EP0307255B1 - Catenary anchor line for a floating device, and apparatus and method for operating this anchor line - Google Patents

Description

The invention relates to a catenary anchor line for a floating object, in particular a very long line allowing anchoring to a depth greater than 300 meters. The invention also relates to a device and a method for mounting and installing the anchor line.

Floating devices used for the research and exploitation of hydrocarbons at sea require the use of anchor lines, the end of which is secured to a marine anchor or fixed to a bored pile, to ensure that they are held in place against the forces causing their drift, for example produced by wind or sea currents.

The needs of exploiting petroleum deposits at ever greater depths require the use of very long lines joining the bottom to the surface.

For anchoring oil platforms, buoys or storage tankers, it may be necessary to have anchor lines at very great depths of water, for example of the order of 300 to 1000 meters.

It is entirely conventional to produce an anchor line in the form of a chain, for example with welded links. However, in the case of a long anchor line, for example greater than 3000 meters, the weight of the chain is too large compared to the traction capacity of this chain.

In fact, a large part of the mechanical strength of the chain is then devolved to the support of the chain itself and not to the retention of the floating object.

The realization of the anchor line in the form of a cable is better suited to long anchor lines, due to the high tensile strength of the cable and a linear weight (at equal resistance ) lower than the chain. However, in the case where very long lengths are required, the manufacture of the cable and its use as an anchor line becomes difficult if not impossible. In particular, the storage and handling of very long lengths of large diameter cable pose problems which are extremely difficult to solve.

It is known to use tubular elements closed at their ends and interconnected in an articulated manner to form long anchor lines. Each of the constituent elements of the line which is closed at its ends in a watertight manner delimits an interior volume filled with air and thus constitutes a float capable of compensating by the buoyancy of Archimedes in the water all or part of the item weight. Such anchor lines have hitherto been used as tension lines, that is to say lines which are in a practically straight position in service, as described for example in US-A-4,471,709.

The horizontal return forces of the floating device, when this device tends to drift under the effect of wind or sea currents, are due either to the elasticity of the line alone, or to a traction device connected to the end of the anchor line located in opposition to the drift of the machine.

When using such tensioned anchor lines which function like guy lines, it is obviously advantageous to reduce the weight. apparent from the line, when it is immersed in water. It can be particularly advantageous to calculate the constituent elements of the line to make its apparent weight in water substantially zero.

In the case of catenary anchor lines, that is to say lines taking a curved shape of chain in service, the problem is very different since the horizontal restoring force of the floating object corresponds to the horizontal projection of the force in the line, at the point of connection between the anchor line and the machine. The effort in the anchor line depends on the apparent weight of the line in the water while the horizontal component of the effort at the anchor point is all the greater compared to the vertical component as the line is closer to horizontal at this point.

The lightened anchor lines known from the prior art would not make it possible to obtain significant horizontal restoring forces and would not be capable of constituting catenary lines of great efficiency.

In the case of very long lines, lightening under the effect of Archimedes' thrust is of great interest, however, since the self-weight of the entire line can be reduced in large proportions.

The object of the invention is therefore to propose a catenary anchor line for a floating object constituted by a plurality of successive elements connected together in an articulated manner and each constituted by a tube closed in a watertight manner to each. of its ends, delimiting an interior volume filled with air, the line comprising a shallow end ensuring the connection with the floating object as long and a very deep anchoring end ensuring anchoring on a seabed, this anchoring line being able to present a great efficiency and being of a great flexibility of use.

For this purpose, the buoyancy of the anchor line is variable along its length, the tubular constituent elements generally having a wall thickness related to their diameter the greater the closer they are to the anchor end. of the line.

• La figure 1 est une vue latérale d'une portion de ligne d'ancrage suivant l'invention.
• La figure 2 est une vue latérale avec coupe partielle d'une articulation de liaison de deux éléments successifs de la ligne d'ancrage, représentée sur la figure 1.
• La figure 3 est une vue latérale d'une portion de ligne d'ancrage, suivant l'invention et suivant un second mode de réalisation.
• La figure 4 est une vue en élévation de l'extrémité à faible profondeur d'une ligne d'ancrage suivant l'invention assurant sa liaison avec une plateforme de forage semi-submersible.
• La figure 5 est une vue agrandie du détail 5 de la figure 4.
• La figure 6 est un diagramme montrant la décomposition de l'effort exercé dans la ligne représentée sur les figures 4 et 5, à son point de liaison avec la plateforme.
• La figure 7 est une vue schématique de la forme prise en service par une ligne d'ancrage suivant l'invention et suivant un premier mode de réalisation.
• La figure 8 est une vu schématique de la forme prise en service par une ligne d'ancrage suivant l'invention et suivant un second mode de réalisation.
• Les figures 9, 10, 11 et 12 illustrent divers modes de mise en oeuvre et d'utilisation d'une ligne d'ancrage suivant l'invention.

In order to clearly understand the invention, we will now describe, without limitation, with reference to the attached figures, several embodiments of an anchor line according to the invention as well as a method and a mounting and fitting device and uses of this anchor line.

• Figure 1 is a side view of a portion of anchor line according to the invention.
• FIG. 2 is a side view with partial section of a connecting articulation of two successive elements of the anchor line, shown in FIG. 1.
• Figure 3 is a side view of an anchor line portion, according to the invention and according to a second embodiment.
• Figure 4 is an elevational view of the shallow end of an anchor line according to the invention ensuring its connection with a semi-submersible drilling platform.
• FIG. 5 is an enlarged view of detail 5 of FIG. 4.
• Figure 6 is a diagram showing the breakdown of the force exerted in the line shown in Figures 4 and 5, at its point of connection with the platform.
• Figure 7 is a schematic view of the form taken into service by an anchor line according to the invention and according to a first embodiment.
• Figure 8 is a schematic view of the form taken into service by an anchor line according to the invention and according to a second embodiment.
• Figures 9, 10, 11 and 12 illustrate various modes of implementation and use of an anchor line according to the invention.

In FIG. 1, we can see a section of an anchoring line according to the invention generally designated by the reference 1.

The anchor line 1 constituted in articulated form comprises successive segments 2 connected together by articulations 3.

As can be seen in all of FIGS. 1, 2 and 3, each segment 2 is constituted by a tube 2a closed at each of its ends by a closure and connecting piece 2b.

The parts 2b are forged parts comprising a bottom 4 for closing the tube and a prominence 5 for connection. The solid bottom 4 has a cylindrical rim with a diameter equal to or greater than that of the current part of the tube 2a and which is welded, for example by friction, to the end of the tube, along the connection zone 6. The end of the tube 2a on which the connection is made has a thickness greater than the current thickness of the tube.

Each of the tubes 2a closed at its two ends by the bottoms 4 of the parts 2b delimits an interior volume filled with air that is completely airtight vis-à-vis the outside environment.

Each of the elements 2 of the anchor line thus has significant buoyancy when it is immersed in a liquid such as sea water.

The two embodiments of the anchoring line shown in FIGS. 2 and 3 differ only in the constitution of the articulated connection device 3 between the successive elements 2.

In the case of the embodiment shown in FIG. 2, the articulated connecting device 3 is constituted by two shackles 8a and 8b engaged one inside the other and each connected to the connecting end 2b of a tube 2a , the two successive tubes 2a thus being connected in an articulated manner. Each of the shackles is connected to the corresponding prominence 5 by a pin 10 engaged in aligned openings of the prominence 5 and of the two protuberances secured to the ends of the branches of the shackle. The pin 10 is fixed, after assembly, by a pin 11.

The successive protrusions 5 are arranged at 90 ° relative to each other.

In Figure 3, there is shown a second embodiment of the articulated connection between the successive elements 2 of the line. For each of the elements 2, one of the end pieces 2b constitutes a double yoke 12 in which a hinge pin 13 is fixed and which comprises in a symmetrical position relative to the axis of the line 1 and in a direction parallel to axis 13, a hole 14 smooth in one part of the yoke and tapped in the other part.

The other end piece 2b of each element 2 is formed in the form of a yoke 15 between the branches of which an axis 16 is fixed.

A jumper 17 is mounted articulated on the axis 13 of the double yoke 12 by one of its ends and has a hole 18 at its other end.

The jumper 17 can be placed in an open position 17 ′ (in phantom) by tilting outwards. In this position, the yoke 15 and the axis 16 of a part 2b of a first section can be placed in an assembly position near the yoke 12 of a part 2b of a second section 2 to be assembled .

The rider 17 is then folded back into its closed position (in solid lines), around the axis 16 and so that the branch of the rider 17 comprising the opening 18 comes to engage in the part of the yoke 12 comprising the tapped hole 14. The assembly is completed by engaging a screw 19 in the holes 14 and 18 in alignment and by performing the screwing in the tapped part of the hole 14.

In the two embodiments described and shown, the connecting device 3 acts as a universal joint and allows any relative orientation of the two successive tubes. In addition, the two tubes have a relative axial displacement latitude which can be relatively large, in the case of the connection by shackles.

In both cases, the assembly and the connection of the two successive tubes can be carried out very easily and very quickly.

In the case of an anchor line used in the field of drilling or petroleum exploitation, at sea and at very great depth, the tubes 2a may have a length of the order of 9 to 12 meters.

These steel tubes may be constituted by drill rods, drill collars, casing elements or any other tubular steel element commonly used in the petroleum technique and available from tube manufacturers.

It is quite obvious that by varying the ratio between the diameter and the wall thickness of the tube, it will be possible to obtain compensation for the weight of the tube by the buoyancy, in a determined proportion. According to the invention and as will be explained below, the anchor line has a variable buoyancy along its length, its constituent elements not all being identical and having wall thicknesses related to their variable diameters. In general, the constituent elements placed at greater depth will have a greater wall thickness than the elements placed at shallower depth.

In Figure 4, we see a part of a semi-submersible drilling platform 20 partially submerged below sea level 21 and the shallow part designated as a whole by the reference 22 of an anchor line 23 according to the invention. Part 22 of line 23 connects it to platform 20, a few meters below sea level 21.

The platform 20 carries on its submerged upper part, a winch 24 to which a traction cable 26 is connected and, in its submerged part, a fairlead 25 ensuring the guidance and the return of the cable 26 to a submerged mobile muffle 27 connected to the end of the line 23. The muffle 27 ensures the return of the cable 26 to a fixed anchor point 29 on the submerged lower part or pontoon 28 of the platform 20. The muffle 27 is maintained in a suitable orientation by a float 30 which can be connected to a surface buoy allowing the location of the muffle.

As can be seen in FIG. 5, the last tubular element 32a of the anchoring line 23 has an end portion 33 which is tubular and closed by a piece 34 internally threaded. An element 35 hingedly connected to the block 27 also has a threaded end portion.

A connection piece 36 comprising two threaded parts allows the assembly of the pieces 33 and 35 and therefore of the line 23 and of the muffle 27 connected to the platform by the traction cable 26. The threaded parts of the piece 36 are screwed into the threaded parts 34 and 35 for assembly.

The line is tensioned by the winch 24 via the cable 26 and the block 27.

The tubular element 32a is connected to a tubular element running from the anchoring line 23 by an articulation means 33 as shown in FIG. 3 and generally designated by the reference 3.

The tubular elements 32 of the anchor line 23 are constituted in the same way as the tubular elements 2 shown in FIGS. 1 to 3.

These elements constitute floats and ensure at least partial compensation of their weight by the buoyancy, when immersed in water. This compensation can be variable, as a function of the ratio of the total volume of the tubular element corresponding to the volume of water displaced, to the volume of the mass of steel of this tubular element, that is to say in fact as a function from the va their ratio of the wall thickness of the tubular member to the outside diameter of the member. It is thus possible to use tubular elements of determined buoyancy to constitute the different parts of the anchor line.

We will now refer to FIG. 6 to explain the general principle on which the design of the anchor line according to the invention is based.

In Figure 6, there is shown the force or tension F in the anchor line 23 at the point of connection of this anchor line with the platform 20, that is to say at the end of this line connected to the muffle 27. FIG. 6 shows the horizontal component F _H and the vertical component of the force F _V , the vertical component F _V making an angle ϑ with the direction of the force F, that is that is, the direction of the line at its connecting point.

The useful return force of the platform 20 is constituted by the horizontal component F _H of the force F. The vertical component F _V is a parasitic force, since this variable load is exerted at the expense of the stability of the float.

It is therefore desirable to make the vertical component F _V as small as possible, that is to say, for a given force F, to have an angle ϑ as large as possible. This result can be obtained by using lightened tubular elements to constitute the shallow part of the anchor line 23.

For the return force F _{H to} be significant, it is also necessary that the axial force F is itself significant.

A uniformly lightened anchor line over its entire length would not achieve this result, since the total apparent weight of the line in water would be low, in the case of a catenary anchor, this is ie an anchor in which the line has a significant curvature and provides a restoring force depending on its apparent weight in the water.

According to the invention, the anchor line 23 has a buoyancy that varies along its length, the tubular elements located at a shallower depth generally having a smaller wall thickness than the tubular elements located at greater depth. This design makes it possible to reconcile the contradictory imperatives mentioned above, since the greatly reduced upper tubular elements 32 make it possible to increase the angle ϑ at the connection point and that the lower tubular elements located at great depth having an apparent weight in the high water allow the effort F to be increased sufficiently to obtain a satisfactory return force F _H.

In addition, in the case of anchoring at great depth, constituent elements with a thick wall in the lower part of the anchoring line make it possible to increase the pressure resistance of this line.

The anchor line 23 being lightened thanks to the presence of the tubular elements, it will be possible to use a very long line, which makes it possible to reduce the average inclination of this line relative to the horizontal plane and to increase the restoring force.

In Figures 7 and 8, there is shown schematically the shape of an anchor line 23 (or 23 ′) according to the invention ensuring the anchoring of a semi-submerged platform 20 under the surface 21 of the sea. The anchoring line 23 (or 23 ′) is connected by its upper end at shallow depth 22 (or 22 ′) to the platform 20 and by its lower end 40 (or 40 ′) to a bored pile 41 (or 41 ′) fixed to the bottom 42 of the sea.

By adjusting the buoyancy of the different parts of the line along its length, between its end 40 and its end 22 (or 40 ′ and 22 ′), it is possible to obtain different shapes of this anchor line during its immersion.

This adjustment of the buoyancy of the line is obtained by varying the wall thickness / diameter ratio of the constituent elements 32 or 32 ′ of the anchor line, from its end 40 to its end 22.

In general, the constituent elements 32 located in the vicinity of the end 40 (or 40 ′) will have a wall thickness greater than that of the elements 32 (or 32 ′) located in the vicinity of the end 22 (or 22 ′) .

Generally, as will be explained below on exemplary embodiments, the anchor lines will consist of successive sections comprising identical constituent elements, these sections having increasing buoyancy from the bottom to the surface.

The anchor line shown in Figure 7 has the shape of a chain whose curvature varies continuously. This shape is obtained with a line comprising successive sections whose buoyancy increases continuously from the bottom to the surface.

The anchor line shown in Figure 8 has a complex S shape with two inflection points. This line includes very deep sections having a significant apparent weight in water, shallow sections having a very low apparent weight in water and an intermediate part made up of relatively short sections whose buoyancy increases rapidly.

The profile shown in Figure 8 reduces the length of line to deploy and therefore the costs of realization and implementation of this line.

We will now describe two embodiments of a tubular catenary line according to the invention allowing the anchoring of a semi-submersible platform by 600 meters of bottom (first example) and by 1000 meters of bottom (second embodiment).

In both cases, the float constituted by the semi-submersible platform undergoes external forces due to wind, currents and swell which contribute to causing its displacement on the surface of the sea. In order to reduce the horizontal displacement of this float , in any direction, an anchoring device is used comprising ten catenary lines according to the invention regularly distributed around the float, that is to say angularly spaced by 36 ° relative to each other.

In each of the cases envisaged, the performance of the catenary anchor line according to the invention will be compared to the performance of a traditional line constituted in part by a chain and in part by a cable having the same breaking strength.

In both cases, the horizontal displacement of the float must be limited to 6% of the anchoring depth.

Example 1 : Anchoring at a depth of 600 meters:

The maximum authorized displacement of the float is 36 meters in any direction.

A conventional platform anchoring device would include eight lines each consisting of 1,500 meters of cable and 1,500 meters of 7.5 cm (3 "(inch)) diameter chain. Each of the lines would be claimed to be 600 KN to support the external efforts in operation.

In such a configuration, the maximum horizontal tension reached in operation in a line is 1266 KN and the associated vertical tension of 650 KN, the angle at the fairlead being 27.2 ° relative to the horizontal. The vertical force exerted on the float by the eight anchor lines is therefore close to 5200 KN.

An articulated tubular anchor according to the invention allowing the anchoring by 600 meters of the bottom of the platform and the recall of this platform under the given conditions will be constituted by eight lines in tubular links with external diameter 27cm (10 3/4 inches) and having an apparent weight increasing with the depth of immersion. Each line of length equal to 4000 meters will be made up of three sections of respective length specified below. Each section will consist of tubular elements having a given apparent weight in water, that is to say a constant wall thickness / outside diameter ratio. The apparent weight in water of the different sections decreases from the bottom to the surface as indicated in the table below:

For the same external forces imposed on the float as those indicated in the case of the catenary anchor of traditional design and the same initial pre-tension forces, the maximum horizontal tension reached in a line in operation is 1887 KN and the vertical tension of 442 KN, the angle at the fairlead then being 13.2 ° relative to the horizontal.

The vertical force exerted on the float is therefore close to 3536 KN. This value should be compared to the vertical force of 5200 KN exerted by the traditional type anchoring on the platform. The difference between these two values, ie 1664 KN, makes it possible either to increase the variable payloads on board the float, or to reduce the stability reserve of this float and therefore its cost.

Example 2 : Anchoring by 1000 meters of bottom:

The maximum horizontal displacement allowed is 60 meters in any direction. The anchor is constituted as before by eight lines regularly distributed around the platform.

In the case of a line of traditional design made up of 1500 meters of cable and 1050 meters of 9.5 cm (3 "3/4 (inch)) diameter chain, the maximum vertical tension reached in operation is of 1297 KN for an angle at the fairlead of 31.7 ° relative to the horizontal. The maximum vertical force exerted on the float by such an anchoring device is therefore close to 10,376 KN.

This traditionally designed line will be compared to a tubular link line of 5,300 meters in length made up of four sections of respective lengths specified below. The apparent weight in the sections of water increases with depth, as indicated in the table below:

The maximum vertical tension reached in a line is 808 KN with a fairing angle of 23.2 ° relative to the horizontal. The maximum vertical force exerted on the float by such an anchoring device is close to 6470 KN. This value is to be compared with the value of 10376 KN obtained in the case of the device of traditional design. We therefore obtain a reduction in the vertical force of 3906 KN, which allows to obtain advantages such as those mentioned above in the case of anchoring by 600 meters of bottom.

In all cases, the tubular anchoring line according to the invention may consist entirely of standard elements each obtained by welding identical forgings at the end of sections of tubes having an desired outside diameter and wall thickness.

The connection devices articulated between the line elements may be either specially manufactured or constituted by standard elements available commercially. The manufacture of the anchor line can therefore be limited to the production of forged end pieces and their butt welding at the ends of the tubes, for example by friction. These operations can be easily automated.

The tube steel used in the petroleum technique can have a breaking strength of the order of 1000 MPa; this resistance compares favorably to the resistance of steels for chain which never exceeds 900 MPa.

On the other hand, the cables are made of steel wires whose resistance can reach 1900 MPa after treatment.

However, the lightening due to the Archimedes' push of the anchor line according to the invention during its use makes it possible to obtain performances much higher than those of the best anchor cables known at present.

In addition, as will be explained below, the anchoring line according to the invention can be deployed and implemented, easily, thanks to its modular constitution.

In Figure 9, there is shown a first mode of use and implementation of a line anchor 23 according to the invention, in the case where it is desired to anchor, on the sea floor, a semi-submersible platform 50.

A barge 51 equipped with a crane 52 is brought near the semi-submersible platform. We loaded onto the barge 51 all the elements necessary for the constitution of the anchoring line 23 of the platform. These elements can be constituted by several tens of tubular elements such as elements 2 or 32 which have been described above and by several tens of articulated connection devices such as devices 3 or 33. One realizes, at a mounting and assembly installation 53 (for example a casing table), thanks to the crane 52, the assembly of sections, of the anchoring line each constituted by a tubular element 32, in order to obtain a certain length of this anchor line. The first segment 55 of the anchor line is connected to the platform 50, in the usual way.

The mounting of the anchor line is continued, from the barge 51, until the anchor line has reached a sufficient length. The end of the line obtained is then fitted with a sea anchor which is then submerged or the final end of the line is connected to a bored pile, using suitable equipment known in the art.

The assembly and the deployment of the line were carried out using the crane 52 and the device 53, according to a technique known from the prior art, for the constitution of a drill string or a casing. The device 53 can consist of any drill rod assembly table or any casing table.

It is also obvious that other means commonly used in drilling techniques or laying pipelines may be used. Such means can be constituted by lifting winches, wedges or various means of stacking tubes.

The barge 51 may advantageously be constituted by a barge for handling and laying pipelines or by a barge supplying drilling platforms allowing the transport and supply of heavy packages.

FIG. 10 shows a second embodiment of an anchor line 23 according to the invention, the end of this anchor line 23 being connected via a chain 61, of classic link structure, to a bored pile 60 fixed in the bottom of the sea 62.

The chain was formed and deployed using the barge 51 used in the embodiment of FIG. 8. The anchoring line 23 is assembled from the barge 51 then connected to the pile 60 and finally the barge 51 ensures thanks to its crane 52 connects the upper end of line 23 to the floating object for which it is desired to anchor, for example a semi-submersible platform or a buoy.

In Figure 11, there is shown a third embodiment of the implementation of an anchor line 23 according to the invention.

A drilling platform 65 of dynamic type carries a derrick 66 and a handling crane 67.

Several dozen elements intended to constitute the anchor line 23, namely the segments 2 or 32 and the articulation elements 3 or 33, were stored beforehand on the platform 65.

The handling and operating means mentioned are used to mount and deploy the anchor line 23 at the end of which is fixed a marine anchor 68. When the anchor line 23 reaches a sufficient length greater than the water depth at the level of the platform 65, the anchor 68 secures the platform to the seabed.

In Figure 12, there is shown a fourth embodiment and use of an anchor line 23 according to the invention. The anchor line is assembled and deployed from a storage tanker 70. Handling and assembly means 71 and 72 fixed to the tanker's superstructure allow the line 23 to be mounted element by element, as in the previous cases.

A marine anchor 73 allows the anchoring of the tanker when the line has reached a sufficient length.

In the case of implementation according to one of Figures 11 and 12, it is obvious that the end of the anchor line 23 can be fixed to a bored pile such as the pile 60 shown in the figure 10, instead of anchoring from a marine anchor.

The anchor line according to the invention is highly efficient, simple to produce and at low cost. In addition, its manufacture and implementation can be carried out by simple operations carried out using equipment existing in the field of drilling and petroleum exploitation.

The mechanical resistance and buoyancy characteristics of the line can be easily adapted to each particular case and the total length of the anchor line can be, by its modular design and the compensation of its weight by Archimedes thrust, brought to a very high value, higher than it is for all the currently known embodiments. This length could for example be between 4000 and 10000 meters.

The invention is not limited to the embodiments which have been described.

We can consider a law of any variation of the buoyancy of the anchor line along its length depending on the desired results.

It is possible to use sections of any length, greater or less than the lengths given above. Any connecting device articulated between the tubular elements of the anchoring line can be used, from the moment when these connecting devices can be easily assembled, under existing operating conditions.

One can envisage the realization of an anchor line not only by assembling end to end tubular constituent elements according to the invention but also by arranging such constituent elements in a spaced manner and by connecting them by sections of anchor line. according to the prior art.

Finally, the anchor line according to the invention can be used in fields other than oil research and exploitation.

Claims (8)

1. Catenary anchorage line for a floating structure (20), consisting of a plurality of successive elements (2, 32) connected to one another in an articulated manner and each formed by a tube closed in a watertight manner at each of its ends, defining an internal volume filled with air, the line (1, 23) having an end at shallow depth (22) providing the connection with the floating structure (20) and an anchorage end at great depth (40) providing the anchorage on a sea bed (42), characterised in that the buoyancy of the anchorage line (1, 23) is variable over its length, the tubular constituent elements (2, 32) generally having a wall thickness in relation to their diameter which is the greater the closer they are to the anchorage end (40) of the line.

2. Anchorage line according to Claim 1, characterised in that it consists of successive sections of constant buoyancy, each consisting of identical tubular elements (32).

3. Anchorage line according to any one of Claims 1 and 2, characterised in that it consists of tubular elements (2, 32) having a length ranging between 9 and 12 metres.

4. Anchorage line according to Claim 3, characterised in that the said tubes (2, 32) consist of tubular elements used in oil exploration or exploitation, such as drill stems, drill collars, or casing elements.

5. Anchorage line according to any one of Claims 1 to 4, characterised in that in service it has a catenary shape, the curvature of which varies continuously from its anchorage end (40) to its connecting end at lesser depth (22).

6. Anchorage line according to any one of Claims 1 to 4, characterised in that in service it has an S-shape with two bending points, having an anchorage end (40′) at great depth and a connecting end (22′) at shallow depth inclined slightly with respect to the horizontal.

7. Anchorage line according to any one of Claims 1 to 6, characterised in that it has a part at greater depth consisting of tubular elements (32) having thick walls and a high apparent weight in the water, and a part at shallow depth consisting of tubular elements (32) with thin walls and having a low apparent weight in the water.

8. Anchorage line according to any one of Claims 1 to 7, characterised in that it has a length between its anchorage end (40) and its connecting end at shallow depth (22) ranging between 4,000 and 10,000 metres.

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