GB2165663A

GB2165663A - Bend limiter

Info

Publication number: GB2165663A
Application number: GB08525298A
Authority: GB
Inventors: Peter David Jenkins
Original assignee: British Telecommunications PLC
Current assignee: British Telecommunications PLC
Priority date: 1984-10-16
Filing date: 1985-10-14
Publication date: 1986-04-16
Also published as: GB2165663B; GB8525298D0; GB8426138D0

Abstract

A bend limiter, which can also be used to carry tensile load without undergoing extension, comprises a tube whose wall has the form of a bellows in that it has alternating flared and wasted portions. The diameter of each flared and each waisted portion is fixed but the portions are free to move relative to one another to a limited extent, in a direction perpendicular to the axis of the tube. The latter freedom of movement allows the tube to bend but the fixed diameters prevent elongation. In one embodiment the wall is constructed out of a plurality of springs 1 which are mounted on circular hoops 2 so as to lie parallel to one another. The circular hoops 2 are of alternating size along the tube and provide the fixed diameters of its flared and waisted portions. The bend limiter finds particular application in the field of optical fibre submarine cables. <IMAGE>

Description

SPECIFICATION Bend limiter The present invention relates to bend limiters and finds particular application in the field of optical fibre submarine cables.

It can be the case that an element such as a cable can only tolerate being bent to a certain minimum radius of curvature. Beyond that minimum radius, the element is susceptible to damage. A bend limiter is a device which will impose such a minimum radius.

When an optical fibre submarine cable is being laid or recovered, a tensile load of up to about 15 tons may be imposed on the cable.

Particularly if a winch drum is being used, any rigid portion of the cable can lead to this tensile load being applied in a direction to bend the cable at quite a sharp angle. A repeater housing represents such a rigid portion.

The bending action is introduced because, as the rigid portion passes around the winch drum, its profile will not conform to the profile of the winch drum. As a result its ends will protrude from the surface of the winch drum.

As th drum rotates, the body of the rigid portion will lie at a changing angle to the direction in which the tensile load acts. The load on the cable will as a result act to bend the cable in the region where it meets the rigid portion. Under certain circumstances, that bending load can be magnified by leverage to a value of up to twice the 1 5 tons mentioned above.

A known type of bend limiter which may be mounted at the end of a repeater housing is in the form of a bell-mouth tube. The cable passes from the end of the repeater housing into and through the bell-mouth tube. As a result it can only be bent as far as the internal surface of the bell-mouth tube will allow.

A bell-mouth tube however is not convenient in general handling of a cable, being prone to interfere with the seabed for instance. Further the necessarily wide diameter of the mouth compared to the diameter of the cable can be considerably inconvenient and it is necessary for instance to provide a tapered buffer to ensure smooth transmission of the cable through cable laying machinery.

It is an object of the present invention to provide a bend limiter which, in use, can contribute to a relatively smooth cable profile.

It is a further object of the present invention to provide a bend limiter which retains considerable flexibility up to its point of maximum curvature.

According to the present invention there is provided a bend limiter comprising a tube whose wall has substantially the form of a bellows, having alternating flared and waisted portions, wherein retaining means are provided to maintain constant the cross section of the tube at each flared portion, and at each waisted portion, such that bending of the tube can occur up to a predetermined minimum radius of curvature by means of movement of the flared and waisted portions relative to one another in a direction transverse to the tube axis, but such that extension of the tube is substantially prevented.

An advantage of bend limiters according to embodiments of the present invention is that they may sustain a considerable degree of flexibility until the minimum radius of curvature is reached.

A further advantage of bend limiters according to embodiments of the present invention is that they can be designed to carry substantial tensile loads without undergoing extension.

They can then replace portions of rigid loadcarrying members such as repeater housings, both reducing the effect of a tensile load on a cable which is being winched and making the cable easier to handle.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying Figures in which: Figure 1 shows a cross section of a bend limiter according to a first embodiment of the invention.

Figure 2 shows a plan view of a spring for use in the bend limiter of Fig. 1.

Figure 3 shows a bend limiter equipped to be mounted at the end of a complete, independent repeater housing.

Figure 4 shows a portion af the bend limiter of Fig. 1, bent to its minimum radius of curvature.

Figure 5 shows a cross-section of part of a bend limiter according to a second embodiment of the invention.

Referring to Fig. 1, a bend limiter according to a first embodiment of the invention comprises a plurality of springs 1 mounted to lie parallel to each other by means of circular hoops 2. Together the springs 1 form the wall of a tube. Each spring 1 comprises a series of substantially coplanar loops 3, offset from one another in alternate directions transverse to the length of the spring 1.The loops 3 of adjacent springs 1 are aligned in a circumferential direction around the tube so that each hoop 2 can extend through a loop 3 from each spring 1. The ends 4 of the springs 1 are secured in two collars 5, one collar 5 lying at each end of the tube, and a moulded rubber casing 6 is provided over the tube as a whole.

The bend limiter will now be described in more detail.

Referring to Figs. 1 and 2, each spring 1 is constructed out of a single steel rod. Each loop 3 is formed by approximately two turns of the relevant steel rod, all such turns having the same handedness. Hence, when looked at from the side, all the loops 3 of the same spring 1 will be seen to be wound in, for instance, a clockwise direction. Adjacent loops 3 of each spring 1 are offset from one another, as mentioned above, such that if a line were to be drawn joining the centres of consecutive loops 3 it would form a zig-zag.

The offset between loops 3 in a direction perpendicular to the length of the spring 1 (transverse offset) is 30 mm and the distance between the centres of adjacent loops 3 in a direction parallel to the length of the spring 1 (longitudinal offset) is 24.5 mm. The steel rod has a diameter of 6.3 mm.

Twenty springs 1 are arranged to extend in parallel, evenly spaced around the circumference of a tube that they form. The planes in which the loops 3 of each spring 1 lie extend radially and longitudinally with respect to the tube and the loops 3 of each spring 1 are aligned with the loops 3 of each adjacent spring 1 in a circumferential direction around the tube. The loops 3 are directed outwards, with respect to the tube.

The springs 1 are mounted on hoops 2.

Two series of hoops 2 are provided, the hoops 2a of one series being of a different diameter to the hoops 2b of the other series.

Each hoop 2 is threaded through all the loops 3 of a circumferentially aligned set of loops 3.

Clearly, as shown, the larger hoops 2a are threaded through loops 3a which lie radially outward in relation to the tube while the smaller hoops 2b are threaded through loops 3b which lie radially inward in relation to the tube.

The steel rod of each spring 1 extends, at each end of the spring 1, into a receiving hole 7 in one of the collars 5. Each receiving hole 7 is 28 mm deep and has a diameter designed to provide a sliding fit with its respective end of a spring 1. The collars 5 are each fixed to the hearest hoop 2 on which the springs 1 are mounted by means of a number of bolts 8. The bolts 8 extend through countersunk, plain bores 9 in the collar 5, in a direction parallel to the axis of the collar 5, and are screw-mounted at their ends into threaded recesses 10 in the nearest hoop 2.

Twenty bolts 8 are used to fix each collar 5, being regularly spaced around the collar 5 in alternation with the receiving holes 7 which take the ends of the springs 1.

Between the collars 5, the springs 1 and hoops 2 are encased in, and so protected by, a moulded rubber sleeve 6. The sleeve 6 is provided with circumferential grooves 11 (only one shown), extending between the aligned sets of loops 3 through which the larger hoops 2a are threaded. These grooves 11 act to reduce any stiffening of the bend limiter which may be caused by the sleeve 6. The sleeve 6 can also facilitate handling of the bend limiter, in use, by smoothing its external profile.

The design of the collars 5 will be affected by the use to which the bend limiter is to be put. For instance, the bend limiter may be used to provide an end portion of a repeater housing, which end portion contains a jointing chamber wherein co.mponents of the cable are jointed to components which extend to the repeater apparatus. Such a jointing chamber may be capable of withstanding a submarine environment but not of transferring tensile load, which may be up to 20 tons for an unarmoured cable, from the cable to the repeater housing. In this case, the bend limiter may be used to transfer the tensile load from the cable to the housing and the two collars 5 will be designed for mounting on the main section of the repeater housing, and for providing attachment to the load carrying elements of the cable, respectively.

Alternatively the bend limiter may be used only to limit any bend imposed on a cable where it joins a repeater housing. In this case the bend limiter may not be required to carry any tensile load. The two collars 5 may then be designed for mounting on the end of the repeater housing, and to ride freely on the cable, respectively.

Referring to Fig. 1, taking the case where the bend limiter is to provide an end portion of a repeater housing, the collar 5a which is to be attached to the main section of a repeater housing comprises a short cylinder equipped at one end with an outturned flange 12. The plain bores 9 for the bolts 8, and the receiving holes 7 for the ends of the springs 1, extend through the endface of the collar 5a which lies adjacent the flange 12. An external thread 13 is provided on the cylinder which thread 13 provides means by which the collar 5a may be attached to the main section of the repeater housing.

The other collar 5b, which is designed to provide attachment to the load carrying elements of the cable, comprises a short cylinder equipped with an inturned flange 14. The plain bores 9 and receiving holes 7 again extend through the endface of the collar 5b which lies adjacent the flange 14 but this time an inter nal thread 15 is provided on the cylinder. The internal thread 15 allows the load carrying elements of the cable to be attached by means of a suitable clamp (not shown).

Referring to Fig. 3, if the bend limiter is to be used in conjunction with a repeater housing just to limit cable bend, and not to carry tensile loads, then one collar Sc may be of the same general design as the collar 5a described above for attachment to a repeater housing. The other collar 5d however may be part of a tapered termination 16 to the bend limit which termination 16 is slidably mounted on the cable. The sliding collar 5d is of the same general design as the collar Sc at the other end of the bend limiter, having an outturned flange 17, but it is provided with a different form of external thread 18.This sliding collar 5d may be of smaller dimensions and strength in general than those of the col lar Sc which can be attached to the repeater housing. This is because, in conventional use, the forces which will be exerted on the repeater housing end of the bend limiter will be much greater than those exerted on the cable end of the bend limiter. As a rigid repeater housing passes onto a winch drum, for instance, tensile load on the cable will be translated into a bending load which increases, due to leverage, along the body of the bend limiter.Conventionally a cable is not bent through more than 90O to a repeater housing and therefore that leverage will be greatest at the joint between the bend limiter and the repeater housing: the tensile load acts there at the greatest angle to the bend limiter and at the greatest distance from the leverage point concerned. At the cable end however the tensile load acts on the cable alone, or only to a relatively minor extent on the bend limiter.

The sliding collar 5d is attached to the bend limiter by bolts (not shown), and receives the ends of the springs 1 in receiving holes 7 in the outturned flange 17, in the manner described above with reference to the other collars 5a, 5b, 5c. It is incorporated into the tapered termination 16 however as follows.

The termination 16 comprises the collar 5d, a tube 19, a spring 20 and a moulded rubber sleeve 21. At rest, the tube 19 is coaxial with the collar 5d, extending in a direction away from the bend limiter. Between the tube 19 and the cylindrical body of the collar 5d is a gap, across which gap extends a coaxial spring 20. The spring 20 is mounted by means of the external thread 1 8 provided on the collar 5d and by means of a similar thread on the external surface of the end region of the tube 19. Encasing the collar 5d, spring 20 and tube 19 is the moulded rubber sleeve 21 which is conveniently integral with the rubber sleeve 6 which encases the springs 1 and hoops 2 of the bend limiter.To provide location between the rubber sleeve 21 and the tube 19, the tube 19 has a profiled circumferential flange 22 and a shouldered nose portion 23, the end of the rubber sleeve 21 being moulded around the flange 22 and in abutment with the shoulder 24 so provided. The rubber sleeve 21 is smoothly tapered externally, being of greatest diameter at the collar 5d, and the nose portion 23 of the tube 19 continues the taper to an external diameter which is not substantially different from that of the cable in the region where it enters the termination 16. The internal diameter of the termination 16 is the same as that of the bend limiter and together they present a smooth bore through which the cable may extend.

Referring to Fig. 4, in use the bend limiter will bend but not extend. The transverse offset of the loops 3 is controlled by the hoops 2. Under a bending load the longitudinal offset of the outer loops 3c, on the outer side of the resultant curve of the bend limiter, will increase while that of the inner loops 3d, on the inner side of the curve, will decrease. At the same time the transverse offset of the outer loops 3c necessarily decreases while that of the inner loops 3d increases. The hoops 2 can accommodate this movement by means of a transverse shift of all the smaller hoops 2b, relative to the larger hoops 2a, towards the outer side of the curve. The result is that the bend limiter will bend, its central line retaining the same length but its outer side extending and its inner side contracting.

Under a tensile load, in order for extension to occur the longitudinal offset of all the loops 3 of the bend limiter would have to increase.

Consequently the transverse offset of both the inner loops 3d and the outer loops 3c would have to decrease. This is prevented by the presence of the hoops 2 which could only accommodate such changes by changes in their own diameters. Extension of the bend limiter does not therefore occur.

Because all the loops 3 of each spring 1 are wound in the same rotational direction and are directed outwards with respect to the bend limiter, when the bend limiter is bent, all the inner loops 3d will tend to be tightened onto their respective hoops 2. The extent to which the bend limiter will bend is limited because the inner loops 3d will be brought into abutment with each other. Once in abutment these loops 3d, in conjunction with the hoops 2 on which they are mounted, represent a substantially solid beam. Such a construction can offer extremely high resistance to further compression.

Although in the bend limiter described the springs 1 are mounted on two sets of hoops 2, the springs 1 could be mounted on two spiral elements. In this case clearly the loops 3 would have to be aligned along the spiral elements rather than along curcumferences of the bend limiter.

Methods of assembly of the bend limiter described will be fairly self evident. Referring to Fig. 1, it has been found convenient however to mount the springs 1 on the hoops 2 by constructing each hoop 2 in two semicircular parts, threading each part through the relevant loops 3, then connecting the two parts of each hoop 2 by means of pins 26 (only one shown) held by transverse screws 27.

Referring to Fig. 5, without departing from the spirit of the invention the springs 1 could be replaced by a tubular bellows 28. The bellows 28 is threaded between the hoops 2 such that the larger hoops 2a each lie against the inner surface of a respective expanded portion 29 of the bellows 28 while the smaller hoops 2b each lie against the outer surface of a respective waisted portion 30 of the bellows 28. Under a bending or a tensile load, this form of bend limiter will act in much the same manner as a bend limiter constructed using springs 1 as described above. However the choice of material used in the bellows 28, and its physical dimensions, will be constrained by the degree of deformation that must take place between each expanded portion 29 and its neighbouring waisted portions 30 of the bellows 28 when undergoing a bend.

Claims

1. A bend limiter comprising a tube whose wall has substantially the form of a bellows, having alternating flared and waisted portions, wherein retaining means are provided to maintain constant the cross section of the tube at each flared portion, and at each waisted portion, such that bending of the tube can occur up to a predetermined minimum radius of curvature by means of movement of the flared and waisted portions relative to one another in a direction transverse to the tube axis, but such that extension of the tube is substantially prevented.

2. A bend limiter according to Claim 1 wherein the retaining means provided with respect to the flared portions are provided by a first helical member and the retaining means provided with respect to the waisted portions are provided by a second helical member.

3. A bend limiter according to Claim 1 wherein the retaining means provided with respect to the flared portions are provided by a first series of hoops and the retaining means provided with respect to the waisted portions are provided by a second series of hoops.

4. A bend limiter according to Claim 3 wherein the hoops are circular.

5. A bend limiter according to Claim 4 wherein the hoops of the first series all have a common, first diameter and the hoops of the second series all have a common, second diameter.

6. A bend limiter according to Claims 2, 3, 4 or 5 wherein the wall of the tube is provided by a plurality of longitudinal elements which extend in a direction along the tube and which are each mounted on at least two consecutive retaining means along the tube.

7. A bend limiter according to Claim 6 wherein each longitudinal element comprises a length of substantially inextensible rod, mounted on the retaining means by means of integral loops, each loop being constructed out of a portion of the rod which is bent into a shallow spiral circumscribing a portion of the relevant retaining means.

8. A bend limiter according to Claim 7 wherein the loops are all directed outwards with respect to the tube.

9. A bend limiter according to any one of Claims 6, 7 or 8 wherein each longitudinal element extends the whole length of the tube.

10. A bend limiter according to any one of Claims 7, 8 or 9 wherein the loops are a sliding fit on the retaining means.

11. A bend limiter according to any one of Claims 7 to 10 wherein the shallow spiral comprises in each case more than one complete turn of the relevant rod.

12. A bend limiter according to any one of Claims 7 to 11 wherein the rod is constructed out of metal.

13. A bend limiter according to any one of Claims 7 to 12 wherein, when the tube is bent to its minimum radius of curvature, adjacent loops along any longitudinal element which lies innermost with respect to the bend of the tube abut one another.

14. A bend limiter according to any one of the preceding Claims for use in limiting bend in an optical fibre submarine cable.

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

16. A repeater housing assembly, for use with a submarine communication cable, comprising a tubular housing which is adapted to carry the tensile load which may be taken by the cable in the region of the assembly during installation or use, the housing being provided with a cable jointing arrangement in the region of each end, wherein a portion of the- tubular housing is constructed in the form of a bend limiter according to any one of the preceding Claims.

17. A spring member for use in a bend limiter according to any one of Claims 1 to 15 comprising a metal rod bent to form a series of loops, each loop comprising at least one spiral turn of the rod, wherein all the loops lie substantially in a common plane, consecutive loops being offset from one another in alternate directions transverse to the length of the spring member, and wherein the spiral turns which the loops comprise all have the same handedness.