost exclusively used at the cutting edges since they provide higher penetration rates and longer drill bit life than metal edges. The following is the list of drill bits that are shown numbered in Figure 1.4:

  1. This coring bit was used for soft rock formations and its main advantage was that the cutting structure could be restored to the original shape by merely replacing the hardened metal on the worn-out teeth. Note the four cutters that are perpendicular to the circumference of the core drill and four additional cutters that are placed along the circumference of the core drill. The function of the latter cutters was to cut out the core, whereas the former cutters were designed to excavate the annular area and to sweep the cuttings produced to the outside.
  2. This bit was designed for medium-to-hard formations. It had hardened steel teeth and slanted drilling fluid channels (also called "junk" slots) on its outside surface to allow for easy removal of drilled cuttings.
  3. This drill bit was also designed for penetrating medium-to-hard formations. This bit could be restored to its original shape by replacing the hardened metal on the worn-out teeth.
  4. This bit was designed for hard formations. It is a simple core-drill bit with hardened metal cutting edges.
  5. This bit is only suitable for soft formations and it has two sets of "wings". The four inner ones are actually cutting teeth that drilled the rock, whereas the four outer large wings swept the cuttings to the outside and in turn helped to keep the bottom of the hole clean.
  6. Unlike the previous five drill bits that used fixed cutters, this drill bit is a roller cone bit, whereby cutters are placed on rotating "wheels" around the rim of the drill bit. As the drill bit rotates, these six rollers also rotate in unison. The cutting takes place essentially by crushing the rock underneath from a crushing load imposed by the cutter structure. This is a different drilling mode, since the previous five cutters in fact cut or sheared the rock surface rather than crushed it. These types of bits were used for medium-hard to hard formations, depending on the type of roller teeth used.
  7. This device is not a drill bit but, rather, a core catcher that is inserted into the bit prior to coring operation. It consists of a number of thin plates or wings. Note that unlike all the drill bits, which are shown upside down, this part is shown in its correct vertical operation direction. As the drill is being pulled out of the hole, the wings of the core-catcher wedge into the surface of the core. This action not only helps to break the core off the bottom by developing tensile stresses in the core, but also helps to retain the core inside the coring bit.

The advent of battery technology contributed significantly to the portability of drills and it enabled the development of the first cordless electric drill, which was introduced by Black & Decker in 1961. Cordless technology allowed the drilling by the Apollo 15,16 and 17 astronauts to depths of more than 70 cm (achieved on earlier missions by hammering in core tubes). Between 1971 and 1972, three Apollo lunar missions used a battery-operated rotary-percussive drill to acquire cores and implant sensors into the surface of the Moon (Decker and Black, 1961). The drill, called the Apollo Lunar Surface Drill, was used to produce holes for the deployment of heat probes for measuring of heat flow out of the lunar surface and also for collecting a continuous core oflunar regolith. The Apollo 15,16 and 17 cores of lunar regolith brought back from the Moon (Heiken and Jones, 2007) provided important material for scientific study of the Moon.

Drilling on the Moon was a major challenge and drilling the deepest hole on Earth was even more so. Although the original idea of drilling a hole on Earth with a depth target of 15 000 m (49 210 ft) was first proposed in the former Soviet Union in 1962, the actual drilling project did not commence until 1970, after more than 8 years of preparation. The Kola Peninsula in the northwest Soviet Union (Figure 1.5) was chosen as the best location for this project, dubbed the Kola Superdeep Borehole project. The objective was purely scientific - to learn about the properties of subsurface rocks - and the science payoff was indeed outstanding. To this day, the cores acquired from these great depths continue to be analyzed and are a source of many scientific publications. In addition to discovering that below the 3-6 km granite layer there is a metamorphic layer rather than the basalt rock layer that was previously supposed, scientists found microscopic fossils as deep as 6.7 km below the group surface. In 1989, the drill reached its greatest depth of 12 261m (40 226 ft) and analysis of recovered rock samples has shown them to be 2.7 billion years old (Kola, 1989). Unfortunately, using this drill to reach greater depths was not feasible because of the very high temperatures encountered, in the region of 180 °C (356 ° F), which made the rock more plastic and in turn more difficult to drill. Drilling to these

Kola Well Sg3http://www.geo.uni-bonn.de/members/schulze/)."/>
Figure 1.5 The Kola-SG3 drill-rig enclosure and location of the Kola Superdeep Borehole project which drilled 12 262 m (7.5 miles) into the Earth's crust (based on June 2000, K.C. Schulze, http://www.geo.uni-bonn.de/members/schulze/).

great depths required drilling engineers to invent ever newer drilling technologies. This project still remains one of the greatest drilling achievements ever. In 2008, efforts to dismantle this facility have started to bring this project to its end.

In the USA, a parallel initiative to conduct ultra-deep drilling was taken in 1961. This project, called Mohole, was led by the American Miscellaneous Society and was funded by the National Science Foundation (NSF) (Burleson, 1998). Its goal was to drill a hole through the ocean floor to reach the boundary between the Earth's crust and the mantle. Drilling was done through 3500 m (11 700 ft) of water and then through the seafloor to a depth of 183 m (601 ft) off the coast of Guadalupe, Mexico. Unfortunately, the project was canceled by US Congress in 1967.

The hunger for oil led to the emergence ofdrilling in a non-vertical direction or at a slant, also known as directional drilling. The technology was used as early as the 1920s, where rigs on one property were used to tap into reservoirs in neighboring properties. A key issue related to this technology is the ability to track the drilling direction in relation to the geological area that is being drilled.

Another challenging area for drilling is subsurface penetration of extraterrestrial planetary bodies in order to acquire extraterrestrial samples for detailed analysis. The acquisition of samples from the subsurface of extraterrestrial bodies involves challenges that are far more complex and demanding than similar operations on Earth because of the limitations on mass, power and human control (Further details can be seen in Chapters 7 and 8). On Mars it will be necessary to drill deeper than a few meters to reach locations where oxidation and irradiation by cosmic rays have been minimal, thus suggesting that the possibility of finding evidence of putative extraterrestrial life may be higher. Subsurface samples can also provide evidence of

1.2 Methods of Drilling and Penetration of Objects | 9

past climates and geological events. The acquired samples can either be analyzed on the surface using a suite of in situ instruments or, ideally, brought back to Earth for a more thorough analysis using more sophisticated equipment.

Extraterrestrial drill systems have to be able to penetrate unknown media (this is referred to as geological uncertainty), which could be rocks of various degrees of hardness, compacted soil, pure ice, dirty ice or even frozen carbohydrates as is the case on the Saturn's moon, Titan. Also, a drill system has to be able to operate successfully in dusty environments that may be cold or hot and where the pressure may be high, low or a vacuum. For example, the pressure on the surface of Venus is over 90 times higher than the Earth's atmospheric pressure (or as high as at a depth of 1 km in the Earth's oceans) and the surface temperature can be as high as 480 °C. Another planet with an extreme environment is our Moon, which has no atmosphere and where the surface temperature can vary from —233 °C at night in the polar regions to + 123 °C during the day in the equatorial regions. In addition, the surface that faces the Sun can be significantly hotter than the surface that is facing deep space. The resulting high temperature gradient needs to be addressed to make sure the drilling system can survive these potentially damaging conditions. In addition, planetary missions have strong constraints on the mass and power that can be available. Another factor that affects the operation of drills is the local gravity which provides the necessary capability to pre-load the drill from its support platform (rover or lander). For Mars the gravity is about one-third of that on Earth, and for asteroids and comets local gravity may be many hundreds or thousands times smaller.

Operational autonomy is an obvious requirement for drill systems deployed on the other planetary bodies. The communication delays that approach 20 min each way make remote control operation impractical on Mars. Once a command is sent out, it takes more than 40 min to learn whether the drill indeed started to drill or not and the presence of any problems would be indicated to the operators on Earth only about 20 min after they had occurred.

Methods of Drilling and Penetration of Objects

In general, there are three basic approaches to breaking rocks: mechanical, thermal (thermal spalling, thermal melting and vaporization) and chemical approaches. The focus of this book is on methods of drilling that use mechanical forces to penetrate objects. However, for the sake of completeness, this section provides an overview of the various alternative drilling techniques.

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