Background

The present invention pertains to the connectors, i.e. the

pins and boxes, of tubular drilling members such as drill pipe or

drill collars. It is especially applicable to the type of drill pipe

generally described in U.S. Patent No. 4,548,431 to Hall et al,

and represents an improvement thereover.

Such a pipe has a pin connector at one end and a box

connector at the other end, each connector being adapted to

mate with a connector similar to that at the opposite end of the

pipe -- but on another pipe, to form a tool joint. The type of pin

connector in question comprises a pin tong section of relatively

large outer diameter; a pin base section outwardly adjacent the

pin tong section and of smaller outer diameter, whereby a

primary, generally axially facing, annular shoulder is defined

between the pin tong and pin base sections; a tapered threaded

section outwardly adjacent the pin base section; and a pin nose

section outwardly adjacent the threaded section and of smaller

diameter (by a value c,) than the small end of the threaded

section, the pin nose section having an annular pin end face. The

box connector comprises an internal, generally axially facing,

annular secondary shoulder sized to align with the pin end face

of a similar pipe in a tool joint, a box base section of relatively

small diameter outwardly adjacent the secondary shoulder and

sized to receive the pin nose section; a tapered threaded section

outwardly adjacent the box base section and adapted to mate

with the threaded section of the pin connector; a box counterbore

section outwardly adjacent the threaded section of the box

connector, of larger inner diameter than the trough of the largest

thread of the box connector, and sized to receive the pin base

section, the box counterbore section having an annular box end

face sized to align with the primary shoulder on the pin

connector.

The pipe described in the Hall et al patent is designed to

form tool joints which can withstand higher torque loading than

conventional tool joints. The analysis reflected in this prior patent

was largely limited to the stresses and spring-like qualities of the

pin base, the box counterbore section and the pin nose.

Although testing and finite element analysis were conducted as

a means to confirm the accuracy of the stress analysis, and did

not reveal any potential problems, at least one thread failure of

this type of pipe during service indicated that the torsional

strength of the connection can be limited by the length of the

threaded sections. However, the geometry of the joint is such

that the diameter of the pin nose section, which also effects the

torsional strength, is decreased as the thread length is increased,

thus creating a sort of dilemma.

It was discovered by later tests that, when joints were

designed with the thread length adequate for the smallest

anticipated inside diameter, the cross-sectional area of the pin

nose at the largest inner diameter available was too small. Larger

inside diameters produced a pin nose with inadequate strength

compared to the pin base, the box counterbore section and the

threads. In other words, the joint was not "balanced."

Summary of the Invention

The present invention is directed at the problem of

maximizing the torsional strength of a joint, and also ensuring

that the initial failure mode will be buckling of the pin nose and

box counterbore, rather than shearing of the threads. While

buckling of the pin nose and box counterbore is to be avoided, it

is not catastrophic in that it will not ordinarily cause the drill

string to part. However, if there is a thread failure, the drill string

is likely to part, causing very severe problems indeed.

The present invention accomplishes these objectives by

correlating (a) the length of the threaded section of the pin, (b)

the transverse cross- sectional area of the pin nose, and the

smaller of (c) the transverse cross-sectional area of the box

counterbore, or (d) the transverse cross-sectional area of the last

engaged thread of the pin, so that the torsional strength of the

tool joint is approximately equally dependent on factors (a), (b)

and the smaller of factors (c) and (d) above.

More specifically, these conditions can be satisfied when:

A _t _> 1 .73(A _L + A _N )

where: A _t = thread shear area

A _L = transverse area of smallest last engaged thread A _N = transverse area of pin nose section

The connectors may advantageously incorporate and/or

improve on other features of the prior connectors over which they

improve. For example, the lengths and thicknesses of the box

counterbore, pin base, box base, and pin nose sections are

preferably selected so that, when a tool joint is made up to at

least one half, and preferably 60%, of its yield strength, the box

end face will tighten against the primary shoulder at a force

greater than any force being exerted by the pin end face against

the secondary shoulder, with the pin end face tightening against

the secondary shoulder if additional torque is encountered during

drilling. Likewise, the outer diameter of the box connector may

be reduced along the box counterbore section to provide the

necessary deflectability in that area.

Still other features and advantages of the invention will be

made apparent by the following detailed description, the drawing,

and the claims.

Brief Description of the Drawing

Fig. 1 is a longitudinal quarter sectional view of pin and box

connectors according to the present invention in position for

mating to make up a tool joint.

Fig. 2 is an enlarged detailed view of the area encircled in

Fig. 1 .

Detailed Description

Referring to Fig. 1 , there is shown the pin connector 10 of

one piece of drill pipe in position for mating connection to the box

connector 12 of another piece of drill pipe. It should be

understood that the drill pipes are identical. Therefore, the pipe

carrying pin connector 10 has a box connector similar to 12 at its

other end; likewise the pipe carrying the box connector 12 has a

pin connector similar to 10 at its other end. Except for the

improvements described hereafter, the connectors 10 and 1 2 may

be generally of the type disclosed in prior U. S. Patent No.

4,548,431 , which is incorporated herein by reference.

The pin connector 10 has an enlarged diameter tong section

14. (In the case of a drill collar, the tong section outer diameter

is equal to that of the central section between tool joints.)

Axially outwardly adjacent tong section 14 is a cylindrical pin

base section 16. Section 16 is of smaller transverse dimension,

more specifically smaller outer diameter, than section 14,

whereby a primary annular, generally axially facing shoulder 18

is formed between sections 14 and 1 6. ("Generally axially

facing" herein will mean that the direction in which the surface

faces has a significant axial vector component. In the

embodiment illustrated, shoulder 18 is strictly axially facing, i.e.

no other vector components.) The outer edge of shoulder 18

may be beveled, as shown, and at the inner extremity of shoulder

18, there may be provided a relief groove 20, as explained in

prior U.S. Patent No. 4,548,431 . Axially outwardly of pin base

section 1 6 is the tapered threaded section 22 of the pin

connector. The length of section 22, which will be designated

L^, is shown in Fig. 1 . Axially outwardly of threaded section 22

is a pin nose section 24 which is cylindrical, and has a smaller

outer diameter

than the small end of the threaded section 22 by a value c, (see

c,/2 in Fig. 2). The outer end of nose section 24 defines an

axially facing annular pin end face 26, the corners of which may

be beveled as shown.

The box tong section 31 has a reduced inner diameter 32

equal to the inner diameter of the pin. Box connector 1 2, at the

outer end of section 32, has an internal, annular, axially facing

secondary shoulder 28 sized to align with the pin end face 26

and to abut the pin end face 26 when the tool joint is sufficiently

torqued up. Outwardly adjacent to the shoulder 28 is a

cylindrical box base section 30 of larger inner diameter than the

main body 32, whereby shoulder 28 is formed. The inner edge

of shoulder 28 may be beveled, as shown. Box base section 30

is sized to receive the pin nose section 24.

Axially outwardly adjacent the box base section 30, is the

tapered threaded section 36 of the box connector, which is

adapted to mate with threaded section 22 of the pin connector.

Axially outwardly of the threaded section 36, is a box

counterbore section 38 of larger inner diameter than the trough

of the largest thread of the box connector, and sized to receive

the pin base section 16. The end of the box counterbore section

38 forms an annular axially facing box end face 40 sized to align

with the primary shoulder 18 and to abut shoulder 18 when the

joint is made up.

When the joint is made up, the axial force at abutting

surfaces 18 and 40 will be greater than any such force at

opposed surfaces 26 and 28 (which may or may not abut prior to

the application of additional torque during use). More specifically,

the joint is usually made up to at least about one half of its yield

strength, and preferably to about 60%, and in that condition, the

force at faces 18 and 40 will be greater than the force (if any) at

faces 26 and 28. If additional torque is encountered during

drilling, faces 26 and 28 may further tighten against each other,

but preferably not to a force exceeding that at faces 18 and 40.

As explained more fully in U.S. Patent No. 4,548,431 , these

ends are accomplished by proper selection of the lengths and

thicknesses of the box counterbore 38, pin base 1 6, box base

30, and pin nose 24 sections. It is noted, in particular, that for

purposes of adjusting these relative lengths and thicknesses, the

thickness of the box counterbore section 38 may be reduced by

reducing its outer diameter as indicated at 42. This may allow

the necessary compression of the box counterbore section, as the

enclosed pin base section 16 stretches, in tightening end face 40

against the primary shoulder 18. The aforementioned correlation

of lengths and thicknesses of the various sections can be done,

for example, as per the teachings of prior U.S. Patent No.

4,548,431 .

Indeed, the apparatus, as thus far described, is largely in

accord with prior U.S. Patent No. 4,548,431 , and may also

incorporate other details described therein, such as bench mark

shoulders.

While the features thus far described have been generally

successful in allowing tool joints to handle higher torques, and

achieve other effects described in prior U.S. Patent No.

4,548,431 , at least one thread failure was experienced during

service of such pipe, which indicated that the torsional strength

of the connection could be limited by the length of the threads.

However, the geometry of the connection is such that the

diameter of the pin nose section, which also effects torsional

strength, is decreased as the thread length is increased. It was

found, by further testing, that when connections were designed

with the thread length adequate for the smallest anticipated inside

diameter, the cross-sectional area of the pin nose section at the

largest diameter available was too small. Larger inside diameters

produced a pin nose with inadequate strength compared to the

pin base section, the box counterbore section, and the threads.

In other words the connection was not balanced.

The improvements of the present invention optimize the

thread length and nose diameter for any given inside diameter,

thus maximizing the torsional strength of the connection. These

improvements also ensure that the initial failure mode will be

buckling of the pin nose section and box counterbore section,

rather than shearing of the threads. This is important since

buckling of the pin nose section and box counterbore section are

generally not catastrophic; the drill string will not part because of

such buckling. However, a thread failure is likely to cause the

drill string to part, resulting in extreme difficulties and expense.

Thus, in accord with the present invention (a) the length of

-li¬ the threaded section of the pin, (b) the transverse cross-sectional

area of the pin nose, and the smaller of (c) the transverse cross-

sectional area of the box counterbore section, or (d) the

transverse cross-sectional area of the last engaged thread of the

pin ("smallest last engaged thread" hereafter) are correlated so

that the torsional strength of the tool joint is approximately

equally dependent on each of factors (a), (b) and the smaller of

(c) or (d).

One method of achieving this is to design the connectors so

that:

(I): A _t _> 1 .73(A _L + A _H )

where: A _t = thread shear area

A _L = transverse area of smallest last engaged thread A _N = transverse area of pin nose section

The concept of A _t or thread shear area is best understood

by reference to the enlarged thread profile shown in Fig. 2. The

pitch of the thread profile shown is the length P measured from

a point on one thread to the analogous point on the next adjacent

thread. The pitch line l _p is a line parallel to the thread crest

through the part of the thread where its thickness is equal to half

of the pitch, i.e. P/2. This pitch line is approximately where the

thread would shear if shearing were to occur. A _t , the thread

shear area is the thickness of the thread at the pitch line l _p

multiplied by the total length L _t of the thread at the pitch line l _p ,

i.e. as if the helical thread were straightened and measured as a

straight linear distance.

One way of determining parameters which satisfy

relationship (I), is to begin by postulating relationship (I) as an

equation:

(II) A _t = 1 .73 (A _L + A _N ),

then solving that equation, which may be done by using the

following equations:

(III) A _t = L^ TΓ R.

where R _t = mean thread radius

where c = pitch line diameter at a chosen gauge point between the last engaged thread and the shoulder 18

q = the distance between the gauge point and the last engaged thread

H = untruncated thread height

S _RS = radial dimension of root truncation

T = taper (in inches/ft.)

d = inside diameter (32 in Fig. 1 )

where (VI) D _n = nose diameter

c-2( H

(Note: c, is derived from c,/2 [Fig. 3] for the full diameter of the member)

After solving all of the above, about % in. is added to L^,

satisfying the " > " aspect of relationship (I) and ensuring that

failure will not be by thread shearing.

It will be understood that relationship (I) is set up in terms

of areas. Each of these areas, of course, is functionally related

to other variables, such as corresponding diameters. Thus, other

equivalent relationships can be extrapolated, expressed in terms

of different variables. However, a connector design which

complies with any such equivalent relationship should also

comply with the relationship (I).

An alternate technique for designing a tool joint with the

proper correlation between parameters so that the torsional

strength of the tool joint is approximately equally dependent on

each of those parameters, is by an iterative process.

More specifically, one would iterate by following these

steps:

1 . Start with a small inner diameter for the size pipe in

question. (This will produce a relatively large value for A _N ).

2. Choose a first outer diameter for the pin nose section,

and calculate the needed thread length L^ based on that diameter

using the following relationship:

(VII) L^ π R _t - 1 .73(A _L + A _N ) = 0

3. Using the thread length L^ obtained in step 2,

recalculate (or determine graphically) a new outer diameter for the

pin nose section using relationship (VI).

4. Using the new nose diameter obtained in step 3,

recalculate the necessary thread length by repeating step 2.

5. Continue to alternate these steps until the nose

diameter and thread length reach equilibrium, or near equilibrium,

i.e. when the solution to equation (VII) is O + applicable tolerance

range, as understood by those in the art.

Various modifications of the above embodiments are within

the skill of the art and the scope of the invention, which is limited

only by the following claims.