GB2423778A

GB2423778A - Laying of sub-sea cables, pipes and the like using a plough

Info

Publication number: GB2423778A
Application number: GB0504378A
Authority: GB
Inventors: Timothy William Grinsted
Original assignee: Engineering Business Ltd
Current assignee: Engineering Business Ltd
Priority date: 2005-03-03
Filing date: 2005-03-03
Publication date: 2006-09-06
Anticipated expiration: 2025-03-03
Also published as: GB2423778B; GB0504378D0

Abstract

A plough 10 for laying sub-sea pipes and cables 50 in a trench comprises a main body 12, means for supporting the plough on the sea bed, such as skids 14, 15, means for directing the cable, such as a bellmouth 20, and a trench cutting device, such as a ploughshare 18. The plough 10 further comprises a moveable arm 26, 28 engageable with a soil displacing tool, such as a hydrodynamic (jetting) tool 36 or a bucket scoop or the like, whereby a trench can be formed in locations otherwise not conveniently trenchable by the ploughshare 18. The moveable arm 26, 28 is preferably rotatable about mounting means 30 and includes pivot points E, F. The plough 10 may be used to lay cables and pipes close to offshore structures such as piles (72A, 72B; figures 7-15).

Description

LAYING OF SUB-SEA CABLES, PIPES AND THE LIKE The present invention relates

to the laying of sub-sea cables, pipes and the like (hereinafter cables) and in particular to the laying of sub-sea cables in confined areas, for example in the region of offshore fixed structures such as wind turbine foundations and columns (monopiles).

The laying of cables in short stretches or confined locations presents particular difficulties. Conventionally, the trench required for burying such cables is often created by using a jetting tool. However, such tools are increasingly being viewed as environmentally disadvantageous because of the disturbance which they create to the seabed by displacing large volumes of soil. However, in confined locations using a cable laying plough may also present difficulties.

When using a cable laying plough on the approach to a monopile the plough is usually towed by a surface vessel. As the vessel approaches the monopile it must necessarily adjust its course away from the monopile. This necessarily places a limit on how close the plough can approach the monopile while being towed by the surface vessel. A similar difficulty occurs when the plough is, alternatively, pulled by an underwater tractor. The present invention seeks to alleviate or obviate this problem.

Disturbance of the sea bed near to the monopile may interfere with the operation of ploughs or other trenching devices, such as by preventing the plough from approaching close to the monopile. Such disturbance can occur as a result of the S..

installation process of the monopile. For example, the disturbance can be caused by the jacks ofjack-up barges used in the installation or otherwise. Waves and tidal S. motion can also have a scouring action on the sea bed around the monopile.

Another problem can occur where it is necessary to interrupt the cable laying process, for example if repair or maintenance of the plough is necessary, if damage occurs to the cable or because of adverse weather conditions. If this happens, normally the plough must be recovered to the surface and, if the cable is broken, steps taken to reconnect the cable. The plough is then returned as near as possible to its previous position and laying of the cable continues. However, such an operation normally leaves a length of cable which is not buried by the plough. Action must then be taken to bury the remaining length of cable and this may require the use ofjetting tools supported from a surface vessel andlor the use of divers.

According to a first aspect of the invention there is provided a sub-sea plough comprising: i). a main body: ii). means for supporting the plough on the seabed; iii). a soil cutting device; iv). cable directing means; and v). an arm having a first end attached to the main body and a second end adapted to engage a soil displacing tool, the arm being operatively movable with respect to the plough to a range of operating positions.

According to a second aspect of the invention there is provided a plough as defined in the first aspect of the invention having a soil displacing tool connected to the second end of the arm.

Preferably the soil displacing tool is a hydrodynamic tool. .. I * * S S. *S

Preferably the soil displacing tool is a jetting tool.

* S* S 25:::; In alternative embodiments, the soil displacing tool is a bucket, scoop or the like.

Preferably the soil displacing tool is operatively connected to a surface vessel.

Preferably the soil cutting device is a ploughshare.

According to a third aspect of the invention there is provided a method of laying a sub-sea cable comprising: i). providing a sub-sea plough according to the second aspect of the invention; ii). moving the plough in a forwards direction to plough a trench and lay a length of cable in the trench; iii). using the arm and soil displacing tool to prepare an additional length of trench, and moving the cable into the said additional length.

According to a fourth aspect of the invention there is provided a method of laying a sub-sea cable comprising: i). connecting the cable to a fixed offshore structure and extending the cable from the fixed offshore structure onto the seabed; ii). providing a plough according to the second aspect of the invention; iii). using the arm and soil displacing tool to trench a length of cable extending between the fixed offshore structure and the plough; and iv). moving the plough away from the fixed offshore structure and trenching the cable using the plough.

According to a fifth aspect of invention has provided a method of laying a sub-sea I.'-.' cable comprising: : i). providing a plough according to a second aspect of the invention; ii). using the plough to trench a cable in a direction approaching a fixed offshore * structure; iii). stopping the plough close to the offshore structure; * iv). using the arm and soil displacing tool to trench the cable extending between the plough and the fixed offshore structure. " Preferably the method further comprises the step of v). after stopping the plough in step iii), dismounting the cable from the plough.

Preferably the method further includes the step of vi). after stopping the plough in step iii), attaching the leading end of the cable to the fixed structure.

Preferably the method further includes the step of, prior to step vi), using the arm and soil displacing tool to expose a length of cable behind the plough.

For better understanding of the invention and to show the same may be carried into effect reference will be made by way of example only to the following drawings in which: Figure 1 is a side view of a plough according to the invention; Figure 2 is a plan view of the plough according to the invention; Figure 3 is a side view ofajetting tool for use in the invention; Figure 4 is an alternate side view of the tool of Figure 3; Figure 5 a side view similar to that of Figure 1 showing the plough in operation; * * se...

Figure 6 is a side view of the plough of the invention in operation with a surface vessel; *:* * S * iS ** Figure 7 is a schematic illustration of the laying of a cable between two wind turbine S...

monopiles; Figures 8, 9 and 10 are respectively views showing the burying of a cable close to a wind turbine monopile using the apparatus of the invention; Figure 11 is a plan view of a cable laying apparatus close to a wind turbine monopile; and Figures 12, 13, 14 and 15 are respectively plan views of a cable laying procedure using the apparatus of the invention, close to a wind turbine monopile.

Referring now to Figures 1 and 2, the plough 10 according to the invention comprises a chassis or main body 12 which in the illustrated example is supported on the plurality of skids 14 which contact the surface of the seabed. The skids 14 may be mounted on legs 16 which are adjustable with respect to the body 12 to vary the depth of the french cut by the plough. Other support structures such as wheels or tracks may be used as an alternative to, or in addition to, the skids 14. Additional skids 15 may be provided at the rear of the plough, if desirable, for example if the seabed conditions are particularly soft.

The plough 10 also includes one or more soil cutting means for forming the trench within which the cable is laid. The cutting means in the Figures are ploughshares 18 which are used to cut the trench in the seabed to accommodate the cable (not shown in Figure 1). Other trench cutting means include cutting wheels and hydrodynamic tools such as jetting tools mounted on the body 12, or combinations of these. The plough 10 is towed along the seabed by means of a tow rope (not shown in Figure 1) attached to a surface vessel, or possibly to an underwater tractor. The direction of movement of the plough when towed is indicated by Arrow A in Figure 1. The plough also includes a bellmouth into which the cable to be trenched is received. : The cable passes through the bellmouth and exits the rear of the plough where, conveniently, it may be directed into the trench by suitable means such as depressor S.. 22.

The main body or chassis 12 of the plough may also provide a location for control * equipment, for example for adjusting the height of the skids 14 with respect to the main body 12 and/or adjusting the position of the depressor 22.

The plough of the invention further includes an arm 24 rotatably mounted on the main body 12. The arm 24 is mounted on the main body 12 so as to rotate about a nominally vertical axis indicated in Figure 1 by arrow B and in Figure 2 by point B. Nominally vertical in this sense means that the axis is substantially vertical when the plough 10 is in an horizontal attitude. Thus, the arm 24 can adopt a range of positions with respect to the plough 10 and two such positions are shown in Figure 2.

The first position of the arm 24, which is substantially in line with the direction of movement of the plough, is indicated at C. The second position of the arm 24, which is substantially perpendicular to the direction movement of the plough 24, is indicated at D. The arm may also move beyond position D to positions further towards the leading end of the plough (in relation to its normal direction of movement) and to the corresponding range of positions at the other side of the plough 10.

Preferably, arm 24 comprises first and second sections 26, 28 which are pivotally connected to each other at E. Preferably also arm 24 is mounted on mounting means (which mounting means will normally provide the rotation about axis B) by a pivotal connection shown at F. By means of these connections the arm 24 is able to vary the height and reach (distance from the plough) of the leading end 32 of the arm 24. Although not specifically illustrated in Figures 1 and 2, arm 24 is provided with means for adjusting its position, such as hydraulic actuators. Such adjusting means are controlled by suitable control means which may be mounted on the main body 12 S:1 of the plough 10 and/or on the surface vessel. :...:. S..

The leading end 32 of the arm 24 is provided with engagement means 34 by means of: which suitable tools, in particular a soil displacing tool, and especially hydrodynamic S tools, are connected to the arm 24. The respective soil displacing tools may be * permanently attached to the arm 24 or, more preferably, may be removeably attached *, : to the arm 24. The tools are removeably attached in the sense that one tool may be exchanged for another, depending on the particular work required and particular soil conditions, for example. in the Figures, ajetting tool is attached to the arm 24, but other tools such as buckets, scoops, backhoes and the like can be used. The engagement means 34 provides a pivotal connection between the tool 36 and the arm 24. Further adjustment and control means may be provided for adjusting the angle of the tool 36 with respect the second section 26 of arm 24.

Figures 3 and 4 illustrate in more detail an example of ajetting tool for attachment to the arm 24. The tool 36 comprises inlet pipes 40 which supply water under high pressure from a pump which may conveniently be mounted on the surface vessel.

For removal and displacement of the seabed soil, the tool 36 is provided with a plurality of high-pressure water jets 44 through which water exits substantially normal to the axis of the tool 36. The high-pressure water supply also provides ajet 46 of an eductor pump. After passing through the tool 36 water and any entrained material is discharged through discharge pipe 42.

Figure 5 is a similar to Figure 1 and shows a plough 10 having connected thereto a tool 36 in operation, and corresponding parts are given corresponding numbers.

Figure 5 also shows the course of a cable 50 entering the plough via the bellmouth 20 and being directed by the plough into the trench with the assistance of depressor 22.

In variations of the invention the water supply pipe for the high pressure water may be routed through the plough 10, rather than directly to the tool 36 as illustrated. in these variations, it is preferable that the tool 36 is permanently or semi-permanently mounted on the arm 24. Also, water pumps for supplying the high pressure water may be provided on the plough 10, rather than on the surface vessel. Such water *** pumps may be driven by hydraulic or electric motors, for example. *: :..

Figure 6, which is not necessarily to scale, shows the plough 10 in combination with * a surface vessel 60. The vessel 60 includes a crane 62 which controls a cable 64 to * : which the tool 36 is attached. The tool 36 can be raised and lowered by means of crane 62 and cable 64. The cable 64 and crane 62 and may be (but are not necessarily) principally responsible for carrying the weight of the tool 36 and may also provide approximate adjustment of the position of the tool 36 with respect to the seabed 38. The arm 24 of the plough 10 is then used to provide exact adjustment of the location of the tool 36, in other words, to fine-tune the position of the tool 36.

However, in preferred arrangements the tool is predominantly or entirely supported and guided by the arm 24 when in use for trenching operations. The vessel 60 also supplies arid supports the high-pressure inlet pipes 40 to which the tool 36 is connected.

Figure 7 shows a typical arrangement of a sub-sea cable between two wind turbines.

Each wind turbine 70 is mounted on a monopile 72A, 72B which extends below the sea surface 74 to a foundation in the seabed. The cable 50 extends between the monopiles 72A, 72B. A plough for laying the cable 50 is shown schematically at 10.

Figure 7 illustrates a number of difficulties which occur in trenching a cable 50 in a confined environment such as between two wind turbine monopiles 72A, 72B.

Specifically, Figure 7 illustrates that trenching of the cable proximate either of the wind turbine monopiles 72A, 72B is difficult, firstly because the wind turbine itself obstructs the movement of the surface vessel 60 which tows the plough 10, and secondly because scouring of the seabed can occur around the monopile so that the plough 10 cannot enter the scoured region. Figure 7 also illustrates that problems can occur in the case of a break in the cable which must be repaired, or in the event that the plough must be removed for repair or maintenance or because of some other eventuality such as bad weather. In these respective instances Figure 7 shows that portions of the cable 50 remain on the seabed 38 (that is, not trenched) in respective regions 76, 78 and 80.

SSS

Reference will now be made in particular to Figures 8 to 11 which illustrate the S method and apparatus of the invention when used for trenching a cable in a direction * away from a fixed structure, such as a wind turbine monopile 72A. As shown in e: : particular in Figures 8 and 11, the cable 50 exits the monopile 72A through a J tube 82. In Figures 8, 9 and 10, the cable 50 has previously been paid out from the vessel close to its intended course between the monopiles 72A and 72B. In some arrangements the cable may be paid out from the vessel contemporaneously with the trenching operation and this variation is shown in Figure 11. The plough 10 cannot approach the monopile 72A closely because of the scouring or other disturbance of the seabed in region 84. Accordingly, there is a length of cable 50 which will not be trenched in the ordinary course of events when the plough is towed away from the monopile 72A, simply because this length of cable lies between the monopile 72A and nearest point of approach of the ploughshare 18 to the monopile 72A.

Accordingly, before moving away from monopile 72A, or where necessary as the plough 10 begins to move away from the monopile 72A, the arm 24 is used to ensure proper trenching of the cable 50 (Figure 8). To this end, the arm 24 is used in conjunction with tool 36 to excavate a trench adjacent to the length of untrenched cable so that the cable 50 can be guided into the trench so excavated (Figure 9). At this stage, if it has not already been done, the cable 50 is arranged to pass through the bellmouth 20 and through or alongside the body 12 of plough 10 and into the trench.

The plough 10 can then continue to trench the cable 50 as the plough 10 is towed towards monopile 72B by the surface vessel 60 (Figure 10). The tow rope is indicated at 88.

The approach of a conventional plough to the monopile 72B also poses problems. As noted above, the monopile 72B obstructs the approach of the surface vessel which restricts how closely the plough 10 can approach the monopile 72B. Also, the scouring or other disturbance of the seabed in region 86 adjacent monopile 72B * prevents close approach of the plough 10 to the monopile 72B. Thus, a length of the cable 50 extending between the monopile 72B and the ploughshare 18 at its nearest location to the monopile 72B remains untrenched.

The trenching of the cable 50, in accordance with the invention, on the approach to * monopile 72 B is illustrated in Figures 12 to 15. In Figure 12, the plough 10 is * : shown approaching the monopile 72B. Before reaching the monopile 72B, the plough 10 is stopped and the arm 24 and tool 36 are used to expose the buried cable behind the plough. The cable end can then be pulled into the J tube 90 (Figure 12). The plough can then be moved further towards the monopile 72B. At this stage, the cable 50 is unloaded from the plough 10 so that it lies on the seabed beside the plough 10. The arm 24 and tool 36 are used to continue trenching the cable 50 (Figure 13). The plough 10 can then be moved closer towards the monopile 72B as trenching continues with the arm 24 and tool 36. As the plough 10 approaches the monopile 72B the plough is caused to divert from the line of the cable 50 in order to avoid the scoured region 86. Nevertheless, as shown in figure 15 the arm 24 is extended sufficiently in order to allow trenching of the cable 50 to continue right up to the entry of the J tube 90. * *

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Claims

Claims I. A sub-sea plough comprising: i). a main body: ii). means for

supporting the plough on the seabed; iii). a soil cutting device; iv). cable directing means; and v). an arm having a first end attached to the main body and a second end adapted to engage a soil displacing tool, the arm being operatively movable with respect to the plough to a range of operating positions.
2. A plough as claimed in claim I having a soil displacing tool connected to the second end of the arm.
3. A plough as claimed in claim 1 or 2 wherein the soil displacing tool is a hydrodynamic tool.
4. A plough as claimed in claim 3 wherein the soil displacing tool is a jetting tool.
5. A plough as claimed in claim 1 or 2 wherein the soil displacing tool is a bucket, scoop or the like. * .

****** *
6. A plough as claimed in any preceding claim wherein the soil displacing tool is *..:.

operatively connected to a surface vessel.

25::: :
7. A plough as claimed in any preceding claim wherein the soil cutting device is a I...

ploughshare. S... * *.. * S * *. S
8. A method of laying a sub-sea cable comprising: i). providing a sub-sea plough as claimed in claim 2 or any of claims 3 to 7 when dependant on claim 2; ii). moving the plough in a forwards direction to plough a trench and lay a length of cable in the trench; iii). using the arm and soil displacing tool to prepare an additional length of trench and moving the cable into the said additional length.
9. A method of laying a sub-sea cable comprising: i). connecting the cable to a fixed offshore structure and extending the cable from the fixed offshore structure onto the seabed; ii). providing a plough as claimed in claim 2 to or any of claims 3 to 7 when dependant on claim 2; iii). using the arm and soil displacing tool to trench a length of cable extending between the fixed offshore structure and the plough; and iv). moving the plough away from the fixed offshore structure and trenching the cable using the plough.
10. A method of laying a sub-sea cable comprising: i). providing a plough as claimed in claim 2 or any of claims 3 to 7 when dependant on claim 2; ii). using the plough to trench a cable in a direction approaching a fixed offshore structure; iii). stopping the plough close to the offshore structure; iv). using the arm and soil displacing tool to trench the cable extending between the * plough and the fixed offshore structure.
11. A method as claimed in claim 10 further comprising the step of v). after stopping the plough in step iii), dismounting the cable from the plough. S

S S S...
12. A method as claimed in claim 10 or 11 further including the step of * : vi) after stopping the plough in step iii), attaching the leading end of the cable to the fixed structure.
13. A method as claimed in claim 12 including the step of, prior to step vi), vii).using the arm and soil displacing tool to expose a length of cable behind the plough.
14. A method substantially as hereinbefore described with reference to any of the Figures ito 15.
15. A plough substantially as hereinbefore described with reference to any of Figures Ito 15. * *

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S * . * * . S* 55 5**S * I S... * *.S * . * S* S