The present invention relates to a method and apparatus for forming a trench in a sea floor, and relates

particularly, but not exclusively, to a method and apparatus for installing a cable in a trench in a sea floor .

It is known to install a cable in a trench in a sea floor by using a boulder clearing apparatus to clear debris such as boulders from the region where the trench is to be formed, and then to tow a trenching plough by a ship to form a trench having sidewalls inclined relative to each other . After the cable is laid in the trench, a backfill apparatus is used to push spoil displaced by formation of the trench into the trench to bury the cable .

This known arrangement suffers from the disadvantage that several separate apparatus are required which

significantly increases costs , and in order to bury a cable to a depth of 2.5 meters , the known cable plough operates at a pulling force of 350 tonnes and has a weight of 160 tonnes

European Patent Application EP 2840187 discloses an apparatus for forming a trench and burying a cable in the trench in which certain parts of the apparatus are

interchangeable in order to enable the apparatus to carry out boulder clearing, first pass trenching, full depth trenching and backfilling operations . However, this arrangement suffers from the drawback that a significant number of interchangeable components are required, and the apparatus must be retrieved to the surface in order to enable

conversion between at least some of the different modes of the apparatus to take place . Preferred embodiments of the present invention seek to overcome one or more of the above disadvantages of the prior art .

According to an aspect of the present invention, there is provided an apparatus for forming a trench in a sea floor and/or displacing material on the sea floor, the apparatus comprising : - a body; and

sea floor engaging means mounted to the body for engaging the sea floor and supporting the body;

wherein the sea floor engaging means is adjustable between a first mode of said sea floor engaging means , in which the sea floor engaging means engages the sea floor, and a second mode of the sea floor engaging means , in which the sea floor engaging means engages the sea floor adjacent the trench and at at least one wall of the trench. By providing sea floor engaging means adjustable between a first mode of said sea floor engaging means , in which the sea floor engaging means engages the sea floor, and a second mode of the sea floor engaging means , in which the sea floor engaging means engages the sea floor adjacent the trench and at at least one wall of the trench, this provides the advantage of improved control of the apparatus in the first and second modes of the apparatus , without the

necessity of changing the sea floor engaging means . This in turn reduces the cost of components of the apparatus , while also reducing the cost of operation of the apparatus , by reducing the time during which the apparatus need to be recovered to the surface to exchange the sea floor engaging means . The apparatus may further comprise trench cutting means supported by the body for cutting a trench in a sea floor. The sea floor engaging means may comprise at least one skid.

At least one said skid may have a first portion for engaging the sea floor in said first and second modes , and a second portion, pivotable relative to said first portion, for engaging the sea floor in said first mode and engaging a wall of the trench in said second mode .

This provides the advantage of providing a lower cost , simplified construction of the skid, which is easy to adjust .

The height of the sea floor engaging means may be adjustable relative to the body. This provides the advantage of enabling the depth of the trench cutting means to be adjusted.

According to another aspect of the present invention, there is provided an apparatus for forming a trench in a sea floor and/or displacing material on the sea floor, the apparatus comprising :

a body;

trench cutting means supported by the body for cutting a trench in the sea floor;

sea floor engaging means mounted to the body for engaging the sea floor and supporting the body; and

material displacing means for displacing material on the sea floor as a result of movement of the apparatus along the trench, wherein the material displacing means is

adjustable between a first mode of the material displacing means , in which material removed from the trench is displaced laterally away from the trench, and a second mode of the material displacing means , in which material adj acent the trench is displaced into the trench .

By providing material displacing means for displacing material on the sea floor as a result of movement of the apparatus along the trench, wherein the material displacing means is adjustable between a first mode of the material displacing means , in which material removed from the trench is displaced laterally away from the trench, and a second mode of the material displacing means , in which material adjacent the trench is displaced into the trench, this provides the advantage of enabling separate debris clearing and backfilling operations to be carried out by a single apparatus , thereby reducing component cost , and reducing the cost of operation of the apparatus , by while also reducing the cost of operation of the apparatus , by reducing the time during which the apparatus need to be recovered to the surface to exchange a material displacing means suitable for one of the first and second modes for a material displacing means suitable for the other of the first and second modes .

The material displacing means may comprise a plurality of first material displacement members pivotable between said first and second modes of the material displacement means . The apparatus may further comprise restraining means for restraining outward pivoting of said first material displacement members relative to the body in said second mode of the material displacing means . According to a further aspect of the present invention, there is provided an apparatus for forming a trench and/or inserting material into a trench in a sea floor, the

apparatus comprising :

a body;

sea floor engaging means mounted to the body for engaging the sea floor and supporting the body ; and

material displacing means for displacing material on the sea floor as a result of movement of the apparatus along the sea floor, wherein the material displacing means

comprises a plurality of first material displacement members mounted to the body, and a plurality of second material displacement members mounted to the first material

displacement members in a first mode of the material

displacing means such that a lateral width of the apparatus in said first mode of the material displacing means is larger than the lateral with of the apparatus in a second mode of the material displacing means .

By providing material displacing means for displacing material on the sea floor as a result of movement of the apparatus along the sea floor, wherein the material

displacing means comprises a plurality of first material displacement members mounted to the body, and a plurality of second material displacement members mounted to the first material displacement members in a first mode of the material displacing means such that a lateral width of the apparatus in said first mode of the material displacing means is larger than the lateral with of the apparatus in a second mode of the material displacing means , this provides the advantage of ensuring more efficient operation of the apparatus by

enabling debris clearance and/or backfilling to be carried out over a wider path than is covered by a trenching

operation, thereby reducing the tendency of large debris to be inserted into the trench in the backfilling operation . The apparatus also enables the width of the apparatus during deployment from the surface to be reduced, thereby making deployment from and recovery to the surface easier.

The apparatus may further comprise trench cutting means supported by the body for cutting a trench in the sea floor.

At least one said second material displacement member may be pivotably mounted to a respective said first material displacement member . The material displacing means may further be adapted to displace material on the sea floor, forwards of the sea floor engaging means , laterally outwards of the apparatus .

This provides the advantage of further assisting the debris clearing operation.

The material displacing means may be adapted to

displace material by means of at least one third material displacing member mounted to said body.

The third material displacing member may be removable .

A preferred embodiment 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 perspective view of a trenching apparatus embodying the present invention in a debris clearing and first pass trenching mode thereof ;

Figure 2 is a perspective view of the apparatus of Figure 1 in a full depth trenching mode thereof ;

Figure 3 is a perspective view of the apparatus of Figure 1 in a backfilling mode thereof ;

Figure 4 is a plan view of the apparatus of Figure 1 ;

Figure 5 is a side elevational view of the apparatus of Figure 4 configured for launching from or recovery to a surface vessel ;

Figure 6 is a view corresponding to Figure 5 of the apparatus in a configuration for landing on the sea floor;

Figure 7 is a side view of the apparatus of Figure 4 ; Figure 8 is a plan view of the apparatus of Figure 2 ;

Figure 9 is a side elevational view of the apparatus of Figure 8 configured for launching from or recovery to a

surface vessel ;

Figure 10 is a view corresponding to Figure 9 of the apparatus in a configuration for landing on the sea floor;

Figure 11 is a side view of the apparatus of Figure 8;

Figure 12 is a plan view of the apparatus of Figure 3 ;

Figure 13 is a side elevational view of the apparatus of Figure 12 configured for launching from or recovery to a surface vessel ; and

Figure 14 is a side view of the apparatus of Figure 12.

Referring to Figures 1, 4 and 7 , a universal plough 2 for clearing debris such as boulders from a region of a sea floor where a trench is to be formed, and cutting part of the trench in a first pass, is shown in a debris clearing mode of the plough 2. The plough 2 has a body 4 having a single point lift attachment 6 for connection to a lift wire (not shown) for enabling deployment of the plough 2 to the sea floor from a surface vessel (not shown) , or recovery of the plough 2 from the sea floor to the surface vessel . The plough is controlled by means of electrical power supplied by means of an umbilical cable (not shown) .

The body 4 supports trench cutting means in the form of a plough share 8 and a pair of fixed mouldboards 10 are located above the plough share 8 for displacing soil out of a trench formed by the plough share 8 as the plough 2 is towed forwards . The body 4 also supports sea floor engaging means in the form of a pair of front skids 12 pivotably mounted via links 14 to a beam 16, which is in turn pivotably mounted to the front of the body 4. Each of the front skids 12 can be raised and lowered relative to the body 4 by means of a respective hydraulic actuator 18 , and has a fixed first part 20 , and a second part 22 pivotably mounted to the first part 20 , the operation of which will be described in greater detail below with reference to Figures 3 and 12 to 14. A debris clearing member 24 is removably attached to the front skids 12 , and a towing cable 26 is attached to each end of the beam 16 to enable towing of the plough 2.

Material displacing means includes first material displacement members in the form of a pair of pivotable mouldboards 28 pivotably mounted to the body 4 to the rear of the plough share 8 and the fixed mouldboards 10 , and second material displacement members in the form of a pair of mouldboard extensions 30 pivotably mounted to the pivotable mouldboards 28. The pivotable mouldboards 28 are pivotable relative to the body 4 by means of hydraulic actuators 32 , and the mouldboard extensions 30 are pivotable relative to the pivotable mouldboards 28 by means of hydraulic actuators 34 (Figure 3 ) . The body 4 is further supported on the sea floor by a pair of rear skids 36 (Figure 3 ) .A pair of pivotable thrusters 38 , 40 enable control of the orientation of the plough 2 while it is supported by the lift wire before coming into contact with the sea bed .

The operation of the plough 2 in a debris clearing and first pass trenching mode will now be described . Referring to Figure 5 , in a launch/recovery

configuration, the front skids 12 of the plough 2, together with the debris clearing member 24 , are pivoted relative to the body 4 into a generally vertical orientation by means of hydraulic actuators 42 connected between the front skids 12 and the links 14 , to reduce the overall length of the plough 2. The lateral width of the plough 2 is also reduced by pivoting the mouldboard extensions 30 by means of hydraulic actuators 34 to that the mouldboard extensions are arranged in an orientation 3 OA (Figure 4 ) generally parallel to a longitudinal axis 44 of the plough 2.

The plough 2 is then lowered from the surface vessel to the seabed, and as it approaches the seabed, the front skids 12 together with debris clearing member 24 are lowered to a generally horizontal orientation as shown in Figure 6 by means of the hydraulic actuators 42. The mouldboard

extensions 30 are also pivoted relative to the pivotable mouldboards 28 by means of hydraulic actuators 34 to take up the orientation 30B shown in Figure 4 , so that a front surface 46 of each pivotable mouldboard 28 generally forms a continuous surface with a front surface 48 of the

corresponding mouldboard extension 30. The pivotable

mouldboards are pivoted relative to the body 4 by means of hydraulic actuators 32 so that the pivotable mouldboards 28 and mouldboard extensions 30 are arranged at an angle a to the longitudinal axis 44 , as shown in Figure 4. When the plough 2 , now in the configuration shown in

Figure 6, rests on seabed 50 , the front skids 12 and debris clearing member 24 are raised relative to the plough share 8 by means of hydraulic actuators 18 , so that the plough share 8 penetrates the seabed 50 when the plough 2 is towed forwards , as shown in Figure 7,

The surface vessel (not shown) then tows the plough 2 via towing cable 26, and debris located forwards of the front skids 12 is displaced out of the path of the front skids 12 by the debris clearing member 24. In addition, debris is displaced laterally of the plough 2 by pivotable mouldboards 28 and mouldboard extensions 30 to form first debris heaps 52 on both sides of the plough 2. On completion of the first pass cutting of the trench by the lough 2 in the configuration shown in Figure 7 , the plough 2 is raised from the seabed 50 , returned to its launch/recovery configuration shown in Figure 5 , and is then recovered to the surface vessel where the debris clearing member 24 is removed from the front skids 12. The mouldboard extensions 30 are either removed from the pivotable

mouldboards 28 or are pivoted relative to the pivotable mouldboards 28 by means of hydraulic actuators 34 so that they are located behind the pivotable mouldboards 28. The plough is then in the configuration shown in Figure 9 and is returned to the seabed 50 to the trench formed in the first pass trenching operation described above . As the plough approaches the seabed 50 , the front skids 12 are rotated to a generally horizontal orientation by means of the hydraulic actuators 42 to provide the configuration shown in Figure 10. The front skids 12 are then raised relative to the plough share 8 by means of hydraulic

actuators 18 so that the plough share 8 can penetrate the seabed 50 to a greater depth than in the conf iguration shown in Figure 1 to enable the plough 2 to operate in its full depth trenching mode as shown in Figures 2 and 11.

The operation of the plough 2 in a full depth trenching mode will now be described.

The plough 2 is towed by the surface vessel so that second pass cutting of the trench is carried out in which the plough 8 share cuts the trench to its full depth. Soil displaced from the trench by the plough share 8 is displaced laterally away from the trench by fixed mouldboards 10 and then by pivotable mouldboards 28 to form second debris heaps 54 (Figure 8 ) on both sides of the plough 2. Because the mouldboard extensions 30 do not contribute to material displacement from the trench in the full depth trenching mode, the second debris heaps 54 are located laterally inwards of the first debris heaps 52. At the same time, a cable (not shown) to be installed in the trench is guided into the trench.→¾a-^»ieH*b&ele~¾#i—4 .

On completion of the full depth trenching process , the plough 2 is returned to its launch/recovery configuration as shown in Figure 9 and then recovered to the surface vessel for reattachment of the mouldboard extensions 30 , if they have been removed for the full depth trenching mode, and for reconfiguration of the plough 2 to its backfilling mode as shown in Figure 3. The mouldboard extensions 30 are arranged so that the front surface 48 of each mouldboard extension 30 forms a generally continuous surface with the front surface 46 of the corresponding pivotable mouldboard 28 , and the pivotable mouldboards 28 are pivoted forwards of the body 4 by means of hydraulic actuators 32 so that the pivotable mouldboards 28 and mouldboard extensions 30 are arranged at an angle β to the longitudinal axis 44 of the plough 2, to take up the orientation 28A as shown in Figure 12. The angle β shown in Figure 12 is smaller than the angle a shown in Figure 4. Restraining members 56 prevent the pivotable mouldboards 28 from pivoting outwards relative to the body 4 beyond the orientation 28B shown in Figure 12. The operation of the plough 2 in a backfilling mode will now be described .

The front skids 12 are pivoted to a generally vertical orientation as shown in Figure 13 by means of hydraulic actuators 42 to place the plough in its launch/recovery configuration . The plough 2 is then returned to the seabed in its launch/recovery configuration as shown in Figure 13 , and shortly before arrival at the seabed 50 , the front skids 12 are lowered to a generally horizontal orientation by means of the hydraulic actuators 42 to bring the lough 2 into a landing configuration . The second parts 22 of the front skids 12 are then pivoted relative to the first parts 20 to bring the front skids 12 into the configuration shown in Figure 3. The front skids 12 are then located in the top of the trench so that the first parts 20 engage the seabed 50 adjacent the trench and the second parts 22 engage the upper parts of the sidewalls of the trench to more reliably locate the skids 12 in engagement with the trench as the lough 2 is towed forwards .

The front skids 12 are then raised relative to the body 4 by means of hydraulic actuators 18 to bring the plough into the configuration shown in Figure 14 so that the plough share 8 does not enter the trench to its full depth . The plough 2, supported by the front skids 12 and rear skids 36, is then towed along the trench containing the cable and the pivotable mouldboards 28 together with mouldboard extensions 30 displace relatively finer soil located in the second debris heaps 54 (Figure 12) into the trench to bury the cable in the trench, while leaving the first debris heaps 52 , containing relatively coarser soil, generally undisturbed, since the mouldboard extensions 30 extend laterally outwards from the plough 2 to a lesser extent in the backfilling mode of Figure 3 than in the debris clearing mode of Figure 1 , because the angle β is smaller than the angle a. It will be appreciated by persons skilled in the art that the above embodiment has 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 .