EP0319419B1 - Flexible connector for the tension leg of an oil platform - Google Patents

Description

The present invention relates to a flexible articulation for anchoring line of an oil platform of the type known as taut lines.

Such a platform is an operating or drilling platform, of the floating type, which is moored at the bottom of the sea using tubular anchor lines (or rods) whose ends are articulated in articulation boxes fixed to connectors arranged at the bottom of the sea and in the columns of the platform.

All the lines are constantly subjected to tensile forces (hence the expression of taut line platform corresponding to the English expression "Tension Leg Platform", TLP) because of the floating nature of the oil rig.

The articulation of the ends of each of the anchor lines allows them to follow its horizontal displacements under the action of waves and winds, behaving like a deformable parallelogram.

The essential element of each articulation is constituted by an elastomeric laminated annular structure and with spherical symmetry in which elastomeric layers alternate with curved metallic layers, which justifies the designation of "flexible" articulation given to this type of 'joint.

• transmission des efforts de traction exercés par la ligne d'ancrage,
• autorisation des débattements angulaires (ou coniques) de la ligne d'ancrage provoquée par les déplacements horizontaux de la plate-forme flottante à la surface de la mer, en limitant les moments d'encastrement,
• transmission des effets latéraux, au niveau du connecteur supérieur.

The main functions of a laminated elastomer structure of the aforementioned type, within the framework of a flexible articulation, are the following:

• transmission of the tensile forces exerted by the anchor line,
• authorization of the angular (or conical) deflections of the anchoring line caused by the horizontal movements of the floating platform on the sea surface, by limiting the moments of embedding,
• transmission of side effects, at the upper connector.

Of course, the design of the structure laminate is conditioned by many parameters, such as stress, available space, environment, etc ...; but, in any case, the loads and deflections to which a laminated structure is subjected in each connector must be such that this structure only works in compression and in shear.

However, during the mechanical disconnection of the mooring lines, the aforementioned laminated structure is subjected to an undesired tensile force, which should be avoided so as not to destroy it.

The flexible joint used for an anchor line in the prior art comprises a single laminated structure, with spherical symmetry, such as that represented by the reference F in FIG. 2 appended to the description of the present invention: this laminated structure consists of elastomeric layers alternating with curved metallic layers, having the same center of curvature, and is secured to the cover of an articulation box B fixed to a lower connector E formed in the sea floor or to a connector upper located at the upper end of the anchor line.

However, in this known type of flexible joints, the tensile forces manifest themselves not only when the system is disconnected, but also under normal conditions and are limited (as can be seen in FIG. 2 above) only by a metal ball joint R which constitutes an axial stop centered in the center of curvature of the flexible joint. This design is therefore far from satisfactory and further decreases the life of the joint due to wear due to friction between the end of the anchor line and the ball joint.

• l'articulation flexible n'est soumise à aucune tension et limite la fatigue des structures lamifiées, parce que sa conception permet de maîtriser les efforts transversaux,
• la durée de vie est sensiblement augmentée grâce à l'élimination de toute friction par contact métal contre métal dans l'articulation flexible,
• cette articulation présente un centre géométrique qui coîncide avec le centre d'application des efforts.

EP-A1-0 045 613 also discloses a hinge according to the preamble of claim 1, and it is an object of the invention to provide a flexible hinge for the mooring line of a platform. floating oil tank, of the so-called taut line type, which better meets the needs of the practice as flexible joints aiming at the same goal previously known, in particular in that:

• the flexible joint is not subjected to any tension and limits the fatigue of the laminated structures, because its design makes it possible to control the transverse forces,
• the service life is significantly increased thanks to the elimination of any friction by metal against metal contact in the flexible joint,
• this articulation has a geometric center which coincides with the center of application of the forces.

• un boîtier d'articulation de l'extrémité correspondante qui est solidarisé à un connecteur de fixation et qui est délimité par :
  • . un couvercle pourvu d'une ouverture centrale permettant le passage de la ligne tendue et les déplacements angulaires de celle-ci correspondant aux déplacements de la plate-forme flottante,
  • . une paroi latérale, et
  • . un fond,
  cette paroi latérale et ce fond définissant l'enceinte du boîtier,

ladite articulation flexible, logée dans ledit boîtier, permettant la rotation de l'extrémité correspondante de la ligne tendue et comprenant :

• . une première structure annulaire lamifiée à symétrie sphérique par rapport à un centre de courbure situé à l'intérieur du boîtier d'articulation,et disposée autour de la ligne tendue, laquelle première structure lamifiée, -qui est à l'état précontraint à savoir montée avec précontrainte-, est solidarisée à l'extrémité de la ligne tendue et au couvercle du boîtier et se compose d'une pluralité de couches élastomères alternées à des couches métalliques galbées adhérisées aux couches élastomères et ayant le centre de courbure précité,

laquelle articulation flexible est caractérisée par les points suivants :

• a) elle comprend en outre une deuxième structure annulaire lamifiée également à symétrie sphérique par rapport à un point qui coïncide avec le centre de courbure de ladite première structure lamifiée, -et elle aussi montée avec précontrainte-, et qui constitue donc le centre de courbure de l'articulation flexible dans son ensemble, laquelle deuxième structure annulaire lamifiée est solidarisée à l'extrémité de la ligne tendue et au fond du boîtier d'articulation et se compose, comme la première structure lamifiée, d'une pluralité de couches élastomères alternées à des couches métallliques galbées adhérisées aux couches élastomères,
• b) la hauteur de la paroi latérale de l'enceinte du boîtier d'articulation est définie par rapport à l'encombrement axial de l'articulation flexible et est choisie en fonction du degré de précontrainte souhaité ;
• c) ladite deuxième structure annulaire lamifiée est disposée, par rapport au centre de courbure de l'articulation, du même côté ou se situe ladite première structure annulaire lamifiée ;
• d) le fond du boîtier d'articulation présente une saillie tubulaire centrale (axiale) se prolongeant à l'intérieur du boîtier en direction du couvercle de celui-ci, jusqu'à dépasser le centre de courbure précité, l'extrémité de la ligne tendue présentant un évidement (une concavité) dans lequel vient se loger sans contact une portion de ladite saillie tubulaire ;
• e) ladite deuxième structure annulaire lamifiée est adhérisée entre deux pièces annulaires métalliques dont la première pièce est fixée à ladite saillie tubulaire tandis que la deuxième pièce annulaire est fixée à l'extrémité de la ligne tendue.

The subject of the present invention is a flexible articulation for the mooring line of a floating oil platform, known as a taut line platform, comprising at each end of the line:

• an articulation box of the corresponding end which is secured to a fixing connector and which is delimited by:
  • . a cover provided with a central opening allowing the passage of the taut line and the angular displacements of the latter corresponding to the displacements of the floating platform,
  • . a side wall, and
  • . a fund,
  this side wall and this bottom defining the enclosure of the housing,

said flexible articulation, housed in said housing, allowing the rotation of the corresponding end of the taut line and comprising:

• . a first laminated annular structure with spherical symmetry with respect to a center of curvature located inside the articulation box, and arranged around the tensioned line, which first laminated structure, which is in the pre-stressed state, namely mounted with prestress-, is secured at the end of the taut line and at cover of the housing and consists of a plurality of elastomeric layers alternating with curved metallic layers adhered to the elastomeric layers and having the aforementioned center of curvature,

which flexible joint is characterized by the following points:

• a) it further comprises a second annular laminated structure also with spherical symmetry with respect to a point which coincides with the center of curvature of said first laminated structure, -and it also mounted with prestress-, and which therefore constitutes the center of curvature of the flexible joint as a whole, which second laminated annular structure is secured to the end of the taut line and to the bottom of the joint housing and is composed, like the first laminated structure, of a plurality of alternating elastomer layers to curved metal layers adhered to the elastomeric layers,
• b) the height of the side wall of the enclosure of the articulation unit is defined with respect to the axial size of the flexible articulation and is chosen according to the desired degree of prestressing;
• c) said second laminated annular structure is disposed, with respect to the center of curvature of the joint, on the same side where said first laminated annular structure is located;
• d) the bottom of the articulation box has a central tubular projection (axial) extending inside the box in the direction of the cover thereof, until it exceeds the aforementioned center of curvature, the end of the line tense with a recess (a concavity) in which is housed without contact a portion of said tubular projection;
• e) said second laminated annular structure is bonded between two metallic annular parts, the first part of which is fixed to said tubular projection while the second annular part is fixed to the end of the taut line.

According to a preferred form of this provision, said first annular support piece of said second laminated annular structure is threaded around said tubular projection of the bottom of the housing and comes to bear on a shoulder of the external wall of this tubular projection, said shoulder being formed substantially at the center of curvature flexible joint.

According to another preferred embodiment of the joint according to the invention, the height of the side wall of the joint housing is chosen according to a prefixed degree of prestressing which it is desired to induce in said first and second laminated annular structures of the flexible joint.

According to another advantageous embodiment of the articulation according to the invention, said first laminated structure is secured to the cover of the articulation box by means of a part intended to be embedded in the cover.

According to yet another embodiment of the joint according to the invention, the elastomeric layers of each of said first and second laminated annular structures have different thicknesses.

In addition to the above arrangements, the invention also relates to a method for mounting a beautiful articulation as defined above, characterized in that, in order to pre-stress the laminated annular structures, the enclosure of the articulation housing is fixed to the cover thereof, after having fixed, on the one hand, the first laminated annular structure to the cover of the articulation box and, on the other hand, the second laminated annular structure at the removable end of the mooring line , the height of the side wall of the enclosure of the articulation unit being chosen so as to pass the above-mentioned laminated annular structures from the normal state to the prestressed state.

Advantageously, the prestress is greater than the maximum tension which is induced in any one of said first and second laminated annular structures of the flexible joint, during the transmission of a force contrary to the normal direction of operation of the line. mooring, this effort being due to exceptional conditions, such as mounting or handling, but sufficient to damage any of the annular structures laminated which allows them to work always in compression.

• la figure 1 est une vue schématique d'une plate-forme pétrolière flottante, du type dit à lignes tendues (TLP), à savoir d'une plate-forme qui est amarrée au fond de la mer par des lignes d'ancrage articulées à leurs extrémités inférieures et supérieures par l'intermédiaire de connecteurs fixés au fond de la mer et aux colonnes de la plate-forme, respectivement,
• la figure 2, comme déjà évoqué plus haut, illustre une articulation flexible de l'Art antérieur fixée à un connecteur E ménagé dans le fond de la mer et représenté schématiquement avec arrachements, laquelle articulation flexible est disposée à l'intérieur d'un boîtier d'articulation B et comprend une structure annulaire élastomère lamifiée à symétrie sphérique, F, et une rotule R coopérant avec l'extrémité élargie de la ligne d'ancrage 1 ;
• les figures 3 et 4 représentent un premier mode de réalisation de l'articulation flexible selon l'invention et diffèrent essentiellement en ce qui concerne le mode de fixation au couvercle du boîtier d'articulation d'une des deux structures lamifiées dont se compose l'articulation flexible.

Other provisions of the invention will emerge from the description which follows, which refers to the appended drawings in which:

• Figure 1 is a schematic view of a floating oil platform, of the type known as taut lines (TLP), namely of a platform which is moored at the bottom of the sea by anchor lines hinged to their lower and upper ends by means of connectors fixed to the sea floor and to the columns of the platform, respectively,
• Figure 2, as already mentioned above, illustrates a flexible joint of the prior art fixed to a connector E formed in the sea floor and shown schematically with cutouts, which flexible joint is arranged inside a housing of articulation B and comprises a laminated elastomeric annular structure with spherical symmetry, F, and a ball joint R cooperating with the enlarged end of the anchoring line 1;
• Figures 3 and 4 show a first embodiment of the flexible joint according to the invention and differ essentially as regards the method of attachment to the cover of the joint housing of one of the two laminated structures of which the flexible joint.

It should be understood, however, that these drawings and the corresponding descriptive parts, are given solely by way of illustration of the subject of the invention, of which they do not in any way constitute a limitation.

Figures 3 and 4 show one end 10 of one of the taut lines 1 for mooring a floating platform (not shown: see, however, Figure 2) at the bottom of the sea, which is surrounded by a housing d hinge 2 of the end 10.

• un couvercle 3 pourvu d'une ouverture centrale 4 permettant le passage de la ligne 1 et les déplacements angulaires de celle-ci correspondant aux déplacements de la plate-forme flottante sous l'action des vagues et des vents,
• une paroi latérale 5, et
• un fond 6,

ce fond et la paroi latérale définissant l'enceinte 15 du boîtier.The housing, which is fixed to a connector (not shown: however, see still Figure 2), is delimited by:

• a cover 3 provided with a central opening 4 allowing the passage of the line 1 and the angular displacements of the latter corresponding to the displacements of the floating platform under the action of waves and winds,
• a side wall 5, and
• a background 6,

this bottom and the side wall defining the enclosure 15 of the housing.

• une première structure annulaire lamifiée 20 à symétrie sphérique par rapport à un centre C, disposée autour de la ligne 1, qui est adhérisée à l'extrémité 10 et au couvercle 3 du boîtier 2 et qui se compose d'une pluralité de couches élastomères 8 alternées à des couches galbées 9, de préférence métalliques, adhérisées aux couches 8, le centre de courbure de la structure lamifiée 20 dans son ensemble et des couches galbées 9 en particulier étant défini par le point C,
• une deuxième structure annulaire lamifiée à symétrie sphérique qui est solidarisée à l'extrémité 10 et au fond 6 du boîtier d'articulation 2 et qui se compose, comme la première structure lamifiée 20, d'une pluralité de couches élastomères alternées à des couches galbées métalliques adhérisées aux couches élastomères, le centre d e courbure de la deuxième structure lamifiée et de ses couches galbées coïncidant avec le centre de courbure C de la première structure lamifiée 20.

Inside the housing 2 is housed a flexible articulation 40 (cf. FIGS. 3 and 4) allowing the rotation of the end 10 and including:

• a first laminated annular structure 20 with spherical symmetry with respect to a center C, disposed around the line 1, which is adhered to the end 10 and to the cover 3 of the housing 2 and which consists of a plurality of elastomeric layers 8 alternating with curved layers 9, preferably metallic, adhered to layers 8, the center of curvature of the laminated structure 20 as a whole and curved layers 9 in particular being defined by point C,
• a second laminated annular structure with spherical symmetry which is secured at the end 10 and at the bottom 6 of the articulation box 2 and which is composed, like the first laminated structure 20, of a plurality of elastomer layers alternated with curved layers metallic adhered to the elastomeric layers, the center of curvature of the second laminated structure and its curved layers coinciding with the center of curvature C of the first laminated structure 20.

In FIGS. 3 and 4, the laminated structure 30 is carried by a central (axial) and re-entrant tubular projection 14 from the bottom 6 of the housing 2.

Referring now more particularly to FIGS. 3 and 4, it should be noted that the second laminated structure 30, composed of layers elastomers 12 adhered to metallic curved layers 13, is adhered between two annular parts 16 and 17.

The part 16 is threaded around the tubular projection 14 of the bottom 6 of the housing 2 and comes to bear on a shoulder 18 of the external wall of this tubular projection 14, the fixing of the annular part 16 to the bottom of the housing being obtained by bolts 19.

The annular part 17 is fixed to the end 10 of the line 1 by bolts 21.

To allow cooperation between the end 10 and the tubular projection 14 a recess 25 (concavity) is formed in this end.

The first laminated structure 20 can be adhered to the cover 3 directly (cf. FIGS. 3 and 5)

As regards the preparation of the two laminated structures 20 and 30 prior to the mounting of the various parts of the joint according to the invention, it should be noted that the laminated structure 20 is molded directly and simultaneously on the end 10 of the taut line 1 and under the cover 3 of the housing 2, in the embodiments illustrated in FIGS. 3 and 5, or on the parts 33 intended to be embedded in the housing cover, with regard to the embodiments illustrated in Figures 4 and 6.

The laminated structure 30 is molded between the two annular support and securing parts 16, 17.

• encastrement du couvercle 3 du boîtier d'articulation sur la pièce 33 lorsque celle-ci existe,
• fixation de la deuxième structure lamifiée 30 à l'extrémité 10 de la ligne tendue 1 par l'intermédiaire de la pièce annulaire de raccord 17, et
• fixation de l'enceinte 15 du boîtier d'articulation au couvercle 3 de celui-ci (par l'intermédiaire de boulons 11) et à la pièce annulaire de raccord 16 ou 27, respectivement ; lors de cette opération, les deux structures annulaires lamifiées 20 et 30 sont soumises à une précontrainte dont le degré est réglé en fonction de la hauteur de la paroi latérale 5 du boîtier 2 par rapport à l'encombrement axial de l'articulation flexible 40. En pratique, on impose au montage une précontrainte supérieure à la tension maximale qui est induite dans la première et/ou dans la deuxième structure lamifiées 20 et 30 de l'articulation flexible lors de la transmission d'un effort contraire au sens normal de fonctionnement de la ligne d'ancrage dû à des conditions exceptionnelles (démontage,manipulation, etc...), mais suffisantes pour endommager l'une et/ou l'autre des structures lamifiées. De cette manière, l'éventuelle mise en traction de l'ensemble, notamment lors de la déconnexion mécanique de la ligne d'ancrage, laisse l'élastomère des structures lamifiées toujours en compression.

The various parts are assembled in the following order:

• recessing of the cover 3 of the articulation box on the part 33 when the latter exists,
• fixing the second laminated structure 30 to the end 10 of the tensioned line 1 by means of the annular connecting piece 17, and
• fixing the enclosure 15 of the articulation box to the cover 3 of the latter (by means of bolts 11) and to the annular connection piece 16 or 27, respectively; during this operation, the two laminated annular structures 20 and 30 are subjected to a prestressing, the degree of which is adjusted in as a function of the height of the side wall 5 of the housing 2 with respect to the axial size of the flexible joint 40. In practice, the preload is greater than the maximum tension which is induced in the first and / or in the second laminated structure 20 and 30 of the flexible joint during the transmission of a force contrary to the normal direction of operation of the anchor line due to exceptional conditions (disassembly, handling, etc.), but sufficient to damage one and / or the other of the laminated structures. In this way, the possible pulling of the assembly, in particular during the mechanical disconnection of the anchoring line, leaves the elastomer of the laminated structures always in compression.

It is easy to verify that the articulation in accordance with the present invention meets the aims targeted by it and indicated above, in particular as regards the control of lateral (or transverse) forces, the elimination of any metal contact. / metal and any tension in the laminated structures of which it is composed.

That said, it is clear that the existence of transverse stresses causes the application of a torque - relative to the center of curvature C - to each layer of the first laminated structure 20, which would give rise to a lateral tilting of this laminated structure, and therefore to an instability of the system, if the second laminated structure, 30, which does not provide the necessary stability by taking up (i.e., transmitting) the transverse forces, resulting from the combination of axial forces and argular deflections: it therefore behaves like a laminated "safety" structure.

It should also be emphasized that the rigidities of the flexible joint as a whole are the sum of the rigidities of each component and that the radial stiffness of the second laminated structure, 30, is of the same order of magnitude as the axial stiffness of the first laminated structure 20.

• pour le rapport entre les rigidités axiales Ka₁ et Ka₂ de la première et de la deuxième structure lamifiées, respectivement , les valeurs suivantes :
  
  avec Ka₁ ≧ 4·10⁶kN/m, et
• pour la rigidité en rotation K_r (à savoir,de torsion et conique) des valeurs du même ordre de grandeur par angle de rotation unitaire, à savoir :
  
  ${\text{K}}_{\text{r}}\text{≦ 150kNm/deg.}$
  

The contribution to the transmission of forces from the two laminated structures of the flexible joint according to the invention depends on the stiffness of each laminated structure. In the context of the present invention, it is recommended:

• for the ratio between the Ka rigid and Ka₂ axial rigidities of the first and second laminated structure, respectively, the following values:
  
  with Ka₁ ≧ 4 · 10⁶kN / m, and
• for the rotational stiffness K _r (i.e., torsional and conical) values of the same order of magnitude per unit angle of rotation, namely:
  
  ${\text{K}}_{\text{r}}\text{≦ 150kNm / deg.}$
  

As is apparent from the above, the invention is in no way limited to those of its embodiments and of application which have just been described more explicitly; on the contrary, it embraces all the variants which may come to the mind of the technician in the matter, without departing from the scope, or the scope, of the present invention. In particular, it should be understood that, for practical reasons, the two ends 10 of an anchor line, each equipped with the corresponding flexible joint, are not made in one piece with the anchor line , but they are removable, so that the structures shown in Figures 3 and 4 are related to the actual anchor lines (to which they are fixed by all appropriate means, preferably removable).

In addition, it is clear that in the context of the present invention is defined a method of mounting the flexible joint according to the invention, namely the different parts of which it is composed, and which essentially consists of placing under prestressed, namely in a prestressed assembly, of the laminated annular structures 20 and 30.

Stressing of the laminated annular structures 20 and 30 is carried out when the enclosure 15 of the articulation box 2 is fixed to the cover 3 thereof, and this after having fixed, on the one hand, the first structure laminated annular 20 to the cover 3 of the articulation box 2 and, on the other hand, the second laminated annular structure 30 to the removable end 10 of the mooring line 1. This prestressing is obtained by choosing the height of the side wall 5 of the enclosure 15 of the articulation box 2 so as to pass the above-mentioned laminated annular structures 20 and 30 from the normal state (I) to the prestressed state (II): cf. FIG. 4.

More precisely, the prestress is greater than the maximum tension which is induced in any one of the first and second laminated annular structures 20 and 30 of the flexible joint 40, 50, during the transmission of a force contrary to the normal direction of operation of the mooring line, this effort being due to exceptional conditions, such as mounting or handling, but sufficient to damage any of the annular laminated structures 20 and 30, which allows the latter to always work in compression, as already mentioned above.

Claims (6)

1. A flexible joint for mooring lines of a floating oil-drilling platform, referred to as a taut line platform (TLP), comprising at each end (10) of the line (1):

   - an articulation casing (2) on the corresponding end (10) and rigid with a fixing connector (E) which is bounded by:

   . a cover (3) provided with a central aperture (4) to allow passage of the taut line (1) and angular displacements thereof corresponding to the displacements of the floating platform,

   . a lateral wall (5), and

   . a bottom member (6),

   this lateral wall (5) and this bottom member (6) defining the enclosure (15) of the casing (2), the said flexible joint (40) housed in the said casing (2) allowing rotation of the corresponding end of the taut line (1) and comprising:

   . a first laminated annular structure (20) which is spherically symmetrical in relation to a centre of curvature (C) situated inside the joint casing (2) and disposed around the taut line (1), the said first laminated structure (20) - which is prestressed (II) in other words installed with a mechanical bias - is rigid with the end (10) of the taut line (1) and the cover (3) of the casing (2) and is composed of a plurality of elastomeric layers (8) alternating with shaped metal layers (9) adhering to the layers of elastomeric material and having the aforesaid centre of curvature,

   which flexible joint is characterised by the following features:

   a) it comprises in addition a second laminated annular structure (30), likewise spherically symmetrical with a point coincident with the centre (C) of curvature of the said first laminated structure (20) - and it too is fitted with a mechanical bias - and which therefore constitutes the centre of curvature (C) of the flexible joint (40) as a whole, which second laminated annular structure (30) is rigid with the end (10) of the taut line (1) and the bottom member (6) of the joint casing (2) and, like the first laminated structure (20), is composed of a plurality of layers of elastomer (22) alternating with shaped metal layers (23) adhering to the layers of elastomeric material;

   b) the height of the lateral wall (5) of the enclosure (15) of the joint casing (2) is defined in relation to the axial bulk of the flexible joint (40) and is chosen as a function of the desired degree of prestressing;

   c) the said second laminated annular structure (30) is in relation to the centre of curvature (C) of the joint (40) disposed on the same side as that on which the first laminated annular structure (20) is situated;

   d) the bottom member (6) of the joint casing (2) has a central tubular projection (axial, 14) which extends within the interior of the casing (2) in the direction of the cover (3) thereof, until it passes beyond the centre of the aforesaid curvature (C), the end (10) of the taut line (1) having a recess (a concavity 25) in which there is housed, without contact, a portion of the said tubular projection (14);

   e) the said second laminated annular structure (30) adheres between two metallic annular members (16, 17) of which the first member (16) is fixed to the said tubular projection (14) while the second annular member (17) is fixed to the end (10) of the taut line (1).
2. A joint according to claim 1, characterised in that the said first annular member (16) for supporting the said second laminated annular structure (30) is fitted around the said tubular projection (14) on the bottom member (6) of the casing (2) and bears on a shoulder (18) on the outer wall of this tubular projection (14), the said shoulder (18) being disposed substantially at the level of the centre of curvature (C) of the flexible joint (40).
3. A joint according to either of claims 1 or 2, characterised in that the elastomeric layers (8, 22) of each of the said first and second laminated annular structures (20, 30) are of different thicknesses.
4. A joint according to any one of claims 1 to 3, characterised in that the said first laminated structure (2) is fixed to the cover (3) of the joint casing (2) through a member (33) intended to be fitted into the cover (3).
5. A method of assembling the flexible joint according to any one of claims 1 to 4, characterised in that in order to fit together the laminated annular structures (20, 30) with a mechanical bias, the enclosure (15) of the joint casing (2) is fitted to the cover (3) thereof after having fixed on the one hand the first laminated annular structure (20) to the cover (3) of the joint casing (2) and on the other the second laminated annular structure (30) to the removable end (10) of the mooring line (1), the height of the lateral wall (5) of the enclosure (15) of the joint casing (2) being chosen in such a way as to cause the aforesaid laminated annular structures (20; 30) to change from the normal condition (I) to the pre-stressed condition (II).
6. A method according to claim 5, characterised in that the pre-stressing is greater than the maximum tension induced into either of the said first and second laminated annular structures (20; 30) of the flexible joint (40) during transmission of a stress in opposition to the normal direction of operation of the said mooring line, this stress being due to exceptional conditions such as fitments or handling operations, but sufficient to damage either of the laminated annular structures (20; 30) which allows these latter always to work in compression.

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