EP1957850B1 - Connection assembly for drill collars or heavy drill pipes - Google Patents

Abstract

The invention relates to a connection assembly (100) for drill collars or heavy drill pipes comprising: a male connection element which comprises, in its longitudinal direction, an at least partially threaded conical surface portion which tapers from the first portion towards its free end; a female connection element which comprises an at least partially threaded conical inner surface and a cylindrical inner surface (220), said two connection elements being complementary so that one can be assembled in the other by screwing, characterized in that the male connection element additionally comprises, between its free end and said conical surface portion, another surface portion whose conicity is less than that of said surface portion and which is dimensioned with respect to said cylindrical inner surface (220) such that there is a clearance by virtue of which these two surfaces bear one against the other when the assembly is subjected to a bending stress.

Description

The present invention relates to connectors for tubular or cylindrical elements and more particularly to fittings intended to form a column of elements for drilling wells, in particular oil wells. It relates more specifically to a set with connection for drill collars and / or heavy drill rods.

When drilling a well in the ground, it is necessary to connect portions of tubular conduits or other cylindrical elements to form a column or a train of elements to achieve the desired depth. In the field of oil drilling, aiming at the extraction of liquid or gaseous hydrocarbons, it is essential that these connections guarantee, at the level of the assemblies of the elements, in particular structural strength and pressure resistance.

• tiges de forage (en Anglais : drill pipes)
• tiges lourdes (en Anglais: heavy weight drill pipes)
• masse tiges (en Anglais : drill collars)
• outil de forage.

A drill string consists, generally, respectively from top to bottom, of four types of elements:

• drill rods (in English: drill pipes )
• heavy rods (in English: heavy weight drill pipes )
• mass rods (in English: drill collars )
• drilling tool.

These three types of rods are drilled axially from one side to provide a passage for the fluids to extract or inject during drilling.

• les tiges de forage, sont, à l'unité, les plus légères de ces trois types de tiges. Elles sont généralement un assemblage par soudure d'un tube et de deux raccords (tool joint en Anglais). Les tiges de forage servent de lien entre l'outil et la surface, elles sont généralement en sollicitées en traction et servent à transmettre le couple de rotation et le fluide de forage vers l'outil.
• les tiges lourdes ont une section beaucoup plus importante que les tiges de forage et peuvent être monobloc ; elles sont souvent sollicitées en flexion et ont des renflements extérieurs sur le corps (wear pads en Anglais) afin de réduire les contacts et donc l'usure sur les parois du puits. Elles sont utiles comme éléments de transition entre les masses tiges et les tiges de forage. Elles diminuent considérablement les vibrations générées par la coupe de la roche par l'outil.
• les masse-tiges ont des parois très épaisses (de l'ordre de plusieurs pouces soit plusieurs dizaines de millimètres) ; elles sont souvent usinées en spirale sur leur partie extérieure afin de laisser un passage pour le fluide de forage entre la surface extérieure de la masse tige et la paroi du puits, ceci afin de réduire les collages par pression différentielle. Elles peuvent aussi recevoir des usinages extérieurs pour faciliter leur manoeuvre ou leur mise en oeuvre (« slip and elevator recess » en anglais). Elles contribuent par leur poids à exercer un effort de coupe sur l'outil de forage et à mettre en tension les tiges de forage.

These different types of stems are distinguished mainly by the fact that:

• the drill pipes are, by the unit, the lightest of these three types of rods. They are usually a joint by welding a tube and two connections ( tool joint in English). The drill rods serve as a link between the tool and the surface, they are generally stressed in traction and serve to transmit the torque and the drilling fluid to the tool.
• heavy rods have a much larger section than drill rods and can be monobloc; they are often bending and have external bulges on the body ( wear pads in English) to reduce contact and thus wear on the walls of the well. They are useful as transition elements between the rod masses and the drill rods. They considerably reduce the vibrations generated by cutting the rock by the tool.
• the drill-rods have very thick walls (of the order of several inches or several tens of millimeters); they are often machined in a spiral on their outer part to allow a passage for the drilling fluid between the outer surface of the rod body and the wall of the well, in order to reduce the collages by differential pressure. They can also receive external machining to facilitate their maneuver or their implementation (" slip and elevator recess " in English). They contribute by their weight to exert a cutting force on the drill bit and to put in tension the drill rods.

The set of heavy rods, drill collars and drill bits is sometimes referred to as "Bottom Hole Assembly".

It is the connection of the lower elements of such a drilling column that the present invention aims.

The gravity acting, in particular, on the heavy rods and mass rods makes it possible to use them in compression in order to apply a force of penetration to the cutting elements of the drilling tool at the lower end of the drill string. The amount of heavy rods and rod weights used in a given column depends on the load that is to be applied to the drill end. In addition, the elements of the drill string are rotated to give a cutting movement to the drill bit at its lower end.

The elements of the drill string and in particular the mass rods and heavy rods are therefore, at the same time, stressed in compression and in torsion.

The rods constituting the drill string are provided with connections at each of their ends to assemble them to each other, in order to obtain a desired configuration.

Usually, this type of connection is made using complementary threaded surfaces disposed at the ends of the elements to be assembled, on respectively male and female portions, usually using seals sandwiched by two suitable surfaces of said two portions. In general, these are conical threads having a large pitch (several millimeters) to facilitate quick assembly (a fraction of a turn needed for tightening).

Such tapered threaded fittings for oil rig elements have been standardized in API Specification 7 hereby incorporated by reference (API: American Petroleum Institute).

A fitting as defined by this standard is composed of a male element comprising a shoulder and an at least partially threaded conical portion extending from this shoulder to the free end of this element and a complementary female element having a first threaded conical inner surface having the same conicity as the conical surface of the male member, but shorter than the latter, and extended by another smooth conical inner conical surface of smaller conicity than said first conical surface.

A problem encountered in this technical field and in particular with this type of connection results from the fact that the elements of the drill string, in particular the rods and the heavy rods, have radial dimensions several times smaller than their longitudinal dimensions (typically, for a heavy stem: 900cm long for a diameter of 25cm). In addition, the diameter of the boreholes in which the heavy rods and the rods are used is much greater than their own diameter. In general, the diameter of such a borehole is 17 ½ inches (about 44.5cm).

Therefore, being subjected to compressive stresses, these columns of drilling elements are naturally biased to deform buckling in the drilled hole. This buckling is translated locally by bending forces, especially at the connections between the elements constituting the column. In practice, these connections constitute geometrical discontinuities, (in particular because of section reductions) relative to the rest of the column; they are therefore, in case of buckling thereof, the place of concentration of stress, that consequently significant risks of rupture at this level.

In particular, the torsional forces combined with repeated bending forces at the joints typically result in fatigue failure problems caused mainly by stress concentrations on the joints. the female element, in particular on the boundary circle between said two conical inner surfaces. It has been found, in practice, that breaks caused by fatigue phenomena occur along this circle, portions of which are stressed sometimes in compression, sometimes in tension according to the random angular orientation of the buckling of the column.

A connector assembly according to the preamble of claim 1 is known by US 5,908,212 .

The present invention aims to overcome these drawbacks and relates to a connection assembly for drilling elements, best meeting the API Specification 7 standard but allowing to withstand high internal stresses resulting from torsional forces, compression and buckling of the drill string, and thus prevent the breaking of elements of the connection, in particular by fatigue phenomenon. More particularly, it aims at reducing the maximum value of the local stresses observed at the connection.

• un élément de raccord mâle ayant sensiblement une forme de révolution autour d'un axe longitudinal et qui comporte dans son sens longitudinal respectivement :
  • une première portion sensiblement cylindrique formant épaulement à une de ses extrémités,
  • une seconde portion de surface conique au moins partiellement filetée et s'effilant à partir de la première portion vers l'extrémité libre de l'élément
• un élément de raccord femelle sensiblement cylindrique, complémentaire dudit élément de raccord mâle, et qui comporte :
  • une portion radiale formant une face d'appui pour ledit épaulement du premier élément et, longitudinalement, coaxialement et respectivement, à partir de cette portion,
  • une surface intérieure conique au moins partiellement filetée complémentaire de la surface conique au moins partiellement filetée de l'élément mâle,
  • une surface intérieure cylindrique

lesdits deux éléments de raccord étant destinés à être assemblés l'un dans l'autre par vissage sur plusieurs tours des filetages, caractérisé en ce que l'élément de raccord mâle comporte, en outre, entre ladite deuxième portion et son extrémité libre, une troisième portion de surface de conicité inférieure à celle de la seconde portion de surface conique, et dimensionnée par rapport à ladite surface intérieure cylindrique de manière à ce qu'il existe un jeu grâce à quoi, lors d'une sollicitation en flexion de l'ensemble, ces deux surfaces entrent respectivement en appui.It proposes for this purpose a connection assembly for heavy rods or drill pipe-bodies comprising:

• a male coupling element having substantially a shape of revolution about a longitudinal axis and which has in its longitudinal direction respectively:
  • a first substantially cylindrical portion forming a shoulder at one of its ends,
  • a second conical surface portion at least partially threaded and tapering from the first portion to the free end of the element
• a substantially cylindrical female connector element, complementary to said male connector element, and which comprises:
  • a radial portion forming a bearing face for said shoulder of the first element and, longitudinally, coaxially and respectively, from this portion,
  • an at least partially threaded conical inner surface complementary to the at least partially threaded conical surface of the male element,
  • a cylindrical inner surface

said two coupling elements being intended to be assembled one into the other by screwing on several turns of the threads, characterized in that the male coupling element further comprises between said second portion and its free end, a third conical surface portion smaller than that of the second conical surface portion, and dimensioned with respect to said cylindrical inner surface so that there is a game through which, during a solicitation in flexion of the assembly, these two surfaces bear respectively.

Thus, the present invention makes it possible to reduce the stresses observed at the connections, resulting in particular from the buckling of the elements, and thus to increase the resistance to fatigue fracture at the connections.

Preferably, the female element takes the standard dimensions defined by the API Specification 7 standard and the male element is the only element of the connector assembly to be modified while being compatible with standard female elements according to this standard.

• une première portion sensiblement cylindrique formant épaulement à une de ses extrémités,
• une seconde portion de surface conique au moins partiellement filetée et s'effilant à partir de la première portion vers l'extrémité libre de l'élément,

caractérisé en ce que l'élément de raccord mâle comporte, en outre, entre ladite deuxième portion et son extrémité libre, une troisième portion de surface de conicité inférieure à celle de la seconde portion de surface conique.Thus, the invention also proposes a male coupling element for heavy rods or drill collars intended to cooperate with a female element and having substantially a shape of revolution about a longitudinal axis and which comprises in its longitudinal direction respectively:

• a first substantially cylindrical portion forming a shoulder at one of its ends,
• a second portion of tapered surface at least partially threaded and tapering from the first portion to the free end of the member,

characterized in that the male connector member further comprises, between said second portion and its free end, a third conical surface portion smaller than that of the second conical surface portion.

In fact, it has already been proposed by the document FR-2,508,970 to use cylindrical guide surfaces on each of the elements to be assembled, on either side of the threaded conical portions, said bearing surfaces being dimensioned so that they are nested one inside the other before the threads engage in order to ensure proper alignment of these prior to screwing, to prevent damage to the nets. But this document is located in a field very different from that of oil drilling since it is in the field of surface casing, especially at sea.

The tubular elements described in this document are not actual drilling equipment. They are used in a quasi-static way during the operation of a drilled hole, and are not subject to torsional and compressive forces encountered in the drilling equipment. These elements are only subjected to tensile forces exerted by their own weight. The role of the connections between these elements is limited to ensure the seal between said elements and the external environment. The mechanical stresses that apply to these casing elements are therefore totally different from those of the present invention. In addition, on the type of casing described by the document FR 2,508,970 , the main problem is to guide the thread during the assembly of two elements so as not to damage it during screwing.

Conversely, the connections between drill pipes, object of the present invention, do not generally pose a problem of screwing, and do not require guiding elements.

However, the present invention aims to reduce, at the connections between drilling elements, the stress concentration generated by the buckling resulting compression forces / torsion acting on the column.

It follows from the foregoing that the skilled person seeking to improve the connections for drilling elements has a priori no reason to appeal to the knowledge of diametrically opposite technical field of extraction casing fittings, especially as the standards applicable to drilling are very strict and deter in practice the skilled person to seek to retouch.

Document is known US 5,908,212 A a tool coupling with high torque resistance, with two shoulders, involving the abutting of shoulders located on either side of the threaded zone respectively on the male and female portion, which excludes any possibility of consuming the clearance between the nose of the male element and the inner surface of the corresponding female element. Thus, such a tool coupling is unable to solve the problem of reducing the maximum value of the local stresses observed at the connection.

The same goes for the document WO 2005/095840 A1 , which describes a double-shoulder high-torque tool seal, which also excludes any possibility of game consumption, for the same reasons.

• l'élément mâle comporte une gorge disposée entre ladite seconde portion et ladite troisième portion, de telle manière que, lorsque lesdits deux éléments sont assemblés, et lorsque ledit épaulement se trouve en butée contre la face d'appui radiale, la limite géométrique entre ladite surface intérieure conique et ladite surface intérieure cylindrique se trouve comprise dans l'espace formé par ladite gorge ;
• la troisième portion de surface de l'élément mâle formant portée est sensiblement cylindrique ;
  • ladite portion est partiellement conique à son extrémité libre ;
  • ladite conicité est inférieure à celle de la seconde portion conique au moins partiellement filetée ;
  • la troisième portion a une dimension longitudinale au plus égale aux 2/3 de la dimension longitudinale de la seconde portion ;
  • la troisième portion a une dimension longitudinale comprise entre 10 % et 50 % de son diamètre ;
  • la gorge a un rayon de courbure compris entre % et 3 pouces soit environ entre 12 millimètres et 76 millimètres suivant le diamètre des raccords.
  • les deux éléments sont axialement percés et débouchants.

According to advantageous features of the invention, possibly combined:

• the male element comprises a groove disposed between said second portion and said third portion, such that, when said two elements are assembled, and when said shoulder is in abutment against the radial bearing face, the geometric limit between said conical inner surface and said cylindrical inner surface is included in the space formed by said groove;
• the third surface portion of the male bearing member is substantially cylindrical;
  • said portion is partially conical at its free end;
  • said taper is less than that of the second tapered portion at least partially threaded;
  • the third portion has a longitudinal dimension at most equal to 2/3 of the longitudinal dimension of the second portion;
  • the third portion has a longitudinal dimension of between 10% and 50% of its diameter;
  • the groove has a radius of curvature between% and 3 inches is about 12 millimeters and 76 millimeters depending on the diameter of the fittings.
  • the two elements are axially pierced and open.

Thus, the present invention proposes to distribute the stresses on both sides of the boundary between the two surfaces, without soliciting this critical area directly by mechanical contact.

In other words, for the same bending of the connection, the invention proposes to distribute the forces resulting from this bending on a higher contact surface between the two elements constituting the connection, and thus to reduce the value of the maximum observed stresses. (the constraint is by definition a force per unit area).

In addition, the fact of adding a slightly conical or even cylindrical range beyond this critical zone makes it possible to move a part of the forces on a surface farther from the free end of the female element towards a portion of this female element where the walls have a larger section allowing them to better withstand mechanical stresses.

• La figure 1 est une vue en coupe longitudinale d'un élément femelle classique en soi, conforme à la norme API.
• La figure 2 est vue en coupe longitudinale d'un élément male selon l'invention,
• La figure 3 est une vue en coupe longitudinale des deux éléments précités à l'état assemblé selon l'invention.

The features and advantages of the invention will emerge from the description which follows, given by way of illustration and in no way limitative, with reference to the appended drawings in which:

• The figure 1 is a longitudinal sectional view of a conventional female element in itself, in accordance with the API standard.
• The figure 2 is seen in longitudinal section of a male element according to the invention,
• The figure 3 is a longitudinal sectional view of the two aforementioned elements in the assembled state according to the invention.

As visible in figure 3 , a connection assembly 100 according to the invention is composed of a male element 300 and a female element 200 assembled. These two elements are respectively visible in isolation Figures 2 and 1 .

The male coupling element 300 has a shape of revolution and comprises in its longitudinal direction a first substantially cylindrical portion 302 shaped at one of its ends 301 at its end, and intended, at its opposite end, to cooperate with a column member. drilling (not shown), a conical portion 310 provided with a threading 311, a groove 320 being provided between said substantially cylindrical portion 302 shaped as a shoulder 301 and the conical portion 310, and a substantially cylindrical portion 330 forming the end of the element 300, and connected to the conical portion 310 by a groove 340.

This groove advantageously has a radius of curvature of between ½ and 3 inches depending on the diameters of the connections, or between about 12 millimeters and 76 millimeters.

The connecting element 300 is pierced at the end intended to cooperate with a column element of a substantially cylindrical bore 350 terminated by a widening conical portion 360 to open at the opposite end of the connecting element 300. .

The female coupling element 200 is also open and has a substantially cylindrical external shape, and at its end opposite the end intended to cooperate with a drill string element (not shown), a radial face 201 forming a face support for cooperating with said shoulder 301, and, in the longitudinal direction, coaxially respectively, a conical inner surface portion 210 partially provided with a thread 211 complementary to the conical surface 310 provided with the threading 311, a portion of substantially cylindrical inner surface 220 intended to cooperate with the cylindrical portion 330 in free adjustment and preferably just sliding, a conical inner surface portion 230 of the same conicity as the portion 210 , an inner surface portion 240 of substantially greater conicity than the portion 210, and a cylindrical surface portion 250 opening at the end for cooperating with a drill string element (not shown).

As visible in figure 3 when the two elements 200 and 300 are assembled, the shoulder 301 is in abutment against the radial surface 201, and the range 330 substantially opposite the range 220, a slight clearance, exaggerated for the clarity of the figure, being visible between these two litters. This clearance is advantageously of the order of an adjustment H7g6, for example (according to NF R91-011).

The contact between the shoulder 301 and the radial surface 201 seals the assembly.

The groove 340 is astride between the conical surface 210 and the cylindrical surface 220, substantially opposite the circle 260 forming a boundary between these two surfaces.

Thus, during a flexural biasing of the assembly 100, the two surfaces forming surfaces 330 and 220 come into contact and then support, which makes it possible to distribute the forces, reduce the stresses observed at the level of the female element, without the critical circle and its immediate neighborhood being solicited in contact.

It should be noted that the grooves 320 and 340 can be used for engaging and disengaging a cutting tool for machining the thread 311 of the part 310 they frame. Thread 311 may, however, be manufactured by any other method.

In addition, the two grooves can also serve as relaxation grooves for pressurized fluids to help seal the assembly.

Advantageously, these two grooves can receive seals (not shown), for example O-rings, to improve the sealing of the connection.

Alternatively or in addition to the above, a groove (not shown) can be machined in the scope 330 to accommodate a joint sealing, for example an O-ring, always to improve the sealing of the assembly, especially during an eruption drilling, for example.

Advantageously, the span 330 may not be cylindrical but slightly conical (with a conicity smaller than that of the threaded portions), at least partially, at its free end, to facilitate a linear contact with the bearing 220 in case of flexion of the assembly. , and ensure a better distribution (distribution) of constraints.

These elements are advantageously in accordance with the API Specification 7 standard.

• diamètre intérieur maximal de la portion 210: 153 millimètres environ ;
• diamètre intérieur minimal de la portion 210: 134 millimètres environ ;
• longueur totale de la portion conique 210 : 114 millimètres environ ;
• longueur filetée de la portion conique 210 : 98 millimètres environ ;
• longueur de la portion cylindrique 220 : 51 millimètres environ ;
• diamètre intérieur de la portion cylindrique 220 : 134 millimètres environ ;
• longueur de la portion conique 230 : 51 millimètres environ ;
• conicité de la portion 240 : 30° environ ;
• diamètre intérieur de la portion cylindrique 250: 76 millimètres environ ;
• diamètre intérieur de la gorge 320 : 138 millimètres environ ;
• diamètre extérieur maximal de la portion conique 310: 154 millimètres environ ;
• longueur entre l'épaulement 301 et l'extrémité la plus éloignée de la gorge 340 : 114 millimètres environ ;
• diamètre minimal de la gorge 340 : 128 millimètres environ ;
• largeur maximale de la gorge 340 : 20 millimètres environ ;
• rayon de courbure minimal de la gorge 340 :
• diamètre intérieur de la portion 350 : 76 millimètres environ ;
• diamètre intérieur maximal de la portion conique 360: 83 millimètres environ ;
• jeu sur le diamètre entre les portions 330 et 220 : ajustement H7g6)

More precisely, the elements advantageously have the following dimensions, given by way of example (for an NC50 type connection according to the API standard):

• maximum inside diameter of portion 210: approximately 153 millimeters;
• minimum inside diameter of portion 210: approximately 134 millimeters;
• total length of the tapered portion 210: about 114 millimeters;
• threaded length of the conical portion 210: about 98 millimeters;
• length of the cylindrical portion 220: about 51 millimeters;
• inner diameter of the cylindrical portion 220: about 134 millimeters;
• length of the conical portion 230: about 51 millimeters;
• taper of the portion 240: about 30 °;
• inner diameter of the cylindrical portion 250: about 76 millimeters;
• inner diameter of throat 320: approximately 138 millimeters;
• maximum outside diameter of the conical portion 310: approximately 154 millimeters;
• length between the shoulder 301 and the end farthest from the groove 340: 114 millimeters approximately;
• minimum diameter of throat 340: approximately 128 millimeters;
• maximum width of the groove 340: approximately 20 millimeters;
• radius of curvature of throat 340:
• inner diameter of the portion 350: about 76 millimeters;
• maximum inside diameter of conical portion 360: approximately 83 millimeters;
• play on the diameter between the portions 330 and 220: adjustment H7g6)

A test made it possible to evaluate at approximately 60% the reduction of the stresses thus obtained at 10 cm from the sealing surface, constituted by the contact between the shoulder 301 and the radial surface 201 (this test was carried out with gauges of stresses on the outer surface of the female element at 2, 4, 6, 8 inches, namely 50.8, 101.6, 152.4, 203.2 millimeters, of the sealing surface 201) .

It should be noted that many modifications or variations of the connector assembly described and shown can be easily performed by the skilled person without departing from the scope of the invention.

Claims (10)

1. Connection assembly (100) for drill collars or heavyweight drill pipes, comprising:

   - a male connecting element (300) having a substantially circular section about a longitudinal axis and that includes within its longitudinal direction, respectively:

   - a substantially cylindrical first portion (302) forming a shoulder (301) at one of its ends,

   - an at least partly threaded second conical surface portion (310) tapering from the first portion toward the free end of the element,

   - a substantially cylindrical female connecting element (200), complementary to said male connecting element, including:

   - a radial portion (201) forming a bearing face for said shoulder (301) of the first element and, longitudinally, coaxially and respectively, from that portion,

   - an at least partially threaded conical interior surface (210) complementary to the at least partially threaded conical surface (310) of the male element,

   - a cylindrical interior surface (220),

   said two connecting elements (200, 300) being intended to be assembled together by a screwing action entailing several turns of the threads and the male connecting element further including, between said second portion (310) and its free end, a third surface portion (330) of lower taper than the second portion of the conical surface (310) characterised in that said third surface portion (330) is sized relative to said cylindrical interior surface (220) so that there exists a gap whereby these two surfaces come respectively intro bearing engagement if the assembly is stressed in bending.
2. Connection assembly (100) according to claim 1, characterised in that the male element (300) includes a groove (340) disposed between said second portion (310) and said third portion (330), so that, when said two elements are assembled together, and when said shoulder (301) is abutted against the radial bearing face, the geometrical limit between said conical interior surface and said cylindrical interior surface is in the space formed by said groove.
3. Connection assembly (100) according to either of the preceding claims, characterised in that the third surface portion (330) of the male element forming a bearing surface is substantially cylindrical.
4. Connection assembly (100) according to claim 3, characterised in that said portion (330) is partially conical at its free end.
5. Connection assembly (100) according to claim 4, characterised in that said taper is less than that of the at least partly threaded second conical portion (310).
6. Connection assembly (100) according to any ane of claims 1 to 5, characterised in that the third portion (330) has a longitudinal dimension at most equal to 2/3 of the longitudinal dimension of the second portion (310).
7. Connection assembly (100) according to any one of claims 1 to 6, characterised in that the third portion (330) has a longitudinal dimension from 10% to 50% of its diameter.
8. Connection assembly (100) according to any one of claims 1 to 7, characterised in that the groove (340) has a radius of curvature from 1/2 to 3 inches, i.e. from approximately 12 millimeters to 76 millimeters.
9. Connection assembly (100) according to any one of the preceding claims, characterised in that the two elements (200, 300) are axially bored and open-ended.
10. Male connecting element (300) for heavyweight drill pipes or drill collars intended to cooperate with a female element (200) in order to form an assembly according to any one of the preceding claims and having a substantially circular shape about a longitudinal axis, including in its longitudinal direction, respectively:

    - a substantially cylindrical first portion (302) forming a shoulder (301) at one of its ends,

    - an at least partly threaded second conical surface portion (310) tapering from the first portion toward the free end of the element,

    the male connecting element further including, between said second portion and its free end, a third surface portion (330) of smaller taper than the second conical surface portion.

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