GB2363617A - Method for making a composite tether for an offshore platform and tether obtainable by said method - Google Patents

Abstract

The invention concerns a method which consists in forming subassemblies (6) by assembling pultruded strands (5) consisting of traction resistant fibres embedded in a resin, and coating the assembled strands in a protective plastic jacket (7). Said subassemblies are wound on transport reels (12). On a manufacturing site preferably located on the sea front, the tether (4) is formed by assembling the subassemblies unwound from the transport reels, and by coating the assembled subassemblies in an outer plastic sheath (9). The resulting tethers can then be transported to the platform for anchoring the latter.

Description

METHOD FOR MAKING A COMPOSITE TETHER CABLE, IN PARTICULAR FOR AN OFFSHORE

PLATFORM, AND TETHER CABLE OBTAINABLE BY SUCH A METHOD The present invention relates to the field of anchorings in an offshore environment, in particular for offshore platforms, by means of cables.

Tension-leg platforms (TLP) are usually tethered by means of hollow steel tubes anchored to the bottom of the sea and fixed to the platform These tubes work in tension in order to hold the platform in position They are hollow so as to limit their weight and so as to exhibit a degree of buoyancy When the depth of water under the platform is relatively large (typically more than 1000 m), the pressure exerted on the bottom parts of the tubes becomes large, so that a reinforcement of these tubes is necessary, giving rise to a large additional cost and affecting the buoyancy of the system.

Certain studies have envisaged replacing the steel tubes with composite cables based on carbon fibers (see K.S Kim et al "Application of Composites in TLP Tethers", Proceedings of the 7th International Conference on Offshore Mechanics and Arctic Engineering, Houston, February 1988, Vol 3, pages 1- 7) These cables have the advantage of not being hollow, and therefore of being less affected by hydrostatic pressure than metal tubes They may be subjected to large tensile loads while being very resistant to fatigue and corrosion Furthermore, they make it possible to produce relatively lightweight and compact tethers which are less difficult to install than steel tubes.

In the article "Flexible Tension Members from Composite Materials" (Proceedings of the 6th International Conference on Offshore Mechanics and Arctic Engineering, Houston, 1987, Vol 3, pages 7-14), J M.

Walton et al describe a composite cable comprising pultruded tendons grouped into a bundle of hexagonal geometry surrounded by a plastic sheath For an application of such a cable in a TLP anchoring, one is apt either to increase the number of cables, thereby appreciably complicating the installation, or to make the cable from very many pultruded tendons, thereby posing cable manufacturing and transport problems and difficulties in carrying out the anchoring.

An object of the present invention is to propose a novel technique for making composite cables which is well suited to offshore constraints and which can be applied to other industries.

The invention thus proposes a method for making a composite tether cable, in particular for an offshore platform, comprising the steps of:

forming sub-assemblies by assembling pultruded strands composed of tension-resistant fibers embedded in a resin, and by jacketing the assembled strands in a protective plastic sheath; winding the sub-assemblies onto transport reels; forming the tether cable by assembling sub- assemblies unwound from the transport reels, and by jacketing the assembled sub-assemblies in an outer plastic sheath.

This method provides an appropriate response to the industrial constraints posed by the making of composite cables for the anchoring of TL Ps The sub-assemblies are semi-finished products dimensioned so as to allow their storage on reels whose dimensions are compatible with the means of land transport (a few meters) They can be wound onto their transport reels while twisting them, typically at a rate of one turn every 10 meters approximately.

The sub-assemblies can thus be transported to an installation located on the sea front, where they are assembled and sheathed to form the cable.

This assemblage can be modulated in the sense that, depending on the specific characteristics of each TLP anchoring leg, it will be possible to assemble the required number of sub-assemblies.

Another advantageous modular feature of the cable relates to the means used for anchoring it to the bottom of the sea and/or onto the platform Fixing members can thus be anchored to the ends of the individual sub-assemblies, either when the sub- assemblies are manufactured, or when they are assembled If the anchoring of one of these members gives way when the cable is in service, only one sub- assembly will be affected by the break The method thus affords some degree of tolerance to defects As a variant, the sub-assemblies of the cable can be gathered into various groups, and it is each of these groups which receives a fixing member before the installation of the cable at sea.

The cable may be made buoyant and towed to the site of the platform.

However, preferably, the tether cable can be wound on another transport reel of large dimensions allowing transport on a cargo vessel The diameter of such a reel can be several tens of meters, the cable being twisted during the winding thereof with a twist which in general creates only elastic stresses in the materials, typically at a rate of one turn every fifteen meters approximately.

The sub-assembly constitutes a semi-finished product which can be used on numerous work sites, thereby making it possible to reduce the manufacturing cost thereof Furthermore, bringing the complete cable to the platform greatly simplifies the operations of installation on the offshore site, thereby also helping to make the technique economically attractive, especially for relatively large depths.

Another advantage of the construction of the cable as sub-assemblies is to ensure low overall rigidity to bending, it being possible for the sub-assemblies to glide along one another.

The structure of the cable makes it possible to reduce the micro-movements between the pultruded strands during fatigue loadings These micro-movements take place only in the sub-assemblies whereas in the case of a cable consisting only of pultruded strands, all these strands rub against one another.

Finally, the protective sheath of the sub-assembly ensures double protection in the event of loss of leaktightness of the outer sheath of the cable.

Another aspect of the present invention relates to a composite tether cable, in particular for an offshore platform, comprising an outer plastic sheath and sub- assemblies disposed as a bundle contained in the outer sheath, each sub-assembly comprising, in a protective plastic sheath, a juxtaposition of pultruded strands composed of tension-resistant fibers embedded in a resin.

Other features and advantages of the present invention will become apparent in the description hereinbelow of exemplary non-limiting embodiments, with reference to the appended drawings, in which, Figure 1 is a perspective view showing the structure of a tether cable according to the invention; Figure 2 is a perspective view detailing the structure of a sub-assembly of the cable; Figure 3 is a diagram of an installation for assembling the cable.

The elementary strands, from which the composite cable 4 represented in figure 1 is constructed, are pultruded tendons 5.

These tendons 5 are composed of unidirectional fibers embedded in a resin matrix The unidirectional fibers are preferably made of carbon They can be high- strength fibers (HS) or high-modulus fibers (HM) Given that the anchoring of TL Ps is mainly gauged by the stiffness of the legs, tendons based on HM fibers will preferably be used The resin jacketing the rovings of carbon fibers in the pultruded tendons can be a thermosetting or thermoplastic polymer.

There is advantage in using a pultruded tendon 5 of relatively large diameter With the current pultrusion technologies, tendons of diameter 6 mm are suitable.

The pultruded tendons 5 are assembled into sub- assemblies 6 In the example represented in figures 1 and 2, this assemblage has hexagonal geometry and consists of nineteen tendons The number nineteen seems to be a good compromise for making a cable according to the invention, but hexagonal arrangements of seven or thirty-seven tendons could also be considered.

A sheath 7 made of high-density polyethylene (HDPE) surrounds the tendons of the sub-assembly 6 This sheath, deposited by extrusion on the grouped tendons, has for example a thickness of 2 mm It ensures the mechanical protection of the sub-assembly during manipulations thereof Moreover, a reversible flexible substance 8 is injected into the gaps between the tendons, and into the gaps between the tendons 5 and the sheath 7 This substance 8 can for example be a petroleum wax or any other flexible polymer It serves mainly to fasten the tendons 5 with respect to one another so as to avoid undesirable displacements under the effect of the hydrostatic pressure at the bottom of the sea, and during manipulations of the sub-assembly 6 and of the cable 4.

The sub-assembly 6 is twisted with a pitch of ten meters, so as to ensure storage and transport of the sub-assemblies 6 on reels of average size (for example inside diameter 2 50 m, outside diameter 5 50 m and axial width 2 00 m) This slight torsion of the tendons renders the loads due to the curvature of the sub- assembly on its transport reel uniform.

The sub-assemblies 6 are used to construct the cable 4, whose stiffness rating is determined by the chosen number of sub-assemblies.

The hexagonal general shape of the sub-assemblies 6, due to the hexagonal arrangement of its constituent strands, facilitates their juxtaposition in any number and in various geometries In the particular case represented in figure 1, the cable 4 comprises thirty- seven sub-assemblies 6 juxtaposed in a hexagonal arrangement.

The assemblage of the sub-assemblies is also surrounded by an HDPE sheath 9 The thickness of this sheath 9 (for example 15 mm) is designed to provide effective protection of the cable in a marine environment.

The gaps between the sub-assemblies 6, and between the sub-assemblies 6 and the sheath 9, are filled in with a viscous incompressible lubricant 10 which can for example be a petroleum wax.

For transporting the cable 4 to the platform, it can be floated on the sea and towed.

More conveniently, provision may be made to wind it onto a transport reel whose dimensions, dependent on the cross section and on the length of the cable, are relatively large (for example of the order of 20 m in diameter) Such reels are used in offshore transport.

In this case, the assembled sub-assemblies 6 are twisted with a pitch of the order of 15 m to facilitate winding.

When the lower end of the cable 4 has descended to the bottom of the water under the platform, the torsion applied for storage on the reel vanishes spontaneously.

Moreover, if when the subassemblies 6 are unwound so as to assemble the cable, care has been taken to eliminate the torsion which they had on their transport reels, then practically no torsion exists in the tendons 5 and in the carbon fibers, so that they are in an optimal configuration for taking up tensile loads.

Figure 3 diagrammatically shows a line for manufacturing the composite cable, installed in a factory situated on the sea front.

The sub-assemblies 6 are unwound from their transport reels 12, which are mounted on supports having orbital movement adjusted in such a way as to cancel out the torsion applied when they were wound onto the reels 12.

This orbital movement consists of a rotation about the axis of the reel 12 so as to unwind the sub-assembly, combined with a rotation about the axis of extraction of the sub-assembly so as to eliminate the torsion.

The sub-assemblies 6 pass through a first series of orienting rollers 13 before arriving at the device 14 which injects the lubricant 10 The assemblage is thereafter pressed in a second series of rollers 15, and then supplied to an extruder 16 which deposits the HDPE sheath 9 Downstream of the extruder 16 is a motor 17 which pulls the cable 4, and a third series of orienting rollers 18 preceding the orbital-movement support of the cable transport reel 19 This reel 19 pivots about its axis in order to wind the cable 4, and also comprises a component of pivoting transverse to its axis so as to twist together the sub-assemblies 6 of the cable.

The basic diagram of the line for manufacturing the sub-assemblies 6 is very similar to that of figure 3 (except that the tendons 5 are not unwound from orbital-movement reels) However, of course, its dimensions are much smaller than those of the line for assembling the cable 4 This line for manufacturing the sub-assemblies 6 can for example be installed in the pultrusion factory.

The systems for anchoring the cable 4 onto the platform and to the bottom of the sea can be in part mounted on the cable during its manufacture.

In particular, provision may be made to anchor a fixing member to each end of each sub-assembly 6 of the cable, or else to group these sub-assemblies 6 into several groups, each of these groups receiving a fixing member at its two ends The anchoring of these fixing members onto the sub-assemblies can be carried out in a manner known per se by adhesive bonding The fixing members at the end intended to be situated at the bottom of the sea are arranged so as to be simply fixed, for example by bolting, onto anchoring blocks made at the appropriate locations At the platform end, these members can comprise tension adjusting means, for example of the screw/nut type.

Claims (16)

1 A method for making a composite tether cable ( 4), in particular for an offshore platform, comprising the steps of:

forming sub-assemblies ( 6) by assembling pultruded strands ( 5) composed of tension- resistant fibers embedded in a resin, and by jacketing the assembled strands in a protective plastic sheath ( 7); winding the sub-assemblies onto transport reels ( 12); forming the tether cable by assembling sub- assemblies unwound from the transport reels, and by jacketing the assembled sub-assemblies in an outer plastic sheath ( 9).

2 The method as claimed in claim 1, wherein the sub- assemblies ( 6) are wound onto their transport reels ( 12) while twisting them.

3 The method as claimed in claim 2, wherein the sub- assemblies ( 6) are twisted at a rate of one turn every 10 meters approximately.

4 The method as claimed in any one of claims 1 to 3, wherein the assemblage of the pultruded strands ( 5) in the sub-assemblies ( 6) has hexagonal geometry.

The method as claimed in claim 4, wherein each sub-assembly ( 6) comprises nineteen pultruded strands ( 5).

6 The method as claimed in any one of claims 1 to 5, wherein the formation of each sub-assembly ( 6) comprises the injection of a flexible substance ( 8) filling in the gaps between the pultruded strands ( 5) and the protective sheath ( 7) of the sub-assembly.

7 The method as claimed in any one of claims 1 to 6, wherein the tether cable ( 4) is wound, while twisting it, onto another transport reel ( 19), of substantially larger dimensions than those of the transport reels ( 12) of the sub-assemblies ( 6).

8 The method as claimed in claim 7, wherein the tether cable ( 4) is twisted at a rate of one turn every 15 meters approximately.

9 The method as claimed in any one of claims 1 to 8, wherein the formation of the tether cable ( 4) comprises the injection of an incompressible lubricant ( 10) filling in the gaps between the sub-assemblies ( 6) and the outer sheath ( 7) of the cable.

The method as claimed in any one of claims 1 to 9, wherein respective fixing members are anchored to the ends of the individual sub-assemblies ( 6) or sub-assemblies gathered into groups.

11 A composite tether cable, in particular for an offshore platform, comprising an outer plastic sheath ( 9) and sub-assemblies ( 6) disposed as a bundle contained in the outer sheath, each sub- assembly comprising, in a protective plastic sheath ( 7), a juxtaposition of pultruded strands ( 5) composed of tension-resistant fibers embedded in a resin.

12 The tether cable as claimed in claim 11, wherein the assemblage of the pultruded strands ( 5) in the sub-assemblies ( 6) has hexagonal geometry.

13 The tether cable as claimed in claim 12, wherein each sub-assembly ( 6) comprises nineteen pultruded strands ( 5).

14 The tether cable as claimed in any one of claims 11 to 13, wherein a flexible substance ( 8) fills in the gaps between the pultruded strands ( 5) and the protective sheath ( 7) of each sub-assembly ( 6 j.

The tether cable as claimed in any one of claims 11 to 14, wherein an incompressible lubricant ( 10) fills in the gaps between the sub-assemblies ( 6) and the outer sheath ( 9).

16 The tether cable as claimed in any one of claims 11 to 15, wherein fixing members are respectively anchored to the ends of the individual sub- assemblies ( 6) or sub-assemblies gathered into groups.