Bend limiting device for cables

Details Bend limiting device for cables Bend limiting device for cables

1999-12-22

2002-04-16

Schuberg, Darren (Department: 2872)

Optical waveguides

Optical transmission cable

C385S136000, C385S147000, C174S135000

Reexamination Certificate

active

06374022

ABSTRACT:

The present invention relates to a bend limiting device for cables and in particular to a bend limiting device for submarine cables. Both in the case of an electric cable and in the case of an optical cable, the greatest risks of damage due to bending exist along discontinuous portions of the cable itself, in particular in the region of rigid joining elements. In fact, in the case of bending of the cable, and in particular in the case of bending during traction, since the cable has a relatively poor flexural strength, the curvature of the cable portion leaving the joining element may have a very small radius, which may be such as to result in damage or breakage of the cable itself. Basically, each type of cable has its own limit value for the radius of curvature, below which the functional capacity of the cable itself is adversely affected.
In order to reduce the risks of breakage of a cable in the region of the joining elements, bend limiting devices are generally used. These usually comprise a flexible elongated body to be applied on the outside of the cable in the region of the joining element and able to provide a predefined flexural strength, ensuring that the curvature assumed by the cable itself does not reach critical values. For example, the patent U.S. Pat. No. 5,461,690, in the name of AT&T IPM Corp., proposes an apparatus for limiting the bending of a cable to be used in the region of a connector. The apparatus has an elongated cylindrical shape and is composed of elastic material able to withstand loads within predefined limits. On its external surface, the apparatus has circumferential grooves which delimit one or more weakened portions which are designed to bend under the action of relatively small loads. This apparatus is suitable for cables with small dimensions, for example cables with a single optical fibre, and the loads which can be withstood by the apparatus itself are those which may occur during use of cables of this type.
In some applications, however, the loads to be withstood are very high, for example several tonnes. Loads of this order of magnitude occur for example during the process of laying of submarine cables. Submarine cables are electric or optical cables used for conveying energy or for long-distance telecommunications and designed to be laid on sea or ocean beds. The diameter of these cables may vary between about 14 mm and 50 mm in the case of optical cables and between about 50 mm and 160 mm in the case of energy supply cables.
Submarine cables generally comprise sections of cables of limited length which are joined by rigid joining elements. In the case of telecommunications cables, the lengths of the sections may reach maximum values of a hundred or so kilometers and the joining elements may house signal repeaters.
Laying of submarine cables is performed by means of special cable-laying ships which are provided with cable guiding members, usually pulleys with a suitable diameter (of about 3 m for the laying of optical cables and about 6 m for the laying of energy supply cables) so as to allow laying of the cable in water without damaging it. The cable, during laying, must withstand a tractional force which may reach (depending on the weight of the cable itself and the laying depth) several tens of tonnes (for optical cables, the tractional force may vary between 4 and 35 tonnes, depending on the type of cable). When the joining element passes over the guiding member, a situation arises where the joining element contacts the guiding member only in the region of a central portion thereof, while its ends remain at a distance from the above mentioned surface which depends on the curvature of the surface itself and the dimensions of the joining element. Consequently, the cable portion leaving the joining element is not supported by the guiding member surface and, being subjected to a high tractional force directed transversely with respect to the joining element, is subject to severe bending. If the curvature of the cable at the maximum bending point exceeds a critical value, the cable may be damaged. For example, in the case of an optical fibre cable, breakage of the covering sheath of the cable or damage of the optical fibres may occur.
The submarine cable may be, for example, an optical cable of the SA (Single Armour) type. A typical structure of an SA cable comprises a core containing the optical fibres and several coaxial reinforcing and insulating layers, including a load-bearing steel layer (in order to provide the cable with tensile strength), a copper sheath, an insulating layer of polyethylene and an external steel-wire armouring for mechanical protection. The cable may have, for example, an external diameter of 35.9 mm and a weight of 3 kg/m in the air and 2.5 kg/m in the water.
A submarine cable, with a joining element incorporated, in order to be able to be considered suitable for use, must successfully pass bending tests on pulleys as laid down by international standards. The tests include the passage of the cable provided of the joining element over a pulley of 3 m diameter, in accordance with the following criteria (which simulate the severest conditions which could occur during laying):
a) fifty passages under a normal load, i.e. a NOTS (Nominal Operating Tensile Stress) load which in the case of an SA cable is equal to 180 kN;
b) three passages under a load equal to 90% of the breaking load, i.e. a NTTS (Nominal Transient Tensile Stress) load which in the case of an optical cable of the SA type is equal to 260 kN.
In order to ensure that the cable can withstand these stresses, it may be necessary to apply, to the cable portion connected to the joining element, a device which performs the function of a bend limiting device.
In order to protect submarine cables, or in any case cables which are subject to very high loads, from risks of excessive bending in the region of the joints, numerous types of bend limiting devices have been proposed.
The patent application GB 2142788A, in the name of British Telecommunications, describes a bend limiting device comprising a plurality of annular segments which are adjacent and joined together and which define a cylindrical cavity housing the cable portion connected to the joining element. The annular segments are joined together with play by means of bolts and are movable angularly with respect to one another so as to provide the structure with flexibility. The bend limiting device proposed has a limit radius of curvature which depends on the form and the dimensions of the annular segments and the bolts. This bend limiting device has the drawbacks that it is relatively bulky, heavy and complicated to assemble. Moreover, after a certain period of time spent in sea water, corrosion phenomena may result in permanent seizing of the bend limiting device, thereby rendering it useless in the case of recovery of the cable. A similar operating principle is adopted for the bend limiting device according to the patent EP 226188 in the name of Società Cavi Pirelli S.p.A.
The patent U.S. Pat. No. 4,367,967, in the name of Preformed Line Marine Inc., describes a device for relieving the bending forces acting on a cable, comprising a plurality of sleeve elements which are rigid and coaxial with each other and connected together by means of elastomeric material. The adjacent sleeve elements have co-operating surfaces which interfere with one another so as to limit the curvature of the device. More in detail, each sleeve element comprises two longitudinal portions with different diameters. The smaller-diameter portion of each element extends with play inside the larger-diameter portion of the adjacent element. Flexing of the device is made possible, within predefined limits, by the abovementioned play between overlapping portions until the portions themselves interfere with each other. Such a device is described in the patent U.S. Pat. No. 4,396,797 in the name of Horiba Ltd, in which sleeve elements of the type similar to that described above are separated axially from one another by means of flexible

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