Patent Application
20230107302
The present invention relates to a drill string rod to form part of a drill string having a male spigot portion provided at one end of the rod and in particular, although not exclusively, to a spigot portion having a threaded section and a non-threaded shank configured to minimise stress concentrations.
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 the 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 base rock level.
Conventional male and female threaded couplings are described in U.S. Pat. Nos. 4,332,502; 4,398,756; 1,926,925; 5,169,183; EP 1705415; GB 2321073 and U.S. Pat. No. 4,687,368.
When the male and female threaded ends of neighbouring drill rods are coupled to create the drill string, the joint is typically subjected to bending moment during drilling. These bending moments fatigue the coupling and may lead to 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.
In particular, the transition between the different diameters of the threaded male spigot and the main length of the drill rod (or an annular shoulder at the rod end required for ‘shoulder contact’ couplings) provides a region for potentially high stress concentrations due to bending moments and tensile loads. Conventionally, the outside diameter of the rod at the transition axially between the threaded male spigot and the main length or shoulder is flared radially outward with a curved shape profile having a single radius curvature that is as large as can be accommodated between the two regions. However, for a typical threaded coupling stressed by 200 MPa in tension, the transition region reaches a stress level of approximately 300 MPa. Fatigue and possible breakage are therefore very likely, and the multiple threaded couplings represents a significantly weaker region of the drill string. Drill rods are typically replaced periodically according to their predetermined lifetime to try and avoid fracture of the male spigot during use which would cause significant disruption to a drilling operation. EP2845991 discloses a design to reduce stress in this region wherein the outside diameter of the rod axially between the threaded male spigot and the main length or shoulder is flared radially outward with a curved shape profile having a double radius curvature, however the level of stress in the transition region is still higher than desired. There is therefore a need for a drill rod that addresses these problems.
It is an objective of the present invention to provide a drill string rod having a male threaded coupling part that is optimised to minimise the likelihood of stress concentrations at the transition region between the end of the main length section of the rod and the spigot to extend the operational lifetime of the rod and minimise fatigue and the risk of breakage in use. It is a further specific objective to provide a drill rod that is compatible with existing drilling apparatus and methods that comprises an enhanced capacity to withstand large bending moments and tensile loads.
The objectives are achieved by specifically configuring a transition region positioned axially at the interface with the end of the main length section, or an annular shoulder at the end of the main length section. The present invention provides a drill rod coupling that exhibits reduced stress concentrations compared to known designs at the junction of the male spigot with the main length section resultant from incident bending moments or tensile loads.
According to a first aspect of the present invention there is provide a drill string rod to form a part of a drill string, the rod comprising: an elongate hollow main length section extending axially between a first end and a second end; a male spigot portion provided at the second end having an externally threaded section and a non-threaded shank positioned axially intermediate the main length section and the threaded section; the shank having a transition section positioned adjacent the main length section or a radially projecting shoulder at the second end, the transition section having an outside diameter that increases in a direction from the spigot portion to the main length section or the shoulder; wherein the cross section shape profile of the outer surface of the transition section in the plane of the longitudinal axis comprises a segment of an ellipse having semi-major axis (a); a semi-minor axis (b) and an exponential factor (n) according to the equation:
characterised in that the ratio of the semi-major to semi-minor axes (a:b) is within the range 2b<a<8b.
Advantageously, this provides a male coupling end exhibiting enhanced stiffness and that is more resilient to bending moments and tensile forces. The transition section is configured to eliminate or at least minimise stress concentrations at the section where spigot projects axially from shoulder. If the ratio of the lengths of the semi-major to semi-minor axes are above or below this the stress concentrations increase. Consequently, the risk of breakage is reduced and so the operation lifetime of the rod is increased. Optionally, the transition section may also comprise segments wherein the shape profile is straight and/or different curved profile.
Optionally, the non-threaded shank is divided axially into a straight part, positioned axially closest to threaded section, and a curved transition section, positioned axially closest to the side surface. It may be advantageous to increase the distance between the shoulder and threaded part. In this case it will be beneficial to include a straight section as well.
Alternatively, the non-threaded shank has only a curved transition section extending all the way from the side surface to the threaded section. When the non-threaded shank is shorter it is advantageous that there is only a curved transition section, i.e. no straight section, as this aids in keeping the stress concentration as low as possible.
Preferably, the ratio of the semi-major to semi-minor axes (a:b) is within the range 2.5b<a<6b. Advantageously, within the narrowed ratio range the stress concentrations at the section where the spigot projects axially from the shoulder are further reduced meaning that there is enhanced capacity to withstand large bending moments and tensile stresses.
Preferably, the semi-minor axis (b) is proportionate to the dimension of the threaded section according to the following equation:
wherein Di is the diameter of the threaded section between opposing troughs and Dy is the diameter of the threaded section between opposing helical ridges. Advantageously, the length of the semi-major axis (b) is as large as possible, as this provides an elliptical shape with no sharp ends and therefore having the lowest stress concentration. However, if the length of the semi-major (b) is too high, there would effectively be no shoulder and so energy cannot be transferred effectively between the male and female ends, which would result in the female end of the rod breaking.
Preferably, the exponential factor (n) is in the range 1≤n≤3. Advantageously, this provides a transition section having an elliptical profile with the lowest stress concentration.
Optionally, a vertex of the ellipse is positioned at a tangent with the annular side surface of the shoulder. Alternatively, the vertex of the ellipse undercuts the annular side surface of the shoulder. Different load cases may benefit from different forms of the ellipse.
Optionally, the x-axis of the ellipse is parallel to the longitudinal axis. Alternatively, the x-axis of the ellipse is tilted with respect to the longitudinal axis. Different load cases may benefit from different forms of the ellipse.
Optionally, the profile of the outer surface of the transition section in the plane of the longitudinal axis comprises a quarter segment of an ellipse. Alternatively, the cross-sectional shape profile of the outer surface of the transition section in the plane of the longitudinal axis comprises greater than a quarter segment of an ellipse. Alternatively, the cross-sectional shape profile of the outer surface of the transition section in the plane of the longitudinal axis comprises a less than quarter segment of an ellipse. Different load cases may benefit from different forms of the ellipse.
Within the specification, reference to ‘curvature’ encompasses a smooth or gradual change in surface profile and a plurality of sequential linear increases (or decreases) in diameter that collectively may be regarded as a ‘curved’ shape profile. For example, the term ‘curvature’ encompasses relatively small linear step changes such that an edge or middle region of each step may be considered to collectively define a curve.
Preferably, the rod comprises a shoulder projecting radially from the main length section wherein an outside diameter of the shoulder is greater than an outside diameter of the main length section and the transition section of the shank. Such a configuration allows for the conventional ‘shoulder contact’ coupling between the male spigot and the female sleeve that is preferred over the alternative ‘bottom contact’ due to the larger diameter and surface area contact between the rod ends at the region of the male and female parts.
Preferably, a side surface of the shoulder that is in contact with the transition section comprises an annular radially outer region that is aligned substantially perpendicular to the longitudinal axis. The curved transition section therefore does not continue over the full radial length of the annular side surface to provide a flat annular surface for contact by the annular end face of the female sleeve.
Optionally, the threaded section comprises at least one axially extending helical ridge and groove, wherein an outside diameter of the shank axially between the threaded section and the transition section is substantially equal to an outside diameter of the threaded section at an axial and a radial position corresponding to the ridge of the threaded section. Optionally, the threaded section comprises a plurality of threads formed as a double or triple helix etc. Such configurations can be selected to achieve a desired threaded profile having desired mechanical and physical properties.
Optionally, a cross sectional area of the shank is at least equal to a cross sectional area of the main length section in a plane perpendicular to the longitudinal axis over a full axial length of the shank between the threaded section and the main length section or the shoulder. Optionally, the diameter of the threaded section is slightly smaller than the diameter of the main length section. Accordingly, the shank is configured to be robust during bending moments and tensile loads and to avoid creation of stress concentrations resultant from changes in diameter along the length of the rod.
Preferably, the first end comprises a female hollow portion having an internal threaded section to engage with the threaded section of the male spigot portion of a neighbouring rod of the drill string. Preferably, an internal diameter of the threaded section of the female portion is substantially equal to an outside diameter of the main length section. The present coupling therefore provides a region that is enlarged in diameter and cross-sectional area (perpendicular to the longitudinal axis of the rod) relative to the elongate hollow main length section.
According to a second aspect of the present invention there is provided a drill string comprising a drill string rod as claimed herein.
A specific implementation of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which:
Referring to
Referring to
The threaded section 107, according to the specific implementation, comprises a pair of helical turns 209 that extend axially from shank 109 to spigot end 114. In particular, a pair of helical ridges 207 and troughs 208 extend axially over section 107.
Referring to
The equation of an ellipse is defined by a Lamé curve when n=2:
Wherein:
x is the co-ordinate on the x axis;
y is the co-ordinate on the y axis;
a is the semi-major axis (x);
b is the semi-minor axis (y);
n determines the shape of the curve. n=2 defines an ordinary ellipse. n<2 a hypoellipse and n>2 a hyperellipse.
The elliptical profile 214 is shown on expanded view of the transition section 206 in
In the present invention the ratio of the major to minor axes, (a:b) is within the range 2b<a<8b, preferably, 2b<a<6b, more preferably 2.5b<a<6b, even more preferably 2.5b<a<5.75b.
Preferably, the semi-minor axis (b) is as large as possible. More preferably the semi-minor axis (b) is proportionate to the diameter of the threaded section 107 of the male spigot portion 108 according to the following equation:
Wherein (as shown on
Di=diameter of the threaded section 107 between opposing troughs 208;
Dy=diameter of the threaded section 107 between opposing helical ridges 207.
Preferably, the exponential factor n is in the range 1≤n≤3, preferably 1.8≤n≤2.2, most preferably 2.
The equation of the elliptical profile of the transition section 206 can be measured using a contour measuring machine. The contour measuring machine drags a needle over the surface of the transition section 206, then the equipment will try to fit different geometries and then output the equation of shape profile measured.
At each endpoint of the semi-major axis (x) is a vertex 215 of the ellipse 214 and at each endpoint of the minor axis (y) there is a co-vertex 216 of the ellipse 214. Optionally, the vertex 215 of the ellipse is positioned at a tangent with the annular side surface 203 of the shoulder 110, as shown in
Optionally, the x-axis of the ellipse 214 is parallel to the longitudinal axis 204, as shown in
It should be appreciated that any combination of the position of the vertex 215 can be combined with any orientation of the x-axis with respect to the longitudinal axis 204 as described hereinabove.
The profile of the transition section 206 provides a male coupling end exhibiting enhanced stiffness and that is more resilient to bending moments and tensile forces with respect to conventional couplings. Additionally, transition section 206 is configured to eliminate or at least minimise stress concentrations at the section where spigot 108 projects axially from shoulder 110.
The risk for failure is increased as the value of the Dang van criterion in decreased. Thus, darker colours mean higher risk for failure. By comparing
| Number | Date | Country | Kind |
|---|---|---|---|
| 20162266.9 | Mar 2020 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/056075 | 3/10/2021 | WO |
