One of the main causes of damage to power and telecommunications cables laid on the seabed is fishing activity, and attempts are generally made to protect cables from such interference by burying them under the surface of the seabed. The cables are generally buried by means of a submarine plough, which is towed behind a cable laying ship and picks up a cable, laid on the seabed, into the plough and then buries it in a trench dug in the surface of the seabed by the plough.

Pipelines are trenched in a similar manner to protect them from fishing activity, and also to stabilise them from movement by currents in the water and to reduce heat losses from them.

A conventional plough for burying a cable to a depth of up to one metre in most soil and very weak rock is shown in Figure 1.

The plough includes a plough share 1 which comprises an assembly of parts which cut and move the soil at the deepest part of the trench in the seabed to bury a cable 2 which passes through the plough and is held down by a movable depressor 3.

The plough share 1 is connected to a pair of depth control skids 4 at the front of the plough by means of a long beam 5, the skids 4 being movable up or down relative to the beam 5 by moving skid support arms 6. A tow rope is attached to swivel tow points 7 at the front of the plough which are in turn mounted on a lifting drawbar 8 which pivots on a main frame 9 of the plough.

The cable plough runs at a generally constant depth by means of the long beam principle, which will be well-known to persons skilled in the art. Under this principle, a cutting edge 10 of the plough share 1 cuts a flat bottom to the trench in the seabed, and a heel 11 supports the weight of the rear of the plough and slides along the soil surface cut by the cutting edge 10. Any tendency of the plough to alter the running depth, for example by means of the rear of the plough lifting up by pivoting about the front skids 4, is counteracted by the heel 11 lifting off the soil surface which in turn throws the weight of the rear of the plough on to the share 1, which is then unable to support this additional load. As a result, the plough tends to run deeper, counteracting the movement of the rear of the plough. Conversely, it is difficult for the rear of the plough to go deeper because this requires the heel 11 to push down into the soil cut by the share cutting edge 10.

A conventional plough for trenching pipelines is shown in Figure 2. This plough includes all of the features 1 to 11 of the cable plough of Figure 1, except that the cable 2 is replaced by a pipe 12 supported on rollers at a front 13 and rear 14, and there is no depressor 3 or lifting drawbar 8. The pipe, which is much stiffer than the cable of Figure 1, falls to the bottom of the trench under its own weight. It reaches the bottom of the trench a long way behind the plough, and this requires that a wide trench with stable side slopes is ploughed, in contrast to the narrow slit made by the cable plough. A forecutter 15 is often provided on pipeline ploughs and enables the trench to be cut in two stages, which reduces the required tow forces.

One of the main differences between the two types of plough (i. e. cable and pipeline) lies in the size, weight and magnitude of the pull forces. A standard telecommunications cable plough typically weighs 15 tonnes, trenches 1 metre deep and is pulled with forces up to 50 tonnes. A pipeline plough typically weighs 100 to 200 tonnes, cuts 2 metres deep and is pulled by forces up to 300 tonnes.

The ploughs described with reference to Figures 1 and 2 leave a trench bottom flatter than the surface of the seabed. As shown in Figure 3, when the front skids 4 arrive on top of a bump 16 of height H, the cutting edge 10 will only lift by a height hb/ (a+b). This feature is desirable to ensure that the cable or pipe is laid in a smooth trench bottom.

The prior art submarine ploughs discussed above have been shown to perform well in most types of soil, but not where rock is present, in which case other methods are usually used to protect the cable. These include laying the pipe or cable on the surface of the seabed and dumping rock on to it, or cutting a trench with a mechanical cutting wheel or chain cutter.

Both of these methods are considerably slower and more expensive than burial by means of a plough. In addition, the use of cutting wheels or chain cutters makes inefficient use of considerable amounts of energy to cut rock into small pieces, which in turn results in unnecessary expense and wear and tear of equipment. The effectiveness of these methods is also limited by the maximum practicable weight of the equipment, which needs to be loaded and unloaded from a ship.

It is also known to mount a heavy ripper tooth behind a crawler tractor to break up rock, as shown in Figure 4. The ripper tooth works by fracturing the rock up an inclined plane a-b.

The point of the ripper tooth then runs up this plane to a point c where it breaks into the rock moving forward and down to d, from which a new failure plane d-e breaks out to the surface. This gives rise to a characteristic up and down motion of the ripper tooth in rock and leaves a serrated edge at the bottom of the cut in the rock. The forward face of these serrations cannot be steeper than the back face of the ripper tooth.

Any attempt to operate a long beam plough as a ripper in rock would suffer from the drawback illustrated in Figure 5 in that having risen up to the peak at c, the plough is prevented from diving steeply back into work by the flat bottom of the plough share that forms the heel of the plough share. Even when the plough has moved the whole distance from the share to the heel, the tip will still not have descended to the full depth.

Each serration peak is therefore higher than its predecessor, and the plough thus tends to work itself up out of the rock until it is pitched forward sufficiently to make a sudden downward movement. This results in a trench having an undulating bottom and reduced depth.

Preferred embodiments of the present invention seek to overcome the above disadvantages of the prior art.

According to the present invention, there is provided a submarine plough adapted to be towed along a sea floor, the plough comprising: a plough body; forward support means for engaging the sea floor to support a forward portion of the plough body; at least one plough share connected to the plough body and having a respective cutting edge for cutting a bottom of a trench in the sea floor, and a respective heel portion arranged rearwardly of the cutting edge for engaging the bottom of the trench to at least partially support the plough; and ripper means for penetrating rock when the plough is towed along the sea floor and comprising at least one ripper tooth having a respective tip for penetrating rock, wherein each said tip has a respective working position, wherein the or each said tip in the respective working position thereof is located forwardly of the centre of gravity of the plough, and the plough is adapted so that when supported on a substantially flat surface by said heel portion and at least one said ripper tooth the cutting edge of the or each plough share does not come into contact with the substantially flat surface.

By providing at least one tip which in its respective working position is located forwardly of the centre of gravity of the plough, this provides the advantage of causing a greater proportion of the weight of the plough to be transferred to the ripper means when a tow force is applied to cause it to penetrate rock. This also reduces the influence of the cutting edge and heel portion on the depth at which the plough operates, the depth being as great as is allowed by the ability of the tip to penetrate rock.

In a preferred embodiment, the or each said tip in the respective working position thereof is located forwardly of the centre of gravity of the plough by a distance greater than the height above said tip of an attachment point of means for towing the plough.

This provides the advantage of maximising the towing force that can be applied to the plough before the plough share tends to lift up out of the trench cut by the ripper means.

The plough preferably further comprises height adjustment means for adjusting the height of the or each said tip in the respective working position thereof.

This provides the advantage of enabling the or each tip to be lowered to compensate for wear thereof, and thus reduces the necessary set depth of each tip below the cutting edge. This also provides the advantage of increasing the final effective depth of trench for a given penetration of the working tooth. The plough may comprise a plurality of said tips, wherein one or more successive said tips is adapted to be moved into the respective working position thereof.

This provides the advantage of enabling a tip to be replaced when it becomes worn, and thus reduces the frequency with which the plough needs to be recovered from its working position on the sea floor. In the case where the plough needs to be towed from a ship, the reduction in the frequency with which the plough needs to be recovered to the ship provides significant cost advantages.

The ripper means may further comprise at least one carrier member rotatably mounted relative to the plough body and having a plurality of said ripper teeth, rotating means for rotating the or each carrier member relative to the plough body to move one or more successive said tips into the respective working position thereof, and fixing means for fixing the position of the or each said carrier member relative to the plough body.

Preferably, the fixing means comprises a fixing member having an engaging portion for releasably engaging a said tooth and mounted to move between a fixing position in which the position of the corresponding carrier member is fixed relative to the plough body, and a release position in which said carrier member can rotate relative to the plough body.

The plough may further comprise moving means for moving said ripper means between stowed and working positions thereof.

This provides the advantage of enabling the plough to operate in a conventional manner if the ripper means is not used because rock is not present, without the necessity of recovering the plough from the working location thereof. In the case of a plough to be towed along the sea floor from a ship, this provides the advantage that expensive recovery of the plough to the ship is avoided. The plough preferably further comprises measuring means for measuring wear of the or each tip in the stowed position of said ripper means.

The measuring means may comprise at least one video camera and a respective scale for indicating a degree of wear of the or each said tip.

The plough may further comprise ripper support means interconnecting said ripper means and at least one said plough share for supporting the or each said tip against reaction forces from the sea floor acting in a direction substantially opposite to the direction of travel of the plough.

In a preferred embodiment, the ripper support means is adapted to support the or each said tip against reaction forces acting in a direction transverse to the direction of travel of the plough.

Preferably, one of the ripper support means and the ripper means comprises at least one engaging portion adapted to engage a corresponding respective aperture provided on the other of said ripper support means or ripper means to support the or each said tip against forces acting in a direction transverse to the direction of travel of the plough.

In a preferred embodiment, the ripper support means is integral with at least one said plough share and is adapted to cut a bottom of a trench in the sea floor.

This provides the advantage of maximising the useful work done by the ripper support means as it travels through the sea floor.

The forward support means may comprise at least one wheel. This has the advantage over conventionally used skids of riding over rocky ground more easily.

In a preferred embodiment, the or each said wheel in use pivots about a respective substantially vertical axis.

The or each said vertical axis may be arranged forwardly of the axis of rotation of the respective wheel.

In a preferred embodiment, the plough further comprises springing and damping means acting between said ripper means and the plough body.

This provides the advantage of filtering out high frequency force variations which could otherwise cause damage to the plough.

Preferred embodiments of the invention will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawings, in which:- Figure 1 is a schematic side elevation view of a prior art submarine cable plough, Figure 2 is a schematic side elevation view of a prior art submarine pipeline plough, Figure 3 is a schematic illustration of the operation of the long beam principle in relation to the plough of Figure 1; Figure 4 is a schematic side elevation view of a prior art ripper tooth; Figure 5 is a schematic illustration of why a prior art long beam plough cannot operate to cut rock; Figure 6 is a schematic side elevation view of a submarine plough of a first embodiment of the present invention; Figure 7 is a detailed view of the three toothed ripper and suspension of Figure 6; Figure 8 is a schematic front view illustrating how the ripper teeth of Figures 6 and 7 are brought into their operating position; Figure 9 is a schematic plan view of the ripper teeth and locking and rotating mechanism of the embodiment of Figures 6 to 8 in the stowed position; Figure 10 shows a forward support castoring wheel assembly of a second embodiment of the invention; Figures 11 and 12 show a pipeline plough of a third embodiment of the invention: and Figure 13 is a schematic side elevation view of a submarine plough of a fourth embodiment of the present invention.

Referring to Figure 6, a cable plough 100 for burying a cable (not shown) is adapted to be pulled in the direction of arrow A by means of a tow rope 101 attached to tow points 102. The plough 100 comprises a plough body 103 to a forward portion of which is pivoted a pair of wheels 104. A plough share 105 is mounted to a rear portion of the plough body 103 and has a cutting edge 106 for cutting a trench in the sea bed 107, and a heel 108 provided rearwardly of the cutting edge 106 for partially supporting the plough 100 and enabling the long beam principle to operate, as will be understood by persons skilled in the art. The depth dl of the cutting edge 106 below the surface 107 of the sea bed is adjusted by pivoting wheel arms 109 about pivot points at the forward portion of the plough body 103. The centre of gravity 110 is located underneath fixed lifting point 111.

The plough described so far is the standard plough used for most submarine cable installation throughout the world. Since rock sea bottom forms only a small part of the routes ploughed it is desirable that the rock cutting plough is a modification of a standard plough. Furthermore, the plough must remain capable of ploughing in normal soils between patches of rock. The modifications must also be easily fitted and removed with a minimum of added welded on attachments.

The main features of a rock ripping attachment are shown on Figures 6,7 and 8. The tip of a rock digger is positioned below a line drawn between the share 106 and heel 108 a distance d4 in front of the centre of gravity 110 of the plough and a distance d3 below the attachment point 102 of tow rope 101 where d3 is substantially equal to d4. A working tooth 112 points more steeply downward than the plough cutting edge 106 and is part of an assembly of three teeth 113 which can be brought into use successively as they become worn. This assembly rotates on an axle 114 fixed through a tooth carrying frame 115 which is supported on three double acting hydraulic cylinders 116,117,118.

These cylinders are connected to six hydraulic accumulators so that they act as springs and dampers in a way understood by those skilled in the art. The upper 117 and rear cylinder 116 have a short stroke and their function is only as suspension elements. The front cylinder 118 has a relatively long stroke and is used to lower the ripper tip as it wears to keep it at a distance d2 below a line through the plough cutting edge 106 and the heel 108 as well as acting as a suspension element.

Figure 7 is a schematic side elevation showing the way the ripper tooth 112 in the tooth support frame 115 is supported on the main body 103 of the plough. Figure 8 is a schematic view from the front of the plough showing the same main components and the arrangements for swinging the tooth 112 and support frame 115 up into the out of work position 119 above the surface of the soil 107 when the plough is at its deepest.

The plough has a lifting drawbar as shown at 121 on Figure 6.

This pivots about two very strong pins 122 and 123 which cross two main frames 124 and 125 which make up the plough body 103 in this region. These pins transmit the tow rope force from the drawbar 121 into the plough. They are therefore the best way of providing the main thrust into the ripper tooth assembly 115. A telecom cable 126 passes along a cable support channel 127. The cable 126 is loaded up beside the cable channel 127 as indicated by the arrow 128. This operation is carried out with the plough on the ship.

The tooth support frame 115 pivots on axis 129 from the working position 130 to the stowed position 119. It rotates about a ball joint 131 in the head of the rear cylinder 116 and a ball joint 132 in the end of front support arm 133. The ball joint 131 is supported by a pin 134 which also runs through a clevis 135 which is an integral part of the end of the upper cylinder 117. The rod end of this ram is supported on a pin 136 which lies between the two side plates of the tooth support frame.

When the ripper support frame rotates about axis 129 the piston rod of cylinder 117 is forced to rotate about axis 137 by the pin 136 which in turn forces the ball of ball joint 138 to move in its housing. This latter movement limits the maximum value of the angle between axes 129 and 137.

The ripper tooth support frame 115 is free to move forwards and backwards under the restraints imposed by the forward vertical cylinder 118 and the rear vertical cylinder 116 acting through the transverse support arm 133. The extent of the motion is limited by the stroke of the upper cylinder 117. The two cylinders 116 and 117 have a short stroke and are only used as suspension members to cushion the transmission of shock loads to the main plough. The front cylinder has a longer stroke and is used to adjust the vertical position of the working tooth 112 relative to the cutting edge 106 of the plough as the tooth wears in order to control the dimension d2 on Figure 6. This ram is fitted with a rod position indicator to facilitate this adjustment.

Figures 7 and 8 show the way that the main thrust from the ripper tooth 112 is transmitted via ram 117 into the cross pins 122 and 123 which have the primary task of supporting the lifting drawbar 121. Both the original pins are replaced. The pin 122 on the left side of Figure 7 has a large head with a recess to receive the other pin 123. This is a lengthened version of the original pin. After the telecom cable has been inserted by the route 128 the pin 122 is driven across so that the recess in pin 122 engages with pin 123. The thrust from the tooth is by this means transmitted into both sides of the drawbar.

The three ripper teeth 112 shown in Figures 6 and 7 are mounted on common carrier 113 which rotates freely about pin 114 carried between the two side plates 115. A means is provided to prevent the ripper teeth and their carrier from rotational movement when the lowermost tooth is ripping rock. The mechanism can also release the tooth carrier for free rotation.

A means is provided to rotate the unlocked tip carrier to bring the next unworn tooth into the locking position. The following description refers to Figure 9 which is a schematic view looking vertically down onto the tooth and tooth carrier in the stowed position showing both the locking means in the tooth carrier and the rotating means on the plough body.

A cranked arm 150 of the locking mechanism pivots about pin 151, which is held between side plates 152. The end of the arm 150 carries a housing piece 153, which is shaped to engage the flat sides of the ripper tip, away from the working section of the tip. This part of the ripper tip is not generally subject to extreme wear. A cranked end 154 of the locking arm 150 is connected to a double acting hydraulic ram 155, which pivots about the side plates 152 on pin 156.

To unlock the system, to allow the tooth carrier to rotate, the locking arm 150 is displaced towards the end of the ripper tip by extending the hydraulic ram 155 until the arm reaches position 157. In this position the housing piece 153 clears tip circle 158. The entire tooth carrier 113 can now be rotated to bring another tooth into the position occupied by the previous ripper tip just released. The carrier is then locked by reversing the unlocking process and the housing piece 153 engages with the flanks of the new ripper tip.

The large restraining force required to hold the tooth carrier in the ripping position is transmitted directly by the arm 150 through the pivot pin 151 to the side plates 152. The major ripping forces induce tension in the arm 150 and do not subject it to large buckling forces. The operating hydraulic cylinder is of small capacity adequate to move the locking arm 150 and provide the small force required to hold down the housing 153 against the tooth tip flanks.

Separate means is provided for rotating the tooth carrier when it is unlocked. This operation is carried out when the entire ripper tooth assembly, held between the side plates 152, is rotated about axis 129 into its horizontal position.

The ripper tip rotating mechanism is attached to the front beam of the plough and operates in a plane parallel to and lying above the horizontal stowed position of the tooth assembly. A cranked arm 160 of the rotating mechanism pivots on bracket 161 through pin 162. The carrier bracket 161 is attached to front cross beam 163 of the plough. The outer end of the rotating arm 160 carries a ratchet pin 164 which is free to rotate in one direction about pivot 165. The pin engages with the flat shank of a ripper tip 166. This pin is shown in Figure 9 as moving its operating position under gravity. A spring can be used to assist this action. The other cranked end of arm 160 is connected to a double acting hydraulic ram 167, which pivots on pin 168 located on the front plough beam 163.

The hydraulic cylinder 167 is fully extended before the tooth assembly is moved into its stowed position. Consequently, when the tooth assembly is stowed, the pin 164 lies close to or against the outer flank of the ripper tip 166. Prior to stowing, this particular ripper tip has been in the lowermost working position. The locking mechanism is actuated to release the tooth assembly 113 and the hydraulic cylinder 167 is retracted thus rotating the arm 160 in an anti-clockwise sense.

Ratchet pin 164 contacts the tooth tip 166 and, because it is unable to deflect backwards, rotates the entire tooth carrier along with the arm 160 through an angle of not less than 120° into position 169. The tooth carrier is then locked and the hydraulic cylinder 167 is extended. The ratchet pin 164 disengages the face of the ripper tip and deflects over the top of the new tooth that has been advanced and the arm 160 returns to the starting position shown by full lines. This process can be carried out sequentially to bring the next tooth into the working position and locked.

The locking and rotating mechanism described above is applied to the case when the tooth assembly is stowed on the left-hand side of the plough. The mechanism also operates in a similar manner if the tooth assembly is stowed on the right-hand side of the plough.

Fig 10 shows a castoring wheel arrangement of a second embodiment of the invention mounted on the cross beam of the plough body 203. Parts common to the embodiment of Figures 6 to 9 are denoted by like reference numerals but increased by 100. The wheels replace the standard skids 4 on a standard telecommunications plough (see Fig 1). The wheels allow the plough to ride over rocky terrain with less risk of damage than conventional skids. The wheels are arranged with trailing castor freedom to allow the wheels to move sideways as side loads are induced by the plough steering. A wheel and tyre assembly 270 rotates about an axle 271 which trails from a steering axis 272. A king pin arm 273 moves on a parallel linkage, formed by wheel arm 274 and a wheel restraining link 275, to maintain steering control with a fixed vertical castor axis at all wheel positions relative to the plough body defined by the stroke of wheel depth cylinder 276.

Figures 11 and 12 show the use of two ripper teeth mounted on a 150 tonne pipeline plough of a third embodiment of the invention, in which parts common to the embodiment of Figures 6 to 9 are denoted by like reference numerals but increased by 200. The plough is capable of making a trench up to 2m deep utilising a towing force of up to 300 tonne. Two teeth reaching down more than 2m are required, separated by a distance of about a metre, to ensure enough rock is broken to achieve the required trench width.

The biggest single tooth ripper used behind a crawler tractor is intended to be pushed down with 30 tonne forces, pulled with 50 tonne and cuts a maximum of 1.6m deep. A ripper plough based on a telecom cable plough is therefore well within existing technology. However, to fully exploit the weight and pull of a pipeline plough would require either very massive ripper teeth or some method of retracting the teeth if the ripping force became too large.

Each tooth 312 is pivoted about an axis 314 under the control of a hydraulic cylinder 315. Each cylinder is connected to a preloaded hydraulic accumulator which will allow the tooth to move backwards when the force on the tip exceeds a preset value of, say, 100 tonne. This helps to share the available rock breaking force between the two teeth, as well as to prevent overloads. When there is no rock present the teeth can be swung up out of work by cylinder 380 so that normal ploughing can resume.

As pipeline routes are generally much shorter than telephone cable lines the pipeline plough ripper is not fitted with replaceable tips.

Figure 13 is a schematic side elevation view of a submarine cable plough 400 of a fourth embodiment of the invention.

Parts common to the embodiment of Figures 6 to 9 are denoted by like reference numerals but increased by 300. The plough 400 has a plough body 403 supported at a front part thereof by skids 490, and a plough share 405 having a cutting edge 406 and a heel 408.

The plough share 405 is also provided with a forecutter 491 for cutting through soil and located forwardly of the cutting edge 406. A ripper 412 pivots about a pivot axis 414 between a working position shown in solid lines in the figure and a stowed position 492 shown in dotted line under the action of a hydraulic cylinder 493. The ripper 412 is provided with an aperture 494 which is engaged by tip 495 of forecutter 491 to restrain the ripper 412 against rearward and sideways movement.

Since the forecutter 491 gives support to the ripper 412 in its working position, the hydraulic cylinder 493 does not need to hold the ripper 412 in position and therefore does not need to be particularly large.

It will be appreciated by persons skilled in the art that the above embodiments have been described by way of example only and not in any limitative sense, and that various alterations and modifications are possible without departure from the scope of the invention as defined by the appended claims. For example, it will be appreciated that various features of the different embodiments described above can be combined and/or omitted.