Technical background In the natural-gas and oil extraction industry, when drilling oil or gas wells, tubings or casings having a pre-defined length are used, which must be joined together at their ends to form strings, in order to be able to reach the very large depths at which reservoirs of oil, gas, and liquefiable hydrocarbons are located.

The most commonly used drilling technique is that of drilling wells that start from the surface of the ground or of the sea until the oil field or gas reservoir is reached.

The depth of these wells can attain several thousands of metres. During drilling, the boreholes are lined with metal tubings or casings throughout their length. The segments of metal tubing, which are each some ten metres long, are joined together by means of threaded joints. These casings then form a tubular string having a constant diameter throughout its length, except at the joints, where the external diameter is generally at least 1 inch (25,4 mm) greater than that of the string itself.

In order to line the borehole throughout its depth, a number of strings are used, which have, for reasons of mechanical resistance and the geological characteristics of the formation, decreasing diameters the greater the depth reached by the string, so as to constitute a telescopic structure. It follows that, since the diameter at the bottom of the well is fixed according to the pressure and flow rate of the fluid that is to be extracted, the deeper the well, the larger its diameter at the surface. A disadvantageous consequence of this is that the cost of drilling is high, and the amount of material required for these casings is also high, with consequent high costs. A smaller diameter of the well also enables a reduction in drilling times, material consumption, volume of material extracted, which has to be disposed of, and hence of times for completion of the well.

Consequently, it is important to reduce the diameter of wells to the minimum, and hence also the diameter of the pipes used to make the casings, given the same amount of fluid to be extracted.

Once the drilling operations are completed, into the wall of the borehole thus lined there is inserted another tubular string, which has the function of pumping the gas, oil or liquefiable hydrocarbon out of the underground reservoir. This string, which is sunk to the entire depth of the well, and which can thus reach lengths of several thousand metres, is also formed by joining pipes each of some ten metres in length by means of joints of the type referred to above. Normally, also this second type of string has a constant diameter throughout its length, except at the joints, where the external diameter is generally larger, as in the previous case.

In both of the cases referred to above, the pipes are joined together either by threaded integral joints, in which case one end of the pipe, or pin, has a male thread and the other end, a female thread, or by coupled connections, in which case both ends of the pipe, or pins, have a male thread and are joined together by a threaded sleeve or coupling, having female threads at both ends. Normally, the presence of the joint involves an increase in the external diameter of the string at the joint, and this results in an increase in the overall dimensions of the borehole which are the greater, the greater is the external diameter of the joint with respect to the pipe.

In view of the demand from the petroleum companies to reduce to a minimum the costs for extraction of oil, gas and liquefiable hydrocarbons, considerable efforts have been made to reduce the diameter of the wells, and consequently the diameter of the pipes used.

In order to limit the external diameter of the string, and consequently the costs for drilling and for the material installed, threaded joints having small diametral dimensions are used. These can be divided into three types according to the features required and the maximum overall dimensions allowed. A first type, which is frequently referred to as"semi-flush", is a coupled connection, the external diameter of which exceeds the external diameter of the pipe by not more than 6%.

A second type, generally referred to as"near-flush", is an integral joint, the external diameter of which exceeds that of the pipe by 2-3%. Finally, a third type, referred to as"flush", is an integral joint, the external diameter of which is equal to the external diameter of the pipe.

The choice between the various types of joint is made according to the loads that the string has to withstand, to the pressure that acts inside and/or outside the string, to the length of the string, and to the maximum diametral dimensions allowable in relation to the diametral dimension of the well, as well as according to the factors of safety contemplated, to the geometry of the joint, and to its level of efficiency.

In the past much effort has been directed towards improving the joints and bringing them up to an optimal level of efficiency, endeavouring to achieve a proper balance between the various requirements, which are at times conflicting, of minimal overall dimensions, maximum structural strength, and maximum sealing capacity to resist the pressure of the fluids. The pipes are, in fact, subjected to compressive, tensile and bending loads and to the pressure produced by fluids acting from outside and/or circulating inside the pipes themselves.

The joints must also possess excellent characteristics of resistance to screwing and seizing.

The structural problems and the problems of tightness are frequently aggravated by the temperature of the fluids, their capacity for causing corrosion, and the environmental conditions existing in the areas of extraction.

In the past, various solutions have been proposed for joints aimed at meeting the demands referred to above.

The US patent US-A-5427418 describes a coupled connection for pipes, in which tightness and a low screwing torque are ensured by shaping the thread so that it has a semi-circular or a triangular profile and ensuring the seal by means of grease trapped in the thread. Although a joint of this sort can achieve high values of efficiency, it does not have a shoulder designed to protect the joint from possible excess screwing torque, and hence from excessive stresses that would impair its functionality, and, since it is not provided with a metal seal, it does not guarantee a sufficiently efficient tightness in certain operative conditions.

The patent US-A-5462315 describes an embodiment of a joint having reduced diametral dimensions, which in one variant of the invention may be a coupled tubing or casing connection. The joint has a central shoulder, which bears, both upon the male side and upon the female side, a projection and a slot parallel to the axis of the pipe, with homologous surfaces and such as to mate perfectly with a blocking function for the two members of the joint. Present on the projections of the shoulder are two sealing surfaces. The shoulder separates two portions of thread, of a conical or conical-cylindrical shape, radially staggered with respect to one another.

The above joint is very efficient but has a particularly complex structure as regards its manufacture, with consequent high production costs.

Introduction of invention A primary purpose of the present invention is consequently that of overcoming the drawbacks referred to above, which are presented by the aforementioned joints for pipes, by providing a new joint of the semi-flush type which, given the same overall dimensions as compared to those of other joints of the prior art, does not present the disadvantages mentioned previously.

A particular purpose of the present invention is to provide a coupled connection with limited diametral encumbrance, which has a reduced production cost, at the same time guaranteeing high values of strength and tightness when assembled.

A further purpose of the present invention is to provide a shape that facilitates installation.

The above purposes are achieved by a threaded coupled connection for pipes, which, in accordance with Claim 1, comprises two male tubular members that are coaxial when assembled, each provided, on its outer surface, with a portion having a frustoconical shape, tapered in the direction of its own end and provided with a thread, and a female tubular member coaxial to the male members and provided on its inner surface with two frustoconical portions provided with a thread, set on opposite sides with respect to a central plane orthogonal to the axis and tapered in the direction of said central plane, each of said male tubular members having at least two sealing surfaces co-operating with corresponding sealing surfaces of said female tubular member, which they face once the pipes have been mounted, and forming together pairs of sealing surfaces, the said threaded joint being characterised in that one first of said pairs of sealing surfaces is set in a position corresponding to the end portion of said male tubular members and in the vicinity of the central region of said female tubular member, thus obtaining a seal in regard to the internal environment, and in that one second of said pairs of sealing surfaces is set at the opposite end, in the axial direction, of the threaded portions, in a position corresponding to the end region of the female member and to the starting region of the tapered portion of the male elements, thus providing a seal in regard to the environment external to the pipe.

Thanks to the above embodiment, the joint enables facilitated installation of the tubular string, with reduced risks of damage to the pipes at the joints, at the same time guaranteeing a high strength of the tubular string in said regions both during installation and during operation.

With a combination of the various different characteristics of the joint, such as conicity and shape of the threaded portion, type of thread, and an appropriate arrangement of the sealing surfaces between the pipes and the muff that make up the joint, an optimal result in terms of tightness and strength is achieved.

In embodiments in which pipes of small diameter are used, it is more expedient to use for the external seal a type of seal in which the facing surfaces are of a conical-conical type, whereas in embodiments in which the pipes are of a larger diameter, it is expedient to use sealing surfaces of a spherical-conical type.

In order to reinforce the grip between the elements, it is advantageous to provide, on one of the elements, a short portion of the thread with a non-perfect thread, which faces a portion with perfect thread of the element on which it is screwed when installed.

Other preferred embodiments of the invention are described in detail in the dependent claims.

Brief description of the drawings Further characteristics and advantages of the present invention will emerge more clearly from the ensuing detailed description of a preferred, but non-exclusive, embodiment of a coupled connection, illustrated by way of non-limiting example with the aid of the attached drawings, in which: Fig. 1 represents a sectional view in an axial plane of one part of the connection according to the invention, with two of the three component members in a detached position; Fig. 2 represents an overall view of the entire joint, with the three component members in a joined position; and Fig. 3 represents an enlargement of a detail of the profile of the thread of the connection illustrated in Fig. 1.

Means of putting the invention in hand With reference to the above figures, the coupled connection according to the invention comprises three members: two pipes, 1', 1", the pin members of the connection, and a coupling 2, with two female end portions, the box member of the connection. Since the latter are arranged symmetrically, the coupled connection may be examined considering just one of the two ends, on which there is thus present the pin 1'and the box 2. The coupled connection defines an internal part 20 of the pipe in which the fluid, for example natural gas or oil or a similar fluid, flows, and an external part 30 which may also be filled with fluids, either gases or liquids, of various nature, which are also generally under pressure. The external diameter 3 of the muff 2 in the area of maximum diameter, of length D', of the coupled connection is slightly greater than the external diameter 4 of the pipe 1'in the cylindrical central part distant from the coupled connection, of length D". The length D'of the diameter 3 is greater than the length D"of the diameter 4 by a value generally not higher than 6%.

The female member 2 of the coupled connection has an internal thread 7 with a conical generatrix with respect to the axis of the pipe and with tapering in the direction of its central part. The thread may be perfect throughout its length, or else may have a portion with an imperfect thread 7'. In the embodiment illustrated in Figures 1 and 2, this portion is set at the internal end of the thread of the box 2.

The thread has a conicity of equal degree and of a value of between 6,25% and 12,5%. The range indicated above is optimal because, on the one hand, the choice of lower values would entail making threads that are excessively long, particularly in the case of large thicknesses, with the consequence that it becomes difficult to get the male parts to enter the female part, and, on the other hand, the choice of higher values would mean that too few teeth would be available in the threaded portion, and hence the thread would have an insufficient bearing value.

The box 2 has, at the end of the thread 7, which is set inside it, an annular shoulder 5"set on a plane inclined with respect to the axis of the pipe at a negative angle 5 having a value of between and-10°. The choice of these values is advantageous because a negative angle of the shoulder favours proper engagement of the sealing surface of the male part and the female part.

The box 2 has, in the connecting region between the threaded portion and the shoulder 5", a region 10"with conical surface with respect to the axis of the pipe.

The conicity of this surface is between 12,5% and 25% in order to guarantee a good seal with the corresponding contact surface of the male part, reducing the length of sliding during the screwing step.

The range of values proposed proves optimal in relation to the value of the conicity adopted for the thread and such as to limit the negative effect of the tensile loads.

At its end, the box 2 is provided with a spherical surface 11", which, in its contact with a conical region 11', which it faces after being screwed together with the pin 1, guarantees tightness against the external pressure. In particular applications, the surface 11"can also be produced with a conical shape.

The profile of the tooth of the thread is of the"hooked"type, with the load flank 8 at a negative angle a with values of between 0° and-10° and with the lead-in flank 9 with a positive angle P of between 10° and 45°. These ranges of values afford considerable advantages, whilst at the same time maintaining ease of fabrication of the coupled connection. A load flank with a negative angle enables an effective mating of the two members of the coupled connection and reduces the possibility of opening and separation of the coupled connection on account of high tensile loads. A lead-in flank with a positive angle of a value which is not high and is close to the bottom limit of said range of values enables effective participation of the thread in the resistance to compression loads.

The pin 1'has, in the area of the outer surface which faces the threaded region of the box, a thread 6 set in a perfectly reciprocal way, with portions shaped in a perfectly analogous manner.

The connection region between the external surface of the member 1'and the start of the threaded portion 65 has a conical surface 11'with values of conicity of between 12,5% and 25%. This surface presses against the surface 11", which is conical or spherical according to the embodiment, after screwing of the coupled connection, and the dimensions and tolerances are chosen in such a way that the metal-metal contact guarantees a tightness that will prevent any liquid or gas under pressure that may be outside the coupled connection from penetrating.

Likewise, at its end the pin 1'has a surface 10'of a conical shape, which, after screwing with the box 2, comes to press against the conical surface 10"of the box.

Also in this second region, there is produced a metal-metal contact pressure between the two members, which creates a seal against the pressure of the fluid present inside the pipe.

In an alternative embodiment, it is envisaged that also the sealing surface 10'at the end of the pin member 1'is of a spherical shape.

In appropriate types of application, the choice of a region at the end of the coupled connection where the metal seal is produced, which has facing surfaces having a spherical and conical shape, in accordance with the invention, renders the coupled connection less sensitive to the pressure loads that act upon it, and has proved optimal for pipes of large diameter. In fact, given the slenderness of the end portion on which the sealing surface 11"is made, the pressure outside the pipe, acting on the aforesaid end, is very likely to cause its deflection. Consequently, a spherical sealing surface is able to maintain even so an optimal contact, as compared to a seal of the truncated-cone type, which in this case, on account of the rotation imposed by the deflection of the end, fails to maintain the contact over the entire sealing band.

At the end of the thread 6 set in the outer part of the pipe 1', the pipe 1'has an annular shoulder 5'set in a plane inclined with respect to the axis of the pipe by the same angle as that of the homologous shoulder 5"on box 2. After screwing of the pin and box members together, the shoulder 5"of the box 2 comes to rest against the end face 5'of the pin 1'.

The shape of the teeth of the thread of the pin members 1', 1"is the same as that of the thread of the box member 2, referred to above. Advantageously, the thread has a perfect profile throughout its length.

In a variant according to the invention, it may be envisaged that the end region 6' of the end of the portion of thread 6 of the pin, in the proximity of the annular sealing surface 11', has a thread with a non-perfect profile. The corresponding region 7 of the box on the side 11", which is set facing the portion 6', has a perfect thread. The region 7'at the opposite end of the threaded portion 7, i. e., the region in the vicinity of the sealing surface 10", may also itself have a thread with a non- perfect profile, and the corresponding portion of thread 6 of the pin member facing it has a perfect thread.

From what has been said above, the advantages of the coupled connection according to the invention are evident; in fact, even though the sleeve or box is of diametral encumbrance that is only slightly greater than that of the pipe body, it ensures very high operating performance and efficiency.