Pipe joints or couplings – Particular interface – Tapered
Reexamination Certificate
1998-01-06
2001-03-06
Nicholson, Eric K. (Department: 3626)
Pipe joints or couplings
Particular interface
Tapered
C285S390000, C403S307000, C403S343000
Reexamination Certificate
active
06196598
ABSTRACT:
The present invention relates to drill strings.
Drill strings are used to transmit rotary and percussive forces from a surface drilling machine to a drill bit. The holes bored may be relatively deep, typically 50 feet, and accordingly, the string is made from a series of drill rods that are connected by couplings. Typically, the ends of each rod are threaded and the coupling is an internally threaded sleeve which receives the ends of adjacent rods.
When the strings are used to bore blast holes, for example, it is important that the holes are drilled as straight as possible to preserve the required pattern of the blast holes. Deviation of the hole during drilling will produce a different pattern at the bottom of the holes to that at the surface and the results of the blast will therefore be inconsistent. Deviation of the drill string is caused primarily by bending induced by the vertical loads caused by the feed of the string and the percussive forces on the string. In some cases, the deviation can be extreme, resulting in a deviation of several feet over a 50-foot hole. The effect of this is to increase the production costs as the spoil or “muck pile” created from blasting is of a non-uniform size which may require additional secondary breaking to accommodate the size of hauling equipment. It is a goal of the drilling industry to maintain the deviation to within 1.0% but this has not been achieved on a consistent basis.
One approach to reducing the deviation is to utilize tube drilling. In tube drilling, a thin walled metal tube that has an outside diameter close to the drilled hole size is utilized in an attempt to provide the requisite bending stiffness. However, the thin walled tubing limits the form of thread that can be used to connect the tubes with the result that an unsatisfactory rod life is obtained.
An alternative approach is to avoid percussive loads on the threads by placing the percussive hammer down inside the hole (DTH systems) and not transmitting the percussive shock wave through the connecting string. However, this approach tends to be more expensive.
It is therefore an object of the present invention to obviate or mitigate the above disadvantages.
According to one aspect of the invention. a drill string includes a plurality of drill rods for connection in series and each having at least one thread formed at one end thereof for connection to an adjacent rod. Each drill rod includes a cylindrical body having an outer surface and an internal bore extending between the ends of the rod. The diameter of the rod is greater than or equal to 65 mm and the ratio of the diameter of the body to that of the bore is at least 2:1 and preferably equal to or greater than 3:1.
By increasing the outer diameter of the rod, the bending stiffness of the rod is increased but by maintaining the ratio of the bore within the prescribed limits, an appropriate form of thread can be provided.
An increase in rod diameter generally has been considered to be detrimental to the life of the thread used to interconnect the rods as the larger root area associated with the increased diameter results in an increased region of high stress concentration and thus reduces the fatigue life of the rod. The longitudinal loads are significantly increased if the string is “rattled” during withdrawal to assist in loosening the couplings. Rattling imposes rapidly reversing vertical loads on the string and the stresses induced may cause failure of the threads, particularly the initial thread which is more heavily loaded than the other threads.
Previous attempts have been made to reduce the stress concentrations by adopting specific profiles for the threads. In U.S. Pat. Nos. 5,056,611; 5,060,740; and 5,163,523, the thread is formed with a part elliptical root which, according to these patents, provides a larger effective root diameter and therefore reduced stress concentration. However, in each case, the thread profile is asymmetric and to maintain an adequate wear volume, the ellipses used to define the roots of the male and female threads are of different sizes or configurations. This unduly complicates the production of the thread. Moreover, the fatigue life is still determined by the highest stress concentration in the threaded structure and so a high stress concentration at one root will effectively limit the life of the joint.
There is therefore a need to provide a threaded joint in which the above disadvantages are obviated or mitigated.
According to another aspect of the present invention, there is provided a threaded coupling for a drill string having a pair of cooperating male and female threaded members. Each of the members has a respective thread with a pair of opposed flanks extending between a crown and a root region. The root region is radially spaced from the crown and has a part circular generatrix with the flanks tangential there to.
Preferably, the female threaded member has a tapered outer surface which reduces in diameter towards the distal end of the female member and extends over at least one pitch of the thread.
Preferably, the tapered portion has a half angle of between 3° and 10° and, more preferably, between 4° and 6°.
One factor associated with the life of the thread is the wear volume which is the volume of material between adjacent flanks of the thread that can be sacrificed before the flanks intersect. This volume can be increased by utilizing a single start thread as the spacing between adjacent flanks is then increased. However, wear will inevitably occur and can lead to premature replacement of the rod.
According to a further aspect of the invention, a coupling between adjacent components of the string includes a male threaded member and a female threaded member for interengagement with one another. Each of the threads have oppositely directed convergent flanks longitudinally spaced along the member. The threads contact over at least a portion of the flanks to define respective wear volumes between the oppositely directed flanks. The wear volume defined on one of the members is greater than the wear volume on the other member.
Preferably, the female threaded member is a sleeve and the wear volume on the female threaded member is less than that on the male threaded member so that replacement of the sleeve will be required prior to replacement of the rod.
Preferably, the thread is formed such that the longitudinal dimension of the crest of the female thread is less than the longitudinal dimension of the crest of the male thread to provide an increased wear volume for the male thread. It is preferred that the female thread has a wear volume of between 80% and 95% of the male member, more preferably 89% to 95% and typically, 92%.
According to a still further aspect of the invention, there is provided a threaded coupling for a drill string having a pair of co-operating male and female threaded members. Each of said threaded members has a single start helical thread with a helix angle of between 7° and 10° with a pair of opposed flanks extending between a crest and a root region. The root region is defined by a part circular generatrix with the flanks tangential thereto. Each flank is delimited by a crest extending generally parallel to the longitudinal axis of the thread. The depth of each thread measured radially is greater than 5 mm and flanks of the male thread and female thread overlap radially greater than 3.7 mm.
Preferably, the axial dimension of the crest on the female and male threads differs and as a further preference, the axial dimension of the female crest is less than that of the male crest.
REFERENCES:
patent: Re. 27284 (1972-02-01), Hjalsten et al.
patent: 3537738 (1970-11-01), Fischer et al.
patent: 3645570 (1972-02-01), Johansson et al.
patent: 3717368 (1973-02-01), Czarnecki et al.
patent: 3822952 (1974-07-01), Johansson et al.
patent: 4040756 (1977-08-01), Donegan
patent: 4295751 (1981-10-01), Holmberg
patent: 4332502 (1982-06-01), Wormald et al.
patent: 4687368 (1987-08-01), Eklof et al.
patent: 4760887 (1988-08-01), Jansson et al.
patent: 5056611 (1991-
Boart Longyear Inc.
Covington & Burling
Nicholson Eric K.
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