WO1991008499A1 - Cable laying apparatus - Google Patents

Abstract

An apparatus is disclosed for laying cables below a sea bed or an ocean bed. Energy is supplied from a surface vessel to said apparatus, allowing said apparatus to vibrate with sufficient energy to produce a localised fluidized region. The cable is then placed into the sea bed by directing it into said fluidized region. Although energy is required to produce the fluidized region, the energy required to tow the apparatus and effect the laying processes is substantially reduced, compared to previously known techniques.

Description

CABLE LAYING APPARATUS The present invention relates to a cable laying apparatus for laying cables underground and, in particular, ground covered by water.

The laying of telecommunication cables underwater is a technique which has been carried out for many years and is widely used when a telecommunications link is required across a sea or an ocean. Several such links have been made across the Atlantic Ocean, linking North America with western Europe and the demand for communication channels between such distant continents continues to grow. When the technique was initially adopted, heavily armoured cables were allowed to rest on the sea bed, the armour being required to protect the cable from fishing nets etcetera. Such cables were expensive to produce and were, and continue to be, expensive to lay, requiring the use of specialised cable laying ships. In recent years, however, coaxial and similarly configured electrical cables have been replaced by optical fibre cables, providing a significantly larger bandwidth from a cable having a substantially smaller diameter. Furthermore, the cost of producing optical fibre cables is substantially less than that of producing similarly sized electrical cables, particularly if they are produced without armour. As an alternative to covering cable with armour, it may be protected by burying it below the surface of the sea bed. Known techniques for doing this involve the use of ploughs, or water jets, which cut a trench into the sea bed as they are pulled along by a ship. These techniques require the cable laying ship to have significantly more power, than ships which merely lay the cable on the sea bed, in order to lay cable at a reasonable speed, thus adding significantly to the cost of the laying process and thereby reducing the benefit gained from using less expensive cable.

It is an object of the present invention to provide an improved apparatus for burying cables underground and, in particular, under a sea bed. As used herein, the term "sea" includes seas, oceans, lakes rivers and pools, etcetera and the term "sea bed" includes the beds of seas, oceans, lakes rivers and pools, etcetera.

According to a first aspect of the present invention, there is provided apparatus for laying cables below ground and, in particular, ground covered by water, comprising means for vibrating an area of said ground with sufficient energy to produce a localised fluidized region and means for immersing a cable into said liquified region.

The apparatus produces a localised, fluidized region, into which the cable is inserted. As used herein, the term "fluidized" may be defined as vibrating paniculate matter of ground material, thereby causing it to exhibit fluid-like properties. An advantage of the invention is that drag on the cable is significantly reduced, allowing the cable to be laid at an increased speed while using a less powerful ship. Another advantage of the invention is that the cable laying process does not leave a scar on the sea bed, thereby minimising environmental damage and also rendering the cable difficult to detect.

In a preferred embodiment, the means for vibrating an area of said ground includes an hydraulic vibrating means. Said vibrating means may vibrate at a frequency of between one and one hundred cycles per second and, preferably, said vibrating means vibrates at a frequency of between twenty four and thirty one cycles per second.

Preferably, the immersing means has a substantially circular working surface, which may be in the form of a complete wheel and, in a preferred embodiment, means are provided for detaching said immersing means to facilitate replacement of said means.

According to a second aspect of the invention, there is provided a method of laying cables below a sea bed or an ocean bed, characterised by the steps of vibrating an area of said bed with sufficient energy so as to produce a localised fluidized region, and immersing a cable into said fluidized region. Preferably, an apparatus for fluidizing a region of said bed is towed by means of a surface vessel, wherein said surface vessel supplies cable to said apparatus and energy to said apparatus.

The invention will now be described by way of example only, with reference to the accompanying drawing, which shows a cross sectional view of a cable laying machine for laying cable under the sea bed.

A machine 14 for burying un-armoured optical-fibre cable, under a sea bed, is shown in Figure 1. A main housing 15 is pulled by a tow-rope connected to a ship and, in addition to pulling the machine 14, the tow-rope also supports a power line; the combination of the tow-rope and the power line being referred to as an umbilical 16. Optical fibre 17 may be lowered by the ship pulling the machine 14, or it may be lowered by another ship, in advance of said firs ship pulling ship. The cable is received by an inlet opening 18, which directs it to an immersion-wheel 19. In the operating position shown, the axle 20 of wheel 19 is located just above the surface of the sea bed so that, as the machine advances, the wheel 19 rotates thereby directing the cable 17 into a fluidized region of the sea bed.

As the machine advances across the sea bed, an hydraulic ram 21 oscillates at a frequency of between one and one hundred cycles per second and preferably at about twenty five cycles per second, thereby conveying fluidizing energy to the sea bed via the bottom of the housing 15. The ram 21 is powered by a compressor 22, which in turn receives electrical power from the umbilical 16. During operation, up to forty kilo-watts of power may be supplied to the sea bed, via the ram 21, which, for a common type of bed material, is sufficient to fluidize a region of about one square metre. This ensures that the region in the vicinity of the wheel 19 becomes sufficiently fluidized for the wheel 19 to enter the sea bed under its own weight and then be pulled along, laying the cable 17 as it is pulled. The amount of energy required by a ship to pull the machine 14 may be reduced by as much as 90 percent compared to that required to pull a conventional plough-like apparatus.

The wheel 19 is arranged to pivot against the rear end of the machine 14 so that, on encountering bed rock or a solid obstacle, the wheel merely rides over the obstacle, without causing damage to either the wheel 19 or to the obstacle. The machine may, therefore, be used in situations where one cable crosses over an existing cable, without causing damage to the existing cable. The wheel 19 itself is subject to severe operating conditions, similar to being placed in a sand blasting cabinet and may therefore, be fabricated from very resilient material. Alternatively, and preferably, it is designed so that its working surfaces may be replaced at regular intervals, without causing major disruption to the cable laying process.

During the cable laying process, the immersion wheel 19 pushes the cable into the fluidized bed. The resistance offered by the fluidized bed is minimal, therefore complex aligning mechanisms for the cable 16 are not necessary. The lack of such mechanisms is particularly advantageous, because it allows the machine 14 to be removed and re-introduced to the cable with minimal effort. The ease with which the cable enters into the bed also allows the machine to be constructed from relatively lightweight materials, which in turn facilitates operation at higher vibrating frequencies. Although desirable, reductions in weight cannot always be achieved by reducing the size of the machine because the cable must be bent over a playing means (wheel 19) and the bend in the cable must not exceed a predetermined minimum radius.

The lengths of cable between repeaters continues to increase, particularly with optical fibre cables, and lengths of up to 120 kilometres are not uncommon. However, with large lengths of cable, repeaters must be used at times and provision must be made for burying such devices. The present invention facilitates the burial of repeaters, by the creation of a fluidized region of sea bed. The burial of repeaters may also be improved by providing special immersion wheels.

Machines are known, commonly referred to as ploughs, in which a cutting blade vibrates, to thereby reduce the frictional force applied to said blade as it cuts through the sea bed. The apparatus described herein differs from such devices because it fluidizes an area surrounding the machine 14, thereby reducing the resistance of said material to the placing means and cable being laid. Experiments have shown that a machine operating under the techniques of the present invention requires significantly less energy for it to be towed over the sea bed resulting in significantly less energy being used per unit length of cable laid.

Another significant advantage of the present invention is that it readily facilitates de-burial of a previously buried cable because excessive pressure need not be applied to the cable during the de-burial process. Using conventional techniques, there is a significant risk that the cable may be damaged during the de-burial process, thereby making it unsuitable for re-use. However, employing the techniques described herein, it is possible for the cable to be re-used after de-burial, thereby allowing short term use of cable to be made, whereafter it may be moved to another position. The invention has been described with reference to the burial of telecommunication cables and, in particular, optical fibre cables. It has already been stated that the invention has particular application when burying repeaters and the like and this application may be extended to other devices, such as pipes and other solid structures. The term "fluidization" has been used herein to describe the state of the solid matter, in which each solid particle agitates within the volume of interest, into which energy is being applied by the hydraulic ram 21, or other similar device. In this respect, any vibrating device, operating within the required frequency range, is suitable for the application and, as an alternative to electrical energy being supplied over the umbilical 16, pneumatic or hydraulic energy could be applied, although electrical energy is preferred, because a source of electrical energy is usually readily available from the towing ship.

Although the invention has been described in relation to being towed across the sea bed, it could, alternatively, process its own energy supply and be driven by divers operating on the sea bed or controlled, remotely, from a service ship.

Claims

1. Apparatus for laying cables below ground and, in particular, ground covered by water; characterised by means (21, 22) for vibrating an area of said ground with sufficient energy to produce a localised fluidized region and means (19) for immersing a cable (17) into said fluidized region.

2. Apparatus according to claim 1, characterised in that said ground is a sea bed or an ocean bed.

3. Apparatus according to claim 1, characterised in that said means (21, 22) for vibrating an area of said ground includes an hydraulic vibrating means (21).

4. Apparatus according to claim 1, characterised in that said means (21, 22) for vibrating an area of said ground is arranged to vibrate at a frequency of between one and one hundred cycles per second.

5. Apparatus according to claim 4, characterised in that said vibrating means is arranged to vibrate at a frequency of between twenty four and thirty one cycles per second.

6. Apparatus according to claim 1, characterised in that said means (19) for immersing a cable (17) has a substantially circular working surface.

7. Apparatus according to claim 6, characterised in that said immersing means is a wheel (19).

8. Apparatus according to claim 1, characterised by means for detaching said means (19) for immersing a cable (17) to facilitate replacement of said means (19).

9. A method of laying cables below a sea bed or an ocean bed, characterised by the steps of vibrating an. area, of said bed with sufficient energy so as to produce a localised fluidized region, and immersing a cable into said fluidized region.

10. A method according to claim 9, characterised by towing an apparatus for fluidizing a region of said bed by means of a surface vessel, supplying cable to said apparatus from said surface vessel and supplying energy to said apparatus from said surface vessel.