Linear winches, "caterpillar"track tensioners, and similar types of winching systems are commonly used in numerous offshore deployment and recovery operations. Various types of winch are used to deploy the rope to the seabed. These can be drum winches or traction winches, which involve bending the rope under tension, and linear winches where the rope is held in a set of grippers and fed out without any bending. However, these types of winches are designed for use with steel wire ropes and steel pipe, both of which are highly rigid and not easily deformable, therefore having a high degree of resistance to the compressive forces applied by this type of winch.

The viability of steel wire rope based deployment systems becomes significantly impaired with increasing water depth. This is due to the self weight of the rope consuming the majority of the rope strength. Therefore, as operations are carried out in deeper water depths it becomes increasingly attractive to use lighter weight synthetic fibre rope construction.

High performance ropes are available, for example in a 12 x 12 form, as described in <BR> <BR> US 5,931, 076 and US 5,901, 632 (Ryan/Puget Sound Rope Corp. ). Replacing steel wire rope with lightweight synthetic fibre rope such as this would significantly reduce the self weight of the rope. However, standard deployment winches are not suitable for synthetic fibre rope, as they subject the rope to the compressive forces mentioned above, and the combined effect of high bending and tensile loads, which can significantly impair the fatigue performance of a synthetic fibre rope. Linear winches or track tensioners do not induce bending loads in the synthetic fibre rope, but these types of winches induce a high compressive radial load to the rope, against which synthetic fibre rope does not offer sufficient resistance.

Accordingly, it is an object of the current invention to provide an improved rope suitable for use with standard deployment winches and addressing one or more of the aforementioned disadvantages.

In a first aspect of the present invention, there is provided a braided fibre rope having a first end and a second end, said rope further comprising a core having a low-friction surface, said core being insignificant with regard to a tensile load bearing function, while significantly resisting crushing of the rope. The core enables the fibre rope to resist the compressive loads applied by deployment/recovery apparatus, such as linear and caterpillar track tensioners without being detrimental to, or contributing substantially to, the load carrying capacity of the rope.

Said rope may be formed by a plurality of yams braided together to form a multi-strand sub- rope, and a plurality of such sub-ropes braided together around said core to form a woven rope. Each said yarn may comprise a plurality of filaments twisted or braided together. The invention can be applied in any other braided rope construction which will support a central core.

The core may have a cross-sectional area equal to or greater than the calculated void space of the fibre rope. Where the core is greater than the void space and the construction of the rope allows, the rope simply expands radially to accommodate the core (the core will therefore limit the extension of the rope under tension). Where the rope is formed of sub- ropes, the void space of the rope refers only to the central void. Void space and other physical properties of the rope are calculated under a reference tension in accordance with a U. S. Cordage Institute industry standard.

The core may be unterminated at least one end of the rope, to permit sliding of the end within the braided fibre rope. In a preferred embodiment, the core is not terminated at any end.

The rope may be formed in a plurality of sections, longitudinally each section having a section of crush-resistant core captive within it. This avoids the core sliding so far that a long useless section of rope is formed at the upper or lower end. Care must be taken in use to ensure that compression does not become applied to a part of the rope from which the core has slid out.

Said crush-resistant core may comprise a material having a low surface coefficient of friction, or a core coated with or sheathed by said low friction surface. One example of core material is polyester fibre. Alternatively, the core material may be nylon. The core may be formed from a solid core, or from multiple strands that are enclosed by a single sheath.

Some or all of the strands enclosed by the sheath may be formed into braided rope, for example double-braided rope.

Said braided fibre rope may, as an example, be constructed using twelve sub-ropes. Each said sub-rope may be constructed using twelve yarns. This particular construction has benefits of easy repair, as described for example in US'076, mentioned above. Each said sub-rope may further comprise its own crush-resistant core having a low-friction surface.

The yams of the braided fibre rope may be formed from filaments of a low-elongation UHMWPE (ultra-high molecular weight polyethylene) fibre material. An example of such materials of construction are Plasma fibre, a patented variant of Spectra UHMWPE.

(Spectra and Plasma are trademarked by Honeywell and Puget Sound Rope respectively).

Said yams may have a comparatively small diameter so that no one of said yams has a cross-sectional area greater than about 1/144th of a total cross-sectional area of said rope (excluding core area), so as to minimize the number of stages of twisting which are required to form each of said sub-ropes which is used in said large-diameter woven rope. One method of specifying void area is as a percentage of the total cross sectional area of the rope, under standard test conditions. Typical values for the central void space to rope area are within the range 10% to 15%.

The invention does not require absolute incompressibility of the core. It may be sufficient for example if under compression, the core maintains, say 50%, 80% or 90% of its own area, or for example 5% or 8% or 9% of the uncompressed rope area, under industry standard tension.

Said woven rope may have a circumference in the range of 127mm to 508mm. Each said sub-rope may have a diameter of about 11 mm (7/16 inch) or greater. Each yarn from which a sub-rope is constructed may have a diameter of about 0.8 mm (1/32 inch) or greater. Said yams may be twisted together at a twist factor substantially within the range 134 to 140.

Said yams may be braided together at a pick multiplier in the range from about 1.0 to about 1.4. Said plurality of said sub-ropes may be braided together at a pick multiplier in the range from about 2.0 to about 2.8.

In a further aspect of the present invention, there is provided a method of construction a large-diameter braided rope having a first end and a second end, said method comprising the steps of constructing said rope with a crush-resistant core having a low-friction surface, and with regard to tensile load bearing, ensuring said core is un-terminated at at least one of the ends of said rope.

Said method may comprise braiding together a plurality of yams to form a multi-strand sub- rope, and braiding together a plurality of said sub-ropes around said core to form a woven rope. Said braided rope may, as an example, be constructed using twelve sub-ropes. Each said sub-rope may be constructed using twelve yarns. Each said yarn may be constructed using a plurality of filaments. The core may be formed from polyester fibre, or nylon, in filament or solid form.

Said method may further comprise incorporating into each sub-rope while it is being formed a crush-resistant core having a low-friction surface, said core being, with regard to tensile load bearing, un-terminated at at least one of the ends of said sub-rope.

The method may comprise forming said rope in a plurality of sections, captivating a section of crush-resistant core longitudinally within each section.

The method may further comprise coating or sheathing the core with a low-friction surface, such as polyethylene. Where the core comprises a plurality of filaments, said filaments may be twisted or braided together, prior to coating or sheathing the core.

The yams of the rope may be formed from filaments of a low-elongation UHMWPE (ultra- high molecular weight polyethylene) fibre material.

The method may further comprise the additional step of heat-stretching said yams prior to the step of braiding said yams into a sub-rope. The step of braiding said yams together may comprise braiding said yams together to form said sub-rope without further twisting of said yams after said heat-stretching. The step of heat-stretching said yarn may comprise heat- stretching said yams formed of filaments of low-elongation UHMWPE filament material at an elevated temperature and pressure.

The method may comprise twisting a plurality of filaments together at a twist factor substantially within the range 134 to 140 so as to form a plurality of twisted yarns. Said yams may be braided together at a pick multiplier in the range from about 1.0 to about 1.4.

Said plurality of said sub-ropes may be braided together at a pick multiplier in the range from about 2.0 to about 2.8.

Said yams may have a comparatively small diameter so that no one of said yams has a cross-sectional area greater than about 1/144th of a total cross-sectional area of said rope (excluding core area), so as to minimize the number of stages of twisting which are required to form each of said sub-ropes which is used in said large-diameter woven rope. Said woven rope may have a circumference in the range of 127mm to 508mm. Each said sub-rope may have a diameter of about 11 mm (7/16 inch) or greater. Each yarn from which a sub-rope is constructed may have a diameter of about 0.8 mm (1/32 inch) or greater.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described, by way of example only, by reference to the accompanying drawings, in which: Fig. 1. is a side-elevation view of the construction of a known"12 x 12"braided rope; Fig. 2 provides a cross-sectional end view of the rope of Figure 1 ; Fig. 3 is a side-elevation schematic diagram illustrating the general arrangement of a rope- based lifting and lowering apparatus using a vertical tensioner; Fig. 4 shows an abandonment and recovery system using moveable clamps to hold the rope; Fig. 5. is a side-elevation view of the rope of Figs. 1 and 2 that incorporates a crush- resistant core; and Fig. 6 is an end-view of the rope of Fig. 5, when slack; Figures 7a and 7b are cross-sectional detailed views of the central core with a low friction coating and with a low friction jacket, respectively; and DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS There exist a number of applications that make use of very large diameter braided ropes, such as by the offshore industry, or for tugboats or mooring lines. An example is hereby provided in respect of the offshore industry, however the rope construction, and methods of construction hereby described apply equally to other industries making use of such ropes.

Conventional methods for offshore deployment of heavy loads to the seabed are by use of steel cables or steel wire rope. Deployment of steel wire rope is by using standard equipment such as a linear winch or track tensioner. These devices all operate by inducing

high radial compressive loading directly on the steel wire cable of steel pipe. However, for deep-water applications, the weight of the rope itself has a prohibitive impact upon the operational depth of deployment and load that can be deployed, hence the desire to use lightweight, synthetic rope. However, the synthetic fibre rope must be capable of being used on existing track tensioners or other similar devices, without damage.

Figure 1 provides a side-elevation view of a recent development in the construction of very large diameter synthetic fibre rope and which is the subject of US Patents US 5,901, 632 and US 5,931, 076, mentioned above. This particular"12 x 12"braided rope 10 is made up of a multiplicity of single fibre filaments 12 twisted to make fibre yams 14, which are themselves then twisted or braided together to make up a sub-rope 16, which are themselves then twisted or braided together to make up the fibre rope 10.

Conventional braided rope is not effectively"scaled up"to produce very large diameter ropes, especially when using comparatively new, low-elongation fibre materials, such as polyester, Kevlar, or UHMWPE materials such as Spectral for example. The 12 x 12 construction of very large diameter synthetic rope offers an improvement for offshore applications because it exhibits the following beneficial attributes, when compared with conventional large-diameter ropes: - better strength efficiency relative to 12-strand braided rope, in particular for these large rope sizes; - better bundle coherency; - better translation efficiency (the relationship between the breaking strength of the yarn and the combined breaking strength of the fibres which form the yarn, in terms of a percentage of the latter value); - the rope is easily opened and thus fully inspectable; - the rope is easy to splice and re-terminate if necessary; - individual sub-ropes can be repaired if damaged; and - the rope is torque neutral (although this is true for any braided rope).

The particular embodiment which is illustrated employs a 12-strand, two-over/two-under form of braid, forming a large rope out of 12 sub-ropes. However, the skilled reader will understand that other forms of braid and other numbers of sub-ropes or yams may be used (such as 8-strand construction, for example), if the applicant does not demand all of the special attributes of the 12 x 12 type.

To construct the rope 10, the individual twisted yams 14 are first twisted from the filaments 12 and then braided together to produce the sub-ropes 16 using a braider machine. The braided sub-ropes 16 which are produced are then wound onto second spools and loaded onto another braider machine, by which they are woven together to form the finished rope 10.

Moreover, because the braiding process itself imparts a very limited amount of additional twist to the sub-ropes, the techniques of US'632 and US'076 make it possible (unlike with multiple-stage twisting) to maintain an optimal degree of twist in the yams so as to achieve maximum translational efficiency in the finished rope. For example, the yams can simply be given the optimal degree of twist initially, and this twist will remain largely unaffected by the subsequent braiding steps, or in some cases the yams may be given an initial degree of twist which is just slightly less than optimum, to compensate for a small but predetermined amount of twist which will be added during the braiding process. As a result, braided ropes constructed in this manner provide significantly higher tensile strength than is possible with conventional twisted strand rope constructed to have the same body and coherence, and as such are widely utilised in the offshore industry. The reader is invited to refer to patents US'632 and US'076 for further detail relating to the construction of the braided rope selected for improvement. In summary, however, a rope constructed of UHMWPE filament material using yams formed using a twist factor of approximately 140, sub-ropes formed from yams using a pick multiplier of approximately 1.35, and a rope subsequently formed from sub-ropes using a pick multiplier of approximately 2.7 has been found to provide an outstanding combination of strength and handling/durability qualities for typical marine use, and as such an ideal candidate for improvement for use in lifting and deployment operations in which linear winches and tensioners are utilised.

Figure 2 provides a cross-sectional end view of the rope of Figure 1. The nature of the fibre rope construction suggests a relatively high level of void space 20 when sampling the cross

section of the fibre rope 10. A measurement of total void space 20 can be obtained by measuring the nominal rope diameter using an industry standard method of measurement (load applied = 1. 38D2) and re-measuring the rope after a high compressive load has been applied. For example, within the 12 x 12 strand braided rope of US'632 and US'076, the total void space is calculated to be up to 30%. For information, the 1. 38D2 measurement is a U. S. Cordage Institute standard for determining the reference tension to be used when making rope measurements, etc. The reference load (in newtons) is equal to the rope diameter (in mm) squared x 1.38. The US equivalent to 1. 38D2 is 200D2, in which the reference load (in pounds) is equal to the rope diameter (in inches) squared x 200.

The total void area mentioned above includes smaller voids in the sub-ropes for a braid-of- braids construction such as the 12 x 12 rope. It also includes spaces between the strands within the sub-ropes. Accordingly, for the purpose of the present description, it is more instructive to consider the ratio of the central void area only to the overall area of the rope, still under the under reference tension. This ratio will typically be between 10% and 15% for the types of rope described here, and will be similar for a simple 12-rope braid as for a 12 x 12 braid-of-braids.

The application of high radial loads, as would be found in offshore applications using conventional deployment means, forces the fibre filaments to fill the void spaces, causing the filaments to kink and form loops. After a period of such use, this results in a stiff rope which is difficult to handle, greatly reducing the active service life of the rope due to fatigue bending of the filaments. Examples of such use are now described, with reference to Figures 3 and 4.

Figure 3 is a side-elevation schematic diagram illustrating the general arrangement of a rope-based lifting and lowering apparatus using a vertical tensioner. The system is used to lower and/or retrieve a load 30 to/from the seabed from a ship, barge or other sea-borne vessel 32. Fibre rope 34 is stored in a spooling system 36, which does not serve as a winch for the weight of the load 30, however. A continuous track tensioner 38 engages the rope 34 by friction and or other means and provides the tension for controlled lowering or lifting of the load.

Tracks or the like arrayed around the axis of the rope 34 are pressed radially inward by suitable rams, levers and the like to grip the rope, and to release it again when required.

Each track comprises a series of individual rope-handling shoes, linked together. While two tracks are shown for the sake of illustration in Figure 3, three or four tracks will be more usually provided, spaced at 120° or 90° intervals around the rope axis respectively.

The detailed construction and operation of the structures for supporting these tensioners in vertical and/or inclined positions above the sea surface can be readily envisaged by the skilled person, for example by reference to prior art in the field of pipe and cable laying.

Indeed, it is an advantage of the invention that it can be implemented using existing pipe-or- cable laying tensioners on existing vessels.

In order to deploy or recover fibre rope using a multi-track tensioner equipped with pads on the tracks, the pad design is adapted to the rope, as described in our co-pending application PCT/GB2003/000932 (Agents ref. 64056WO) the content of which is incorporated herein by reference. A rope behaves differently than, say, an umbilical or pipeline (flexible or rigid), when it is fed through a tensioner, compressed by the pads and brought under tension. Unlike pipes and umbilicals, the diameter of the rope can change significantly with increasing load onto the pads as well as with increasing tension to the rope. Furthermore the danger of pinching the rope between the pads is significant. The shoe pads are therefore designed to provide a proper fit of the rope between the pads, regardless of the load to the pads or the tension to the rope.

Figure 4 shows another installation where the track type tensioner is replaced by a movable clamp or preferably a pair of clamps, to deploy or retrieve the fibre rope. This shows a tower 40 with a winch 42 mounted at the top. The fibre rope on this winch 42 is sourced from a spool 44. It is connected to a load (in this case the end of a pipeline 46, via a pipeline <BR> <BR> end termination (PLET) ). Two clamps 50,52 having the novel pad arrangement as described above hold the rope. The clamp may be formed in two, three or four sections.

The same clamps have been used to lay the pipeline, and then adapted by changing their shoes to handle the fibre rope for abandonment of the pipeline. (The same considerations as to shoe design apply to the clamps as to track type tensioner).

During deployment and/or recovery both clamps 50,52 move relative to each other, in a sequential manner to and from the middle of the tower, to hand over the grip on the rope from one clamp to the other. This action results in the paying in or out of the rope, and can be controlled to provide continuous movement. The skilled reader will appreciate that the same result can be achieved using a single movable clamp and a fixed clamp, although only intermittent movement could be achieved.

As mentioned previously, conventional woven rope, or the rope described with reference to Figures 1 and 2, cannot be used on this equipment without incurring long-term damage.

Figure 5,6 and 7a and 7b are respective side-elevation and three cross-sectional end views of the rope of Figures 1 and 2, modified to withstand the effects of the radial compressive loads imposed upon the rope by the apparatus described with reference to Figures 3 and 4, by including within the fibre rope structure a crush resistant core 100 having a diameter equal to or greater than the calculated void space 20 with the fibre rope. The crush resistant core is made from a lightweight material such as polyester fibre, and is built into the rope during manufacture. If a multi-fibre core is being used, then the fibres are sheathed by low- friction material. If this is the case, the friction properties of the fibres from which the core is constructed are of secondary importance.

The method of manufacture is the same as described above for the unmodified rope, but with an additional facility for braiding the rope around the central core 100. The size of the core is dictated by the void space to be filled, which is dependent upon the size of the fibres from which it is formed, and the configuration of the sub-ropes used in the construction of the rope.

The core is designed to fill the void in the 12 x 12 construction, thereby preventing the braided sub ropes from interacting together when the rope is subjected to the radial crush loads and deformation from the combined tension and bending loads as a result of deployment over a sheave. By restricting the range of diameters which the rope can take under tension, pinching of the fibres between pads of the tensioner or clamp can be eliminated, while ensuring a strong grip.

In the improved construction, with reference to the figures, twelve yams 14 are braided to form a 12-strand sub-rope 16, and subsequently twelve of these sub-ropes are braided together around the core to form the final rope. There are thus 144 low-twist yams and a central core in the construction of the rope, rather than 12 high-twist primaries as in a typical 12-strand rope. Each yam is formed from multiple filaments of low-elongation material.

A specific example of a modified rope is a construction of a comparatively large-diameter 12-strand rope having a diameter of at least 76mm, incorporating a crush-resistant core having a low-friction surface. The twisted fibre yams have a comparatively small diameter such that no one of the yams has a cross-sectional area greater than about 1/144th of a total cross-sectional area of the rope (excluding the core), so as to minimize the number of stages of twisting which are required to form each of said yams which is used in said large- diameter rope. The construction of the rope uses a multiplicity of low elongation fibres twisted together to form a plurality of twisted yarns, which are subsequently braided together to form a plurality of sub-ropes having a diameter of about 7/16 inch or greater, which are then subsequently braided together around the central core to form a rope having a circumference in the range of 127mm to 508mm. In the example illustrated, the core comprises a jacketed bundle of polyester fibre having a cross-sectional area substantially matching that of the void area in which it resides. The core is terminated at neither or one of the ends of the rope. The material of the core is selected to accommodate higher elongation than the rope, to ensure that when the rope is loaded, the tensile loading on the core is always kept within its operational limits.

Tests were performed using the 12 x 12 rope construction described above, incorporating the central core 100. Under the 1. 38D2 reference tension the overall rope diameter was measured at 88mm. The diameter of the core inserted into the centre of the rope was 29mm.

The cross sectional area of the resulting cored rope is calculated at 6082mm2. This total cross sectional area measurement includes both rope fibre mass, the central cored area (which would be void space in the known rope) and any void spaces in the sub-ropes. The cross-sectional area of the 29mm core is around 660mm, which is 10.9 % of the total. The tests confirmed that the addition of the central core greatly improved the performance of the rope when used with a linear track tensioner.

Figure 5 shows a side-elevation view of the end of a modified rope, and its construction.

The centrally located central core 100 can be seen, integral to the rope.

Figure 6 shows a cross section of the rope of Figure 5. A small amount of void space 20 still exists, but the majority of the space is now occupied by the central core 100. However, the skilled reader will appreciate that each of the braided sub-ropes may also contain their own core, although this is not illustrated in any of the accompanying figures. In this case, sub- rope void spaces should be included in the calculation of the area ratios used herein to characterise different embodiments of the invention.

When the fibre rope which has been elongated due to the application of a tensile load, the large amount of void space, as shown in Figure 6, is substantially removed, due to the elongation of the rope, and the subsequent narrowing of its diameter, the fibres of the rope being compressed against the central core. The improved rope is constructed to ensure that the core does not carry any of the tensile load of the fibre rope. This is achieved by ensuring that the core is terminated at neither or one of the ends of the rope, that the friction between the load-bearing section of the rope (the fibres around the core) and the core itself is minimised. The material selected for the core can also be selected to accommodate a higher elongation without damage than the rope itself. The central core 100 is provided with a surface 110 that minimises the frictional forces between the core and the rope. The presence of a central core means there is less void space for the rope fibres to fill during use of the rope in the aforementioned deployment equipment, thereby significantly reducing the likelihood of fatigue bending of the filaments.

Figures 7a and 7b are cross-sectional close up views of the central core with a low friction coating (not shown) and with a low friction jacket 120, respectively. The additional coating or extruded low friction jacket 120 are constructed from low coefficient of friction materials, such as polyethylene.

The fibre rope crushes as it is fed through a linear track tensioner, for example. Like in the case of elongation, the small amount of void space around the central core is no longer present, as the fibres of the rope are compressed against the central core. As before, the

presence of a central core means there is less void space for the rope fibres to fill during use, significantly reducing the likelihood of fatigue bending of the filaments.

Furthermore, due to the nature of its construction, the very large diameter synthetic rope is capable of being serviced. The ability to splice the individual braided yams permits cuts, frays, and other damage which occurs in service to be repaired using readily available tools and skills. Although the rope can be made in section, the 12 x 12 construction also permits a very long continuous rope. <BR> <BR> <P>The 12 x 12 rope construction also allows construction with a much looser final braid (i. e. , a lower of number of picks per length) than would be possible using only twisted yarns, which is advantageous for certain applications and types of rope. When using twisted yarns, a very loose braid can result in the yams bunching and spreading apart, exposing the individual fibres to abrasion and damage. With the selected rope, however, the first braid <BR> <BR> (i. e. , the initial braiding of the yarns) can be made sufficiently tight to form sub-ropes which are durable and coherent, and then the final braid can be as loose as desired without impairing the serviceability of the rope. The addition of the central core also helps to provide coherence to the rope, when unloaded.

Supplementary to the exemplary embodiment provided, the skilled reader will appreciate that in further embodiments there may be reduced or additional braiding steps and/or variations in the configuration and quantity of the central core (s) within the rope structure, depending on the ultimate size of the rope, the type of material used, and other design considerations. Numerous variations are possible within the principles of the apparatus or methods described above. Accordingly it will be understood that the embodiments illustrated herein are presented as examples to aid understanding, and that the aforementioned and various other alterations, modifications, and/or additions may be introduced into the constructions and arrangements of parts described above without departing from the spirit or scope of the invention claimed.