WO2016133385A1

WO2016133385A1 - Submersible vehicle for providing a trench in a subsea bottom

Info

Publication number: WO2016133385A1
Application number: PCT/NL2016/050109
Authority: WO
Inventors: Bruno Tack
Original assignee: Tideway B.V.
Priority date: 2015-02-18
Filing date: 2016-02-17
Publication date: 2016-08-25
Also published as: EP3259408A1; NL2014308B1

Abstract

A remotely operated submersible vehicle for preparing a trench in a subsea bottom is disclosed. The vehicle comprises a frame, hoisting and control cables for suspending the vehicle from a surface vessel, thrusters for manoeuvring the vehicle, and an elongated mechanical excavating tool extending across the width of the trench and connected to conveying means for carrying excavated seabed material to a position outside the trench. The excavating tool comprises an elongated support member that extends across the width of the trench and is provided with a number of cutter devices that are actively driven in a rotation direction around an axis extending substantially perpendicular to the longitudinal direction of the support member in the direction of the trench. A method of preparing a trench using the vehicle is also described.

Description

SUBMERSIBLE VEHICLE FOR PROVIDING A TRENCH IN A SUBSEA

BOTTOM

BACKGROUND OF THE INVENTION

The invention relates to a submersible vehicle, in particular to a remotely operated submersible vehicle, for entrenching elongate structures under water, such as pipelines or electrical cables for instance. The invention further relates to an assembly of a surface vessel and a submersible vehicle, as well as to a method of preparing a trench in a subsea bottom. The vehicle and method are particularly suitable for entrenching a sea bed at depths below 300 m, as well as a highly undulated seabed.

Submersed pipelines may for instance be used for transporting crude petroleum and gas from a drilling site over long distances to an onshore or offshore collection location. Electrical cables may be used to connect offshore wind farms to an onshore electricity network. To protect such elongate structures and prevent shifting due to current and/or a highly undulated seabed, pipelines are generally buried or entrenched beneath the seabed. In pre-laying methods, a trench is typically prepared before actual positioning of the elongate structure in the trench. In post-laying methods, an elongate structure is typically positioned on a seabed and then sunk into the seabed by for instance fluidising the seabed along the desired trajectory of the elongate structure.

Known devices for entrenching an elongate structure in a seabed typically involve a vehicle that is positioned onto the seabed and propels itself over the seabed by wheels or tracks for instance. Such vehicles are equipped with some kind of excavating means, such as high pressure water jets that aim at the seabed and fluidise and/or disrupt the seabed along the desired trajectory, or mechanical means such as a bucket.

Offshore wind farms are preferably located in rather windy and sometimes remote areas to generate as much electricity as possible. This means that optimum trajectories for the elongate structures, in this particular example electrical cables, may run through difficult subsea areas. The same holds for oil and gas fields which are increasingly discovered in rough and remote areas. An example is the site of the Ormen Lange field in Norway which is remotely located at enormous seabed depths of between 850 and 1100 meters under the rough Norwegian Sea. In addition to depth, the subsea terrain also presents difficulties and ice-cold currents flow through a highly undulated seabed having high peaks and low valleys with peak-to-valley depth differences easily exceeding 10 m. A further difficulty comprises the seabed material itself which comprises heavy compacted clay with undrained shear strength Su of over 20 kPa to more than 50 kPa.

It is an object of the present invention to provide an improved submersible vehicle and method for providing a trench in a subsea bottom and entrenching elongate structures in the seabed, in particular for seabed depths below 300 m and/or in highly undulated seabed bottoms.

BRIEF SUMMARY OF THE INVENTION To this end, a remotely operated submersible vehicle according to claim 1 is provided. The vehicle comprises a frame, hoisting and control cables for suspending the vehicle from a surface vessel, thrusters for manoeuvring the vehicle, an elongated mechanical excavating tool connecting to the vehicle by a hinged connection and extending across the width of the trench, and conveying means for carrying away excavated seabed material to a position outside the trench, wherein the excavating tool comprises an elongated support member that extends across the width of the trench and is provided with a number of cutter devices that are actively driven in a rotation direction around an axis extending substantially perpendicular to the longitudinal direction of the support member in the direction of the trench.

According to the invention, a method of preparing a trench in a subsea bottom is also offered, comprising the steps of lowering a submersible vehicle in accordance with the invention from a surface vessel towards a position close to the subsea bottom, manoeuvring the mechanical excavating tool of the vehicle through the subsea bottom along a trajectory, and conveying the excavated seabed material to a position outside the formed trench. The submersible vehicle is suspended from the vessel by hoisting cables and the mechanical excavating tool is manoeuvred through the subsea bottom by advancing the vessel and towing the submersible vehicle with the vessel. The invented vehicle therefore does not rest on the seabed, apart from the excavating tool that is connected to the vehicle by a hinged connection. The hinged connection between the vehicle frame and the mechanical excavating tool at least partly allows accommodating vertical movements of the vehicle frame induced by currents and heave of the vessel. Since the vehicle hangs relatively freely from the surface vessel and does not rest on the seabed, it is particularly suitable for entrenching undulating and rough sea beds.

With 'substantial' or 'substantially' is meant in the context of the present application at least 70% of the indicated property, more preferably at least 80% of the indicated property, and most preferably at least 90% of the indicated property.

In a preferred embodiment of the invention, a submersible vehicle is provided wherein two adjacent cutter devices rotate in opposite directions. This embodiment is beneficial in that is substantially limits any lateral and longitudinal reaction forces on the remotely operated submersible vehicle. Lateral reaction forces act in a width direction of the formed trench, whereas longitudinal reaction forces act in a direction along the formed trench. This allows to cut harder bottoms with a substantially free hanging remotely operated vehicle. The thrusters provided on the submersible vehicle are primarily used to head the vehicle in the right direction and do not need to counteract large reaction forces induced by manoeuvring the excavating tool through the sea bed.

In another embodiment, the number of cutter devices is even, and in a more preferred embodiment the number of cutting devices along the support member is from 2 to 8, more preferably from 4 to 6. A further improved embodiment of the invention provides a submersible vehicle wherein the excavating tool comprises a propelling member extending in the

longitudinal direction of the support member and provided with flat penetrating tools that extend in a plane perpendicular to the longitudinal direction of the support member. The propelling roll member is preferably provided upstream from the support member for the cutting devices, and is in another embodiment hydraulically driven. The propelling member is preferably cylindrical and takes the form of a propelling roll member. In use, the excavating tool is lowered onto the sea bed such that the propelling roll member rests on the sea bed with a circumferential surface thereof. The penetrating tools penetrate the soil. A hydraulic motor preferably rotates the propelling roll member and advances the excavating tool over the sea bed by frictional forces incurred between the circumferential surface of the roll member and the sea bed, and by tangential forces generated by the flat penetrating tools in the sea bed. The flat penetrating tools further at least partly prevent movement of the excavating tool in a sideways direction, i.e. in a direction about parallel to the longitudinal direction of the support member, because of the passive reaction forces of the soil on the penetrating tools. Since the propelling member rests on the sea bed, vertical movements of the remotely operated vehicle will have a negligible effect on the thickness of the bottom material layer cut by the cutting devices.

In an embodiment, the excavating tool of the vehicle comprises an elongated support member that extends across the width of the trench and is provided with a number of cutter devices. Another aspect of the invention proposes a submersible vehicle wherein the support member comprises a beam or plate provided with a number of shafts that extend perpendicular to the longitudinal direction of the beam, each shaft being rotatably driven around an axis extending in a longitudinal direction of the shaft. In operation, the shafts are directed towards the sub sea bottom. This embodiment is preferred in even more firm sea bed bottoms.

To further improve on carrying away excavated seabed material, an embodiment is provided wherein the cutting devices comprise a number of cutting tools provided on a cage-like body arranged around each axis and connected thereto. A suitable cage-like body is about cylindrical and comprises a number of side-posts extending from a bottom plane comprising bottom members, arranged in a cross-like manner for instance. The side-posts and bottom members are provided with cutting tools such as teeth.

Since the vehicle according to the invention is free-floating, reaction forces incurred by manoeuvring the excavating tool through the sea bed are preferably kept as low as possible in order for the vehicle to keep its track along the envisaged trajectory. A preferred embodiment thereto provides a submersible vehicle wherein two adjacent shafts rotate in different directions. In such an embodiment, reaction forces incurred by the rotating excavating tools substantially cancel each other. In a further preferred embodiment, the number of shafts is even, and each shaft has a counter rotating counterpart shaft.

The total number of cutting tools may be selected within wide ranges. However, an embodiment wherein the number of cutting tools on each cage-like body is from 10 to 40 is preferred. In yet another embodiment, the number of shafts along the beam is from 2 to 8, more preferably from 3 to 5.

The sea bed materials excavated by the advancing excavator tool are removed at least partly by conveying means that connect to the excavator tool and carry away this material to a position outside the trench. A suitable embodiment of the submersible vehicle has conveying means comprising a duct that extends from the excavating tool to the position outside the trench, and a pump connected to the duct. The duct may for instance comprise solid and flexible piping.

The remotely operated submersible vehicle according to the invention may operate at great depths over 300 m, preferably over 350 m and more preferably over 400 m. In order to avoid down time in having to bring up the vehicle, for instance because of clogging of the cutting devices, an embodiment is provided wherein the excavating tool comprises water jet nozzles directed to the cutting devices and adapted to remove clogged seabed material. To this end, the excavating device may be provided with a water chamber for collecting the necessary water.

The pumps associated with the vehicle, and more in particular with the conveying means for carrying away the excavated sea bed material, can be of any available type or construction. In a preferred embodiment, the pumps comprise a propeller pump. Such propeller pumps deliver a relatively low pressure head but the pressure heads provided are high enough to carry away the material outside the formed trench. On the other hand, propeller pumps offer a relatively high flow which improves the efficiency of the conveying operation. Being able to reverse the rotation direction of a propeller pump and thereby reverse pumping action is another advantage of this embodiment. Indeed, a reversed water flow towards the cutting devices may be used to remove clogged sea bed material from the cutting devices. In another embodiment, the excavating tool and the conveying means are actively driven by a power supply provided on the vehicle. The remotely operated vehicle (ROV) is connected to the surface vessel by means of an umbilical comprising control cables, which provides data communication and electric power for at least the thrusters, and optionally also for the cutting tools, the rotating support member, the propelling roll member and the pumps. In a useful embodiment, the ROV is further equipped with an hydraulic power pack to convert electric power supplied via the umbilical to hydraulic power to drive the thrusters and optionally other components of the device. A further advantage of the invented submersible vehicle is that it is light and does not require a lot of power. In a typical embodiment, the power supply is adapted to deliver a power ranging from 300 to 500 kW only. The submersible vehicle according to yet another embodiment comprises a control means for positioning the vehicle and in particular the excavator tool thereof relative to the seabed. The control and steering of the vehicle is performed on the surface vessel, which makes the vehicle a remotely operated vehicle.

The remotely operated submersible vehicle or ROV is used in a method of preparing a trench in a subsea bottom, comprising the steps of lowering the submersible vehicle from a surface vessel towards a position close to the subsea bottom, manoeuvring the mechanical excavating tool of the vehicle through the subsea bottom along a trajectory, and conveying the excavated seabed material to a position outside the formed trench. The position close to the subsea bottom for instance ranges from less than 1 m to distances in the range of 1-10 m. The advantages of the invented ROV are particularly apparent in a method wherein the seabed comprises heavy clay with an undrained shear strength Su of at least 20 kPa, and/or wherein the seabed is highly undulated with peak- to- valley depth differences exceeding 10 m.

It is explicitly mentioned that the embodiments disclosed in the present application may be combined in any possible combination of these embodiments, and that each separate embodiment may be the subject of a divisional application.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be elucidated in more detail with reference to the accompanying figures, without otherwise being limited thereto. In the figures:

Figure 1 is a perspective view of an embodiment of the submersible vehicle according to the present invention;

Figure 2 is a perspective view from below of an excavating tool for use in the embodiment shown in Figure 1 ;

Figure 3 is a perspective view from above of the excavating tool shown in Figure 2; Figure 4 is a perspective view from behind of the embodiment shown in figure 1 in operational mode;

Figure 5 is a perspective view from the front of the embodiment shown in figure 4; Figure 6 is a perspective view from behind of the embodiment shown in figure 1 in actual operation;

Figure 7 is a perspective view from the side of the embodiment shown in figure 6; Figure 8 is a perspective view of a cutting device for use in the embodiment shown in Figure 1; and finally

Figure 9 is a side view of a submersible remotely operated vehicle (ROV) without the excavating tool of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 9 shows a submersible remotely operated vehicle (ROV) 1 for manoeuvring a chute or channel 10 across a sea bed. The channel 10 may be used to dump materials onto a sea bed. The ROV 1 comprises a frame 2 having frame members extending in a longitudinal direction 15, a width direction 16 and a height direction 17 of the ROV 1. Hoisting cables 3 are connected to the frame 2 for suspending the ROV 1 from a surface vessel (not shown) and controlling the position of the ROV 1 in the vertical direction 17. The ROV 1 comprises a plurality of hydraulic ally driven thrusters 4, a first set of thrusters 4 being adapted for propelling the ROV 1 in a first horizontal direction (for instance the longitudinal direction 15) and a second set of thrusters 4 being adapted for propelling the ROV 1 in a horizontal direction perpendicular to the first direction (for instance the width direction 16). Hydraulic power is provided by means of an hydraulic power pack 5 which receives electric power from the surface vessel by means of an umbilical 6, which may be integrated in one of the hoisting cables 3, and which supplies hydraulic power to a so-called common rail 7. Compensators 8 are provided to reduce, in a manner known per se, the pressure difference over the seals in the various hydraulic devices. The ROV 1 also comprises a dynamic positioning system 9, connected to the surface vessel via the umbilical 6. In some embodiments, the ROV 1 is arranged as the master, whereas the surface vessel is arranged as the slave, i.e. the ROV 1 is operated or programmed to follow a predetermined path and the surface vessel follows the submerged ROV 1. In other embodiments, the surface vessel is arranged as the master, whereas the ROV 1 is arranged as the slave, i.e. the surface vessel is operated or programmed to follow a predetermined path and the submerged ROV 1 follows the surface vessel.

The ROV 1 in the embodiment shown further comprises a channel 10 providing a means to releasably accommodate the end of a fall pipe for instance (not shown). The channel 10 extends through the middle and, in this example, through the centre of gravity of the ROV 1. The channel 10 comprises an upper section 10A converging downwards and a lower section 10B converging upwards, the sections together defining a waist. At this waist, the channel 10 is provided with one or more friction elements, e.g. a plurality of the resilient blocks 11 that can be moved radially inwards to clamp a fall pipe and outwards to release a fall pipe by means of hydraulic cylinders 12 mounted about the outer wall of the channel 10. The present invention does not need to use the channel 10 of the ROV 1, nor a fall pipe end accommodated in said channel 10.

Figure 1 shows a first embodiment of a ROV 1 according to the invention. In all figures except figure 9, important components of the ROV 1 like the thrusters 4 and the power pack 5 have been omitted for clarity. The ROV 1 apart from comprising a frame 2, hoisting cables 3 and control cables 6 for suspending the vehicle from a surface vessel (not shown), and thrusters 4 for manoeuvring the ROV 1, as described above, further comprises an elongated mechanical excavating tool 20 at a rear side of the ROV 1. The rear side is defined with respect to the intended towing direction 50. The excavating tool 40 in the present embodiment comprises a support member in the form of an open box with sidewalls 43 and a bottom beam or plate 44 that bears four cutting devices 47, arranged along the longitudinal direction 46 of the support member. As shown in figure 8, each cutting device 47 comprises a central shaft 470 around which a cage-like body is arranged. The cage-like body is about cylindrical and comprises a number of side-posts 471 that extend from a bottom plane comprising bottom members 472 that connect to the central shaft 470 and are arranged to form a cross. The side-posts 471 and bottom members 472 are spaced apart to leave openings between them along which openings excavated sea bed material may be carried away. The central shaft 470 is at a top end provided with a flange 473 which is attached to a number of shafts 45 rotatably arranged in the bottom plate 44 of the excavating tool 40. A shaft 45 is schematically shown in figure 8 by hatched lines. The side -posts 471 and the bottom members 472 are provided with cutting tools such as flattened teeth 474. The teeth 474 extend from the side-posts 471 and bottom members 472 in a direction corresponding to the intended rotation direction 48 of the cutting device 47. The shafts 45 to which each cutting device 47 is attached extend in a substantially vertical direction, i.e. a direction perpendicular to a longitudinal direction 46 of the beam 44. Each shaft 45 is rotatably driven around an axis extending in a longitudinal direction of the shaft 45 by hydraulic rotary motors 49. The power to be delivered by the hydraulic motors 49 typically ranges between 20-40 kW for each motor. As shown in figures 2 and 3 two adjacent shafts 45 rotate two adjacent cutting devices 47 in different directions 48. Reaction forces incurred by the rotating cutting devices 47 substantially cancel each other, which helps in advancing the ROV 1 along the correct direction 50 along the envisaged trajectory of the trench 131 to be formed.

The excavating tool 40 is further equipped with a hydraulically driven propelling roll member 60 that extends in the longitudinal direction 46 of the excavating tool 40. As best shown in figures 3 and 5, the propelling roll member 60 comprises a cylindrical body 61 provided with a number of flat penetrating tools 62 that extend in a plane perpendicular to a longitudinal direction 66 of the propelling roll member 60. The flat penetrating tools 62 are arranged in a number of transverse planes of the propelling roll member 60. The propelling roll member 60 extends in a direction 66 parallel to the longitudinal direction 46 of the excavating tool 40 and is provided upstream of the cutting devices 47. In use, the excavating tool 40 is lowered onto the sea bed 130 such that the propelling roll member 60 rests on the sea bed 130 with a circumferential surface of the body 61, and the penetrating tools 62 penetrate the soil. A hydraulic motor 68 able to offer a power of about 5-10 kW rotates the propelling roll member 60 and advances the excavating tool 40 over the sea bed 130 by frictional forces incurred between the circumferential surface of the roll member body 61 and the sea bed 130, and by tangentiall forces generated by the flat penetrating tools 62 on the sea bed 130. The flat penetrating tools 62 at least partly prevent sliding of the excavating tool 40 in a sideways direction, i.e. in a direction about parallel to the longitudinal direction 46 of the excavating tool 40.

The excavating tool 40 is attached to the vehicle frame 2 by two paired arms (41a, 41b) that extend on both sides of the ROV 1 and connect to a lower and a forward frame member of the frame 2 by two pairs of hinges (42a, 42b). Each pair of arms (41a or 41b) forms a parallelogram, as best seen in figure 1. The hinged connection (42a, 42b) allows rotating the excavating tool 40 from a position at rest, as shown in figure 1 for instance, to a working position, as shown in figure 4 for instance, in which the excavating member 40 has been lowered to contact the sea bed 130 in order to make a trench 131 therein. Since the pair of arms (41a, 41b) each form a parallelogram, and due to additional hinges (43a, 43b) provided at another end of the arms (41a, 41b), this will prevent rotation of the support member around an axis parallel to the longitudinal direction 46. The cutting devices 47 will in this manner remain oriented in a

substantially vertical direction, irrespective of the rotational position of the arms (41a, 41b). As with the embodiment described above, the excavating tool 40 extends across the width 132 of the trench 131.

The sea bed materials excavated by the advancing excavator tool 40 in the towing direction 50 are removed by conveying means in the form of two ducts (28 a, 28b) arranged on either side of the ROV 1 and extending from four collectors 49 that connect with the cutting devices 47 to two sidecasting positions (51a, 52a) outside the trench 131 via additional ducts (29a, 29b) arranged on a front part of the frame 2. Figures 1 and 3 show the rigid parts of the ducts (28a, 28b, 29a, 29b) only. In operation, the rigid parts (28a, 28b) and (29a, 29b) are interconnected by two flexible pipe segments (30a, 30b), as shown in figure 4 for instance. Conveying of excavated sea bed material through the ducts (28, 29, 30) is driven by propeller pumps (52a, 52b). Such propeller pumps (52a, 52b) do provide the preferred relatively high flow rate at a typical power of 80-150 kW. The rotation direction of the propeller pumps (52a, 52b) can conveniently be reversed to generate a reversed water flow towards the cutting devices 47. Such a water flow may be used to remove clogged sea bed material from the cutting devices 47. To further aid in removing clogged sea bed material, the excavating tool 40 is provided with water jet nozzles (not visible) that are directed towards the cutting devices 47 and are sourced by a water chamber 70 provided on the bottom plate 44.

In order to prepare a trench 131 in a subsea bottom 130, the ROV 1 is lowered from a surface vessel (not shown) by unrolling the hoisting cables 3 from winches provided on the vessel until the ROV 1 is in a position relatively close to the subsea bottom 130. The mechanical excavating tool 40 is then brought in contact with the sea bed 130 by rotating the pair of arms (41a, 41b) around the hinged connections (42a, 42b, 43a, 43b) until the excavating tool 40 hits the sea bed bottom 130 and the propelling roll member 60 rests on the sea bed 130. Rotating the arms (41a, 41b) can for instance be done by suspending the arms (41a, 41b) from cables (not shown) provided on winches (not shown) of the ROV 1.

As shown in figure 5 for instance, the roll member 60 is provided slightly above the underside of the cutting devices 47. When the body 61 of the propelling roll member 60 rests on the sea bed at a level 133 (figures 6 and 7), the cutting devices 47 actually penetrate the sea bed 130 to the level of the trench 131. The ROV 1 is then manoeuvred through the subsea bottom 130 by rotating the cutting devices 47 and the propelling roll member 60 along a trajectory in the towing direction 50 and sea bed material is loosened by the cutting tools 474 provided on the cutting devices 47. Excavated sea bed material is conveyed by the ducts (28, 29, 30) to the sidecast positions ((51a, 51b) outside the formed trench 131. In this process, the ROV 1 does not rest on the seabed 130, apart from the propelling roll member 60. The hinged connection (42a, 42b) between the vehicle frame 2 and the mechanical excavating tool 40 allows

accommodating vertical movements of the ROV 1 induced by currents and heave of the vessel. The thrusters 4 of the ROV 1 are primarily used to head the ROV 1 along the intended trajectory of the trench 131. The relatively limited power of the thrusters 4 is sufficient since propelling of the ROV 1 is largely performed by the vessel and by the propelling roll member 60 provided on the excavating tool 40. The rotation direction 67 of the roll member 60 provides a forward thrust to the excavating tool 40.

The invention is not restricted to the above-described embodiments, which can be varied in a number of ways within the scope of the claims.

Claims

1. Remotely operated submersible vehicle for preparing a trench in a subsea bottom, the vehicle comprising a frame, hoisting and control cables for suspending the vehicle from a surface vessel, thrusters or manoeuvring the vehicle, an elongated mechanical excavating tool connecting to the vehicle by a hinged connection, and conveying means for carrying away excavated seabed material to a position outside the trench, wherein the excavating tool comprises an elongated support member that extends across the width of the trench and is provided with a number of cutter devices that are actively driven in a rotation direction around an axis extending substantially perpendicular to the longitudinal direction of the support member in the direction of the trench.

2. Submersible vehicle according to claim 1, wherein two adjacent cutter devices rotate in opposite directions.

3. Submersible vehicle according to claim 1 or 2, wherein the number of cutter devices is even.

4. Submersible vehicle according to any one of the preceding claims, wherein the excavating tool comprises a propelling member extending in the longitudinal direction of the support member and provided with flat penetrating tools that extend in a plane perpendicular to the longitudinal direction of the support member.

5. Submersible vehicle according to claim 4, wherein the propelling member is hydraulically driven.

6. Submersible vehicle according to any one of the preceding claims, wherein the number of cutting devices along the support member is from 2 to 8, more preferably from 4 to 6.

7. Submersible vehicle according to any one of the preceding claims, wherein the support member comprises a beam or plate provided with a number of shafts that extend perpendicular to the longitudinal direction of the beam, each shaft being rotatably driven around an axis extending in a longitudinal direction of the shaft.

8. Submersible vehicle according to claim 7, wherein the cutting devices comprise a number of cutting tools provided on a cage-like body arranged around each axis and connected thereto.

9. Submersible vehicle according to claim 7 or 8, wherein the number of shafts along the beam is from 2 to 8, more preferably from 4 to 6.

10. Submersible vehicle according to any one of claims 7-9, wherein two adjacent shafts rotate in different directions.

11. Submersible vehicle according to any one of claims 8-10, wherein the number of cutting tools on each cage-like body is from 10 to 40.

12. Submersible vehicle according to any one of the preceding claims, wherein the conveying means comprise a duct that extends from the excavating tool to the position outside the trench, and a pump connected to the duct.

13. Submersible vehicle according to any one of the preceding claims, wherein the excavating tool comprises water jet nozzles directed to the cutting devices and adapted to remove clogged seabed material.

14. Submersible vehicle according to claim 12 or 13, wherein the pump comprises a propeller pump.

15. Submersible vehicle according to any one of the preceding claims, wherein the excavating tool and the conveying means are actively driven by a power supply on the vehicle.

16. Submersible vehicle according to claim 15, wherein the power supply is adapted to deliver a power ranging from 300 to 500 kW.

17. Submersible vehicle according to any one of the preceding claims, comprising a control means or connectable to control means for positioning the vehicle and in particular the excavator tool thereof relative to the seabed.

18. Assembly of a surface vessel and a submersible vehicle according to any one of the preceding claims suspended from the vessel by hoisting and control cables.

19. Method of preparing a trench in a subsea bottom, comprising the steps of lowering a submersible vehicle in accordance with any one of claims 1-18 from a surface vessel towards a position close to the subsea bottom, manoeuvring the mechanical excavating tool of the vehicle through the subsea bottom along a trajectory, and conveying the excavated seabed material to a position outside the formed trench.

20. Method according to claim 19, wherein the seabed comprises heavy clay with an undrained shear strength Su of at least 20 kPa.

21. Method according to claim 19 or 20, wherein the seabed is highly undulated with peak-to- valley depth differences exceeding 10 m.