GB2142788A - Bend limiter for cables - Google Patents

Abstract

A bend limiter 4 suitable for use on submarine cables comprises a plurality of annular segments 7 coupled together to form a tube. The annular segments 7 are coupled by means of bolts 8 whose shafts 16 run substantially along the length of the bend limiter 4 rather than transverse to it. The arrangement is such that the head 14 of any bolt 8 lying outermost of the curve of the bend limiter 4, when the bend limiter 4 is at maximum bend, is fully seated. Embodiments of the invention find particular application in limiting the bend of a submarine optical fibre cable 1 in the region of a joint with a repeater 2. <IMAGE>

Description

SPECIFICATION Bend limiter The present invention relates to bend limiters and particularly, though not exclusively, to bend limiters for use with submarine telecommunication cables.

Cables, particularly optical fibre cables such as those used in telecommunications, may be susceptible to damage if they are bent at too small a radius of curvature. A particular risk of excess, localised bending occurs during handling of a cable which is joined to a structure substantially more rigid than itself. An example is a submarine cable joined to a repeater.

Further, bending under considerable load may occur. Submarine optical fibre cables can be expected to take tensile loads of up to 20 tons when they are being laid or taken up. As a repeater passes over a winch drum, these loads will act to bend the cable, particularly in the region of the cable to repeater joint.

Methods which have been used to determine the minimum bend radius of a cable which will occur at a joint with a rigid element include the following: i) increasing the thickness of the cable over an extended region adjacent the rigid element; and ii) providing resilient support extending from the rigid element along the cable (such as a coil spring wound around the cable).

A disadvantage of these methods is that an extended region of the cable is made considerably less flexible, which is undesirable, particularly in the case of submarine cables as cable laying is made more difficlt by the extended inflexible sections.

An object of the present invention is to be able to impose a minimum bend radius on a cable while maintaining the cable's flexibility above that radius.

According to the present invention there is provided a bend limiter, for use with submarine cables, comprising a plurality of annular segments coupled together by means of connection elements to form a tube, each connection element having a shaft and a head, the head being retained in an aperture in a segment, which aperture extends in a direction which lies at an angle to the axis of said segment of up to 20g.

Preferably, the aperture is provided with an annular seating against which the head is adapted to be supported, the head and the seating being provided with faces adapted to mate when the associated connection element lies outermost of the bend limiter under the condition of maximum bend.

An advantage of bend limiters according to embodiments of the invention is that they may be designed to contribute to a smooth external profile when mounted, for instance, at a cable to repeater joint, while retaining satisfactory load bearing capability.

A further advantage of bend limiters according to embodiments of the invention is that they may provide means to clamp the armour of an armoured cable at a point separated from a repeater by a flexible element.

By way of example only, a bend limiter according to an embodiment of the invention will now be described with reference to the accompanying drawings, in which Figure 1 is a diagrammatic representation of a bend limiter according to the present invention, installed on a submarine cable.

Figure 2 is a plan view of a segment of a bend limiter according to the present invention; Figure 3 is a cross-section of a pair of segments, coupled together; Figure 4 shows the pair of segments of Fig.

3, subject to a bending load; and Figure 5 shows the bend limiter of Fig. 1, mounted on a winch drum.

Referring to Fig. 1, the bend limiter 4 is mounted on a cable 1 where the cable 1 is joined to a repeater 2. The bend limiter 4 comprises a series of annular segments 7 coupled together with bolts 8 to form a tube and the cable 1 extends through the bore of the tube. At one end the bend limiter 4 is attached to the repeater 2 and at the other end it is attached to a retaining collar 5 which is slidably mounted on the cable 1. A resilient sleeve 1 5 is provided over the segments 7 for protection and ease of handling.

Referring to Figs. 1 and 2, each segment 7 comprises a plate of circular cross-section with a central circular hole 9. The central hole 9 of each segment 7 contributes to form the bore 6 of the bend limiter 4. The segments are all of similar thickness, but range in overall diameter from the largest, with a diameter close to that of the repeater 2, to the smallest, which has a diameter close to that of the cable. The segments are arranged so that the overall diameter of the tube 4 gradually decreases between the repeater 2 and the cable 1. This gradual diameter change facilitates handling of the cable 1 with the repeater 2 and bend limiter 4 installed.

Each segment 7 is coupled to each of the segments adjacent to it by means of four bolts 8. The bolts 8 allow the segments 7 to tilt relative to one another (as shown in Figs. 4 and 5) such that the bend limiter 4 can adopt a curved configuration.

Each bolt 8 has a head 14 and an externally threaded shaft 1 6. The head 14 is retained in an aperture 1 2 in one of the segments 7' and the shaft 1 6 is screwthreaded into a receiving hole 1 3 in the adjacent segment 7". Each segment 7 is provided with four apertures 1 2 and four receiving holes 1 3 therefor, arranged symmet rically around its central hole 9.

The aperture 12 has a circular cross-section and extends along an axis which is tilted at 3" 36' to the axis of the central hole 9' of the segment 7'. The diameter of the aperture 1 2 shows a step change which provides a seating 1 7 for the head 14 of the bolt 8. Said head 14 is tapered at an angle of 7' 30' to the axis of the bolt's shaft 16, the larger end of the head 14 providing a planar annular face adjacent the shaft 16. The receiving hole 13 in the adjacent segment 7" lies parallel to the axis of the central hole 9" of said segment 7".

The segment of the bend limiter 4 which lies adjacent to the repeater 2 is bolted to a fixing ring 3 which in turn is attached to the repeater 2. The ring 3 may be bolted to the repeater 2 or may be attached to the repeater 2 in any other suitable manner. For instance, in an alternative arrangement, the ring 3 may be held into the end of the repeater 2 by means of an externally threaded stop member (not shown) which can be screw mounted into a corresponding thread inside the end of the housing of the repeater 2.

Referring to Figs. 4 and 5, the bend limiter 4 can bend to a limiting minimum radius of two feet. This limiting radius is, as will be clear, determined by the dimensions and relative positioning of the bolts 8 and the segments 7. Bending to this extent may occur when the weight of a cable section is supported by the repeater 2 as it passes onto a winch drum 1 8 during cable pick-up. It can be seen that the bend limiter 4 bends in response to the bending load exerted by the weight of'the cable section until the point of maximum bending when the segments 7 touch on the inner side of the bend and the heads 14 of the bolts 8 on the outer side of the bend are seated against their respective seatings 1 7. Up to the point of maximum bending the bend limiter 4 moves freely.To this end, the resilient sleeve 15 may be filled with a lubricant as well as with a corrosion inhibitor. The sleeve 1 5 itself may be constructed out of rubber.

The dimensions and material of the individual bolts 8 are at least partially determined by the maximum load they can be expected to carry. The maximum load for each bolt 8 will occur if the bolt 8 is lying on the outermost side of the curve of the bend limiter 4 when the latter is under load at its limiting minimum radius. Under those conditions, the bolt 8 in question will be carrying the whole force of the load which is acting to separate the relevant pair of segments 7, on its own. As mentioned above, a submarine optical fibre cable 1 is likely to experience a pull of up to 20 tons, due to its own weight, while being laid or taken up by means of a winch drum 1 8. When the bend limiter 4 is at its limiting bend, it acts as a solid, curved beam.The 20 tons pull is magnified by leverage and the tensile load experienced by bolts on the outermost side of the curve can be considerably greater than 20 tons. If the 20 tons act at 90 to the length of the repeater 2, the outermost bolt 8' which lies adjacent the repeater 2 may, individually, see a tensile load of up to 47 tons. However, the load experienced by the other outermost bolts decreases with increasing distance of the bolt concerned from the repeater 2. The bolts 8 which will lie adjacent the repeater 2 can therefore be selected to have the highest yield strength of all the bolts, the necessary yield strength of the rest of the bolts 8 decreasing along the bend limiter 4 away from the repeater 2.

This feature of decreasing yield strength can be particularly advantageous since the bend limiter 4, as mentioned above, tapers in a direction away from the repeater 2. Because a lower yield strength is required for bolts 8 which are further from the repeater 2, the size of the bolts 8 used can correspondingly decrease. This allows all the bolts 8 to be easily accommodated in spite of the reducing diameter of the bend limiter 4.

Under the condition of maximum bend, the bend limiter 4 described above is designed such that the head 14 of any bolt 8 which lies on the outside of the curve will be fully seated on its respective seating 1 7. This gives such a bolt 8 maximum support in the position where it can experience the greatest tensile load.

A second type of load, imposed on the bolts 8 which lie on the inside of the curve of the bend limiter 4 when at maximum bend, can be a shear load. The shear load is exerted because any pull on the cable 1 will act to slide the segments 7 past each other. However, this shear load is not magnified by leverage and, at worst, will only reach the 20 tons which may be carried by the cable 1.

During bending, the innermost bolts 8 of the bend limiter 4 described will be tilted in the same direction as the outmost bolts 8 by the segments 7 in which their shafts are mounted. It will be clear that the apertures 1 2 which receive the heads 14 of the bolts 8 must provide sufficient clearance for this to occur. It is to facilitate the tilting motion, without increasing the size of the apertures 1 2 unnecessarily, that the heads 14 of the bolts 8 may be tapered at an angle, such as described above.

The minimum radius which the bend limiter 4 described above can bend is selected because it is intended for use with submarine optical fibre cables. Such cables are generally susceptible to damage if bent at smaller radii.

However, if a bend limiter is to be put to a different use, a different minimum radius may be required. The design of the bend limiter components will clearly be largely determined by whatever minimum bend radius is re quires.

If the bend limiter 4 is to be used on armoured submarine cable, it can be advantageous if the cable armouring is terminated at the retaining collar 5. This can reduce the length of the single rigid member constituted by the repeater 2 and so facilitate handling of the submarine cable. Said length may be reduced because if the armour is clamped to the end of the repeater 2, the clamping arrangement generally causes a rigid extension to the repeater 2: by clamping the armour at the retaining collar 5 instead, that rigid extension is avoided.

In this type of arrangement, with the armour terminated at the collar 5, the bend limiter 4 may be required to carry any tensile load normally carried by the armour. Such loads may include a pull of up to, say, 70 tons exerted by, for instance, a trawler fouling the cable. These loads generally occur under conditions wherein the cable is free to pull straight and as a result the load the bend limiter 4 may have to carry will be distributed amongst all the bolts between each pair of segments. This imposes the design condition, however, that if four bolts are provided between each pair of segments, even the smallest of the bolts should be capable of carrying a tensile load of 17+ tons.

In order to protect the unarmoured cable of the above type of arrangement, that is the cable which runs between the collar 5 and the repeater 2, it is suggested that a flexible pressure hose should be provided inside the bend limiter, through which the unarmoured cable can pass.

The segments 7 of the bend limiter 4 may conveniently be made from a metal such as cast iron, steel or a corrosion resistant aluminium alloy. Any metallic components may be provided with a corrosion resistant coating.

Although in the description of the bend limiter the use of bolts 8 which screw into threaded receiving holes in the segments is referred to, it is not essential that such a system is used. The bolts may, for example, be held in unthreaded receiving holes by means of nuts. In this case the unthreaded receiving holes may be at a slight angle to the central hole of their respective segment in the same manner as the apertures receiving the bolt heads are at an angle. A tilted seating may be provided for the nuts, in a substantially similar manner as a seating is provided for the bolt heads, such that the nuts are also fully seated at maximum bend. Another alternative arrangement is that both the shafts and the heads of the bolts are retained in receiving holes or apertures which are tilted with respect to the central holes of their respective segments. The angle of tilt of the apertures in which the heads are retained may then be reduced.

It will be clear that the clearance allowed by all the bolts between each pair of segments should generally be the same, particularly where the bend limiter may have to withstand high tensile loads when straight. Otherwise the tensile load will not be evenly distributed amongst the bolts.

Any of the quantities or materials mentioned above may clearly be varied to suit the use to which a bend limiter according to an embodiment of the invention is to be put.

Claims (6)

1. A bend limiter, for use with submarine cables, comprising a plurality of annular segments coupled together by means of connection elements to form a tube, each connection element having a shaft and a head, the head being retained in an aperture in a segment, which aperture extends in a direction which lies at an angle to the axis of said segment of up to 20 .

2. A bend limiter according to Claim 1 wherein the aperture extends at an angle to the axis of said segment of up to 5 .

3. A bend limiter according to Claims 1 or 2 wherein the connection element is a bolt.

4. A bend limiter according to any preceding claim wherein the aperture is provided with an annular seating against which the head is adapted to be supported at more than point when the associated connection element lies outermost of the bend limiter under the condition of maximum bend.

5. A bend limiter according to Claim 4 wherein the head and the seating are provided with faces adapted to mate when the associated connection element lies outermost of the bend limiter under the condition of maximum bend.

6. A bend limiter substantially as described hereinbefore with reference to the accompanying Figures.