Method and apparatus for percussion drilling

A method for maintaining a penetration rate of a drilling operation, where the drilling operation is carried out by a drilling rig having a hammer drill apparatus. A reference penetration rate for the hammer drill bit is chosen. A starting depth of cut is computed from the reference penetration rate, the measured hammer frequency, and the drill bit parameters. The system measures the actual penetration rate of the drill bit and computes an actual depth of cut. From the actual depth of cut a target bit rotation speed is computed. The target rotation speed is compared to the actual rotation speed, and the rotation speed is adjusted up or down to set the bit rotation speed at a value that will maintain the target rotation speed, notwithstanding changes in hammer frequency or ground conditions.

BACKGROUND

Technical Field

This disclosure relates to methods and apparatus for drilling boreholes in the earth in general, and more specifically, to methods and apparatus for percussion drilling of blast holes of the type commonly used in mining and quarrying operations.

Background

Various methods and apparatus for drilling boreholes are known in the art and have been used for decades in a wide variety of applications, for example, from oil and gas production, to mining, to quarrying operations. In mining and quarrying operations, such boreholes are typically filled with an explosive that, when detonated, ruptures or fragments the surrounding rock. Thereafter, the fragmented material can be removed and processed in a manner consistent with the particular operation. When used for this purpose, then, such boreholes are commonly referred to as “blast holes,” although the terms may be used interchangeably.

A number of factors influence the effectiveness of the blast, including the nature of the geologic structure (i.e., rock), the size and spacing of the blast holes, the burden (i.e., distance to the free face of the geologic structure), the type, amount, and placement of the explosive, as well as the order in which the blast holes are detonated. Generally speaking, the size, spacing, and depth of the blast holes represent the primary means of controlling the degree of rupture or fragmentation of the geologic structure, and considerable effort goes into developing a blast hole specification that will produce the desired result.The present disclosure generally relates to the technology of percussion drilling with a down-the-hole drill (called here, a “DHD”). Typical DHDs involve a combination of percussive and rotational movement of the drill bit to drill or chip away rock and are often referred to as “hammer drills”. Such DHDs are typically powered by a rotatable drill string attached to a drilling platform, that supplies rotation and high-pressure air for percussive drilling. In percussive drilling, rock cutting is a result of percussive impact forces rather than shear forces. In other words, rotation of the DHD merely serves to rotationally index the drill bit to fresh rock areas after the drill bit impacts the rock surface, rather than to impart shear cutting forces to the rock surface. The present disclosure relates to down-the-hole drilling that comprises indexing of the drill-bit buttons in conjunction with depth-out-cut control.

There is a desired ratio of penetration rate per drill bit revolution where the drill-bit carbides penetrate and fracture the rock efficiently, resulting in desirable drilling speed and bit-wear characteristics. This ratio is referred to as the desired depth of cut (DOC). A rate of penetration (PR) can be calculated by multiplying the rotation speed (that is, the indexing of the bit) by the DOC. Prior-art methods have used a simple feedback loop to adjust the rotation rate applied to the bit to maintain an assumed optimum penetration rate. However, these methods do not efficiently adjust drilling for variations in hammer frequencies, or bit diameter, or the relative hardness of the rock. What is needed is a method of monitoring and adjusting these parameters to achieve optimum drilling efficiency by maintaining a penetration rate, depending on local drilling conditions.

Although this application is focused on solving problems in percussive blast-hole drilling operations, the disclosure and claims are equally applicable to the drilling of boreholes in other fields, such as oil and gas drilling.

SUMMARY

I disclose a method for maintaining a penetration rate of a drilling operation, where the drilling operation is carried out by a drilling rig having hammer drill apparatus. A reference penetration rate for the hammer drill bit is chosen. A starting depth of cut is computed from the reference penetration rate, the measured hammer frequency, and the drill bit parameters. The system measures the actual penetration rate of the drill bit and computes an actual depth of cut. From the actual depth of cut a target bit rotation speed is computed. The target rotation speed is compared to the actual rotation speed, and the rotation speed is adjusted up or down to set the bit rotation speed at a value that will maintain the target rotation speed, notwithstanding changes in hammer frequency or ground conditions.

DETAILED DESCRIPTION

Referring toFIG. 1, in one embodiment, the drilling apparatus100may comprise a drilling rig110having a mast or derrick120configured to support a drill string130having a pneumatic hammer drill140provided on the end thereof, which pneumatic hammer drill140is connected to a drill bit300. Drilling rig110may also be provided with various systems for operating the drill string130to form boreholes180. For example, in the embodiments shown and described here, drilling rig110may also comprise a drill motor system150, a drill hoist system160, as well as an air injection system and a water injection system (not shown inFIG. 1). The term “hoist system” as used here refers to any system or actuator for raising and lowering the drill string, which may include a conventional pulley and sheave hoist system or actuator motors.

The drilling apparatus100comprises a control system170that is operatively associated with the drilling rig110, as well as with the various systems thereof, e.g., a motor system150, a hoist system160, or an air injection system and water injection system (not shown inFIG. 1). As will be explained in greater detail below, control system170monitors various drilling parameters generated or produced by the various drill systems and controls them as necessary to form the borehole180into the surface of the formation190.

The drill motor system150is connected to the drill string130and may be operated by a control system170to provide a rotational force or torque to rotate the drill bit300provided on the end of the pneumatic hammer drill140. The drill motor system150may also be provided with various sensors and transducers, as described below, to allow the control system170to monitor or sense the hammer frequency, as well as the rotational speed and rate of penetration of the drill bit300.

FIG. 2schematically shows the control system170referred to above at a high level.FIG. 2is not limiting, and the control system170may comprise other and further components relevant to its function. The control system170includes a computer200that is typically a programmable digital computer comprising a read-only memory, a non-transitory computer readable storage medium for storing instructions executable by a processor (such as a random-access memory), a central-processing unit or processor, and a hard drive or flash memory or the like for further storage of programs and data, as well as input and output ports.

InFIG. 2, the drill hoist system160and the drill motor system150are shown schematically as operatively connected to the computer200of the control system. Also present in practical drilling systems, and also operatively connected to the computer200, may be an air-injection system230and a water-injection system240, which systems may also include various sensors and transducers to allow the control system170to monitor or sense the amounts or flows of injected fluids.

The control system170also may include a display210with a graphical user interface, and an operator's control console220, connected to the computer200to receive inputs from an operator during a drilling operation, and provide information to the operator. The operator's console220may include a keyboard, keypad, joystick, mouse, or other input device. In this application, the collective input mechanisms of the operator's console220and the display210may be referred to generally as a graphical user interface, or GUI. The display210of the GUI may of course provide one or more pages of information and input fields to an operator. The operator console220may not necessarily be located on the drilling rig110, but may be remotely connected to the control system.

FIG. 3illustrates a typical hammer-drill bit300. The drill bit300has a bit diameter310and carries a plurality of hardened buttons320, each typically made of a cemented carbide. Each button320has a button diameter330. The pneumatic hammer140drives the impact of the drill bit300at some operating frequency. The drill motor system150rotates the drill bit300at a certain rotational speed to index the next strike of the buttons320away from the previous impacts to newly exposed rock between strikes.

FIG. 4illustrates the travel of the drill bit300as it is rotated for successive impacts in soft and hard rock.FIG. 4Ais a view looking upward at example drill bit300having buttons320as the same is rotated at a speed sufficient to obtain indexing equal to one button diameter330per hammer blow in soft rock.FIG. 4Ashows the position325of a button320moved exactly one button diameter330by rotation of the drill bit300.FIG. 4Bis a side view of the same buttons320, showing a rotation or indexing sufficient to move the position325of each button320one diameter330in soft rock340.FIG. 4Cis a view looking upward at an example drill bit300having buttons320as the same is rotated at a speed where the bit300is indexed less than one button diameter330per hammer blow in hard rock350.FIG. 4Dis a side view of the same buttons320, showing an indexing sufficient to move the position325of each button320some amount less than one diameter of the button320(as measured at the base of the button320), but equal to the diameter355of the portion of the button320below the rock face350. The actual rotation speed is some fraction equal to or less than an ideal rotation speed, to maintain a set penetration rate, as is calculated as described in the following paragraphs.

A database250is provided as a part of the control system. The database250may have predetermined settings and parameters for achieving optimum performance of the drilling apparatus or system100. Such settings and parameters can include physical characteristics, such as diameters of the pneumatic hammer drill, the drill bit300, and, in some cases, the diameter of the drill bit buttons320. The operating air pressures and hammer frequency in beats per minute for particular pneumatic hammer drills140may also be available in the database250. In the operation of one embodiment of the drilling apparatus or system100, an operator selects a display of information about the bit being used from a dropdown menu on the operating system GUI of the control console220. From these inputs, calculations are performed as described below, and the optimum operating range for the bit chosen is used for automatic control of drilling, and also displayed as a reference for manual drilling. In some cases, where a needed parameter is missing, this can be entered by the operator in the GUI of the display210.

First, the embodiment of system and method disclosed here calculates an ideal rotation speed as a target so as to rotate the bit300sufficiently so that a button320on the bit300has moved exactly one diameter around the circumference of the bit300between hammer strikes. This ideal indexing rotation speed (called RS here) is then assigned to a certain reference penetration rate to determine a depth of cut (called DOC here). As a non-limiting example a penetration rate of 1500 mm/minute would be reasonable for many drilling applications. For a bit rotation speed of, for example, 45 rpm, these values set the DOC at 1500 mm/min/45 RPM, which equals 33 mm/rotation of the bit300.

The actual penetration rate of the bit300is measured, such as by taking the time derivative of the drill head position, which position may be measured using, for example, a rotary encoder165on a sheave in the drill feed system. As discussed below, the drilling control system170is programmed to adjust the rotation speed of the bit300based on the measured penetration rate to maintain the 33 mm/rotation ratio, as stated in the example in the previous paragraph, even though rock conditions may change during drilling.

Stated procedurally, we illustrate the foregoing calculation inFIG. 5. At step500, a reference penetration rate PR for the hammer drill bit300is chosen. The reference penetration rate PR can be chosen for ideal or average rock conditions, as, for example, known in the field. Then drilling is initiated as some initial rotation speed, where the rotation speed is reported by the drill motor system150on the drilling rig110. The initial rotation speed may be chosen by reference to hammer-drill manufacturer's specification, and may be programmed into the database250of the drilling apparatus or system100. (See, e.g., the “Quantum Leap Technical Manual” for hammer drills, published by Atlas Copco Secoroc AB, of Fagersta, Sweden.) Such specifications will typically include a nominal hammer frequency (“beats/minute”) for a given air pressure. (In general, the hammer frequency will vary dynamically during drilling.)

At step510an initial hammer frequency HF is measured. This initial hammer frequency can alternatively be estimated from published hammer-drill specifications, as discussed in more detail below.

Step520computes a depth of cut DOC for the drill bit300according to the relationship DOC=PR/(HF*Dbutton/(Dbit*π)), where Dbuttonis the diameter330of the bit button320, and Dbitis the diameter310of the bit300. At step530, the system measures the actual penetration rate PRmas explained above and, at step540, computes a target rotation speed RS=PRm/DOC.

At step550the system measures the actual rotation speed RSa. If RSa>RS, then control passes to step570, where the rotation speed RS is adjusted downward by the control of the drill motor system150; else, a comparison is made again at step580. After this comparison, if RSa<RS, then control passes to step590, where rotation speed RS is adjusted upward; else, if RSa=RS (within the limits of measurement accuracy), then the rotation speed RS is maintained at step595. Finally, control from steps570,590, or595passes to step510, where the current hammer frequency HF is again measured, and this value is passed to step520to re-compute the desired depth-of-cut DOC from the reference penetration rate PR and the current hammer frequency HF. The control loop continues after step520so long as drilling continues. In this way, the pre-determined penetration rate, PR, is maintained, although the hammer frequency HF and ground conditions may vary. In one embodiment, the target rotation speed is limited by maximum and minimum reasonable rotation speeds stored in the database250.

Note that in this calculation, the rotation speed, RS, will be increased in proportion to an increase in the rate of penetration PR, the hammer frequency, HF, or the diameter330of the button, Dbutton(if the bit were changed to one with differently-sized buttons, for example). The rotation speed RS will be decreased in proportion to an increase in the diameter, Dbitof the bit300. The diameter of the bit300and the diameter of the buttons320may be taken from manufacturer's publications, or, if necessary, measured in the field. Correspondingly, the rotation speed RS will be decreased in proportion to a decrease in the rate of penetration, or of the hammer frequency HF.

The above exemplary calculations imply a means for measuring the rate of penetration PR, and the hammer frequency HF, and transmitting such data to the control system170. Hammer frequency may be measured by a accelerometer145attached to the pneumatic hammer140, or by an acoustic pickup175on the drilling rig110, or by fluctuations in drilling air pressure. A starting hammer frequency HF can be estimated from published hammer-drill information relating hammer frequency to air or hydraulic pressure driving the pneumatic hammer140, which values are available in published manufacturer's manuals, as next discussed.

As an example of establishing a hammer frequency value, where no initial direct measurement is available, we can estimate the frequency from this example calculation:
HF=(m_press*hammer_diameter+b_press)*(bit_air_pressure+(m_frequency*hammer_diameter+b_frequency).

In the above equation, bit_air_pressure is measured by the pressure transducer (not shown) typically included on the drilling rig110; this air pressure will influence the hammer frequency as air pressure increases or decreases while drilling. The “hammer diameter” in this case is not the same as the bit diameter310used in this application, but is a parameter available in the manufacturer's documentation. This “hammer diameter” will be input to the control system170through the GUI210by the operator. The coefficients m_press, b_press, m_frequency, and b_frequency can also be determined from manufacturer's published tables for hammer operating frequency of different diameter hammers through a range of pressures. These variables as described are only examples of relevant parameters to hammer frequency, and do not limit the claims.

The actual rate of penetration PRm, in the calculation above can be determined by taking the time derivative of the drill head position, which position is measured using, for example, a rotary encoder165on a hoist sheave160in the drill feed system. A ideal or reference rate of penetration PR can be determined by field testing of particular machines with particular hammer drill systems, in particular rock formations, to arrive at a starting value where there is 100% indexing of the drill bit300; that is, with no overlap of strikes by the buttons320. Normal drilling speeds will usually be somewhat less than this reference value due to hardness of the rock, although the reference value is not limiting, and faster rates of penetration will be available in softer ground.

None of the description in this application should be read as implying that any particular element, step, or function is an essential element which must be included in the claim scope; the scope of patented subject matter is defined only by the allowed claims. Moreover, none of these claims are intended to invoke 35 U.S.C. Section 112(f) unless the exact words “means for” are used, followed by a gerund. The claims as filed are intended to be as comprehensive as possible, and no subject matter is intentionally relinquished, dedicated, or abandoned.