Hydraulic and earth engineering – Subterranean or submarine pipe or cable laying – retrieving,... – Submerging – raising – or manipulating line of pipe or cable...
Reexamination Certificate
1998-11-04
2001-04-24
Taylor, Dennis L. (Department: 3673)
Hydraulic and earth engineering
Subterranean or submarine pipe or cable laying, retrieving,...
Submerging, raising, or manipulating line of pipe or cable...
C037S465000, C405S161000
Reexamination Certificate
active
06220786
ABSTRACT:
The present invention relates to a method and apparatus to enable the underwater burial of an elongate member, such as a cable or pipe.
Post lay burial of underwater telecoms cables is normally carried out in soft silt or sand soils (hereinafter “soil”) by means of water jetting. This method is employed in preference to mechanical soil cutting devices as low pressure water jets will not damage the polythene outer covering of the cable and has the advantage of requiring little mechanical contact with the cable. There are various types of water jetting tools in use but they all work on essentially the same principle. Jetting swords or nozzles are directed at the seabed on each side of the cable and fluidise the soil allowing the cable to be imbedded. The disadvantage of water jetting systems is that they are inefficient. For every kW of water power directed at the seabed an equal amount has to be directed in the opposite direction to balance the reactive forces. Typically, powers of greater than 75 kW are absorbed by the tool alone to bury a cable in soft soils and sand at an acceptable rate. The neutrally buoyant Remotely Operated Vehicle (ROV) to which the tool is attached requires at least the same amount of power again in order to effectively transport it in deep water.
Mechanical soil cutting devices are only used in soils too hard to effectively fluidise, such as consolidated clays of greater than 50 kPa shear strength, chalk and rock. These tools are generally toothed wheels or chain cutters and in use require that the cable is first “loaded” into a protective chute and depressor to prevent contact between the cutting surfaces and the cable. The fact that the cable needs to be loaded creates the need for complicated subsea robotics both for loading and unloading the cable as well as mechanisms for automatic ejection, in the event of loss of vehicle power. Because of the size and weight of conventional mechanical cutting tools and their associated equipment, they can only be deployed from heavy subsea tractors, typically weighing greater than 12 tonnes. Whilst necessary for burial of pre-laid cables in hard materials, they are of limited use in areas of soft seabed as they tend to sink into the soil.
Both of the solutions above are of necessity powerful and hence expensive. Consequently they are both, together with their launching and control systems, very large (in excess of 100 tonnes), they are expensive to transport and can only be launched from cableships and offshore vessels with strong, spacious decks.
The emerging single span and festoon cable systems business however is dictating the need for lower cost shallow water cable deburial, retrieval and reburial solutions. The maintenance requirements for such low cost cable systems will not stand the cost of permanently based vessels of size necessary to deploy a modern 250 hp, 130 tonne cable maintenance ROV nor indeed will they support the cost of such an ROV on standby.
It would therefore be advantageous to devise a method for imbedding cable in soft soils using low power.
In accordance with a first aspect of the present invention there is provided apparatus for locally disturbing an underwater bed in a line so as to enable the burial of an elongate member, the apparatus comprising a mounting, resilient means coupled to the mounting, and means for moving the resilient means with respect to the mounting, wherein the resilient means is constructed such that on moving the mounting along the line while moving the resilient means with respect to the mounting, the underwater bed is disturbed by the resilient means to enable the elongate member to be buried.
In most cases the resilient means will move soil mechanically but it has been found that in addition the resilient means agitates the water and creates an eddy effect which at least partially disturbs and fluidises the bed (which may be a seabed, riverbed, lakebed etc.) in the region of a cutting face. In some cases the eddy current effect could be used alone with no contact between the resilient means and the bed.
It has been recognised that jetting with low power such as 9 kw is not practical to trench depths in excess of 600 mm which leaves some form of mechanical method as the only approach. It has also been recognised that a conventional mechanical tool would not be feasible because the associated loading/unloading and ejection equipment would be large and costly.
It has been found that the disadvantages of the conventional mechanical approach (which uses rigid toothed wheels or chain cutters) can be avoided by the provision of resilient means. The resilient means is generally stiff enough to cause disturbance of the water adjacent the cutting face and apparent fluidisation of the disturbed soil at or adjacent the advancing cutting face, whilst being flexible enough to contact the elongate member, such as a cable, without causing any damage.
The resilient means may comprise any suitable resilient member or members. For instance the resilient means may comprise one or more strips of rubber which extend from a rotating axle. Preferably however the resilient means comprises a plurality of elongate resilient members such as brush filaments. The density, length and flexibility of the resilient members can be suitably chosen for the type of bed material, type of elongate member (e.g. telecommunications cable), cutting depth and cutting speed required. The brush filaments may be formed from any suitable material such as metal or stiff plastic. The stiffness of the filaments of a conventional varnish stripping brush has been found to be suitable. In one example nylon brush filaments are used with a diameter in the range of 1-2 mm, and a length in the range of 20-30 mm.
The resilient means may be oscillated with respect to the mounting, or mounted on a conveyor belt. However preferably the resilient means are rotated with respect to the mounting, and typically extend from an axle rotatably coupled to the mounting. In this case the means for moving the resilient means with respect to the mounting comprises drive means for rotating the axle. This provides a simple low power solution. Where the resilient means comprises a plurality of elongate resilient members such as brush filaments the filaments preferably extend radially from the axle.
The axle may be inclined or perpendicular to a vertical plane passing through the line of disturbance. However preferably the axle is mounted and deployed (for instance from an ROV) such that, in use, the axle lies substantially parallel to a vertical plane passing through the line of disturbance. This ensures that when the elongate member is buried at the same time, it is not damaged by rigid rotating components.
The resilient means may be uniformly distributed about the axle but preferably is arranged as a plurality of arms circumferentially spaced around the axle. We believe that the spaced arms are more efficient in effecting fluidisation of the underwater bed material by means of a “fanning” effect, resulting in a lower power requirement.
In one example the resilient means is arranged as a plurality of groups axially spaced along the axle. In a preferred embodiment the resilient means is arranged as twenty axially spaced groups. In an alternative example, the resilient means may be in the form of one or more spirals extending along the length of the axle. The radial length of each group (eg the length of the brush filaments) and the axial spacing (eg. the spacing between the groups or the pitch of the spiral) can be suitably chosen for the depth of trench required and the angle of deployment of the axle. In preferred examples the groups are spaced by 50-100 mm and the spiral has a pitch of 18-38 mm.
Typically the resilient means extends downwards in use into the underwater bed, and in a preferred embodiment the resilient means is non-uniformly distributed such that, in use, the density of the resilient means increases downwards into the underwater bed. For instance the axial spacing between groups of resilient means or the pitch of the
Ollason Andrew Stuart
Peterson Kevin Clyde
Wallace Graham Peter
Cable & Wireless PLC
Pillsbury Madison & Sutro LLP
Taylor Dennis L.
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