A continually increasing number of elongate products such as pipes and cables lie on the sea bed, particularly on the continental shelves. It is often necessary to lay and/or bury such products in trenches to protect them against mechanical damage from fishing gear, anchors or the like, to provide stability against currents, or to provide thermal insulation.

There are three main types of such elongate product to be buried, i. e. flexible cables such as telephone cables and power cables, rigid pipes such as steel pipelines, and so-called flexible pipes, which possess a considerable degree of stiffness.

Rigid pipes are buried by digging a trench at the desired location on the sea bed and allowing the pipe to fall into the trench under its own weight. A conventional method of laying a rigid pipe in a trench involves cutting a V-shaped trench by means of up to four inclined chain cutters. The sides of the trench are angled at generally 60 degrees to the vertical.

This prior art method suffers from the drawback that because a very large volume of soil is broken up and excavated by the chain cutters, a very large power input is required, and rapid wear of the chain cutters occurs, which necessitates frequent recovery of the apparatus to the surface to replace the cutters. In addition, if an insufficient volume of soil is excavated to achieve the desired trench profile by the first pass of the apparatus along the intended trench position, a second pass of the apparatus is very difficult to achieve because the width of the trench results in the spoil material excavated from the trench during the first pass impeding the tracks of the trenching apparatus during the second pass.

As a result, trenching vehicles to carry out the method are often very large and complex, and have long, light duty chains in order to cut inclined trench walls. This results in very flexible chains which make the vehicle unsuitable for even very soft rock. Even with four chains, soil, stone and broken rock tends not to be adequately removed from the trench, so that the pipe is often not properly buried. The inclined chains are also frequently jammed by stones falling on to their upper surface.

The above prior art method relies on avoiding touching the pipe with any part of the digging machine. This necessitates driving the trenching vehicle so that the cutters are generally equidistant from the pipe on both sides thereof to ensure that the pipe falls approximately at the centre of a wide trench.

However, this results in the method being unsuitable for flexible products such as cables and so-called flexible pipes, since such products tend to flex considerably as the soil is dug from beneath them, resulting in so much lateral movement of the product that they are frequently destroyed by the cutters.

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

According to an aspect of the present invention, there is provided a vehicle for laying an elongate product in a trench in a sea floor, the vehicle comprising: a vehicle body adapted to travel along the sea floor; trench cutting means provided in use below said vehicle body for cutting a trench having substantially vertical side walls in the sea floor; and support means provided on said vehicle body for supporting the elongate product in use above said trench cutting means prior to insertion of the elongate product in the sea floor, and substantially without displacement thereof in a substantially horizontal direction transverse to the axis of the elongate product when said trench cutting means remains within a predetermined distance in that direction of an intended trench position.

By providing trench cutting means adapted to cut a trench having substantially vertical walls, this gives the advantage of minimising energy waste by reducing the volume of excess material removed from the trench while facilitating a second pass of the vehicle because of the reduced width of the trench compared with the prior art. By supporting the product above the trench cutting means substantially without displacement of the product in a direction transverse to the axis of the product, this provides the advantage of avoiding accidental damage to the product by the trench cutting means, while at the same time avoiding transverse displacement of the product which may otherwise result in failure to correctly locate the product in the trench. This is particularly problematic with prior art methods of laying a product comprising a rigid pipe because of the considerable distance between cutting of the trench and laying of the pipe into the trench.

Preferably, the support means has an upper surface for supporting the elongate product thereon such that the elongate product is within predetermined limits substantially free to move relative to the upper surface in a direction lateral to the axis of the elongate product and/or pivot about a substantially vertical axis.

In a preferred embodiment, the coefficient of friction in use between the elongate product and the support means in a direction transverse to the axis of the elongate product is as low as practicable.

The support means may comprise first and second support portions and is adapted to support the elongate product at respective first and second locations along the length thereof, the first and second support portions being connected by a rigid beam adapted to pivot about a substantially horizontal transverse axis substantially equidistant from the first and second support portions.

By providing first and second support portions connected by a rigid beam which can pivot about an axis equidistant from the support portions, this gives the advantage of ensuring that both support portions remain in contact with the elongate product to exert substantially equal loads thereon, minimising stress in the elongate product.

The first and second support portions are preferably located as far apart as practicable.

The support means may comprise at least one roller adapted to rotate in use about a respective substantially horizontal transverse axis.

By providing one or more rollers which can rotate about a respective axis, this gives the advantage of minimising frictional forces acting on the product in the direction of the axis of the product.

In a preferred embodiment, the support means comprises a plurality of said rollers.

This provides the advantage of maximising the radius of curvature about which the product is forced to bend, thus minimising stresses which may otherwise damage the product.

Preferably, at least one said roller comprises a respective wheel rotatably mounted to a shaft and adapted to slide axially along said shaft, the wheel having a circumferential groove for receiving the product when supported thereon.

In a preferred embodiment, the support means is pivotable relative to the vehicle body to provide a substantially horizontal support surface in a transverse direction for the elongate product.

This provides the advantage of enabling the support means to support the elongate product substantially without lateral displacement thereof while the vehicle is travelling along an inclined surface.

In a preferred embodiment, the support means is movable in use relative the vehicle body in a direction transverse to the axis of the elongate product.

This provides the advantage of enabling the flexible product to be supported substantially without transverse displacement for a wide range of movement of the vehicle, while enabling a compact construction of the support means.

The support means may be movable relative to the vehicle body between a working position and a stowed position.

This provides the advantage of facilitating loading of the product onto the support means.

The vehicle may further comprise restraining means for restraining displacement of the product relative to the support means in a direction transverse to the axis of the elongate product when the trench cutting means exceeds said predetermined distance from the intended trench position.

This provides the advantage of preventing unintended disengagement of the product from the vehicle and/or unintended contact between the product and other components of the vehicle.

In a preferred embodiment ; the restraining means comprises at least one restraining member arranged on each lateral side of the support means.

The or each said restraining member on at least one side of the vehicle may be displaceable to allow passage of the product.

This provides the advantage of facilitating location of the product onto the support means and/or emergency removal of the vehicle from the elongate product in the event of breakdown.

In a preferred embodiment, the trench cutting means is movable between a working position and a retracted position thereof.

This provides the advantage of enabling the vehicle to be located onto a product with the trench cutting means in its stowed position and the trench cutting means to be deployed into its working position after correct location of the vehicle relative to the product. This minimises the risk of damage to the product by the trench cutting means.

Preferably, the support means and the trench cutting means are mounted to a support member, the trench cutting means is pivotable about an axis on the support member between its working and retracted positions, and the support member is pivotable about an axis on said vehicle body to move the support means between the working position and stowed position thereof.

This provides the advantage of simplifying the movement of the support means between the working and stowed positions thereof, and of the trench cutting means between the working and retracted positions thereof.

The vehicle may further comprise handling means for displacing the product relative to the vehicle body in a substantially vertical direction.

This provides the advantage of facilitating loading of the product on the support means.

The handling means may comprise at least one powered grab.

In a preferred embodiment,-the elongate product is only supported by said support means when the trench cutting means is in operation.

The vehicle may further comprise jetting means supported by said vehicle body for providing one or more jets of water in the sea floor to fluidise material therein.

The jetting means preferably comprises at least one respective jetting leg adapted to be arranged in use on each side of the product, and wherein each said jetting leg is adapted to produce a plurality of jets of water.

In a preferred embodiment, the jetting means is pivotable about a substantially vertical axis relative to the vehicle body.

This provides the advantage of enabling the jetting means to achieve the optimum position relative to the elongate product.

The vehicle may further comprise detector means for detecting the position of the elongate product relative to the vehicle body.

This provides the advantage of enabling the vehicle to be controlled to keep the product located generally in the middle of the support means.

In a preferred embodiment, the detector means comprises at least one detector member biased into contact with the product in use.

According to another aspect of the invention, there is provided a method of laying an elongate product in a trench in a sea floor, the method comprising the steps of: passing a vehicle body of a vehicle along the sea floor; cutting a trench having vertical side walls in the sea floor by means of trench cutting means provided on the vehicle body; and supporting the product on the vehicle above the trench cutting means prior to laying of the product in the trench, wherein the product is supported substantially without displacement thereof in a substantially horizontal direction transverse to the axis of the product when the trench cutting means remains within a predetermined distance in that direction of an intended trench position.

The method may further comprise the step of supporting the elongate product above the trench cutting means only when the trench cutting means is operating.

The method advantageously further comprises the step of detecting the position of the elongate product relative to the vehicle body.

The product may comprise a substantially rigid pipe.

Alternatively, the product may comprise a flexible cable or a semi flexible pipe..

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 of a pipe trenching vehicle embodying the present invention; Figure 2 is a side elevation of a trench cutter and pipe support means of the vehicle of Figure 1; Figure 3 is a schematic end view of the vehicle of Figure 1 with trench cutting means and support means thereof in respective working positions thereof; Figure 4 is a detailed view from the front, corresponding to Figure 3, of the vehicle of Figure 1 with the trench cutting means and support means in respective stowed positions thereof; Figure 5 is a view from above of part of the vehicle of Figure 1; Figure 6 is a cross-sectional end view of a first embodiment of the pipe support means of the vehicle of Figure 1; Figure 7 is a cross-sectional end view of a second embodiment of the pipe support means of the vehicle of Figure 1; and Figure 8 is a schematic end view, corresponding to Figure 3, of the vehicle of Figure 1 travelling along an inclined surface.

Referring to Figure 1, a submarine trenching vehicle 1 for laying a rigid or large semi-flexible pipe 2 in a trench 3 on the sea floor 4 comprises a vehicle body 5 which can move along the sea floor 4 by means of tracks 6, as will be known to persons skilled in the art. A powered grab 7 for lifting the pipe 2 is provided on the vehicle body 5, and is movable along the vehicle body 5 to adjust the position of the pipe 2 supported by the grab 7.

A pipe support roller system 8 is also provided in the vehicle body 5 and comprises one or more rollers which can rotate about respective axes arranged generally transversely to the axis of the pipe 2. The end rollers of roller support system 8 are arranged as far apart as, practicable and the entire roller support system is mounted to a walking beam arrangement. This ensures that both end rollers are always in contact with the pipe 2, such that the pipe 2 exerts equal loads on both end rollers, thus minimising stress in the pipe 2.

A trench cutter comprising a heavy duty chain cutter 9 includes a series of cutting teeth 10 driven by means of a chain (not shown) around a pair of wheels 11 in order to cut the sea floor 4 to form the trench 3 such that the trench 3 has generally vertically extending side walls. The chain cutter 9, which has a working position as shown in Figure 1 is arranged below the pipe support roller system 8 such that the pipe 2 is supported above the chain cutter 9 and cannot come into contact with the cutter 9.

A jetting machine 12 comprises a pair of jetting legs 13, each of which has a series of jets 14 arranged in a row for ejecting pressurised water to fluidise particulate material in the trench, and is arranged to straddle the pipe 2 such that a jetting leg 13 lies on each side of the pipe 2. The jetting legs 13 of the jetting machine 12 are attached to a support 15 which can pivot about a generally vertical axis 16 passing through the vehicle body 5 at a position midway between the tracks 6 of the vehicle 1. As a result, any variation in the position of the pipe 2 from the centre of the trench 3 can be accommodated by the jetting machine 12.

The jetting machine 12 serves to remove and/or fluidise any soil in the trench 3 after it is formed by the trench cutter 9, or as a result of collapse of the side walls of the trench 3.

The vehicle 1 may also be provided with an eductor system (not shown) for sucking loose sand out of the trench, in a manner which will be familiar to persons skilled in the art.

Referring now to Figures 2 to 4, the chain cutter 9 is mounted at the lower part of a carrier arm 17 so that it can pivot about an axis 18 between a working position A and a retracted position B as shown in Figure 2, under the action of a hydraulic cylinder 19. The carrier arm 17 is in turn mounted to upper cross frame members 20 of the vehicle body 5 so that the carrier arm 17 can pivot about axis 21, extending generally transversely to the axis 18., under the action of hydraulic cylinder 22 between a working position as shown in Figures 1 to 3 and a stowed position as shown in Figure 4.

The pipe support roller system 8 and chain cutter 9 can be located under the pipe 2 by locating the vehicle 1 with a track 6 on each side of the pipe 2 and with the roller system 8 in its stowed position and the chain cutter in its retracted position, lifting the pipe 2 vertically by means of the powered grab 7, pivoting the carrier arm 17, with the chain cutter 9 still in its retracted position B, about pivot axis 21 in the direction of arrow D in Figure 4, and then lowering the pipe 2 onto the support roller system 8. The chain cutter 9 can then be pivoted about pivot axis 18 into its working position A.

Referring now to Figure 5, the support roller system 8 is provided with a pair of generally upright posts 23 on the side thereof towards the stowed position of the roller system 8, and a pair of posts 24 on the opposite side. The posts 23 are fixed in position, while the posts 24 can be lowered by means of hydraulic cylinders 25 to permit loading or removal of the pipe 2 from the support roller system 8.

The posts 23,24 prevent the pipe 2 from falling off the support roller system 8 and coming into contact with other components of the vehicle 1. The strong frame of the roller system 8 when bearing working loads provides a convenient mounting for the posts 23,24.

A pair of transducers in the form of feelers 26 are spring biased towards the pipe 2 moving over the support roller system 8 relative to the vehicle 1 to detect the position of the pipe on the support roller system 8. This produces an output signal dependent upon the position of the feelers 26, in response to which movement of the vehicle 1 can be controlled to maintain the pipe 2 generally at the centre of the pipe support roller system 8.

Figure 6 shows in greater detail an embodiment of the support roller system 8 and comprises a series of generally cylindrical rollers 27, each of which is rotatably mounted to a respective shaft 28 carried on a support frame 29. Each of the rollers 27 has a pipe support surface covered by a soft rubber tyre 30 which enables the pipe 2 to creep sideways under relatively small loads, thus minimising any tendency for the vehicle 1 to cause lateral displacement of the pipe 2 provided that the pipe does not come into contact with the upright members 23 or 24.

Figure 7, in which parts common to the embodiment of Figure 6 are denoted by like reference numerals but increased by 100, shows a second embodiment of the support roller system 108.

The embodiment of Figure 7 is particularly suitable for small pipes 102 and comprises one or more pulleys 127 having a V- shaped circumferential groove 131 for carrying the pipe 102.

Each pulley 127 is mounted to shaft 128 carried by frame 129 by means of a low friction bearing 132 such that the pulley 127 can slide along shaft 128 with very low lateral friction.

The operation of the vehicle 1 described with reference to Figures 1 to 7 will now be described.

The pipe 2 to be buried in the sea floor initially lies on the sea floor 4 over the intended burial site. The vehicle 1 is lowered from a ship onto the pipe 2 with the chain cutter 9 in its retracted position B and the support roller system 8 in its stowed position such that a track 6 of the vehicle lies on each side of the pipe 2. As a result, the risk of damage to the pipe 2 by the chain cutter 9 is minimised.

The movable upright members 24 are then lowered by means of hydraulic cylinders 25., The pipe 2 is lifted generally vertically by means of the powered grab 7, and the support arm 17 is pivoted about pivot axis 21 under the action of hydraulic cylinder 22 to bring the support roller system 8 to its working position underneath the pipe 2, the cutter 9 remaining in its retraced position. The pipe-2 is then lowered by means of the grab 7 until it rests on the roller system 8, and the displaceable upright members 24 are moved to the upright position thereof under the action of the hydraulic cylinders 25. The chain cutter 9 is then activated and pivoted about horizontal axis 18 from the retracted position B to the working position A thereof under the action of hydraulic cylinder 19.

The chain cutter 9 is activated, and because the pipe 2 is supported on roller system 8 above the chain cutter 9, the risk of damage to the pipe by the cutter is minimised. At the same time, the vehicle 1 is driven forwards on its tracks 6 (i. e. in the direction of arrow C shown in Figure 1) so that the trench 3 is formed and the pipe 2 leaving the rear of the vehicle body 5 falls into the newly formed trench 3 under its own weight.

The jetting machine 12 straddles the pipe 2 as it enters the trench 3 and fluidises and/or removes any soil or other particulate material in the trench 3 to enable the pipe 2 to pass to the bottom of the trench 3.

Referring now to Figure 8, side slopes pose problems to prior art mechanical trenching machines in that the trench cutter cuts at an incline, and one side of the trench therefore overhangs, causing a danger of collapse. With the vehicle 1 of the present invention, the support roller system 8 is maintained in a horizontal orientation under the pipe 2, and the chain cutter 9 maintains a vertical orientation. The orientation of the roller system 8 and trench cutter 9 are controlled by pivoting carrier arm 17 about pivot axis 21 by means of hydraulic cylinder 22 in response to signals received from the feelers 26 shown in Figure 5 and/or control signals from a remote control apparatus (not shown) to produce a trench 3 having generally vertical side walls.

Referring again to Figures 3 and 4, if the jetting machine 12 fails to remove or fluidise sufficient particulate materials in the trench 3 on a single passing to enable the pipe 2 to be adequately located at the bottom of the trench 3, a second pass of the vehicle 1 along the trench 3 is made. Because the sides of the trench 3 are sufficiently steep, spoil heaps 33 caused by the trenching operation during the first pass do not impede the vehicle tracks 6.

When the trenching operation is complete, or if the vehicle 1 needs to be removed from the pipe 2 as a result of an emergency caused by mechanical breakdown, the chain cutter 9 is lifted to its retracted position B, the moveable posts 24 are lowered by means of the hydraulic cylinders 25, and the roller system 8 and chain cutter 9 are removed from the pipe 2 by pivoting carrier arm 17 about axis 21.

It is found that the vehicle 1 of the present invention is also suitable for laying flexible products such as cables, as well as rigid and semi-flexible pipes. Movement of the cable as a result of digging of the trench 3 under the cable does not cause significant risk of damage to the cable by the cutter 9, since the cable passes over the cutter 9 and is separated from it by the support roller system 8. For laying flexible cables, it is preferable to provide a powered grab 7 at each end of the vehicle body 5 to assist in handling the cable.

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.