Technical field

The present invention relates to drill string rods for use with drill bits for percussion rock drilling. Specifically, the invention relates to improving the reliability and longevity of such drill string rods.

Background

Percussion drilling is used to create a long borehole via a plurality of elongate drill string rods coupled together end-to-end by interconnected male and female threaded ends. The well-established technique breaks rock by hammering impacts transferred from a rock drill bit, mounted at one end of the drill string, to the rock at the bottom of the borehole. Typically, the energy required to break the rock is generated by a hydraulically driven piston that contacts the end of the drill string (via a shank adaptor) to create a stress (or shock) wave that propagates through the drill string and ultimately to the rock. Conventional male and female threaded couplings are described in US 4,332,502; US 4,398,756; US 4,687,368 and DE 2800887.

The male and female threaded ends of neighboring drill rods are coupled to create the drill string and the joint is typically subjected to large forces during drilling. These forces fatigue the coupling and lead to wear and breakage within the threaded portion of the joint. Typically, it is the threaded male spigot that is damaged and determines the operational lifetime of the coupling. US 6,767,156 discloses a threaded joint between two percussive drill rods having conical guiding surfaces provided at the leading axial ends of the male and female portions in an attempt to achieve a secure coupling and prevent damage to the threads.

EP2845992B1 by the present applicant aims to mitigate the above- mentioned drawbacks.

Modern rock drilling increasingly demands drilling of larger holes and therefore a demand exists for provision of drill strings capable of handling larger drill bits, such as drill bits with a diameter of 130 mm and larger. Conventional drill string rods are too weak to carry larger drill bits and it has shown that merely up-scaling conventional drill string rods is not an option since they tend to crack in the spigot or in the sleeve portion rather than permitting use until the threads of the spigot/sleeve portion are worn out.

Hence, there is a need for a large-size drill string rod with improved reliability and longevity, still allowing for efficient flushing of drill cuttings.

Summary

An object of the invention is to provide a drill string rod enabling use of larger-than-normal percussion drill bits with improved reliability and longevity of the drill string rods. According to a first aspect of the invention, this object is achieved by the inventive drill string rod as defined in the appended independent claim 1 , with alternative embodiments defined in the dependent claims. The drill string rod is to form part of an assembly of connected such drill string rods. The drill string rod comprises an elongate central rod portion extending axially between a male end and a female end. The central rod portion is hollow-cylindrical defined by an inner first diameter (drod) and an outer second diameter (D _r0 d). The male end comprises a spigot, wherein the spigot comprises a base projecting axially from a shoulder that axially separates the spigot and the central rod portion. The female end comprises a sleeve portion configured to fit to the spigot. Also, the base is provided with an outer thread and the sleeve portion is provided with an inner thread, wherein the inner thread corresponds to the outer thread such that the inner thread of the sleeve portion is attachable to the outer thread of the base of the spigot of a further drill string rod of the assembly. In a radial plane to the longitudinal axis of the drill string rod, the base of the spigot is defined by an outer third diameter (Dspigot) and an inner fourth diameter (dspigot) and the sleeve portion is defined by an outer fifth diameter (Dsieeve) and an inner sixth diameter (dsieeve). The present invention is limited to drill string rods with an outer second diameter (Drod) larger than 60 mm, thus being suitable for use with larger and heavier percussion drill bits. Csieeve and Cspigot are related to the diameters of the drill string rod as defined by the following formulas: Csieeve should be larger than 2.31 or Cspigot should be larger than > 0.68.

Drill string rods and their use are known in the art. However, drill string rods with an outer diameter Drod larger than 60 mm are not common. These larger drill string rods are very robust and suitable for larger rock drill bits. While normal drill string rods with rod diameter smaller than 60 mm tend not to have material failures but to be discarded and replaced only when the threads of the spigot and sleeve portion are worn out, larger diameter drill rods tend to break by cracks in the sleeve portion or spigot. Mitigating formation of cracks merely by increasing the thickness of goods is generally not successful, since it would either weaken some other part of the drill rod or reduce available space for flushing fluid and drill cuttings to move through. The skilled person understands that the diameters cannot be chosen freely. For example, the outer diameter of the spigot is naturally limited by the inner diameter of the sleeve portion. Also, the outer diameter of the sleeve portion is typically limited by the size of the bore drilled and the requirement for enough space for the flushing away of drill cuttings past the sleeve portion between the sleeve portion and the inner wall of the bore. Similarly, the inner diameter of the spigot limits the rate at which flushing fluid can be pumped through the drill string rod, and thus should not be too small. The design of these large-diameter drill string rods is clearly a difficult balancing act and the present invention guides the skilled person to the selection of a combination of parameters enabling reduced risk of material failures in the drill string rod with sustained flushing performance. To solve this problem has shown to be more difficult than it would seem.

The inventors of the present invention have realized that the drill string rod failures are due to the combination of the increased stiffness of the central rod portion and forced bending of the drill string induced by harder layers or cracks in the rock. The bending of the drill string rod happens while the rod is rotating and thereby creates both additional stress due to bending, and fatigue due to the constant change of bending axis caused by the rotation of the rod in the bore whilst in its bent condition.

The proposed solution is to dimension the spigot and the sleeve portion such that the calculated maximum bending stress in the spigot and the sleeve portion respectively is related to the calculated maximum bending stress in the central rod portion. However, the inventors have realized that the calculated maximum stresses of the sleeve portion and the spigot

respectively should not be equal to the calculated maximum bending stress of the central rod portion but rather relate to the calculated maximum bending stress of the central rod portion multiplied by the factor Csieeve or Cspigot, respectively, in order to account for commonly occurring differences in strength occurring for example due to local material variations likely occurring due to uneven hardening and/or machining at manufacturing of the drill string rods.

In some embodiments, Csieeve > 2.31 and Cspigot > 0.68. This balance of the first, second, third, fourth, fifth, and sixth diameters involved provide for high reliability and longevity of the drill string rod.

In some embodiments, the base of the spigot is conical.

In some embodiments, the base of the spigot is cylindrical.

In some embodiments, the first diameter (drod) is 50 mm, wherein the second diameter (Drod) is 80.5 mm, wherein the third diameter is (D _spig ot) is 82 mm, wherein the fourth diameter (d _spig ot) is 50 mm, wherein the firth diameter is (Dsieeve) is 120 mm and wherein the sixth diameter (dsieeve) is 91 .2 mm.

In some embodiments, the drill string rod is suitable for use with a drill bit having an outer seventh diameter (Dhoie), wherein the fifth diameter is < 0,90 ^* the seventh diameter (Dhoie).

A further aspect relates to a system comprising a plurality of drill string rods according to the first aspect described above.

In some embodiments, the drill string rods of the system are of the type mentioned above which are suitable for use with a drill bit having an outer seventh diameter (Dhoie), wherein the fifth diameter is < 0,90 ^* the seventh diameter (Dhoie), and wherein the system further comprises the drill bit.

The difference in diameter between the hole and the outer diameter of the sleeve portion enables efficient flushing of drill cuttings past the sleeve portion, whilst providing robust drill string rods capable of handling the forces involved.

In some embodiments, the seventh diameter is > 130 mm. Drill string rods having a second diameter > 60 mm are suitable for such large-diameter drill bits, wherein the specified constraints of the involved diameters of the drill string rods governed by Csieeve and Cspigot provide for a robust drilling system.

Description of drawings

Fig. 1 shows a perspective view of two identical connected drill string rods.

Fig. 2 shows an enlarged perspective view A of the male end of the drill string rod as indicated in Fig. 1.

Fig. 3 shows an enlarged perspective view B of the female end of the drill string rod as indicated in Fig. 1 , which is connected to the male end of the other drill string rod.

Fig. 4 shows a cross-sectional side view of the sleeve portion and the spigot of the two connected drill string rods also shown in Fig. 1 , said cross- section taken in a plane through the longitudinal central axis of the drill string rod.

Detailed description

A first embodiment of the invention is shown in Figs. 1 -4. As shown in the figures, a plurality of identical drill string rods 1 , 1 b are connectable to form a system/assembly 10. The drill string rod 1 comprises an elongate central rod portion 2 extending axially between a male end 3 and a female end 4. As shown in Fig. 4, the central rod portion 2 is hollow-cylindrical defined by an inner first diameter drod and an outer second diameter Drod. The male end 3 comprises a spigot 5, and the spigot 5 comprises a base 6 projecting axially from a shoulder 7 that axially separates the spigot 5 and the central rod portion 2. The female end 4 comprises a sleeve/sleeve portion 8 configured to fit to the spigot 5. The base 6 is provided with an outer thread and the sleeve portion 8 is provided with an inner thread, wherein the inner thread corresponds to the outer thread such that the inner thread of the sleeve portion 8 is attachable to the outer thread of the base 6 of the spigot 5 of a further drill string rod of the assembly. In a radial plane P to the longitudinal axis of the drill string rod the base 6 of the spigot 5 is defined by an outer third diameter (Dspigot) and an inner fourth diameter (dspigot) and the sleeve portion 8 is defined by an outer fifth diameter (Dsieeve) and an inner sixth diameter (dsieeve). It should be noted that in Fig. 4, Dspigot and dsieeve are both mentioned in connection with the same arrow although in reality the diameters are slightly different with dsieeve being larger than Dspigot. The scale of the drawing is such that the difference in diameter cannot be shown in Fig. 4 using separate arrows. Although the diameters may vary along the length of the spigot and the sleeve portion respectively, the radial plane P

which by necessity runs through both the spigot and the sleeve portion, is used to define the relationship between the diameters in an unambiguous way. The present invention is limited to drill string rods with an outer diameter

larger than 60 mm, thus being suitable for use with heavier percussion drill bits, such as drill bits larger with larger diameter than 130 mm. Hence, the second diameter (Drod) is > 60 mm. Csieeve and Cspigot are derivable

from/related to the diameters of the drill string rod using the following formulas: These formulas stem from the calculation of section modulus for hollow-cylindrical beams/bodies which provide a good approximation of the present involved cross-sectional shapes. Specifically, the section modulus equations for hollow-cylindrical cross-sections is:

Our assumption is that:

And Similarly that Ssleeve=C _sl eeve ^* Srod ->

In this embodiment, the first diameter drod is 50 mm, wherein the second diameter Drod is 80.5 mm, wherein the third diameter D _{s igot} is 82 mm, wherein the fourth diameter d _spigot is 50 mm, wherein the firth diameter D _sieeve is 120 mm and wherein the sixth diameter d _sieeve is 91 .2 mm.

This embodiment of the drill string rod 1 , 1 b is suitable for use with a drill bit (not illustrated) having a specified diameter (the diameter not including the drilling inserts) of 140, 152, 165,172 or 178 mm. The actual diameter of the holes drilled is slightly larger since the drill bits protrude radially.The first through sixth diameters may in other embodiments be chosen differently, as long as the second diameter Drod, which defines the outer diameter of the central rod portion 2, fulfills the above constraint of being at least 60 mm, and the other diameters fulfil the constraints that Csieeve is larger than 2.31 or that Cspigot is larger than 0.68. Preferably, Csieeve is larger than 2.31 and Cspigot is larger than 0.68.

The drill string rods are made of a suitable material, such as steel, and are hardened as needed.

The drill string rod 1 , 1 b is dimensioned based on the size of the drill bit for which it is to be used. However, care must be taken to provide enough space around the sleeve portion 8 for drill cuttings to be flushed past the sleeve portion 8. To this effect, the drill string rod 1 , 1 b may in some embodiments be suitable for use with a drill bit having an outer seventh diameter Dhoie, wherein the outer fifth diameter Dsieeve of the sleeve portion 8 is less than 0,90 ^* the seventh diameter Dhoie. A plurality of identical drill string rods 1 , 1 b may be provided together as part of a system/assembly 10 of the drill string rods 1 , 1 b. The system 10 may alternatively comprise a drill bit. The drill string rods 1 , 1 b of the system 10 may have an outer seventh diameter Dhoie, wherein the fifth diameter is <

5 0,90 ^* the seventh diameter Dhoie, and wherein the system further comprises the drill bit.

The difference in diameter between the hole and the outer fifth diameter Dsieeve of the sleeve portion 8 enables efficient flushing of drill cuttings past the sleeve portion 8, whilst providing robust drill string rods 1 , 1 b 0 capable of handling the forces involved.

In some embodiments, the seventh diameter Dhoie may be 1 30 mm or greater. As mentioned above, the drill string rods of the present invention all have a second diameter greater than 60 mm and are suitable for such large- diameter drill bits. The specified constraints of the involved diameters given 5 by Csieeve and Cspigot provide for a robust system/assembly 10.