ARRANGEMENT FOR BURYING A LINE IN THE SEA BED

Arrangement for burying a line in the sea bed.

This invention relates to a device for burying a line, for instance a cable, a pipeline or similar in the sea bed, comprising a number of rearward-facing nozzles (viewed in the direction of digging ) each of which is connected to a pressure liquid hose and positioned to either side of the centre line of the device.

Several different types of trenching device for burying a line in the sea bed are known which incorporate nozzles. Examples of these are to be found in American Patents 3.217.499 and 3.505.826.

It is necessary to place very high demands on efficiency, operational reliability, simplicity and ease of handling under the special conditions encountered when digging a trench on the sea bed. Thus any device which is used for digging a trench on the sea bed must be uncomplicated, since problems will exist, for instance, in relation to the adjustment of the device on the sea bed. The device must also be robust, since repairs and improvements are difficult to perform on the sea bed and the working site may be such that repairs are difficult to effect even on land. There is a self-evident requirement for the device to be light in weight, since it will often be necessary to carry the device to the point at which it is to be used or to transport it in small boats. The device which is the subject of this invention is therefore intended to be an improvement on the existing devices of this kind and is characterized in that the nozzles are arranged in pairs, with one nozzle of each pair being inclined downwards at an angle and the other being inclined upwards at an angle. In one preferred design of the invention, the angle between a plane running parallel to the pressure liquid hose and the two nozzles is at least approximately equal in each case. In individual instances, it may be appropriate to set the angle between the said plane and the downwardpointing nozzle somewhat smaller than the angle between the same plane and the upward-pointing nozzle. One practical design is characterized in that each pair of nozzles is attached to a pipe stub which is connected to a common holder which in turn may be easily coupled to the common pressure liquid hose. One practical design of this is characterized in that it has a holder in the form of a socket which is arranged so as to facilitate coupling with the pressure liquid hose, and the socket has attached to it two parallel pipe stubs separated from eachother by a distance which is rather larger than the line which is to be buried. The invention will be described in greater detail below with reference to the illustration in which Fig. 1 is a diagram of the device which is the subject of the invention in use; Fig. 2 shows the device as seen from the side; and Fig. 3 shows a section along the line 3-3 in Fig.

It will be seen from the illustration that the device which is the subject of the invention may be executed in an extremely simple manner. It consists in principle of a hose containing liquid under pressure 1 to which nozzle arrangements 2 are attached to either side of the centreline of the hose. The nozzles are arranged in pairs 3 with one nozzle 4 being inclined upwards and backwards at an angle and the other nozzle 5 being inclined backwards and downwards at an angle. Each pair of nozzles 3 is located at the end of a pipe stub 7 which is suspended from a holder 6 in the form of a socket. The holder 6 in the form of a socket is easily coupled to the hose 1 . In this way it is easy to extend the entire digging device by the attachment of one or more additional lengths of hose and one or more holders 6 with arms 7 and pairs of nozzles The length of the device, i.e. the number of pairs of nozzle will depend on the nature of the sea bed in which the trench 8 is to be dug. If the sea bed is of a light material such as silt, then it will be possible to manage with one or two sets of holders with .arms and pairs of nozzles. The length of the device, i.e. the number of pairs of nozzles, may be increased in proportion to the hardness of the sea bed. As may be observed from the illustration and from the above description, the equipment is extremely simple and robust. It consists of nothing more than a hose containing liquid under pressure fitted with holders for pipe stubs 7 with pairs of nozzles 3 . Before digging commences, a trench must first be excavated by some other means which is of sufficient length to allow a section of the line 9 to be placed at the bottom of the trench at the required depth. Then the device which is the subject of the invention with the arms 7 to either side of the line is laid in the trench and water is introduced under pressure into the line 1 . The source of pressure for the water, e.g. a powerful pump, may be at the surface in a boat, or may be moved along on a sled immediately ahead of the digging device. The whole may be controlled by a diver or with the aid of a television camera. The nozzles which are inclined downwards at an angle 5 will blow away the material from the bottom and throw it backwards. The turbulence produced within this fluidized mass will cause the particles contained in the mass to be carried upwards and into the vicinity of the nozzles 4 which will ensure that the fluidized mass is then carried away upwards and backwards. Thus the material will be kept in suspension in the water until the line 9 has reached the specified depth, and once the material passes outside the effective range of the rearmost pair of nozzles it will then come to rest and will fall back into the trench again, filling it. If the sea bed is of a light material, then of course some of the material which has been stirred up will be carried away in the water, for instance by currents, with the result that the trench will not be filled up immediately. However, the trench will fill up of its own accord relatively quickly.

The device which is the subject of the invention is cheap to produce, easy to transport and erect on-site, reliable and efficient in use and simple to adjust to suit the different types of material found on the sea bed. A prototype of the device which is the subject of the invention has been tested with excellent results laying electric cables in the fjords of northern Norway.