Threaded joint for drill rod elements

A threaded joint for coupling two drill rod elements includes a first drill rod element having at least one end thereof a male tapered helical threading with a cone opening angle greater than 1.degree. and less than 3.degree.. The threading has a constant pitch determining an angle of inclination of the decreasing helix as greater than 1.degree. and less than 3.degree. at the beginning of the helical threading. A second drill rod element has corresponding female tapered helical threading. Each tapered helical threading has primary and secondary shoulders on opposite ends thereof and a stand-off between the primary shoulders when the male and female tapered helical threading are manually fitted together. The male tapered helical threading has discharge grooves in an unthreaded zone for deflection of lines of force. The primary and secondary shoulders have a truncated cone shape, and the ratio between the cone opening angle and the angle of inclination of the helix at the beginning of the tapered helical threading is less than 2.

BACKGROUND OF THE INVENTION 
The present invention relates to a threaded joint for coupling, by screwing 
at the end, drill rod elements, one of which is equipped at least one end 
with tapered male threading having a cone opening angle of less than 
3.degree. and a constant threading pitch with a double screwing motion 
stop and of which the other is equipped with corresponding tapered female 
threading. 
It is essential for the strength of threading used for drilling purposes to 
correspond at least to the static and dynamic forces of the line of rods 
in the hole to be drilled and to the additional stresses required during 
manoeuvres. 
Thus, knowledge of the coefficient of safety allows the thread to be worked 
to its ultimate limit without difficulty when rescuing the line of drill 
rods. 
Drilling can currently be carried out to a depth of 14,000 m. 
SUMMARY OF THE INVENTION 
The present invention proposes screw threading which is better adapted to 
the above-mentioned requirements in that it has, in particular, better 
performance under torque, thrust and traction and fatigue under simple or 
compound stresses. 
The appearance in drilling machinery of new cutting tools such as 
diamond-studded plates or polycrystalline diamond-studded cutting elements 
has led to the use of turntables which are more powerful and are capable 
of transmitting higher torques both in size and speed of rotation, 
sometimes despite considerable deviations imposed by guided drilling 
operations. 
These new requirements impose extremely harsh stresses on the drill rods 
and, in particular, on the screw threaded joints, both with regard to the 
stresses of shearing, torsion, traction, bending and compression and with 
regard to the phenomena of fatigue of the screw-threaded member. 
Truncated cone shaped rod elements provided with threading and comprising 
at least one screwing motion limiting stop, against which the end of a 
male element rests when tightened, are currently used when drilling for 
oil. 
The distance kept between the external male stop for limiting the screwing 
motion and the external female stop for limiting the screwing motion, 
after manually tightening the male thread, constitutes the stand-off. The 
end of the thread is optionally provided with a discharge groove intended 
to deflect the lines of force. 
This distance is calculated such that an excessive tightening torque 
produces a rigid assembly without causing permanent deformation either in 
the male connecting piece or in the female element. 
Threading having slight conicity allows a large stand-off in relation to 
the cone angle. 
The truncated cone shaped threading of the male element and that of the 
female element having a stand-off act, by their nature, as hoops. By 
screwing one in the other, radial stresses are exerted in the region of 
the threading and tend to reduce the diameter of the male element 
threading and to increase the diameter of the female element threading. 
Various factors, for example, the machining tolerances, the mechanical 
characteristics of the grease and the length of the stand-offs, influence 
the torque which has to be applied to the rods in order to bring into 
contact the screwing motion limiting stops. 
The recommended torque load is that which allows the screwing motion 
limiting stops to be brought against one another with the desired 
pressure. 
This known threading, as does all the standardised threading used nowadays 
for transmitting a moment in drilling, respects the concepts defined by 
the A.P.I., the American Petroleum Institute, in a simple manner. 
Various types are very well known to oil drillers, in particular the 
Regular, Full Hole and Internal Flush types which comprise approximately 4 
to 6 threads per inch (T.P.I.). They resist very high pulling forces and 
have a simple stop and cone opening angle of about 8.degree. to 15.degree. 
and a relatively small pitch, selected such that the ratio between the 
cone opening angle and the average angle of inclination of the helix is 
between 8/1 and 18/1. This standardised A.P.I. threading has the 
disadvantage of concentrating the moment-transmitting stresses on a piece 
of material of which the length does not exceed two pitches. 
It should be noted that other types of threading also exist in oil and 
mineral drilling, in particular those used in core barrels. These types 
satisfy the A.P.I. recommendation of having the greatest possible conicity 
compatible with the rod thickness and of having a relatively small pitch. 
To safeguard this A.P.I. recommendation, the designers have selected 
pitches of between 6 and 8 T.P.I. 
The threads standardised by the A.P.I. as well as the threads of core 
barrels have the significant feature of being interchangeable with one 
another. While the commercial criterion of interchangeability prevails, 
the only method of increasing performance in terms of strength relative to 
a moment of a standard thread is to select a steel having a higher 
strength, that is to say a higher yield stress and break point. However, 
this choice contributes only a negligible improvement and consequently 
causes a reduction in the elongation and therefore a clearly reduced 
resistance to fatigue, in other words, the rod tolerates slighter 
deviations and will have a shorter service life. 
The majority of the damage observed in lines of rods with A.P.I. threading 
under the harsh conditions of use demanded by the new drilling methods is 
localised in the immediate vicinity of the screwing motion limiting stops 
and is manifested in one of the following ways: 
1. bulging of the bore zone of the female element in the vicinity of the 
external stop and simultaneous annular reinforcement of the male base by 
creep; 
2. clean break of the brittle type of the male connecting piece body in the 
region of the last turn or of the bore zone adjacent to this turn and to 
the external stop; 
3. clean break of the sleeve of the female element in the region of the 
starting turn of the threading or of the bored portion of the tube 
adjacent to the external stop; 
4. clean break of the male connecting piece in the region of the beginning 
of the threading; 
5. clean break of the female element in the region of the bored zone at the 
base of the threading; 
6. deformation of the thread roots due to excessive tension or fatigue. 
The object of the present invention is to overcome the above-mentioned 
disadvantages. For this purpose, it proposes a tapered, screw-threaded 
connection having the benefit of unexpected performance data. The gain in 
resistance to torsion results from the combination of a small cone angle 
of the order of 1 to 3 degrees and a large pitch. However, care should be 
taken to keep the tangent of the helix smaller than the coefficient of 
friction of 0.08 generally set forth by the American Petroleum Institute 
to ensure that the parts cannot be unscrewed and to increase the permitted 
load torque value of the joint. 
The invention is based on the fact that, when the inclination of the 
threading is increased, the contribution of the threading to the 
transmission of a torque is also increased because the stops are less 
stressed. 
By increasing the inclination of a threading, a larger volume of material 
is brought into play for a given threading profile. 
These features and details of the invention as well as others will appear 
in the course of the following detailed description which refers to the 
following drawing illustrating a particular embodiment, given as a 
non-limiting example, of the invention.

DETAILED DESCRIPTION 
By moving away from the A.P.I. theory and following a logic closer to 
current drilling requirements, a thread 2 having much higher torque and 
also higher resistance to fatigue has been created in the same thickness 
of rod 1, while keeping the other properties (pull capacity, 
imperviousness, etc.) at least equal to their present level. 
The threading according to the invention can be of any profile, for example 
triangular, trapezoidal or round. This trapezoidal threading allows better 
centering due to the inclination of the flanks. 
The combination of a small opening angle .alpha. and a large pitch p 
relative to the diameter D of the rod 2 gives the threading unexpected 
performance data which are surprising in view of the weight of the rods. 
EQU p/D=tg.beta. 
EQU .beta.=1 to 3.degree.. 
It allows screwing at the end of the rod elements at the recommended load 
torque in only two or three turns. 
It also allows the provision of stops 3 having a load bearing surface area 
sufficient to take up the high forces of pressure. Slight conicity allows 
better control of the pressures due to the hooping effect of the stand-off 
which rests on the flanks of the thread profiles. 
Suitable choice of the profile allows the threads to rest better on the 
flanks 4 and, for certain applications, on the root 5 of the threads. In 
fact, a hooping effect is induced on the upper face of the profile during 
the screwing motion while effects of pressure are created on the stops by 
the flanks of the profile. The flanks 4 facing the stops thus help to 
transmit the load torque. This load torque induces over the entire length 
of the threading a mechanical stress bias which prevents the phenomena of 
fatigue due to the repeated bending of the rods. The thread 2 also has a 
crest portion 9, as shown in FIG. 1. 
It can easily be understood that a large pitch allows the stops to be 
stressed less for the same torque. 
The combination of a small opening angle .alpha. and a large pitch allows 
the more demanding performance data required for current drilling 
operations to be attained. 
Furthermore, a double stop 3 machined on either side of the threading in 
the limits of machining precision allowed by current machine tools and of 
elasticity of the thread is selected so as to double the surface area of 
the stops for limiting the screwing motion, in order to be able firstly to 
increase the mechanical performance of the line of rods, in particular the 
torque transmitted by the line of rods, and not to increase the 
impermeability of said line of rods. 
Owing to the above-mentioned combination of an opening angle .alpha. of the 
order of 1.degree. to 3.degree. and an inclination .beta. of the helix of 
the order of 1.degree. to 3.degree. at the beginning of the threading, the 
torque strength value can be increased by more careful use of the material 
available to the thread and by distributing the forces over a larger 
volume of material. The performance of the new threading according to the 
invention is thus improved, relative to the known threading, in a 
proportion which may be as high as 270%, as illustrated by the following 
comparative example. 
The comparative Table of a new 63/4 thread is given by way of example. 
__________________________________________________________________________ 
NEW 
STANDARDISED 
THREAD COMATIVE 
THREAD A Single Beginning 
FACTOR 
__________________________________________________________________________ 
Recommended 
1300 kg.m. 
3490 kg.m. 
270% of the 
load torque standardised 
Breaking moment 
3055 kg.m. 
8200 kg.m. 
270% of the 
standardised 
Maximum pull 
125 T 193 T 154% of the 
standardised 
__________________________________________________________________________ 
This Table allows the new performance data to be evaluated. The 
measurements are the result of investigations to compare research and 
tests (analyses by finished elements, stress and deformation gauges . . . 
). The most accurate and most high-performance measurements were taken on 
a torque testing stand. 
The selection of a non-threaded introduction zone 6 having a length of at 
least two pitches between the first thread and the stop allows an increase 
in the quantity of steel working at a high level of stress and involving 
severe dynamic problems and significant inopportune locking. 
The discharge grooves 8 provided in the dead zones 7 and in the rods in the 
vicinity of the connections and illustrated only in FIGS. 1 and 2 allow 
the fatigue stresses to be filtered and allow the thread to the protected, 
if they have suitable dimensions.