WO2001049947A1

WO2001049947A1 - Submarine plough and plough share for such plough

Info

Publication number: WO2001049947A1
Application number: PCT/GB2000/004743
Authority: WO
Inventors: Alan Richard Reece
Original assignee: Soil Machine Dynamics Limited
Priority date: 2000-01-05
Filing date: 2000-12-12
Publication date: 2001-07-12
Also published as: AU2194601A; GB0000050D0

Abstract

A plough share for a submarine plough for cutting a trench in a sea floor is disclosed. The plough share has a cutting edge (104) for fracturing the sea floor to cut a bottom of the trench, and an orifice (122) is arranged adjacent the cutting edge (104) for directing a stream of high pressure water towards a crack formed in the soil by the cutting edge (104) to assist the cutting edge in fracturing the sea floor.

Description

SUBMARINE PLOUGH AND PLOUGH SHARE FOR SUCH 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 cables, -such as power or telecommunication cables, laid on the sea bed is fishing activity. Attempts are therefore generally made to protect such cables from damage by burying them under the surface of the sea bed by means of a submarine plough towed behind a cable laying ship. A cable laid on the sea bed is picked up by the plough, which digs a trench in the surface of the sea bed and buries the cable in the trench.

An example of a known type of cable burying plough is disclosed in European patent application 0088190 and is shown in Figures 1 and 2. A plough 1 comprises a plough share 2 having a vertical cutting face 3 and a horizontal cutting edge 4. A rotating disc 5 arranged forwardly of the plough share 2 provides a further cutting edge. The cutting edges 3 and 5 cut the sides of a wedge of earth, and the cutting edge 4 cuts the bottom of a trench formed by the wedge of earth. The plough share 2 also has an inclined face 6 which lifts the wedge of earth cut by the cutting edges. The plough depresses a cable 7 into the trench, and after the plough 1 has passed, the wedge- of soil falls back into the trench to bury the cable 7.

Almost all of the world's fleet of cable laying ships is equipped to deploy and recover this type of plough in a size capable of placing a cable 1 metre below the seabed. As a result, these ships tend to exert tow forces of up to 40 tonne and can lift onboard up to 30 tonne of weight of plough and tow rope . However, although these known types of plough have previously generally provided adequate protection over much of the sea bed in which cable is buried, they suffer from the disadvantage that they are incapable of providing adequate protection in areas of soft mud and shifting sand because the cables cannot be buried sufficiently deeply. To enable known types of cable plough to bury cables deeper, or to bury cables in the hard or fixed soil beneath weak and shifting soil, an increase in plough weight, size and required tow force is necessary. The cost of producing cable laying ships with this capability is excessive .

Figure 3 illustrates the principle of operation of the prior art plough of Figures 1 and 2. This shows the continuous unbroken furrow slice 8 lifted up to illustrate its relation to the main components of the plough soil moving apparatus. The plough aims to define the side of a furrow by means of the disc 5 and then to wedge the soil sideways and upwards by means of the cutting edges 3 and 4 and inclined face 6. This wedging aims to bend the tall narrow furrow slice 8 in a vertical plane rather than cause it to fail along a series of shear planes. This bending process is, in strong soils, much more efficient than shearing. It puts the top of the furrow slice into compression and the bottom into tension. Underwater sand is able to withstand such tension due to the development of suction in the pore water. This wedging action also creates tension cracks 9 in the soil in front of the cutting edges 3 arid 4. As shown in Figure 3, the space 10 in front of the cutting edge 4 and inclined face 6 is sealed from the water surrounding the plough 1. As a result, the plough of Figures 1 and 2 suffers from the drawback that the pressure of any water in space 10 falls to a level significantly lower than that of the ambient water pressing down on top of the slice of soil, which in turn significantly impedes the ploughing action of the cutting edge 4 and inclined surface 6. Preferred embodiments of the present invention seek to overcome the above disadvantages of the prior art.

_According to an aspect of the present invention, there is provided a plough share for a submarine plough for cutting a trench in a sea floor, the plough share comprising:-

a cutting edge for fracturing the sea floor to cut a bottom of the trench, and fluid directing means arranged adjacent said cutting edge for directing at least one stream of liquid under pressure towards a region of operation of said cutting edge to assist said cutting edge in fracturing the sea floor.

By providing a plough share having fluid directing means arranged adjacent the cutting edge for directing at least one stream of liquid under pressure to assist the cutting edge in fracturing the sea floor, this provides the advantage that the earth fracturing capability of a plough equipped with the plough share is increased without necessitating an increase in towing force. This is based on the very surprising discovery that fluid directing means can be used to assist the earth fracturing process of the plough share. Although it is known to use jetting means emitting high velocity jets of liquid at ambient pressure to fluidise the soil of the sea floor in order to enable a cable to be buried, such prior art methods are highly inefficient since the jetting means uses energy in breaking up the soil into small fragments.

The present invention, on the other hand, utilises a completely different process whereby the fluid directing means assists in fracturing the soil to cut a wedge or slice of soil, but does not waste energy in breaking up the wedge or slice. There has hitherto been a strong prejudice in the art against using jetting means in connection with ploughs to be towed by cable ships, since it is widely believed that the process of towing a plough having jetting means would cause the jets to become blocked by soil and thus become inoperative. The cutting edge may be replaceable.

In a preferred embodiment, said fluid directing means comprises at least one aperture for emitting a respective stream of liquid therefrom, and the plough share further comprises closure means for closing the or each said aperture.

This provides the advantage of minimising ingress of soil into the apertures when streams of liquid are not being directed out of the apertures .

The closure means may comprise a respective retractable elongate member for blocking the or each said aperture.

The plough share may further comprise a displacing portion for displacing earth from said trench.

The displacing portion preferably includes at least one upwardly directed surface arranged rearwardly of the cutting edge.

The plough share may further comprise a heel portion rearwardly of the cutting edge.

According to another aspect of the present invention, there is provided a submarine plough including a plough share as defined above .

The plough may be adapted to insert a flexible elongate member into the trench.

According to another aspect of the invention, there is provided a method of forming a trench in a sea floor by means of a submarine plough comprising a plough share having a cutting edge for fracturing the sea floor to cut a bottom of the trench, the method comprising the steps of: towing the plough to cause the cutting edge to fracture the sea floor to cut a bottom of the trench; and

directing at least one stream of liquid under pressure towards a region of operation of said cutting edge to assist said cutting edge in fracturing the sea floor.

The method may further comprise the step of burying an elongate flexible member in the trench.

The elongate flexible member is preferably a cable.

In a preferred embodiment, the plough includes fluid directing means comprising at least one aperture for emitting a respective stream of liquid therefrom, and the method further comprises the step of closing the or each said aperture.

The or each said aperture may be closed by means of a respective retractable elongate member.

A preferred embodiment of the present 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;

Figures 2a, 2b and 2c a respective cross sectional views of the soil before, during and after passage of the plough of Figure 1;

Figure 3 is a schematic illustration of the principle of operation of the plough of Figures 1 and 2;

Figure 4 is a schematic perspective view, corresponding to Figure 1, of a cable burying plough embodying the present invention; Figure 5 is a detailed cross-sectional elevation view cf a cutting edge of the plough share of the plough of Figure ;

Figure 6a is a schematic illustration of the formation of a crack in soil by the plough share of Figure 4 without the application of hydraulic pressure to the upper face of the plough share; and

Figure 6b_ is a schematic illustration, corresponding to Figure 6a, of the formation of a crack in soil by the plough share of Figure 4 with the application of hydraulic pressure to the upper face of the plough share.

Referring to Figures 4 and 5, in which parts common to the prior art plough of Figures 1 and 2 are denoted by like reference numerals but increased by 100, a cable burying plough 101 has a plough beam 120 supporting a plough share 102, the plough share having a vertical cutting face 103 and a cutting edge 104 for cutting the bottom of a trench in the sea bed. A heel portion 121 for supporting the weight of the plough 101 is arranged rearwardly of the cutting edge 104, and an inclined surface 106 is provided rearwardly of the cutting edge 104. A passage is provided for passing a cable 107 into a trench created by the plough 101.

A freely rotating disc 105 is arranged forwardly of the plough share 102 and provides a cutting edge for cutting a side of the trench, the side being inclined to the vertical, the other side of the trench being cut by the cutting edge 103. The bottom of the trench is partially formed by the cutting edge 104 at the front of the plough share 102 fracturing soil at the bottom of the trench in a manner similar to the prior art.

Referring in detail to Figure 5, however, an orifice 122 is arranged adjacent the cutting edge 104 for directing a stream of high pressure water towards a crack formed in the soil by the cutting edge 104. This therefore assists the earth fracturing effect of cutting edge 104.

The cutting edge 104 is formed by a replaceable tip which is a push fit onto an adapter 123 formed of hard steel, where it is held in position by means of a retaining pin 124. The orifice 122 is connected to a conduit 125 drilled into the front of the adapter 123, and is formed by a specially shaped ceramic insert (not shown) screwed into the outer end of the conduit 125. The conduit 125 communicates with a larger borehole 126 which extends to the rear face of the adaptor 123 and to which is bolted a double acting actuator 127. The actuator 127 includes a thrust bar 128, the front of which forms a needle which can be moved forwards along the conduit 125 blocking the orifice 122 completely. A hole 129 is provided in the tip of the cutting edge 104 to allow the stream of liquid to pass through the hole, and water is supplied via a pipe 130 connected to a pump (not shown) on top of the plough 101.

The operation of the plough shown in Figures 4 and 5 will now be described.

As the plough is towed forwards in the direction of arrow A by a ship (not shown) , the rotating disc 105 cuts one side of a trench in the sea bed, and the plough share 102 cuts the other side and the bottom of the trench as it passes through the sea bed. As the cutting edge 104 fractures soil to form the bottom of the trench a high pressure flow of water is directed from the orifice 122 and is directed into the crack in the soil formed by the cutting edge 104 fracturing the soil. The pressure from the orifice 122 is rapidly transmitted to the water in the crack in front of the cutting edge 104, which causes further fracturing of the soil, and the wedge of soil is then separated from the base of the trench by means of the cutting edge 104 and inclined surface 106. As a result of the rapid transmission of localised pressure into the water in front of the cutting edge 104, significantly more effective fracturing of the soil at the bottom of the trench takes place than would occur as a result of the cutting edge 104 alone.

Figures 6a and 6b illustrate the effect of applying hydraulic pressure in front of the plough share to increase ploughing speed. The task is to propagate a crack 9 and bend a large baulk of soil upwards (and sideways) .

Figure 6a. shows what happens without hydraulic pressure with the application of a force of 20 tonne taking account of a friction angle of 25 degrees on both faces of the plough share. The force that applies a bending moment to the baulk of soil is the normal component of the force on the upper face of the share. This force is approximately 17 tonne.

Figure 6b shows what happens when water under pressure is introduced into the space 110 in front of the cutting edge 104 through the conduit 122. The pressure acts on the top and bottom of the crack 109 and acting together with the mechanical wedging action of the cutting edge 104 causes the crack to propagate forward. The crack extends further in front of the cutting edge increasing the leverage of the forces available to bend the furrow slice 108 upwards. The higher the hydraulic pressure the less mechanical force is required. The tow force from the ship will be reduced or, if it is maintained, the plough will move forward more quickly.

The pressure in front of the cutting edge 104 will travel along the metal faces of the top and bottom of the share as shown in Figure 6b, gradually reducing towards the free water behind the plough. This will partially lift the lower surface of the cutting edge 104 off the bottom of the furrow and the soil off the upper surface. If the hydraulic pressure is sufficient, lift off becomes complete, the friction angle becomes zero and the wedging action of the share approximately doubles as shown by the triangle of forces on Figure 6b. This reduction in friction between share and soil has the very important advantage of reducing the wear on the cutting edges 103 and 104.

An approximate estimate of the pressure required to completely lift the soil off a 200mm x 200mm plough share cutting surface is 340kN divided by 0.04m² = 8500kN/m². This is a very high pressure emphasising that a jetting process is not involved. If less pressure is available, then a reduced effect will be achieved.

A further advantage of the hydraulic fracturing arrangement described above is that the furrow slice is bent upwards in a more gentle fashion, bending on a larger radius resulting in lower internal stresses and lower ploughing force. The share is a 30 degree wedge, the sharpest that is practicable in order to provide enough strength to the cutting edge. The pressurised water in front forms a long streamlined cutting edge.

If the furrow slice is made of sand, it only has a resistance to bending if the sand has a tensile strength in the region above the crack. This is provided by pore water suction engendered by the action of bending at high speed. The bending strength therefore can be expected to be approximately proportional to the speed of advance of the plough. A doubling of the bending moment would suggest an approximate doubling of forward speed. This is generally in agreement with experimental observations .

There is a secondary, but possibly equally important, effect. That is, the high pressure on the tensile side of the baulk of soil will reduce pore suction and hence bending strength.

Flow is required as well as pressure because water will flow into the dilating sand changing the pore pressure from negative to strongly positive along the surfaces from the tip of the fracture to some way back along the share. The more porous the sand the greater will be the flow required.

On a submarine plough the power available for this process is limited by the cost of supplying it from a ship on the surface down an umbilical cable to the electric motor driving the pump on the plough. If the pump used is of a constant delivery type then it would be desirable to drive it though a variable speed transmission so that it could increase, flow at reduced pressure to maintain at least part of the effect as the sand porosity increases. This effect can also be achieved by using a centrifugal pump with a steeply sloping head-against-flow curve .

It should be noted that as the porosity of the sand increases, this hydraulic fracture process will become less effective for a given power. However, the basic ploughing process also becomes easier.

The hydraulic fracture process has proved very effective in clay as well as sand, even though there is less friction on the sliding contacts between share and soil.

It will be appreciated by person 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. In particular, the hydraulic fracture process has been described in its application to an arrangement of the type disclosed in European patent application 0088190 designed to avoid breaking up of the soil. However, it will be appreciated that the plough may well also be applied to other types of ploughs of similar shape .

Claims

C AIMS

1. A plough share for a submarine plough for cutting a trench in a sea floor, the plough share comprising:-

2. A plough share according to claim 1, wherein the cutting edge is replaceable .

3. A plough share according to claim 1 or 2 , wherein said fluid directing means comprises at least one aperture for emitting a respective stream of liquid therefrom, and the plough share further comprises closure means for closing the or each said aperture .

4. A plough share according to claim 3, wherein the closure means comprises a respective retractable elongate member for blocking the or each said aperture .

5. A plough share according to any one of the preceding claims, further comprising a displacing portion for displacing earth from said trench.

6. A plough share according to claim 5, wherein the displacing portion includes at least one upwardly directed surface arranged rearwardly of the cutting edge.

7. A plough share according to any one of the preceding claims, further comprising a heel portion rearwardly of the cutting edge.

₈. A plough share for a submarine plough for cutting a trench in a sea floor, the plough share substantially as hereinbefore described with reference to Figures 4 to 6 of the accompanying drawings .

9. A submarine plough comprising a plough share according to any one of the preceding claims.

10. A plough according to claim 9, wherein the plough is adapted to insert a flexible elongate member into the trench.

11. A method of forming a trench in a sea floor by means of a submarine plough comprising a plough share having a cutting edge for fracturing the sea floor to cut a bottom of the trench, the method comprising the steps of:

towing the plough to cause the cutting edge to fracture the sea floor to cut a bottom of the trench; and

12. A method according to claim 11, further comprising the step of burying an elongate flexible member in the trench.

13. A method according to claim 12, wherein the elongate flexible member is a cable,

14. A method according to any one of claims 11 to 13, wherein the plough includes fluid directing means comprising at least one aperture for emitting a respective stream of liquid therefrom, and the method further comprises the step of closing the or each said aperture.

15. A method according to claim 14, wherein the or each said aperture is closed by means of a respective retractable elongate member.

16. A method of forming a trench in a sea floor by means of a submarine plough comprising a plough share having a cutting edge for fracturing the sea floor to cut a bottom of the trench, the method substantially as hereinbefore described with reference to Figures 4 to 6 of the accompanying drawings.