GB2245287A - Tethers - Google Patents

Abstract

A tether, e.g. a cable, rod or tube for a structure such as a tension-leg platform, a cable-stayed bridge, pre-stressed concrete beam or rigging system comprises an elongate carbon fibre composite material made from a plurality of elongate carbon fibres 27 bound together with a matrix of thermoplastics resin. Each carbon fibre extends along the length of the tether and is substantially continuous for the greater part of that length, which may be from 50 m to 1.5 km. The thermoplastics resin may be a poly-ether-ether-ketone. The properties of the tether may be varied along its length, e.g. by varying the tether thickness and the number and nature of the carbon fibres. Regions of the tether may be heat-softened and moulded to a required shape. A protection sheath 28, e.g. a woven or wrapped carbon fibre layer in the same matrix, may be provided. Sensing fibres 29 may be incorporated. <IMAGE>

Description

Tethers This invention concerns tension members, and relates in particular to the use of carbon fibre composites for the construction of elongate tension members, referred to hereinafter as tethers, suitable for securing or reinforcing structures such as floating oil platforms, cable-stayed bridges and pre-stressed concrete assemblies as well as rigging lines and stays on marine vessels, aerials and masts, and the like.

It is common in many fields for there to be a need for an elongate tension member, along the general lines of a rod or rope, to hold a structure together. For example, certain types of floating body, such as seaborne oil platforms, need to be anchored, or tethered, to the sea bed (they are called tension leg platforms), while certain types of bridge, such as cable-stayed bridges, require to have one portion (the bridge deck) secured by a tension member to a load-bearing pylon or tower. Also, pre-stressed concrete structures such as a bridge beam have elongate members under tension within a concrete coat.

All these types of tension member may have considerable length (an oil platform tether can be as 0.5km - long) - and may take a number of physical forms. Typically they will be either pipe-like tubes similar to oil well caising, for example - in relatively short sections either screwed or welded together (this variety is especially useful in the case of tethered oil platforms), or multi-stranded (spiral strand) ropes, and in general they are made of a metal, commonly a steel, theoretically capable of standing up to the ambient conditions the designer expects to be encountered. Though in principle such tethers should perform quite well, in practice they are beset by a multitude of different problems for which as yet no satisfactory solution has been found.Thus, despite the best endeavours of the designers and metallurgists the presently-used steel tethers suffer seriously from chemical corrosion, stress corrosion and metal fatigue, while being at the same time heavy, difficult and costly to install, service and maintain. Moreover, with very long tethers - especially those of one and a half kilometres and more in length (as is increasingly necessary for tension leg oil platforms) - a serious difficulty arises from the natural longitudinal resonance frequency of the tether (which frequency is partly dependent upon the material of which the tether is formed, and especially its elastic modulus) being comparable to the frequency of the alternating stresses the tether experiences as the structure of which it is a part is subjected to the various natural forces of wind, wave action, and so on.

The invention suggests that all these problems associated with tethers might be alleviated, if not totally removed, by constructing the tether out of a completely different material, not previously considered suitable for this purpose, namely a carbon fibre thermoplastic composite. More specifically, the invention suggests such a composite where for the most part the carbon fibres are themselves elongate to a degree comparable to the tethers elongate nature, and the composite employs a matrix (the fibre-binding material) of a thermoplastic material such that the composite can be formed as a continuous structure by an extrusion-like process.

In one aspect, therefore, this invention provides a tether in the form of an elongate structure and comprising a carbon fibre composite material itself comprised of a collimated plurality of elongate carbon fibres extending along the length of the structure and bound together with a matrix of a thermoplastic resin.

It has been established that a tether of this invention may be expected to display excellent properties for the intended uses; of the many properties such carbon fibre composites exhibit their remarkably good fatigue resistance in fluctuating tension, very low creep, exceptional resistance to corrosion and stress corrosion cracking in salt water environments, and very high strength and stiffness to density ratios appear to make them ideal for the purpose.

In principle the tether may take a physical form similar to that of the known tethers. Thus, it may be a long rod or tube - typical cross-section dimensions are 10 to 40 cm outer diameter (and, for a tube, up to 30 cm inner diameter) - or it may be a "wire" rope made from a number of small-diameter (1 to 5 mm) "wires" either helically wound around each other or parallel laid (a typical such rope will contain 261 "wires" wound into a single giant bundle about 20 cm in diameter).

The tether is in the form of an elongate structure, and by "elongate" is here meant an object tens, hundreds or even thousands of metres long. A very long body may be an integral body - continuous along its entire length, which is preferred - or it may be constructed of a number of smaller parts or sections each continuous along its entire length and all joined together in some appropriate way. Thus, for example, a 1.5km tether might be built up from five 0.3km tethers.

For a deep sea oil platform, then, the tether might be an integral body of a half to two kilometres in length, while as a stay in a cable-stayed bridge the tether might be 50 to 500m long. A tether of this invention is extremely useful for this purpose, because it can be manufactured from the carbon-fibre composite in one piece (and is therefore extremely strong) and to have exactly the required length.

The elongate carbon fibres used in the tether are for the most part of a length corresponding to that of the tether as a whole. In this connection it should here be noted that in practice in a bundle of many tens of thousands of fibres there will be some broken fibres, and if the bundle is long enough probably most if not all the fibres therein will have at least one break. Experimental evidence exists demonstrating that the apparent strength of carbon fibre composites decreases with specimen length, reflecting the increasing probability of a given number of fibre breaks being encountered at a single cross-section.

Thus, the reason for choosing "continuous" fibres for use in the invention rather than "chopped" fibres (as is most commonly encountered in carbon fibre composites) is to reduce the probability of an injurious number of fibre breaks occurring at any one cross-section. The carbon fibres are of a length corresponding to that of the tether as a whole - but by that it is not meant that they are actually of the same length but merely that they are of the same general order of length. By way of example, it is considered that in a 1.5km tether individual fibres of 1,000m, or even of 500m are of the same order, and are thus "correspondingly elongate", while fibres of a mere lm or lOm are not.Put another way, "correspondingly elongate" fibres are fibres of such a length that in any one bundle thereof the chances of finding more than a few fibres ending (or broken) at any particular point along the cable are small. It is difficult to be precise about the meaning of "correspondingly elongate", but in general it will mean fibres of not less than one tenth the length of the tether itself.

The reason for choosing long fibres is, of course, to improve the overall strength of the tether. In a composite material such as is here used the ultimate tensile strength depends to a considerable extent on both the tension forces withstandable by each single fibre and the statistical probability of a combination of fibre ends occurring at a common location. The former are very high - a single carbon fibre is immensely strong in tension - whereas the latter are by comparison rather low - dependent on the fibre length.

The shorter the length the higher the probability of a large number of such ends occurring simultaneously. It follows that the shorter the fibres the weaker the tether - and, conversely, the longer the fibres the stronger the tether. Long fibres - even fibres as long as the whole tether - are therefore very much preferred.

The plurality of carbon fibres within the composite should be collimated to extend along the length of the tether. Thus, the fibres lie generally parallel one to the other along the structure, so as to provide the tether with the maximum possible tensile strength.

The number of fibres used in the matrix depends on the diameter of individual fibres (generally around 5 to 7 microns), their properties (a tensile strength of around 2,200 to 7,500 mPa and a stiffness of 230 to 600 GPa, depending on the grade), and on the stiffness or strength required in the final composite article (which naturally depends on its application). In general, though, it will be a very large number, possibly in the region of several millions, with the proportion of fibres to thermoplastic matrix by volume being typically in the range of 100% to 10%.

The carbon fibres, which carry the applied load, are bound within a matrix of a thermoplastic resin.

The purpose of the matrix is to protect the fibres from chafe and abrasion, bind the fibres together, and distribute the load amongst the fibres. Plastics that is, natural or synthetic resins - are used for the matrix because they can be selected for their resistance to corrosion and chemical attack, and for the ease with which the tethers may be manufactured from the resultant composites, and then handled (plastics matrices tend also to have a low density, so the composite is relatively light). The thermoplastics resins used in this invention provide considerable advantages for the production of the tether by an extrusion-like process.Moreover, thermoplastic matrices make it very much easier to form the tether terminations, transition pieces, transverse bearings, or other components, by re-moulding to any desired profile, so simplifying stress distribution, handling and installation.

Of the many thermoplastics suitable for use as the matrix material, that comparatively recent polymer available from ICI and known as PEEK (for Poly Ether Ether Ketone), which has excellent toughness and resistance to abrasion, is particularly desirable.

A tether of this invention may be manufactured by a continuous hot forming operation incorporating proprietary processes believed to be similar to extrusion and pultrusion processes. The specific details of these processes are of no concern here, but in general they require the plurality of fibres to be fed into a melt of the thermoplastics matrix material and then pulled through a consolidating and forming system, the whole taking the form of the desired composite as it emerges.

This invention enables the stiffness, strength and mass of the tether to be tuned to suit the dynamics of the intended use. Thus, by adjusting the fibre content and the fibre type tethers of different physical characteristics may be formed. Moreover, such an adjustment may be carried out within a single tether, so that different parts of it have different properties. For example, a portion of a tether could be thickened (and so stiffened, and made stronger) either by adding in more fibres as the formation of that length proceeds, or by changing the type of the fibres over that length, or by subsequently adding/bonding extra layers of composite, or even by "forging" - heat-softening and re-shaping - of the formed tether into a different form. This adjustment will be particularly suitable for constructing a tether containing transition pieces and transverse bearing portions.

As noted above, use of a thermoplastic matrix material allows the tether of the invention to be moulded in many ways, including into terminations, transition pieces and transverse bearings (for example to transfer a horizontal load to the tether) Naturally, these parts may have incorporated therein reinforcing or other components such as metal rings, as required.

Most desirably a formed tether is thereafter provided with an outer coating, or protection sheath.

This may be formed of some sacrificial material, and a typical one is woven or wrapped carbon fibre in a matrix of the same binding plastics as used for the tether per se.

Use of tethers of the invention enables the alleviation of many of the problems presently experienced with the known tether. Thus, as regards tension leg platforms it at least partly deals with the vertical heave resonance problem, so allowing greater utilization of such platforms, especially at sea depths of a kilometre or more. It also overcomes the concerns regarding fatigue resistance, corrosion and stress corrosion, resulting in more efficient and reliable structures. The lightness and strength also offer installation advantages.

Moreover, in connection with cable-stayed bridges, the problems of corrosion of the cables where they approach their lower terminations adjacent the bridge deck - severe corrosion conditions are caused by the salts used for de-icing and by the exposed maritime atmosphere around many such structures - are overcome by utilizing materials inert to the effects of the atmosphere and corrosive salts.

Again, many large concrete structures are presently pre-stressed using materials that are subject to creep and degradation due to corrosion, limiting the safe life of the structures of which they form a part, and the present invention overcomes this, for the creep in the carbon fibres is small in comparison with high tensile steel, and the material is unaffected by corrosion or atmospheric degradation.

In addition, it should be noted that the use of a continuous composite tether, itself containing continuous fibres, enables a variety of techniques to be used to monitor the condition of the tether from one or other end (such as on a tethered platform). This may be achieved by monitoring, for example, the acoustic or electrical properties of the carbon fibres, or also by incorporating dedicated sensing fibres of appropriate materials into the composite.

The invention extends to a structure such as a tension leg platform, a cable-stayed bridge, a prestressed concrete beam, a marine vessel, tower, aerial, mast or the like rigging system whenever constructed using, as a tension member, a tether of the invention.

The invention is now described in more detail, though only be way of illustration, with reference to the accompanying drawings in which: Figure 1 shows a perspective view from above and one side of an example of a tension leg platform in situ; and Figure 2 shows a tether as used for the platform of Figure 1.

Figure 1 shows a tension leg platform (TLP) held in place by tethers according to the invention. The platform itself (generally indicated at 10) has a buoyancy unit 10a comprising four columns 12 and hulls 12a supporting a rectangular deck 11. Attached to the buoyancy unit 10a and extending down through each column 12 to an anchoring block 13 on the sea bed, are a multitude of tethers 14 of the invention, by which the platform 10 is moored to the sea bed.

Details of a tether 14 are shown in Figure 2, which illustrates the upper, "middle" and lower portions of the tether 14. The top of the tether has an adjustable termination (generally indicated at 20) with a load-bearing pad 21, and held in place by a locking device 22. Below this the tether passes through a wider pipe-like shaft 23 in the TLP column 12, where it is widened at 24 to form an upper transverse bearing and flexible joint. As it exits the column it narrows down, via a transition piece 25, to the tether 26 proper, only a short length of which is shown, but which might be one and a half kilometres long. As can be seen from the inset, this is a tubular construction 27 of a matrix of collimated - i.e., parallel - carbon fibres within a thermoplastic binder, wrapped in a protective sheath 28. There are in addition special "sensing" fibres 29 built thereinto during forming. At the bottom end the tether widens via another transition piece 30, and ends in a lower termination and flexible joint 31.

Claims (13)

1. A tether in the form of an elongate structure and comprising a carbon fibre composite material itself comprised of a collimated plurality of substantially continuous elongate carbon fibres extending along the length of the structure and bound together with a matrix of a thermoplastic resin.

2. A tether according to claim 1, wherein the elongate structure is in the form of a relatively long rod or tube.

3. A tether according to claim 1 or claim 2, wherein each carbon fibre in a majority of the fibres is correspondingly elongate to the length of the tether, as a whole.

4. A tether according to any of the preceding claims, wherein the proportion of fibres to thermoplastic matrix by volume is in the range of 100% to 10%.

5. A tether according to any of the preceding claims, wherein the thermoplastics matrix material comprises poly- ether-ether-ketone.

6. A tether according to any of the preceding claims, wherein the properties of the tether are varied along the length of the tether, by varying one or more of the tether thickness, the number of carbon fibres incorporated in the matrix, and the nature of the carbon fibres.

7. A tether according to any of the preceding claims, wherein extra layers of reinforced matrix are added to the completed tether, at locations where transition pieces or transverse bearing portions are required.

8. A tether according to any of the preceding claims, wherein the tether includes regions of increased crosssectional area formed by heat-softening a localised region of the completed tether and forging the heat-softened region to the required shape.

9. A tether according to claim 7 or claim 8, wherein the regions of increased cross-sectional area include added reinforcement.

10. A tether according to any of the preceding claims, wherein an outer coating or protection sheath is provided on the formed tether.

11. A tether according to claim 10, wherein the protection sheath comprises a woven or wrapped carbon fibre layer incorporated in a matrix of the same thermoplastics matrix as is used for the tether per se.

12. A tether according to claim 1 and substantially as hereinbefore described, with reference to the accompanying drawings.

13. A tension leg platform, a cable-stayed bridge, a prestressed concrete beam, a marine vessel, tower, aerial, mast or the like rigging system whenever constructed using, as a tension member, a tether according to any of claims 1 to 12.