EP0521032B1

EP0521032B1 - Improvements in fluid-based excavating

Info

Publication number: EP0521032B1
Application number: EP91905990A
Authority: EP
Inventors: Trevor John Smith; William Preston Croager
Original assignee: Cable and Wireless PLC
Current assignee: Cable and Wireless PLC
Priority date: 1990-03-22
Filing date: 1991-03-14
Publication date: 1996-05-08
Anticipated expiration: 2011-03-14
Also published as: WO1991014834A1; ES2089200T3; CA2078688C; ATE137833T1; EP0521032A1; JPH05505222A; DE69119411T2; DE69119411D1; CA2078688A1

Abstract

A method and apparatus for excavating cohesive material comprises directing a jet of fluid (12), for example water, at a cutting face (13) of the material and along spaced apart tracks to cut spaced apart slots and thereby to create a kerf between the two slots. The invention is particularly applicable to the excavating trenches for sub-sea cables and pipes to be laid in.

Description

This invention relates to fluid excavating. The invention is particularly applicable to fluid excavating trenches for burying cables or pipelines in the seabed.
In laying undersea pipes or cables it is advantageous to bury them beneath the surface of the sea bed to afford protection. To do this it is known to use water emitted from a nozzle to fluidise a non-cohesive or soft cohesive material making up the sea bed. In this way the fluidised material can be dredged away allowing the pipe or cable to be lowered into the trench thus created. However, fluidisation is only practicable when used on non-cohesive or soft cohesive materials, such as sand and very soft clays.
It is also known to use water emitted from a nozzle, to cut into cohesive material making up the sea bed to define more easily removable blocks as part of a procedure for cutting a channel in which an undersea cable or pipe is to be lowered. The blocks are removed by a dredging unit.
US Patent 1688109 describes a device intended to collect dirt, sand and gravel from the sea bed. The apparatus is mounted on a surface floating vessel and includes a flexible conduit extending to the sea bed. The conduit comprises a pair of concentric tubes, the outer tube being used for supply of high pressure water. The outer tube may be conveniently separated into a number of high pressure blast tubes. At the end of the tube adjacent the sea bed is a special head designed to direct the water out under pressure to the surrounding areas to loosen the sea bed material which is then sucked away through the inner passage of the concentric tubes. The inner suction passage conveys spoil to the surface vessel from where it may be processed or discharged.
In US Patent 2956354 a dredging device for use off a boat is also described. In this device a centrally located suction bell is placed near the ocean bed to remove loosened spoil. On each side of the suction pipe are located a pair of arms carrying at their ends jetting nozzles. The arms are rotatable about a vertical axis, and the force for rotation is provided by use of trailing position of the jetting nozzles. The rotating arms are provided with paddles which are adjustable in plane to provide adjustment for the speed of rotation of the arm. The arms and jetting nozzles are arranged to sweep out a wide area of material, including an overlapping central area to loosen sea bed debris which is then carried away in central suction pipe.
It is an object of the present invention to provide an improved excavating method and apparatus particularly for use on stiff cohesive materials as well as non-cohesive and soft cohesive materials.
According to a first aspect of the present invention there is provided a method of laying undersea pipes or cables the method comprising: moving a first jet along the line of a desired trench while causing the jet to move laterally relative to the material of a cutting face to cut a first lateral kerf in the cutting face and thereby excavating material; directing a second jet of fluid at the cutting surface and moving the second jet along the line of the desired trench while causing the second jet to move laterally relative to the material to cut a second lateral kerf in the cutting face and thereby excavating material wherein the first and second kerfs are vertically displaced from each other and are separated by a castellation, whereby the trench is formed upon removal of the material forming the castellation; and laying a pipe or cable in the trench.
The invention also extends to apparatus for laying undersea pipes or cables according to the method of the first aspect of the invention which apparatus includes a carrier and a plurality of nozzles, said nozzles being movably mounted on the carrier wherein the nozzles are vertically spread apart to cause jets to move along vertically spaced apart tracks to provide vertically spaced apart lateral kerfs separated by a castellation.
The acute angle ot impingement may be parallel to a plane which is normal to the direction of movement of the jet or at an angle thereto. By directing the jet at an angle with respect to the said plane, such that the outlet lags the point of impingement of the jet on the cutting face, the jet may also serve to expel material from the excavated area.
By passing the jet in a succession of runs, or a series of spaced jets in a single pass, a deeper channel may be formed than that simply created by the single jet alone. The material between the kerfs may not be completely detached but be connected at its root. Thus, preferably, a second pass of the jet, or a second jet, preferably parallel to the first, serves to create the second kerf and to sever the first in one action. Thus, successive passes of the jet or jets will create a kerf with a width that is proportional to the jet spacing.
In this way, a channel may be created either by longitudinal runs of the jet or jets with respect to the line of the channel or, alternatively, lateral runs.
Preferably, the jet or set of jets is moved at a constant rate relative to the material to be cut. This rate in part determines the depth of cut in a material of constant density. On the other hand a non-constant rate may be imparted to the jet, for example sinusoidal.
The movement may be linear. However, any other path or movement may be adopted in order to create the kerf. Another factor governing the depth of cut of the kerf is the distance of the outlet from the surface. Thus, it is also preferable that the nozzle is maintained at a small and substantially constant distance from the material being cut.
As mentioned above, any pattern of movement of the jet or jets may be adopted as required. One particular way is to sweep the jet in an arc or parabola across the cutting face of the material. In this case, the axis of the sweep is conveniently generally vertical when excavating, for example, a trench in a horizontal seabed.
Proposals for using such a jetting technique also include cutting slits in the cohesive material to make them more easily removable as comminuted lumps by means of a following plough arrangement. The cutting of slits therefore breaks up the consistency of the seabed in advance of the plough arrangement, hence reducing tow forces required to pull the plough.
It is found that removal of the kerf is most suitably achieved in the excavating process by directing the jet at an angle of about 30° to the surface of the material being cut into. In suitable materials, such as clays, each kerf is in the manner of a scallop or scroll of material similar to the shaving created by a chisel. In harder materials the kerf may fragment as it is forced to curl out of the path of the jet. In any case, it is the effect of the stagnation pressure at the root between the parted material and the newly created surface which forces the waste kerf away from the excavated area.
The parameters which determine the successful removal of a kerf, instead of simply cutting into the clay, are presently considered to be the pressure of the cutting fluid, the flow rate, the nozzle profile, the vertical angle of the jet and the speed at which the jet travels across the surface.
When a set of jets are used the cutting characteristics are also dependent on the number of jets and the spacing of the jets both in the direction of movement and normal to that direction.
In one form, the invention comprises directing a plurality of oscillating fluid jets at the surface to be excavated at the acute angle to sheer off a succession of kerfs to form the trench. The channels may be formed longitudinally with respect to the overall lie of the trench being cut or be formed laterally with respect thereto.
In another form the cut may be achieved by means of a set of nozzles arranged in a helical pattern on a rotatable drum. The drum may be horizontally or vertically disposed. In either case it is necessary that the jets impinge on the material at an acute angle with respect to the material to create the kerf.
The present invention can be put into practice in various ways some of which will now be described by way of example with reference to the accompanying drawings in which:

Figures 1A) and B) are schematic representations of an excavating arrangement according to the present invention;
Figure 2 is an illustration of a nozzle drum assembly for use in one embodiment of the present invention;
Figure 3 is a valve arrangement used in one embodiment of the invention;
Figures 4A and 4B are end and side views of a components of the valve of Figure 3;
Figures 5A, 5B, 5C and 5D are side and end views of another component of the valve of Figure 3; and
Figure 6 is a side view of a modified excavating assembly.
Figures 7A), B) and C) are illustrations of parts of alternative forms of the invention;

Referring to Figures 1A) and B) and 2, a trenching apparatus comprises a submersible frame (not shown) having hydraulically driven positioning and driving propellers which are powered by a hydraulic power motors source (also not shown) on the frame. A hydraulic motor also rotates a 36mm diameter nozzle drum 10 of the excavator head about its axis which is generally near vertically disposed when the frame is arranged on a horizontal surface. In general, the frame is adapted to orientate the drum 10 so that its axis of rotation is substantially normal to the attitude at which the frame rests. The angle of the nozzles may be more or less than 30°, for example between 20° (or less) and 40°. Each of a set of nozzles 12 in the drum is orientated to direct a jet of water from the drum downwardly at an angle of 30° with respect to a cutting face 13 of the cohesive material which the trench is to be dug. Commonly, on a horizontal seabed this will result in a substantially vertical axis of rotation. The nozzles have a 2mm outlet diameter and are angularly spaced, with respect to the drum axis, at a 30mm pitch over an axial length of 600mm on the drum.
The nozzles 12 are arranged on the drum in a helical pattern. This presents an overlap of the cutting effect which each individual nozzle presents in order to provide an overall effective cutting width equal to the spacing between the upper and lower-most nozzles 12a and 12b on the drum.
Referring particularly to Figures 1a) and b) it is the purpose of the jet of water from each nozzle to cut into the cutting face at least to create a kerf. It may be that a single nozzle could be used with sufficient water pressure to create and sever its own kerf. However, using a plurality of nozzles, as depicted in Figure 1B), the pass of nozzle 1 has cut the kerf beneath it away. At the same time it defines the lower surface of the next kerf to be removed. As nozzle 2 then passes its jet penetrates above the lower surface defined by nozzle 1 and forces the kerf defined between the planes of penetration of the jets away from the cutting face.
As the nozzles rotate a complete layer of the cutting face is removed. On the next pass of the nozzles the same action takes a further layer off and so on. In this way the trench cutting progresses.
The nozzles 12 are fed with water pumped from a hydraulically driven pump and filter arrangement on the submersible frame to a series of conduits in the drum leading to the nozzles. Clearly, the rotating nozzles will only be used for a limited amount of each turn of the drum. Thus, a kidney valve arrangement 14 is used to interrupt the flow of the water to the nozzles so that fluid is passed only to the nozzles in the relevant cutting portion of each turn of the drum, i.e. when they are adjacent the cutting face 13.
The kidney valve is shown in Figures 3, 4 and 5. It comprises a circular valve plate 16 which is secured to the frame and a distribution member 18 which bears on and is rotatable relative to the valve plate 16. The valve plate 16 is formed on one mating side 25 with a pair of radially spaced, angularly coincident curved channels 20 set in annular raised guides 22. The channels 20 are referred to in this description as kidney ports. The kidney ports are coaxial with the axis of rotation of the distribution member which, in turn, is coaxial with the axis of rotation of the drum itself. Both kidney ports communicate with outlet ports 24 which open on to the other side of the plate. The 26 nozzles 12 communicate with the kidney ports in upper and lower groups of 13. Thus, by directing water to one or both ports the active region of the drum is selectable.
The distribution member 18 is formed with a plurality of distribution ports 26 which extend from its mating face 28 to the other side. The distribution member is also formed with an annular flange by which the member is secured relative to the one end of the drum to rotate with it to distribute the pumped water to the nozzle heads. The mating face 28 of the distribution member 18 is sealingly engaged with that of the valve plate 16.
The drum and attached distribution member are rotated together by means of a conventional hydraulic motor (not shown). In so doing, a selection of the distribution ports is in registry with the adjacent kidney port. Thus, nozzle water is supplied only to those distribution ports in registry for as long as they remain so. By correctly adjusting the orientation of the valve plate, nozzle water is fed only to those nozzles within the effective working part of each cycle corresponding to the period when a particular nozzle is at the cutting face. In this way, the amount of pumped water required is considerably reduced.
Following on from the general description of the excavating apparatus of Figure 2 a modified arrangement is illustrated in Figure 6. The drum 10 is powered by a hydraulic motor and gearing arrangement not specifically illustrated in Figure 6 but which is enclosed in a housing 29.
The drum 10 is also provided with a spoil scavenging arrangement comprising a trailing suction pipe 30, having an inlet towards the base of the drum 10 and communicating with a venturi ejector 32. The arrow in Figure 6 denotes the direction of travel of the drum when cutting a trench. The suction pipe 30 is formed with a shroud 33 which consists of an open metal box structure in which the sides defining the open face conform generally to the adjacent curved surface of the drum 10 but leaving a small gap along all edges of about 25mm between the edges of the shroud and the drum. The ejector 32 has an ejector water inlet pipe 34 and a back flushing pipe 36. Both of these pipes 34 and 36 are attached to a further sea water pump system on the submersible frame respectively for creating the ejector vacuum to create suction at the open end of the suction pipe 30, adjacent the drum 10, and to flush out blockages if they occur in the ejector.
The scavenged spoil drawn into the scavenging arrangement is exhausted through an outlet pipe 38 which is rotatable relative to the fixed suction pipe and ejector assembly to direct the spoil as required out from the area of the cutting face. The outlet pipe 38 is orientatable about an upright axis by means of a worm drive and a hydraulic motor 40 which moves an engaged gear wheel 42. This orientatability is particularly useful in sub-sea applications in which the excavator and frame are remote controlled using video cameras. Strong currents can be encountered and by directing the spoil to flow with the current the chance of it drifting back into the trench is removed and the problems associated with clouding up the water, thus obscuring the view, can be avoided.
The device is mounted on the submersible frame by means of a mounting block 44, and a pair of locating pins 46. Preferably, the orientation of the excavator with respect to the submersible frame is adjustable. In a particular situation the angle of the axis of the drum may be better tilted away from the vertical.
In this embodiment, the drum is designed to rotate to produce a linear speed of about 14 metres/sec. The water is pumped to the nozzles at 200 litres/min to develop 210 bar at the lower 13 nozzles for cutting 400kPa shear strength clay or at 300 litres/min to develop 137 bar at all 26 nozzles for cutting 200kPa clay. Using the arrangement the excavator is able to cut a trench 300mm deep using the lower 13 nozzles alone or a trench 600mm deep using all 26 nozzles.
By actuating the motor by hydraulics and pumping the filtered water to the nozzles, via the distribution valve, the helicai arrangement of nozzles will cut a succession of adjacent kerfs from a wall of cohesive clay or the like. each nozzle jet except the top-most or bottom-most nozzles impinges on the wall constituting the cutting face opposite the root 34 of the previously formed kerf. The force of the jet makes and shears off a kerf in a sliver at the same time as the base of a new kerf is made. A large amount of the spoil created by the excavating operation is forced out of the way by the pressure of water. However, some will tend to fall back into the created trough. This loose material is removed by means of the following spoil scavenging system.
Alternatively, referring to Figures 7A), B) and C) the cutting fluid constituting the jet is fed to a column 50 of axially spaced outlet nozzles 52 or groups of outlets. The column 50 is oscillatable through an arc 54 to effect a cut. More than one oscillatable column can be used to effect the cutting of a trench. The spacing of cutting jet outlet nozzles 52 on the column is such that the penetration of the jet of one outlet extends past the point of initial penetration of the lower adjacent nozzle. In this embodiment the nozzles 52 create and cut kerfs contemporaneously and not in succession as the columns oscillate in antiphase.
When sets of nozzles are most closely spaced there is a region between the columns 50 that is not as agitated as that directly in line with the nozzles. Thus it is advantageous to install wash jet outlets 56 between the cutting jet nozzles. The wash jets are directed at the space between the cutting jets and of the cutting face to agitate the water and assist in removal of spoil.
The number of columns of jets can be varied to suit the width or trench to be cut. Similarly, the amount of overlap between adjacent jets in a column can be varied to accommodate, for example, different densities of clay to be removed.
It is found that the dislodged spoil can clog the oscillating mechanism for the columns 50. To overcome this the column and nozzles 52 are enclosed in a metal shroud shown in Figure 7C). The shroud has groups of arcuate apertures 57 which allow the jets to impinge on the cutting face throughout their sweeps. The shroud can equally well be used with one column or any number of columns disposed side by side.
The method of this invention relates to the use of jets of fluid (water) for excavation, e.g. of trenches, on the sea-bed.
The direct effect of a jet causes thorough disintegration of the material onto which the jet impinges and, in accordance with practice as established by the prior arts, jets have been used to disintegrate substantially all the material lying in the region to be excavated.
This invention differs from this prior usage in that it uses a jet or jets to create castellations, e.g. slabs of material. The castellations are removed, but the material forming the castellation is not directly disintegrated by the jet nor is it disintegrated to the same extent as material upon which the jet impinges. In order to create a castellation, it is necessary to cut kerfs on both sides thereof, or, in the case df a castellation at the surface, on the side away from the free surface. This invention uses a jet or jets to cut the kerf by preferentially directing the jet or jets at the region where kerfs are intended (and preferably avoiding the regions where castellations are intended). The cutting of the kerf or kerfs creates the castellations but a jet directed into the kerf creates a high-pressure therein, and this high pressure produces a mechanical load which causes the castellation to break up. It will be appreciated that directing a jet into the kerf necessarily creates a corresponding outflow from the kerf and the detritus, e.g. both material comminuted by the jet and material arising from the breakup of the castellation, is removed from the slot by said outflow. It is usually convenient to provide a suction inlet near the workface to assist in this removal. If necessary, the detritus can be deposited a substantial distance from the working region.
Apparatus according to this invention includes means for moving a jet or jets along spaced apart tracks. This produces spaced apart kerfs along the spaced apart tracks and the castellations are formed between the kerfs. A preferred arrangement for producing the spaced apart jets comprises nozzles which are helically arranged about the axis of a rotatable drum.
(Note "KERF" means a cutting into solid material, e.g. the cut made by a saw, axe or similar instrument).

Claims

A method of laying undersea pipes or cables the method comprising: moving a first jet along the line of a desired trench while causing the jet to move laterally relative to the material of a cutting face to cut a first lateral kerf in the cutting face and thereby excavating material; directing a second jet of fluid at the cutting surface and moving the second jet along the line of the desired trench while causing the second jet to move laterally relative to the material to cut a second lateral kerf in the cutting face and thereby excavating material wherein the first and second kerfs are vertically displaced from each other and are separated by a castellation, whereby the trench is formed upon removal of the material forming the castellation; and laying a pipe or cable in the trench.
A method according to claim 1, wherein the angle at which the jet impinges on a cutting face (13) of the material is 20° - 40°.
A method as claimed in either claim 1 or 2, wherein the second kerf is adjacent the root (34) of the first kerf.
A method as claimed in any one of claims 1, 2 or 3 in which the jet or jets are moved relative to the material to be cut (13) at a constant rate.
A method as claimed in any one of claims 1 to 4 in which the jet or each is emitted from a moving nozzle (12) which nozzle is maintained at a substantially constant distance from the material being cut (13).
A method as claimed in any one of the preceding claims in which the jet or jets move linearly with respect to a supporting frame across the cutting face (13) of the material.
A method as claimed in any of claims 1 to 5 in which the jet or jets sweep in an arc across the cutting face (13) or the material.
A method as claimed in either claim 6 or claim 7 in which a plurality of the jets are applied successively or contemporaneously to the cutting face to excavate at least one kerf perpendicular to the movement of the jets.
Apparatus for laying undersea pipes or cables according to the method of any one of the preceding claims which apparatus includes a carrier and a plurality of nozzles (12), said nozzles being movably mounted on the carrier wherein the nozzles are vertically spread apart to cause jets to move along vertically spaced apart tracks to produce vertically spaced apart lateral kerfs separated by a castellation.
Apparatus according to claim 9, wherein the orientation of the nozzles is such that each subtends an angle 20° and 40° with respect to the cutting face (13).
Apparatus as claimed in either claim 9 or claim 10 in which the nozzles (12) are mounted in staggered relationship with respect to their direction of movement.
Apparatus as claimed in claim 10, in which the nozzles (12,12a,12b) are helically arranged about the axis of a rotatable drum (10).
Apparatus as claimed in claim 12 including valve means (14) arranged to supply fluid only to those nozzles adjacent the cutting face (13).
Apparatus as claimed in either claim 12 or 13 which also include adjacent spoil scavenge means (30, 32, 33, 34).
Apparatus as claimed in claim 14 in which the scavenge means comprise suction means (32) and a suction pipe (30) having an inlet near the nozzles (12).
Apparatus as claimed in claim 15 in which the scavenge means also comprise a shroud (33) having an open face defined by edges conforming generally to the swept path of the nozzles, the opening in the suction pipe communicating with the interior of the shroud.
Apparatus as claimed in any one of claims 14, 15 or 16 in which the scavenge means include a rotatable outlet pipe (38) for directing scavenged spoil away from the cutting face.
Apparatus as claimed in any one of claims 14, 15 or 16 in which the suction means are an ejector (32).
Apparatus as claimed in either claim 9 or 10, in which a set of fluid outlets (52) are arranged in a column (50).
Apparatus as claimed in claim 19 in which at least a pair of columns of outlets are arranged side-by-side.
Apparatus as claimed in claim 20 in which the sets of outlets (52) are arranged to sweep through predetermined arcs (54) in antiphase.
Apparatus as claimed in either claim 20 or 21 in which a set of wash outlets (56) are disposed between the adjacent columns (50) and orientated to direct wash jets therebetween.