WO2001075236A1

WO2001075236A1 - Submarine plough

Info

Publication number: WO2001075236A1
Application number: PCT/GB2001/001198
Authority: WO
Inventors: John Peter Reece
Original assignee: Soil Machine Dynamics Limited
Priority date: 2000-04-05
Filing date: 2001-03-20
Publication date: 2001-10-11
Also published as: GB0008237D0; AU2001240893A1

Abstract

A submarine plough (101) for burying a cable in a trench in the seabed is disclosed. The plough (101) includes a plough share (102) having a cutting edge for cutting a bottom of the trench in the seabed, and a plough body (107) connected to the plough share (102) and having attachment means for enabling the plough to be towed by a ship. A pair of skids at least partially supports the plough, and a jetting leg (120) is located forwardly of the plough share (102) in use for producing plurality of jets of pressurised water to at least partially break up and fluidise the seabed forwardly of the plough share (102). The jetting leg (120) is displaceable from a working position to a stowed position out of the way of the cutting edge (103).

Description

SUBMARINE PLOUGH

The present invention relates to submarine ploughs, and relates particularly, but not exclusively, to submarine ploughs for burying cables under the surface of the sea bed.

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

A known plough for burying a cable in soil on the sea bed is shown in Figure 1. A plough 1 includes a plough share 2 having a cutting edge 3 for cutting the bottom of a trench in the sea bed, as well as an assembly of parts for moving the soil to bury a cable 4 which passes through the plough and is held down by a movable depressor 5. The plough share 2 is connected to a pair of depth control skids 6 at the front of the plough by means of a long body 7. The skids 6 are movable up and down relative to the body 7 by moving skid support arms 8 to adjust the depth of the plough share 2 relative to the plough body 7. The plough 1 is pulled along the sea bed by a tow rope 9 which joins the plough to a cable ship (not shown) laying cable on the sea bed in front of the plough.

Almost all of the cable ships available in the world are equipped to deploy and recover this type of plough in a size capable of placing a cable two metres below the surface of the sea bed. However, they do not have sufficient towing capability to bury more than one metre deep in any but the weakest soil. Such ships can exert tow forces of up to 40 tonne and can lift on board up to 30 tonne of weight of plough and tow rope. However, such ships suffer from the drawback that they generally provide inadequate protection in areas of soft mud and shifting sands, and are unable to bury cables deeper under such circumstances without a significant increase in tow force supplied, especially if penetration of hard or firm soil below the soft or mobile layer is required. Such operations would generally require an increase in plough weight, size and required towing force, as a result of which more powerful ships would need to be provided at excessive cost.

Many attempts have been made to reduce the tow force required by the plough by pumping jets of water out through the face of the share, as shown in Figure 2. This shows an efficient forward raked plough share 2 with jets 10 pointing out generally perpendicularly to the share surface 11. To be efficient, the jets have to use large volumes of water at a relatively low pressure. This pressure is much less than that exerted on the share by the soil if the plough is pulled hard by the towing ship. The result is that in practice, the jets block up with soil and cease to function.

An attempt to solve this problem is shown in Figure 3, in which the plough share 2 is raked backwards with jets 10 pointing generally vertically downwards in an attempt to avoid the jets becoming blocked regardless of how hard the plough is pulled. However, this shape of plough share 2 is very inefficient when used as a plough without jetting. The consequence of this is that the plough must use jetting constantly instead of only occasionally when particularly hard soil is encountered.

Jetting is used successfully on light buoyant vehicles, known as ROV's (remotely operated vehicles), which are moved by propellers which can only develop very low thrust. These vehicles move at a speed dictated by the rate at which the jets erode and fiuidise the soil, as a result of which there is generally no problem with the jets becoming blocked.

Although it may be expected that a jetting plough could work with an efficient shape of plough share by pulling it very gently, this is not the case, because the plough is heavy and there is considerable friction between the plough and the soil. Furthermore, it is desirable to use all of the pull of the cable ship to plough as much as possible, leaving the jetting to take care of any excess requirement.

Preferred embodiments to the present invention seek to overcome the above disadvantages of the prior art. According to the present invention, there is a provided a submarine plough for inserting an elongate flexible member into a trench in the sea floor, the plough comprising:

a plough share having a cutting edge for cutting a bottom of a trench in the sea floor;

a plough body connected to the plough share and having attachment means for enabling the plough to be towed by a ship;

sea floor engaging means for at least partially supporting the plough; and

jetting means located forwardly of the plough share in use for producing a plurality of jets of pressurised water to at least partially break up and fiuidise the sea floor forwardly of the plough share, wherein the jetting means is displaceable in use from a working position to a stowed position out of the way of said cutting edge.

By providing jetting means displaceable out of the way of the cutting edge, this provides the advantage that the plough share can be designed to have an optimum shape, without compromising the shape of the plough share to avoid blocking of the jetting means. Furthermore, if jetting is not required, the jetting means can be moved out of the way of the cutting edge to avoid impeding ploughing action of the plough share.

The plough share may be adapted to displace material from the sea floor to one lateral side of the plough share.

The jetting means may be mounted to the plough body.

The jetting means may be pivotable relative to the plough share between said stowed position and working positions thereof.

In a preferred embodiment, the depth to which the jetting means works is adjustable. This provides the advantage of enabling the ratio of the depth of trench made by jetting and by ploughing to be varied according to the soil characteristics to optimise performance. For example, clay soil is easy to plough and difficult to jet, so that the jetting means would only create a small proportion of the upper part of the trench. Sand, on the other hand, is easy to jet and difficult to plough, so that the jetting means can be adjusted to create most of the depth of the trench.

The jetting means may be adapted to be displaced from a working position thereof when the force exerted by the floor exceeds a predetermined level.

This provides the advantage that the jetting means does not need to be designed to withstand large ploughing forces if it encounters an obstacle such as a rock in the sea floor.

The plough may further comprise at least one actuator for displacing said jetting means relative to the plough share.

The jetting means preferably comprises a plurality of apertures for producing said jets of pressurised water.

The orientation of said apertures relative to the plough share may be adjustable.

In a preferred embodiment, the jetting means is adapted to project a plurality of jets of pressurised water substantially perpendicularly to the direction of movement of said apertures relative to the sea floor.

This provides the advantage of minimising the risk of blockage of the apertures.

The jetting means may comprise at least one elongate member provided with said apertures.

In a preferred embodiment, the or each said elongate member comprises a respective guarding means, and said apertures are located rearwardly of the corresponding guarding means. This provides the advantage of further minimising the risk of blockage of the apertures.

The or each said guarding means may comprise a rib.

The plough may further comprise means for determining the force required to maintain the jetting means in a working position thereof.

This provides the advantage of enabling the pull exerted by the ship to maximised and the proportion of trenching carried out by applying and jetting to be optimised.

The plough preferably further comprises means for measuring water pressure in said jetting means.

This provides the advantage of giving an indication that the jetting means is beginning to become blocked, as a result of which the pull and speed of the ship can be reduced.

The jetting means may be adapted to at least partially break up and fiuidise the sea floor to a lateral width greater than that of the cutting edge.

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 schematic perspective view of a prior art cable burying plough;

Figure 2 is a schematic side elevation view of a prior art application of jetting to an efficiently shaped plough share;

Figure 3 is a schematic side elevation view of a prior art application of jetting to an inefficiently shaped plough share in an attempt to avoid blockage of the jets of the plough;

Figure 4 is a schematic side elevation view of a cable plough embodying the present invention and having jetting means in a working position thereof; Figure 5 is a schematic side elevation view of the plough of Figure 4 with the jetting means in the stowed position; and

Figure 6 is a schematic front elevation view of the jetting means of the plough of Figures 4 and

5.

Referring to Figure 4, a cable burying plough 101 has a plough share 102 with a cutting edge 103. Jetting means including a jetting leg 120 is pivotable about an axis 121 transverse to the direction of ploughing and which passes through two brackets 122 on the plough body 107. Such brackets 122 are often provided on conventional plough bodies to carry a rock ripping leg. The jetting leg 120 has a lever arm 123 fixed to it which can be moved between the fully down position shown on Figure 4 and the fully up or stowed position shown in Figure 5 by means of a hydraulic ram 124 attached to a pivot 125 on the plough body 107. The hydraulic ram 116 can be locked in any position between the fully up and fully down positions by means of hydraulic controls (not shown) which will be familiar to persons skilled in the art.

The hydraulic system can be set to allow the ram 124 to collapse if the moment exerted on it by loads exceeds a predetermined value. As will be familiar to persons skilled in the art, this can be achieved by means of a relief valve (not shown) which can be adjusted remotely by means of a plough control system on the towing ship. The position of the jetting leg 120 can also be determined by means of a displacement transducer (not shown) in the ram 124, and the moment of the loads on it measured by means of a pressure transducer measuring the hydraulic pressure across the piston of the ram 124. As a result, the jetting leg 120 does not need to be designed to withstand significant ploughing forces if it should encounter obstacles such as rocks in the sea bed.

The jetting leg 120 is provided with jets 126 mounted along its forward face, each of the jets 126 being mounted in a respective recess 127. The recesses 127 lie behind a central rib 128 which protects the jets 126 from damage if the jetting leg 120 should strike an obstacle such as a rock. The jets 126 can be directed downwards at right angles to the direction of relative movement of the soil passed the plough to minimise the risk of the jets becoming blocked by soil. The effective length of the jetting leg 120 can be adjusted by pivoting the leg 120 about axis 121 by means of the ram 124. This enables the depth of the trench created by the jetting means 120 to be adjusted, depending upon the depth of ploughing required, and the pull available from the towing ship.

The water pressure in the jetting leg 120 behind the jets 126 can also be measured. If this pressure rises above a predetermined value, it is an indication that the jets 126 are beginning to block, as a result of which the pull and speed of the ship can be reduced to prevent blockage of the jets.

Figure 6 shows a schematic front elevation view of the jetting means and plough share of the plough of Figures 4 and 5. It can be seen from the Figure that the jetting leg 120 consists primarily of a large diameter steel tube bent down at one end to form the jetting leg, and is attached to a pressure type swivel 129 at the other end thereof. This connects to an elbow and flange 130 through which the jetting water is supplied. The jets 126 on the face of the jetting leg 120 break up the soil to a width greater than the width of the leg 120 in order to provide sufficient space for the mixture of broken soil and jet water to travel past the leg 120 into the space behind it. The plough share 102 following behind then has the relatively easy task of forcing this water and soil mixture upwards and out of the jetted trench.

The plough cutting edge 103 and plough share 102 cut and move strong unjetted soil outwards and upwards to the space filled by the jetted slurry displacing it upwards. The jetting leg 120 can therefore be at least as wide as the plough share 102 and be fitted with enough jets 126 of sufficient diameter to excavate the trench about twice the width of the plough share.

The plough shown in Figure 6 is fitted with a share 102 of assymetrical section, viewed in plan, which aims to move the soil to one side only. As a result, the jetting leg 120 is mounted to one side of the share centre line by a distance d of at least half a share width.

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. For example, the jetting leg 120 need not create a trench as wide as the cutting edge 103 of the plough share 102, and more than one jetting leg 120 may be provided, on more than one side of the plough share.

Claims

CLATMS

1. A submarine plough for inserting an elongate flexible member into a trench in the sea floor, the plough comprising:

sea floor engaging means for at least partially supporting the plough; and

jetting means located forwardly of the plough share in use for producing a plurality of jets of pressurised water to at least partially break up and fluidise the sea floor forwardly of the plough share, wherein the jetting means is displaceable in use from a working position to a stowed position out of the way of said cutting edge.

2. A plough according to claim 1, wherein the plough share is adapted to displace material from the sea floor to one lateral side of the plough share.

3. A plough according to any one of the preceding claims, wherein the jetting means is mounted to the plough body.

4. A plough according to any one of the preceding claims, wherein the jetting means is pivotable relative to the plough share between said stowed position and working positions thereof.

5. A plough according to any one of the preceding claims, wherein the depth to which the jetting means works is adjustable.

6. A plough according to any one of the preceding claims, wherein the jetting means is adapted to be displaced from a working position thereof when the force exerted by the floor exceeds a predetermined level.

7. A plough according to any one of the preceding claims, further comprising at least one actuator for displacing said jetting means relative to the plough share.

8. A plough according to any one of the preceding claims, wherein the jetting means comprises a plurality of apertures for producing said jets of pressurised water.

9. A plough according to any one of the preceding claims, wherein the orientation of said apertures relative to the plough share is adjustable.

10. A plough according to any one of the preceding claims, wherein the jetting means is adapted to project a plurality of jets of pressurised water substantially perpendicularly to the direction of movement of said apertures relative to the sea floor.

11. A plough according to any one of the preceding claims, wherein the jetting means comprises at least one elongate member provided with said apertures.

12. A plough according to claim 11, wherein the or each said elongate member comprises a respective guarding means, and said apertures are located rearwardly of the corresponding guarding means.

13. A plough according to claim 12, wherein the or each said guarding means comprises a rib.

14. A plough according to any one of the preceding claims, further comprising means for determining the force required to maintain the jetting means in a working position thereof.

15. A plough according to any one of the preceding claims, further comprising means for measuring water pressure in said jetting means.

16. A plough according to any one of the preceding claims, wherein the jetting means is adapted to at least partially break up and fluidise the sea floor to a lateral width greater than that of the cutting edge.

17. A submarine plough for inserting an elongate flexible member into a trench in the sea floor, the plough substantially as hereinbefore described with reference to Figures 4 to 6 of the accompanying drawings.