EP0287442B1 - Precompressed steel tube, especially adapted for anchor lines used in tension leg platforms, method for handling and positioning of such tubes and platform using them - Google Patents

Description

The present invention relates to a steel tube for constituting a line subjected in service to a tension exerted between opposite support points this tube being terminated by support connectors allowing the application of said tension, in particular for the constitution of an anchor line for an offshore production platform of the stretched line type, as well as a method of handling and placing such a tube.

This type of platform, which allows drilling or production in the deep sea, is a floating platform stowed at the bottom of the sea by means of lines anchored to bases or weights arranged at the bottom of the sea; the anchor lines, which generally consist of a bundle of steel tubes or cables, are provided at each of their ends with a flexible or articulated connector, which allows them to always work in tension, with torques very low bending despite the efforts undergone by the floating platform and transmitted to these lines.

Such a type of stretched line platform, as well as its method of installation, are in particular described in FR-A-2,554,780.

An anchor line is known which consists of a series of tubular sections of steel prestressed longitudinally individually by an inner cable and assembled step by step by threaded parts (publication EP-A-0 191 992).

In such an anchoring line, the screwed portion of the sections is not prestressed and the previously prestressed portion of the sections is only prestressed at a compression force lower than the servid tension of the line.

The present invention aims to overcome this drawback.

This is achieved, according to the invention as defined in claim, by constituting the anchoring line by a tube formed of a series of tubular steel sections and terminated at each end by a support connector, these sections being longitudinally prestressed by a bundle of cables which is common to all the sections, the prestressing being adjusted to a value greater than the value of the tension of the line in service so that in service, the length of the tube between the supports is maintained in compression.

The invention therefore allows the steel to no longer work in tension but still in compression, hence very good fatigue behavior.

The cables are in particular made of glass fibers, carbon or other mineral materials.

In fact, mineral fibers such as glass fibers and carbon fibers have a tensile strength much higher than that of ordinary steel, generally of the order of 3000 MPa, or about seven times the resistance of a steel.

However, such fibers could not be considered alone for the anchoring of platforms with taut lines, taking into account the importance of the variations in elongation under the effect of the swell; in fact, their modulus of elasticity is generally lower, and in any case never higher, than that of steel.

The principle of the invention therefore makes it possible to combine the respective advantages of mineral fibers and steels, taking advantage of the very high tensile strength of the fibers to prestress the steel, which can then be stressed at a stress rate much lower than that of fibers, resulting in a significant decrease in the lengthening of the anchor lines.

It was also known (publication GB-A-2 085 939) to constitute an anchor line by a series of tubular sections and a prestressed longitudinal common cable, but, in contrast to the present invention, it is on the common cable which is exerted the operating tension and the modulus of elasticity of the tubular sections must be lower than that of the cable. This is the reason why, in this embodiment, these sections are made of concrete.

In a first embodiment, the cables are blocked by a fiber coating resin.

In a second embodiment, the cables are blocked by mechanical centralizers.

In the case where it must be submerged, the tube forms a hermetically sealed volume, and the outside diameter of the steel tube is preferably chosen so that the apparent weight of the tube in water is substantially zero.

In a preferred embodiment, the tube is made up of sections which are only held assembled end to end by said prestress.

Thus, the fibers constitute a means of assembling the tube, when the latter is formed of sections arranged end to end, and avoid the use of mechanical or weld assembly means, which is always difficult to implement in an environment. marine.

The invention also relates to a method of handling and placing such a tube consisting of sections maintained and assembled by prestressing according to claim 8, this method being applied to the constitution of an anchoring line of a flat - offshore production form with tight lines.

• préassembler, à terre, les tronçons de tube reliés entre eux par les câbles non tendus ainsi que les éléments connecteurs inférieur et supérieur, puis enrouler cet ensemble sur un tambour de manutention de contour polygonal de côté sensiblement égal à la longueur d'un tronçon de tube,
• dérouler l'ensemble dans le puits d'ancrage de la plate-forme,
• appliquer une tension aux câbles, de manière à précontraindre le tube,
• relier l'élément connecteur inférieur à une embase placée au fond de la mer,
• mettre le tube en tension par des méthodes classiques de tensionnement et relier l'élément connecteur supérieur à la plate-forme.

According to the invention, this process is characterized by the steps consisting in:

• preassemble, on the ground, the sections of tube connected together by the non-tensioned cables as well as the lower and upper connector elements, then wind this assembly on a handling drum with a polygonal outline of side substantially equal to the length of a section of tube,
• unroll the assembly in the platform anchoring well,
• apply a tension to the cables, so as to prestress the tube,
• connect the lower connector element to a base placed at the bottom of the sea,
• tension the tube by conventional tensioning methods and connect the upper connector element to the platform.

• la figure 1 est une coupe longitudinale, selon la ligne I-I de la figure 2, d'un premier mode de réalisation d'un tube selon l'invention,
• la figure 2 est une coupe transversale de ce même tube, selon la ligne II-II de la figure 1,
• la figure 3 est une coupe longitudinale, selon la ligne III-III de la figure 4, d'un second mode de réalisation d'un tube selon l'invention,
• la figure 4 est une coupe transversale de ce même tube, selon la ligne IV-IV de la figure 3, et
• la figure 5 montre l'ensemble formé par le tube et ses deux connecteurs inférieur et supérieur reliant la plate-forme flottante à la fondation préparée au fond de la mer.

Other characteristics and advantages of the invention will appear on reading the detailed description below, made with reference to the appended drawings in which:

• Figure 1 is a longitudinal section along line II of Figure 2, of a first embodiment of a tube according to the invention,
• FIG. 2 is a cross section of this same tube, along line II-II of FIG. 1,
• FIG. 3 is a longitudinal section, along the line III-III of FIG. 4, of a second embodiment of a tube according to the invention,
• FIG. 4 is a cross section of this same tube, along the line IV-IV of FIG. 3, and
• Figure 5 shows the assembly formed by the tube and its two lower and upper connectors connecting the floating platform to the prepared foundation at the bottom of the sea.

In the embodiment of Figures 1 and 2, the tube consists of elements of small diameter and thick.

The ends of the tubular steel elements 10 are provided with cutouts 11, 12 machined to allow self-centering, good contact and good alignment.

In addition, if a perfect seal under high hydrostatic pressure is required, an appropriate glue or resin, or a metal joint can be provided.

These various elements are held in position and prestressed by means of prestressing cables 20 made of mineral fibers coated with resin.

So that these prestressing cables are rigorously positioned and maintained to avoid any relative transverse movement of the fibers and of the steel and to ensure axial prestressing, they are placed contiguously (cf. FIG. 2) and maintained in this position by means of 'a central concentric tube 30, not prestressed, made of steel or synthetic material.

The adjoining wire harness pe t u also optionally be embedded in a suitable embedding resin.

In the embodiment of FIGS. 3 and 4, on the contrary, a tube with elements of large diameter and small thickness is provided.

The steel elements 10 are assembled in the same way as in the previous embodiment; only their dimensions have been changed.

On the other hand, due to the much larger internal volume of the tube, the prestressing cables 20 are no longer contiguous, but held in position by means of a guide device consisting, for each section of tube, of two disks 40 perforated in 41 to allow the prestressing cables to pass and remain in place 20.

These two discs 40 are connected together and held by a central tube 42, not prestressed, in steel or in synthetic material, to which they are secured.

This guide device (disc 40 and central tube 42) slides with little play inside the tubular element 10 made of steel, a gap is provided at 43 between the successive guide devices, so as to be able to absorb without transmission. of stress the different stresses that the tube will undergo.

Finally, perforations 44 are provided to ensure the communication of the volumes located between two successive discs, so as for example to allow the emptying of the entire interior volume of the tube.

FIG. 5 illustrates an application of such a tube to an offshore production platform with tight lines.

Indeed, the tubes of the invention can be very advantageously used whenever the problems of significant elongation, long clean period or fatigue of the metal are critical when conventional steel systems are used.

This is particularly the case with anchor lines for offshore production platforms of the stretch line type.

In this case, the tube corresponding to the embodiment of FIGS. 3 and 4 is preferably used.

In fact, the diameter of the steel elements can then be chosen so that the apparent weight of the prestressed steel tube in the water is practically zero after the water contained inside the elements has been drained.

In FIG. 5, the tube 10 is provided at the bottom with a connector 50 terminated by a hemispherical end piece 51 which is received in a member 100 secured to the anchoring foundation.

The connection between the lower connector 50 and this element 100 forms a ball joint allowing the transmission of tension forces from the line to the foundation, and which must induce only very low bending torques. This is achieved by means of blocks 101 in laminated neoprene, or hemispherical ports in graphite bronze.

The fiber prestressing cables are embedded in resin 52 inside the element 50.

Finally, a drainage tube 53 is provided with a non-return valve at the top of this element 50, communicating with the interior volume of the prestressed tube 10.

At its upper end, the tube is provided a prestressing box 60 connected to the floating platform 200.

This prestressing box comprises an element 61 of cast steel forming the bottom of the box and the male part of the ball joint for anchoring to the platform; this part comes to bear on a crown 201 in molded steel by means of blocks 202 of laminated neoprene or hemispherical ports in graphite bronze so as to minimize the transmission of bending torques, as in the case of the lower connector.

Spacers 62 make it possible to diverge the prestressing cables at the outlet of the steel tube and direct them towards an anchoring ring 63, to which they are conventionally fixed by means of conical keys and resin injection. The operating voltage of the prestressing cables is continuously monitored by control systems, so as to monitor their behavior.

The box 60 is closed by a watertight cover 64 provided with a nozzle 65 to which a flexible hose will be connected for emptying the tube 10 by means of an air or nitrogen flush, the water then being discharged by the drainage tube and the non-return valve 53 of the lower connector.

This emptying device could even remain permanently connected for emptying any leaks in service, or pressurizing the tube to check its tightness.

The box 60 finally comprises a set of ears 66 allowing the attachment of the devices 67 for handling and tensioning the tube.

The prestressing box is installed inside a well 203 enlarged to the right of the box to allow its movement. This well can be put under air pressure during the installation and maintenance phases so that the prestressing box and the support device are out of water.

The crown 201 of the ball joint bears against the bottom of this well by means of a system of adjustable wedges 204; the diameter of the opening 205 in the bottom of the well must be sufficient to allow the passage of the lower connector 50, the anodes and any floats.

The platform may include, for each leg, an anchoring line formed from seven to eight tubes thus produced and put in place.

Claims (10)

1. A steel tube for constituting a line which is subjected in service to tension exerted between opposite thrust points, said tube being terminated by thrust connectors enabling the application of said tension being constituted by a sequence of lengths of steel which are longitudinally prestressed by a bundle of cables, characterized in that the bundle or prestress cables is common to the entire set of lengths and in that the prestress is adjusted to a value which is greater than the value of said tension such that in operation the tube between the thrust points is maintained under compression.
2. A tube according to claim 1, characterized in that it is prestressed by cables (20) tensioned along the tube, inside or outside the tube, and which are prevented from moving transversely relative to the tube, said cables being constituted by fibers having a breaking strength which is greater than that of steel.
3. A tube according to claim 2, characterized in that the cables (20) are made of fibers of glass, carbon, or other inorganic materials.
4. A tube according to claim 2 or 3, characterized in that the cables are locked by a resin coating the fibers.
5. A tube according to claim 2 or 3, characterized in that the cables are locked by mechanical centering devices (40).
6. A tube according to claim 2 or 3, characterized in that the tube forms a hermetically closed volume with the outside diameter of the steel tube being selected in such a manner as to ensure that the apparent weight of the tube in water is substantially zero.
7. A tube according to any one of claims 1 to 6, characterized in that it is constituted by lengths (10) which are held assembled end-to-end by said prestress.
8. A method of handling and installing a tube according to claim 7, in particular for constituting an anchor line for a taut line production platform of sea, the method being characterized by the following steps :
   
      preassembling, on land, the lengths (10) of tube which are interconnected by the cables (20) under no tension, together with the bottom connector component (50) and the top connector component (60), and the winding said assembly on a handling drum having a polygonal contour whose side is substantially equal in length to the length of each length of tube ;
   
      unwinding the assembly into the anchor well of the platform ;
   
      applying a tension of the cables so as to prestress the tube ;
   
      connecting the bottom connector component (50) to a base (100) placed at the bottom of the sea ; and
   
      putting the tube under tension by conventional tensioning methods and connecting the top connector component (60) to the platform (200).
9. A method according to claim 8, characterized in that after the anchor line has been put under tension, the inside volume of the tube is emptied.
10. A taut line type production platform at sea, characterized in that the lines comprise tubes according to any one of claims 1 to 7.

Images