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A plough for excavating a subsea channel and a sea-going vessel comprising a plough. Field of the Invention

This invention relates to subsea seabed trenching or ploughing, in particular to apparatus and a method for ploughing the seabed so that cables or oil or gas pipelines may be buried below the surface of the seabed. Background of the Invention

UK Legislation requires that oil and gas pipelines laid on the seabed must be buried below the surface of the seabed if the diameter of the pipe is less than approximately 40cm.

It is also desirable to bury offshore subsea oil or gas pipelines because the burial of the pipelines underneath spoil provides the pipeline with protection against damage which can be caused by fishing boat dredges as well by anchor cables. Furthermore, for oil or gas pipelines, the oil or gas that is pumped through pipelines is usually much hotter than the ambient temperature of the seabed as well as the seawater near the seabed. This causes the pipeline to expand and can lead buckling of the pipeline in localised areas. Burying the pipeline provides support for the pipeline and helps to avoid buckling. Burial of pipelines also has the advantage that the pipeline is thermally insulated from the sea environment.

It is known to bury pipelines and cables using mechanical trenchers and deepwater trenching vehicles. Such systems usually operate remotely on the seabed, and are controlled by a support vessel. The mechanical trenchers and deepwater trenching vehicles are usually large and can weigh up to 180 Tonnes. Furthermore, such trenching vehicles are expensive and can cost up to £10 million. Since the trenching vehicles are remotely operated from the vessel, hydraulics and electrical power and monitoring systems are required to control the direction of the vehicle and also to provide monitoring of the trenching vehicles from the support vessels. This technology is complicated, and is particularly prone to breakdown in the harsh sea environment. Furthermore, expensive support vessels are required which are capable of lifting heavy trenching vehicles. This leads to high mobilisation costs of the support vessel and trenching system, and are heavy and complex to install on ships.

Moreover, such trenching systems and support vessels are not available globally, and transporting support vessels and trenching systems to remote parts of the world is very expensive. Summary of the Invention According to a first embodiment of the present invention, there is provided a plough for excavating a subsea channel comprising an anchor and a spacing arm, the anchor comprising a shank and one or more blades or shares rigidly connected to the distal end of the shank wherein the spacing arm is pivotally connected to the proximal end of the shank and which, in use, maintains the proximal end of the shank at a substantially constant distance from the ploughing surface, thereby controlling the depth of the trench cut in the ploughing surface by the blade(s) as the plough travels over the ploughing surface.

According to a second embodiment of the present invention, there is provided a method of modifying an anchor into a plough, the anchor comprising a shank and one or more blade(s) rigidly connected to the distal end of the shank, the method comprising pivotally connecting a spacing arm to the proximal end of the shank of the anchor such that the spacing arm, when in use, maintains the proximal end of the shank at a substantially constant distance above the ploughing surface, thereby controlling the depth of the trench cut in the ploughing surface by the blade(s) as the plough travels over the ploughing surface. This embodiment has the advantage that an anchor readily available close to where the trenching operation is to be performed may be modified into a plough, without the high mobilisation costs of transporting trenching equipment to where the pipeline or cable is being laid. For example, for trenching in North West Australia, anchors available in Singapore may be converted into a plough without the need to transport the trenching equipment from the United Kingdom.

According to a third embodiment of the present invention, there is provided a sea-going vessel comprising: a plough, propulsion means for moving and controlling the direction of the plough and the vessel; and means for connecting the vessel to the plough, the plough comprising a modified anchor and a spacing arm, the anchor comprising a shank and one or more blade(s) rigidly connected to the distal end of the shank wherein the spacing arm is pivotally connected to the proximal end of the shank and which, in use, maintains the proximal end of the shank at a constant distance above the ploughing

surface, thereby controlling the depth of the trench cut in the ploughing surface by the blade(s) as the plough travels over the ploughing surface.

Embodiments of the invention provide an improved plough which may excavate trenches in which fibre optics cables, electrical cables, pipes and hoses may be laid. In addition, bundles of any types of cables, known as umbilical cables may also be laid in these trenches as well as piggyback lines

Piggyback lines are small diameter pipelines piggybacked onto a carrier pipe. The carrier pipe can carry oil or gas or water, the piggyback usually carries ethanol or some other chemical for re-injecting into the well or for corrosion inhibition. The lines are piggy backed so as to be able to be laid at the same time as the carrier pipe. Brief Description of the Drawings

The invention will now be described in detail by way of example with reference to the accompanying drawings, in which:

Figure 1 is photograph showing an unmodified anchor;

Figure 2 is a side view of an anchor modified according to an embodiment of the invention;

Figure 3 is a view of an embodiment of the invention from above in which the skid further comprises a cut-away rear section;

Figure 4 is a perspective view of another embodiment of the invention further including mould boards;

Figure 5 shows a perspective view of a furrow (trench) cut using an embodiment of the invention; Figure 6 shows another perspective view of a furrow (trench) cut using an embodiment of the invention;

Figure 7 shows another perspective view of a furrow (trench) cut using an embodiment of the invention;

Figure 8 shows a perspective view of an embodiment of the invention in which the mould boards further comprise skids;

Figure 9 shows a front view of the plough in use;

Figure 10 shows a cross section the front skid in an embodiment in which the front skid is substantially v-shaped in cross section;

Figure 11 is a schematic diagram of a perspective view of an embodiment of the invention further comprising a fore ripper and stability pole;

Figure 12 is a schematic diagram of an embodiment of the invention comprising a sea-going vessel towing a plough according to another embodiment of the invention; and

Figure 13 is a schematic diagram of an embodiment of the invention in which the sea-going vessel further comprising a remote observation vehicle;

Figure 14 is a schematic diagram of a further embodiment of the invention in which the sea-going vessel further comprises a remote observation vehicle;

Figure 15 is a schematic diagram of a further embodiment of the invention in which the sea-going vessel further comprises a remote observation vehicle; and

Figure 16 is a schematic diagram of an embodiment of the invention in further comprising a second sea-going vessel connected to the remote observation vehicle.

Detailed Description of the Preferred Embodiments of the Invention

An unmodified, conventional anchor 101 , a drag embedment anchor of the Plastimo plough anchor type, is shown in figure 1. It comprises a shank 103 with one or more blades(s) 105 rigidly connected to the distal end of the shank. The blade(s) are positioned on the shank such that the pointed end of one of the blades(s) penetrates the surface of the seabed as the anchor is pulled across the seabed. The blade(s) 105 are also known as share(s). The anchor shown in figure 1 has one or more flukes 102, which are enlarged parts of the blade(s) which are broadened and flattened out so that the anchor has greater resistance to pulling forces when it is embedded in the seabed. As the anchor fully or partially penetrates the seabed, the anchor resists pulling forces by the resistance of the soil in front and above the anchor. Drag embedment anchors such as that shown in figure 1 primarily are designed and used to resist horizontal tension forces, that is forces which are applied substantially parallel to the seabed. They can however resist some vertical forces, but to a lesser extent. An eye 107 is provided at the proximal end of the shank which allows the anchor to be connected to a rope or chain which is, in use connected to a seagoing vessel (not shown).

Figure 2 shows a side view of the plough 104 according to an embodiment of the invention. The proximal end of the shank of the anchor 101 is pivotally connected to a spacing arm 201. It is not necessary that the anchor 101

must have flukes 102 on the blade(s) (share(s)) 105. It is sufficient that the blade(s) fully or partially embed into the seabed as the plough is towed across the seabed. However, the flukes 102 improve the width and depth of the trench cut in the seabed because they move the spoil further away from the trench cut by the blade(s).

In the embodiment shown in figure 2, the proximal end of the shank is pivotally connected to the spacing arm using a nut and bolt arrangement, however, any connecting means which allows the pivotal movement of the shank relative to the spacing arm 201 may be used. Preferably, the spacing arm is made out of high quality steel, for example roller quenched and tempered steel or from British Standard 4360G 5OD steel.

Preferably, the shank 103 has a substantially diamond shaped cross section in order to permit upward movement of seabed material as the plough is drawn across the seabed. A connector 205, for example, a towing cable, chain or rope is attached

207 to the proximal end of the shank 103 or to the spacing arm so that the plough 104 can be towed across the ploughing surface by the sea-going vessel (not shown).

The embodiment shown in figure 2 has a number holes 203 spaced along the length of the spacing arm 201. The holes 203 allow the proximal end of the shank to be pivotally connected to the spacing arm 201 so that distance of the proximal end of the shank is maintained at a substantially constant distance from the ploughing surface as the plough is towed across the seabed. Alternatively, one or more pup pieces (spacing pieces) can be bolted or welded together to form a spacing element. The pup pieces allow the distance of the pivotal connection point 211 above the seabed to be varied by rigidly connecting a number of these spacing units or pup pieces together by welding or otherwise.

The spacing arm 201 can be connected to the towing cable, chain or rope at a point calculated to give the correct depth of anchor 101 penetration in the seabed allowing the shank 103 to be shorter. This provides improved handling when deploying the plough from a ship equipped with a stem roller.

The position of the pivotal connection point 211 of the spacing arm 201 and the shank 103 is adjusted such that the tip of the anchor is in contact with the ploughing surface, while the rear end portion of the blade 209 is not in contact with the ploughing surface. Then, as the plough 104 is towed across the

ploughing surface, the self-weight of the anchor causes the tip of one of the blade(s) 105 of the anchor to embed in the ploughing surface. The towing of the plough causes seabed material to move up the side of the blade 105, and is thereby moved away from the line along which the plough is drawn. As this occurs, the self-weight of the anchor and the forces cause by the excavation of the soil, cause the anchor to further rotate about the connection point 211 of the shank 103 and the spacing arm 201.

As the anchor becomes more deeply embedded in the ploughing surface, the angle the angle α between the blade 105 and (the originally flat) ploughing surface decreases, and eventually, the angle α is so reduced that the plough does not become more deeply embedded in the ploughing surface as the plough is drawn across the ploughing surface. In this way the depth of the trench cut by the plough may be controlled so that when the plough reaches its maximum embedment depth in the ploughing surface as it is towed across the surface, the depth of trench cut is substantially constant as the plough is further towed across the ploughing surface. Therefore, the depth of trench cut can be controlled so that it is shallower than would be obtained if the anchor alone were towed across the seabed without the spacing arm 201. Without the skid the anchor penetrates deeper and deeper into the seabed. Two theories have been used to determine the towing force F required to pull ploughs according to embodiments of the invention. These are the Plough Theory for Sand Soils and the Plough Theory for Clay Soils.

Plough Theory - Sand

The tow force, F, required to pull a plough on sand soils is given by: F = W χ tan£ + K ₁ D + K ₂ χ Dχ ι/ (1.1) where F is the tow force in Tonnes, W, is the submerged weight of the plough (Tonnes), δ is the soil/steel interface friction angle (degrees), D is the trench depth (metres), K^ is a share configuration coefficient (Tonnes/m ³ ), K ₂ is a dynamic factor, a function of grading and relative density and v is the ploughing velocity (metres per hour). Plough Theory - Clay

The tow force, F, required to pull a plough on clay soils is given by:

F =F _W +C _C x S _υ x D x (1 + C _d v) (1.2) where Fvv is the adhesion on the underside of the skids and share to soil when ploughing (Tonnes) Cc is a coefficient similar to the bearing capacity, S _u is the un drained shear strength (Tonnes per m ² ), D is the trench depth (metres) and C _d is the coefficient relating the strength of the soil at normal shear strain testing rates to the strength of the soil at ploughing rates of strain and v is the plough velocity (metres per hour).

Using these equations, the towing force required to pull conventional pipeline ploughs and ploughs according to embodiments of the invention has been calculated and the results are shown in table 1.

Table 1 : Calculated towing forces for different speeds of pipeline ploughs and ploughs according to embodiments of the invention for a trench of 1.2 metres in depth for a range of soil types.

These results show that embodiments of the invention are able to provide improved ploughs which require less tow force in order to plough a trench of 1.2m in depth, than the tow forces required with pipeline ploughs.

These calculations also show that the force required to tow the plough across the seabed is dependent upon the penetration depth of the anchor in the ploughing surface. Therefore controlling the depth of trench cut in the ploughing surface as the plough it towed across the seabed allows the towing force needed to be controlled and adjusted according to the degree of penetration of the plough by adjusting the height of the connection point 211 from the seabed. The towing force required can therefore also be controlled by selecting the appropriate connection point 211 of the shank and the spacing arm or by using a different pup piece as previously described.

The initial angle α ₀ between the blade(s) 105 and the ploughing surface when the plough is resting on the ploughing surface, without any embedment of the tip of the anchor in the seabed (before towing) may be adjusted by selecting a different hole 203 along the length of the spacing arm 201 to pivotally connect the shank 103 to the spacing arm 201.

The choice of angle α depends on the topology of the seabed surface being ploughed as well as the type of material being ploughed, on the depth of trench required and the maximum towing force.

For example, if the soil at the seabed is particularly soft for example very soft clays or sands, so that there is potentially a large amount of penetration of the plough in the seabed, then the choice of angle α _o will be smaller. Then the maximum depth of trench cut by the plough is shallower, so that the force required to tow the plough is less than towing an unmodified anchor across the seabed. The choice of angle α ₀ for soft soils is approximately 10-15 degrees.

On the other hand, if the soil at the seabed is particularly hard, so that there is potentially little penetration of the plough in the seabed, then the choice of angle αo will be larger so the maximum depth of trench cut is deeper than it would otherwise be. For harder materials such as hard clay and sands, the choice of ploughing angle is larger for example, approximately 20 degrees or more so that the tip of the plough embeds as the plough is towed across the ploughing surface.

Selecting a hole 203 further away from the ploughing surface reduces the angle α ₀₎ while selecting a hole closer to the ploughing surface increases the angle α ₀ , with the maximum angle α _M being when the connection point 211 of the shank 103 and the spacing arm 201 is as close to the ploughing surface as possible.

In a further embodiment not shown in the drawings, the series of holes 203 are joined together to form a substantially oblong slot. The spacing arm 201 is then preferably connected to the distal end of the shank 103 using a nut and bold arrangement, and this allows the position of the connection 211 of the spacing arm on the shank 103 to be continuously adjustable along the length of the spacing arm. Other connection arrangements known to the skilled person, such as pup pieces can be used provided they pivotally connect the proximal end of the shank 103 on the spacing arm so that the connection point 211 is maintained at a constant distance from the ploughing surface as the plough is towed over the ploughing surface.

The embodiment shown in figure 2 comprises a skid 213. In this embodiment, the front most portion of the skid 214 is curved upwards away from the ploughing surface. Alternatively, the skid may be substantially flat.

In a further embodiment shown in figure 3, the rear portion of the skid may comprise a cut away section 303. Figure 3 shows an embodiment of the invention from above. The cut away section is of a substantially parabolic profile. Alternatively, the cut away section of the skid can be substantially delta shaped or chevron shape. This embodiment has the advantage that it allows the soil to be removed without hindrance by the skid. This allows the overall length of the plough to be shorter and reduces tow forces and prevents the necessity for multiple skids joined by a bridge which saves weight.

Preferably, the skid is made out of high quality steel, for example roller quenched and tempered steel or from British Standard 4360G 5OD steel. However, it is not necessary that all embodiments must include a skid. It is sufficient that the spacing arm does not embed below the ploughing surface. For some ploughing surfaces which are sufficiently hard, the spacing arm can be in direct contact with the ploughing surface and does not embed because the ploughing surface is sufficiently hard. For other ploughing surfaces that are softer, it may be necessary to use a skid so that the spacing arm does not penetrate and embed in to the surface as the plough is towed across the

ploughing surface. However, other embodiments may use wheel arrangements or caterpillar track arrangements to prevent the spacing arm from embedding in the ploughing surface. Alternatively, a pole fixedly attached to the spacing arm, which is substantially perpendicular to the spacing arm may be used to prevent the spacing arm from penetrating the ploughing surface. Attaching the spacing arm to a pole distributes the weight of the spacing arm and the weight of the anchor transmitted onto the spacing arm over a greater area and therefore prevents the spacing arm from penetrating into and becoming embedded in the ploughing surface. In this way the proximal end of the shank is maintained at a substantially constant distance from the ploughing surface, thereby controlling the depth of the trench in the ploughing surface by the blade(s) as the plough travels over the ploughing surface.

Preferably, the spacing arm 201 is arranged to prevent the shank of the plough from penetrating the ploughing surface. This has the advantage that a reduced towing force is required because of reduced friction since the shank does not become embedded in the ploughing surface and so there are no or much reduced frictional forces between the ploughing surface and the shank.

Preferably, the spacing arm 201 further comprises a skid 213. Preferably, position of the connection 215 of the spacing arm 201 on the skid 213 is adjustable along the length of the skid. In the embodiment shown in figure 2, the adjustable connection is formed by a series of holes 217 positioned at different positions along the length of the skid. The skid 213 is connected to the spacing arm using a nut and bolt arrangement which passes through one of the holes 217 in the skid as well as a hole in one end of the spacing arm 201. Any other connection means can be used. In some embodiments, the position of the connection point 215 is permanently fixed, for example using, welding or any other fixing means.

In a further embodiment, the series of holes are joined together to form a substantially oblong slot. The spacing arm 201 is then preferably connected to the skid 213 using a nut and bold arrangement, and this allows the position of the connection of the spacing arm on the skid to be continuously adjustable along the length of the skid. Other connection arrangements known to the skilled person can be used provided they fix the connection point 215 as the plough is towed over the ploughing surface.

The Connection point 215 can be made at a number of points along the length of the skid and so doing varies the action of the plough by increasing the distance from the plough tip to the skid. This makes the plough more stable in variable soils. It is not essential for the skid to be directly connected to the rigid arm. Embodiments of the invention may comprise the rigid arm connected to a chain, rope or wire link, which is in turn connected to the skid.

Figure 4 is a perspective view of an embodiment further comprising a pair of mould boards 301 rigidly attached to the flukes 102 for moving spoil away from the trench cut by the plough. This embodiment has the advantage that the mould boards move the spoil further away from the trench cut in the ploughing surface and so maximises the amount of material that is removed from the trench and reduces the amount of material falling back or transported by seabed currents into the trench.

This is particularly advantageous for multipass operations (described in further detail below) in which a plough according to embodiments of the invention is repeatedly passed along a subsea trench so that deeper trenches can be cut than is possible with a single pass operation. In this embodiment, the mould boards are set for a multipass operation such that they push the spoil sufficiently away from the trench such that the trench can be re-ploughed without spoil re- entering the already ploughed trench.

Figures 5, 6 and 7 are various schematic perspective views of trenches cut in the ploughing surface by an embodiment of the invention comprising a plough with two mould boards.

Figure 8 shows an embodiment of the invention in which each mould board further comprises a skid 701. This embodiment has the advantage that the mould boards more easily pass over the ploughing surface and so a smaller towing force is needed to cut the trench. Furthermore, a cleaner trench is cut by embodiments of the invention comprising skids mounted on the mould boards because the skids mounted on the mould boards flatten the ploughing surface neighbouring the trench so that less spoil is accidentally pushed back into the trench.

Figure 9 is a schematic view of an embodiment of the invention from the front in use, and is shown without the ploughing surface in order to see the position of the anchor when the plough is in use. It can be seen that the anchor

101 is below the ploughing surface, thereby removing material from the ploughing surface and creating a trench in the ploughing surface.

Figure 10 is a schematic view of the front skid of an embodiment of the invention in which the front skid has a substantially v-shaped cross section 901. Having a front skid with a v-shaped cross section allows the plough to follow the trench cut by another plough a number of times in what is known as a multipass trench ploughing. This allows for deeper trenches to be cut than is possible in a single ploughing operation and the v-shaped front skid guides the direction of the plough along the pre-cut trench, thereby controlling the direction of the plough. Furthermore, it also allows for trenches to be cut in hard ploughing surfaces by repeating the ploughing operation a number of times using a multipass operation.

In this way, embodiments of the invention can be used in multipass operations and constantly clears spoil from the trench as it is drawn across the seabed. Embodiments of the invention comprise a skid with a substantially v- shaped cross section to ensure good location in the previous trench.

Furthermore, embodiments of the invention join trenches together from either direction and can be deployed in an existing trench.

Figure 11 is a schematic perspective view of an embodiment of the invention further comprising a fore ripper 1001 and a stability pole 1003. The fore ripper comprises an elongated portion 1111 which, in use, digs into the seabed.

In the embodiment shown in figure11, the fore ripper is of a substantially rectangular shape when viewed along the line of the trench, but is substantially triangular in cross section when viewed from the side. This construction provides the fore ripper with the necessary strength to pre cut a trench. Two spikes 1113 are provided at the extremity of the rectangular portion of the fore ripper closest to the seabed. The rectangular portion of the fore ripper is rigidly connected a rear pin 1115, which may comprise the stability pole 1003. The rear pin 1115 allows the fore ripper to be connected to the towing cable chain or rope via a series of chain links. The rear pin 1115 is rigidly connected to a front pin 1117 by one or more ripper connecting arms 1119. The ripper connecting arms are connected to the towing cable chain or rope by a front pin (as well as the rear pin). This configuration prevents the fore ripper from being pushed away from its upright position as the plough is towed by the towing cable, chain or rope. The fore ripper is located in front of the skid 213 or spacing arm 201. Preferably, the fore ripper is double sided so that there are substantially elongated portions

projecting both below and above the plane in which the chain lies. This embodiment has the advantage that which ever way round the fore ripper lands, at least one of the two elongated portions rips into the ploughing surface.

The fore ripper 1001 pre cuts a small trench in the ploughing surface and is particularly useful when ploughing dense sand and hard surfaces because it breaks up the surface allowing the anchor to more easily penetrate the ploughing surface and therefore this embodiment allows for deeper and wider trenches to be cut than is possible with conventional ploughs. It is possible to use a forecutter or ripper in this arrangement as there this is a pre-cut operation and there is no product on the seabed that can be damaged.

Figure 12 is a schematic diagram of an embodiment of the invention in which a seagoing vessel 1101 comprises a plough. The seagoing vessel is connected to the plough 104 by a flexible connector 1103 such as a cable, chain or rope. The seagoing vessel 1101 comprises propulsion means 1105 for providing a towing force to the connector 1103 and for also controlling the direction of the towing force. The vessel 1101 is preferably equipped with a sonar system 1107. The sonar system can detect previous trenches or pipelines or other seabed features so that the ploughing system can be correctly positioned. For example, it may be desirable to re-plough an existing trench using the multipass operation. Use of the sonar system allows the plough 104 to be correctly positioned over an existing trench or trench. Furthermore, the ploughing operation can be monitored by using the sonar system, which shows successful ploughing operations. As in all embodiments, preferably, the plough further comprises a fore ripper 1001. Preferably, the vessel 1001 also comprises conventional surface positioning systems such as Global Positioning Systems

(GPS) which allows the absolute position of the ship to be precisely determined in three dimensions.

Figure 13 is a schematic diagram of an embodiment of the invention in which the seagoing vessel 1101 further comprises a reference platform or remote operating vehicle (ROV) 1201 to monitor, for example by using an onboard camera, visual signals showing the trench or furrow and its surroundings. The ROV is attached to the portion of the plough 104 not in contact with the seabed, and comprises a cable 1203 to send electrical or optical signals back to the seagoing vessel 1101 , where the images are converted using techniques known to those skilled in the art, into images which are displayed on a monitor. Using

this, the quality of the trench can be remotely monitored and the direction of the seagoing vessel and therefore the direction of the towing force transmitted by the connector 1103 can be controlled appropriately. The ROV may also comprise propulsion means so that it can help to control the direction of the plough, so that it is not just the direction of the towing force that controls the position of the trench cut in the seabed. The ROV can act as a portable 'pluggable' power pack. In embodiments comprising a reference platform 1201 , the reference platform is suspended under the vessel 1101 and the reference platform 1201 does not have the ability to independently control its position. Preferably, the vessel 1101 also comprises conventional surface positioning systems such as Global Positioning Systems (GPS) which allows the absolute position of the vessel 1101 to be precisely determined in three dimensions. In a further embodiment, the plough may comprise position determining means such as Global Positioning System (GPS) which allows the absolute position of the plough 104 to be determined in three dimensions.

Figure 14 is a schematic diagram of an embodiment of the invention in which the seagoing vessel 1101 also comprises a remote operating vehicle (ROV) 1201 to monitor the position of the plough, for example by using an onboard high-resolution sonar device 1305. However, in this embodiment, the ROV, is not attached to the plough, and is separate from the plough and is used for remote monitoring of the plough and its surroundings. The ROV is connected to the vessel 1101 using a tether 1309, for example a chain or cable which allows the ROV to be deployed from the vessel 1101. Preferably a tether management system (TMS) 1311 decouples the motion of the ship from the ROV. In this embodiment, the ROV has propulsion means so that it can get navigate around the plough and its environment so that it can remotely monitor the cut trench and neighbouring seabed topology. The subsea plough 104 is deployed and tracked from the same vessel. Data, for example positional data is sent from the ROV to the vessel 1101 via optical or electrical cables incorporated in the tether 1309 connecting the ROV to the vessel 1101.

Hydrophonic Position Reference (HPR) beacons 1303 are mounted on the vessel 1101 , on the ROV and on the plough 104 and allow high-resolution sonar equipment 1305 mounted on the ROV and vessel 1101 to determine the position of the ROV 1201 and the plough 104. This allows the position of the plough 104 to be precisely monitored and controlled via the direction of the

towing force on the tether 1103. Preferably, the plough has a drogue anchor 1306 attached to it. The drogue anchor 1306 controls the speed of the anchor as it is deployed from the sea vessel 1101 and also provides directional stability to the plough. The drogue is released after plough is in position to plough or ready to commence ploughing, and it can be released by acoustic coupling, by breaking a weak link tether as it is towed, or by ROV cutting the tether. A sonar reflector 1307 is preferably mounted on the drogue anchor so that the position of the anchor can be detected by the high-resolution sonar equipment.

Alternatively, the ROV can be connected by a tether to an additional support vessel (not shown). The positional and surrounding topological data of the plough and ROV determined by the high resolution sonar mounted on the ROV and additional support vessel is transmitted to the additional support vessel via electrical or optical cables incorporated into the tether connecting the ROV to the additional support vessel. In this case, data gathered by the ROV is transmitted from the additional support vehicle via a wireless data link to the vessel 1101. The ROV is not attached to the plough and remotely operates using propulsion means.

Alternatively, the ROV can be replaced by a sensor platform comprising a high resolution sonar and acoustic beacon. The sensor platform does not have propulsion means and is simply a monitoring station which is tethered to the additional support vessel, submerged in the sea. An acoustic beacon mounted on the plough allows the high-resolution sonar mounted on the sensor platform to determine the position of the plough. This data is fed to the additional support vessel via an electrical or optical cable incorporated into the tether attaching the additional support vessel to the sensor platform. Data is transmitted to the support ship 1101 via a wireless link. The sensor platform can also be equipped with an acoustic beacon so its position can be monitored by one of the support vessels.

Figure 15 shows an embodiment of the invention similar to that of figure 14, except that a bi-directional acoustic data link is established between the ROV

1201 and the plough 104 using bi-directional acoustic data link transmitters and receivers 1401 mounted on the ROV and on the plough. Position data or / and plough status data are sent using the bi-directional acoustic data link. A sonar reflector 1307 is also mounted on the plough to allow the high-resolution sonar to detect the position of the plough. Also in this embodiment, the vessel 1101

preferably comprises conventional surface positioning systems such as Global Positioning Systems (GPS) which allows the absolute position of the vessel 1101 to be precisely determined in three dimensions. In a further embodiment, the plough may comprise position determining means such as Global Positioning System (GPS) which allows the absolute position of the plough 104 to be determined in three dimensions. Hydrophonic Position Reference (HPR) beacons 1303 are mounted on the vessel 1101 and on the ROV and allow high-resolution sonar equipment 1305 mounted on the ROV and vessel 1101 to determine the position of the ROV and the plough 104. This allows the position of the plough to be precisely monitored and also allows monitoring of the topology of the seabed surrounding the ROV.

In this embodiment, the position of the plough and information about the topology neighbouring the plough is sent from the ROV to the support vessel 1101 via electrical or optical cables embedded in the tether 1309 connecting the ROV 1201 to the vessel 1101, preferably via a TMS 1311.

Alternatively, the ROV can be replaced by a sensor platform comprising high-resolution sonar and HPR beacon and bi-directional acoustic data link transmitters and receivers mounted on the sensor platform (or on the vessel 1101) and on the plough. The sensor platform does not have propulsion means and is a submerged monitoring station. This data is fed to the additional support vessel via an electrical or optical cable incorporated into the tether attaching the additional support vessel to the sensor platform.

Alternatively, the sensor platform can be connected to an additional support vessel (not shown). Positional data and other data can be sent from the plough to the platform and to the additional support vessel via acoustic beacons mounted on the plough and on the sensor platform and on the additional support vessel. The data is then transmitted from the additional support vessel to the vessel 1101 via a wireless data link.

In this way, the position of the plough (both the distance astern and cross course), as well as the depth of the plough may be determined determined by the ROV sending acoustic signals to the receiver 1401 on the plough 104. Signals are then transmitted by the acoustic data link transmitter 1401 located on the plough back to the receiver 1401 located on the ROV 1201. This allows the position of the plough relative to the ROV to be determined. This position data

may be transmitted to the vessel via transmission means using electrical or optical cables incorporated into the tether 1309 or wirelessly to the vessel 1101.

Figure 16 is a schematic diagram of a further embodiment of the invention in which a separate survey sea going vessel 1301 is provided, and which is connected to the ROV 1201 (preferably via a TMS 1311 ) by a cable 1203 so that the monitoring signals can be sent to the survey vessel. The ROV is fixedly attached to the plough 104. The ROV comprises high-resolution sonar 1305 and acoustic beacons 1503 so the position of the ROV can be determined by sonar and so that the ROV can monitor conditions on the seabed or track a previously cut trench that is being deepened. Data is sent from the ROV to the survey vessel

1301 via electrical or optical cables incorporated in the tether 1203, connected to the survey vessel 1301 preferably via a Tether Management System 1311 , as previously described. Alternatively, data can be sent from the ROV to the additional support vessel via the acoustic beacons 1503 mounted on the ROV and on the additional support vessel 1301. Data is sent from the survey vessel

1301 to the seagoing vessel 1101 by data link transmitters and receivers 1501 mounted on the support vessel 1101 , and survey vessel 1301.

Once the subsea channel has been excavated to the required depth, the cable(s) or pipe(s)s can be laid in the excavated channel using conventional techniques known to the skilled person. Once this is complete, the spoil can be pushed back into the channel using techniques known to the skilled person or alternatively, the channel can be left open, with the action of sea currents covering the laid pipe(s) or cable(s) with- the excavated spoil. In this way, embodiments of the invention provide pre-cut trenching which is suitable for the laying and burial of cable(s) and pipeline(s).

Embodiments of the invention can use most types of anchor. Examples of anchors which may be used are Class C anchors such as Stevin, Stevefix, Stevmud, or Flipper Delta anchors with open crown hinge with short shanks and side stabilisers. Class D anchors may also be used, for example Danforth, LWT Moorfast-

Stato-Offdrill or Boss anchors with hinges and stabilisers and longer shanks than class C, also with stabilisers.

Since embodiments of the invention provide a method of modifying an anchor into a plough, it is preferable that the anchor can be easily obtainable from a source close to where the pipeline or cable is being buried. This minimises

mobilisation costs that would be incurred if conventional trenching vehicles were used.

The method of modifying an anchor into a plough comprises pivotally connecting a spacing arm to the proximal end of the shank of the anchor such that the spacing arm, when in use, maintains the proximal end of the shank at a substantially constant distance above the ploughing surface, thereby controlling the depth of the trench cut in the ploughing surface by the blade(s) as the plough travels over the ploughing surface.

Typically, conversion of anchors into ploughs according to embodiments of the invention takes about 8 weeks. This conversion time is much less than the time required to manufacture new trenching vehicles which are usually in excess of 26 weeks. Furthermore, such trenching vehicles require a large stock of spares which are required on the support vessel, and which can be difficult to obtain whereas conversion of anchors into ploughs according to embodiments of the invention requires mainly off the shelf components. The anchor needs to be towable so that a trench is ploughed as it is dragged across the seabed. A further advantage of embodiments of the invention is that they can be operated over the stern roller of a medium sized anchor handling vessel without the need for expensive handling systems. Embodiments of the invention can also be deployed from any vessel which can lift approximately 20-25 Tonnes. This means a wide choice of support vessels are available.

For the avoidance of doubt, individual embodiments of the invention may be used together in a single embodiment or on their own.