WO2004076753A1

WO2004076753A1 - Apparatus for recovery of buried underwater cables

Info

Publication number: WO2004076753A1
Application number: PCT/DK2004/000135
Authority: WO
Inventors: Per Villadsen; Lennie Frederiksen
Original assignee: Alcatel Submarine Networks Marine A/S
Priority date: 2003-02-28
Filing date: 2004-02-27
Publication date: 2004-09-10
Also published as: DK175610B1; DK200300317A

Abstract

The invention relates to a flushing tool having a hook (8) mounted on an arm (9), which hook (8) may be controlled via a release mechanism from the vessel on which the tool is mounted. The flushing tool consists of a wire (10), and mounting of the hook (8) via a pivotable link (7) allows a cable (4) to be controlled, gripped and placed correctly relative to the flushing tool (5, 6).

Description

APPARATUS FOR RECOVERY OF BURIED UNDERWATER CABLES

The invention relates to a flushing tool comprising a hook with a release mechanism on an underwater vessel which is remote-controlled from a surface vessel, said hook being adapted to grip a cable buried beneath a seabed after the flushing tool has exposed the cable.

Subsea cables are typically used for off-shore transmission of high voltage supply or data signals. The cables are frequently flushed or ploughed into the seabed, frequently at a depth of a couple of metres. When subsea cables are lowered into the seabed, this normally takes place by the use of an underwater vessel, a so-called ROV, which stands for Remote Controlled Vehicle. A ROV is thus an underwater vessel which is controlled from a surface vessel, and which, via two controllable arms provided with water-based high pressure nozzles, is capable of flushing the desired depth in the seabed at which the subsea cable is to be placed.

It is known to lift subsea cables when these exhibit malfunction. In that case, the cables are lifted to a cable ship where the repair of the cable takes place.

The method used for lifting subsea cables according to the prior art comprises the use of a so-called grapnel, which may be described as a ploughlike hook mounted between two wheels or a set of runners which is dragged after a surface vessel via a wire. The grapnel hook ploughs the seabed in the area in which the cable is presumed to be present, and when the grapnel hook passes the buried cable, it is caught by the grapnel hook and can subsequently be lifted to the surface of the sea frorh the cable ship dragging the grapnel.

However, it has been found that the technique used till now is associated with some drawbacks. It is frequently a great problem for the grapnel hook to plough deep enough into the seabed to reach the buried subsea cable. Of course, the problem is greatest in areas with a hard seabed, but since this is the case over large areas of the earth, the problem is very great in practice.

The drawbacks of the prior art manifest themselves in that it may be a very time-consuming and thus also costly process to have the subsea cables lifted with the grapnels.

It will frequently take several weeks of dragging of the grapnel to succeed in having lifted the defective subsea cable.

Further, WO 02/20908 discloses a flushing tool having a hook, where the hook is secured to the end of the flushing tool. In the operation of the flushing tool the hook will contact the seabed, involving the risk that it will get stuck between rocks, lost trawls, etc., necessitating implementation of operations to release the hook, if this is possible at all.

Accordingly, an object of the invention is to ensure that the hook is not liable to grip undesirable objects, while the flushing tool is in operation, resulting in operational stoppages.

The object of the invention is achieved by a flushing tool of the type defined in the introductory portion of claim 1 , which is characterized in that the hook is secured at its one end to an arm which is pivotally mounted on the flushing tool relative to the remote end of the hook, and that the other end of the hook is connected with one end of a wire, while its other end is connected with the surface vessel.

Hereby, it is possible in an effective and rapid manner to get down into the seabed to the depth where the undersea cable is present, and to hook the undersea cable to be lifted, without any risk of the hook gripping undesired objects and getting stuck.

In practice, the time it takes to recover an undersea cable may be reduced from days to hours, thereby significantly reducing the costs of lifting the undersea cable.

When, as stated in claim 2, the wire is connected with the hook via a pivot- able link provided on the hook, it is possible to orient the hook toward the cable when the hook is to grip it, and, when the cable is to be pulled up to the surface of the sea, to orient the hook so that the cable does not slide off the hook.

When, as stated in claim 3, the arm is pivotable about an axis perpendicular to the flushing tool, the positions of the hook between the seabed and the underwater vessel may be controlled in a simple manner, depending on the conditions under which the operations take place. In addition, the axial direction of the hook will always to transverse to the cable to be exposed.

A particularly expedient way of configuring the release mechanism is, as stated in claim 4, that the release mechanism is formed by a hollow on a shaft part of one end of the hook, and that the arm is formed with a hole having dimensions which correspond to the outer dimensions of the shaft part, and that inside the hole a transverse hole is provided, having a diameter which is larger than the diameter of the hollow, and that the transverse hole accommodates a piston with a flange, said flange having a diameter which corresponds to the diameter of the transverse hole, while the remaining part of the piston has a diameter which corresponds to the di- ameter of the hollow, and, as stated in claim 5, that the movement of the piston is provided by liquid pressure, supply pipes for the supply of pres- sure, such as liquid pressure, being arranged on each side of the flange of the piston.

It is stated in claim 6 as a further characteristic feature that the hook is pro- vided with means, such as one or more metal detectors, to record a hooked undersea cable.

This makes it possible to get information on the hooking of the subsea cable from the controlling surface ship immediately when the hooking has taken place, which in turns contributes to ensuring an effective and resource-saving recovery.

As stated in claim 7, it is moreover characteristic of the invention that the hook is provided with a blocking device, such as a spring-loaded pawl, to fix a hooked undersea cable.

This provides the advantage that an undersea cable which has been hooked does not unintentionally slide off the hook, which would mean that the searching process for the cable would have to be repeated.

It is stated in claim 8 that the hook may be characterized by being provided with tools, such as knives or scissors, to sever a hooked undersea cable.

When the hook is provided with tools which may optionally be remote-con- trolled from the cable ship, for severing a hooked undersea cable, this may be severed, following which the cable may be lifted on both sides of the severing point.

In practice, of course, a deep-lying cable can be lifted only if it has been severed beforehand. When, as stated, the hook may be released from the ROV, typically via a remote-controlled command from the cable ship, the undersea cable may be lifted, e.g. by a wire connected between the hook and the cable ship, independently of whether the ROV remains on the seabed.

The inventing will now be explained more fully with reference to the drawing, in which:

fig. 1 shows an ROV provided with a hook to grip an exposed under- sea cable, said ROV dredging the seabed in a direction toward a buried undersea cable,

fig. 2 shows an ROV which has exposed an undersea cable, and where the hook for gripping the undersea cable, is moved down toward the cable,

fig. 3 shows an ROV mounted with a hook which has surrounded an exposed undersea cable,

fig. 4 shows an exposed undersea cable secured in a hook which is released from an ROV, and where the hook and thereby the undersea cable are pulled up to a surface vessel,

fig. 5 shows a hook securing an undersea cable,

fig. 6 shows a hook provided with a cable severing tool,

fig. 7 shows a hook with a hooked undersea cable, where the hook is secured in a holding mechanism,

fig. 8 shows a hook with a hooked undersea cable where the hook is placed in a holding mechanism, where a locking pawl is opened, and

fig. 9 shows a hook with a hooked undersea cable, where the hook is released from a holding mechanism, where a locking pawl is opened.

In fig. 1 , the numeral 1 designates an underwater vessel, a so-called ROV, which moves on the seabed 3 in the direction 2. The ROV is controlled from a surface vessel via the shown screw propellers 11. The ROV is provided with a flushing tool, two so-called jetter legs 6, which are arms provided with high pressure water nozzles 5 capable of digging or rather flushing a trench in the seabed. The flushing of the seabed is performed in order to expose the undersea cable 4.

The ROV is moreover provided with a pivotable arm 9 which comprises a holder 7 with a hole in which a hook 8 is secured.

The hook 8 is connected by a wire 10 with the process-controlling surface vessel or a float.

Fig. 2 shows the ROV 1 which moves on the seabed 3 and has exposed an undersea cable 4, which is to be lifted by the use of the hook 8, via the water nozzles on the flushing tool 6.

In fig. 2, the hook 8 is moved toward the exposed undersea cable 4.

Fig. 3 shows a situation where the undersea cable 4 exposed by the ROV 1 has been caught by the hook 8, which is connected by the wire 10 to the controlling surface vessel. Fig. 4 shows a situation where the hook 8 having gripped the undersea cable 4 has been released from the ROV 1 and is pulled via the wire 10 up to the controlling surface vessel.

Fig. 5 shows a close-up of a hook 8 which has gripped an undersea cable

4, and where a blocking mechanism 17 prevents the undersea cable 4 from sliding off the hook 8. The blocking mechanism 17 is spring-loaded, so that, at rest, it is positioned against the hook 8, as shown. The blocking mechanism 17 thus allows a cable to be gripped by the hook 8 and subsequently ensures that the grip is maintained. The hook 8 is provided with a cable detector 18, e.g. in the form of a metal detector, which communicates with the controlling surface vessel via a signal cable or a wireless transmission.

The hook 8 is provided with an attachment mechanism 12 to a wire which may be connected with the surface vessel, and via which the hook may be lifted or released from the holding mechanism 7.

To have the best possible pull in the hook 8 from a connected wire, the wire mounting mechanism 12 is mounted in a pivotable link 13 on the hook 8.

In the embodiment shown in fig. 5, the holding mechanism 7 consists of a piston 14 having a flange which is pressed down into a hollow on the hook shaft which is placed in the holder, whereby the hook 8 is secured in the holder 7. As will be seen, the holding mechanism is formed by inter alia a hole in the arm.

The piston 14 with the flange is controlled by a pressure provided via the shown supply lines 15 and 16.

If the pressure from the supply line 15 is greater than the pressure from the supply pipe 16, the piston 14 is pressed down against the hollow in the hook shaft, whereby the hook 8 is secured in the holder 7.

If the pressure from the supply pipe 15 is smaller than the pressure from the supply pipe 16, the piston 14 will be sucked away from the hook shaft, whereby it may be released from the holder 7.

Fig. 6 shows a sketch of a hook 8 provided with a cable severing tool 19 which may optionally be remote-controlled from the controlling surface vessel.

Fig. 7 shows a hook 8 secured in a holder by means of a depressed piston 14.

Fig. 8 shows the same as fig. 7, but now with the piston 14 raised, whereby the hook 8 may be released from the holder.

Fig. 9 shows how the hook 8 may be released from the holding mechanism by displacing the piston 14 out of the hollow 20. When the piston is pressed into the hollow 20, the hook 8 is secured in the holding mechanism, as shown e.g. in fig. 7.

In the shown examples of the hook and the holding mechanism securing the hook to the ROV, the holding mechanism is controlled by pressure differences on each side of the piston which secures the hook in the holder.

In practice, the piston may be controlled by a liquid pressure, e.g. generated in water.

If the pressure drops because of malfunction, the piston will not be capable of securing the hook, which may therefore be pulled up to the controlling surface vessel. The holder, which secures the hook to the ROV, may also be manufactured with other locking mechanisms, of course, such as electromagnetic ones, which will also be comprised by the present invention.

Claims

PATENT CLAIMS

1. A flushing tool comprising a hook with a release mechanism on an underwater vessel which is remote-controlled from a surface vessel, said hook being adapted to grip a cable buried beneath a seabed after the flushing tool has exposed the cable, cha racte rized in that the hook is secured at its one end to an arm (9) which is pivotally mounted on the flushing tool relative to the remote end of the hook, and that the other end of the hook is connected with one end of a wire (10), while its other end is connected with the surface vessel.

2. A flushing tool according to claim ^characterized in that the wire is connected with the hook via a pivotable link provided on the hook.

3. A flushing tool according to clam 1 or 2, characterized in that the arm is pivotable about an axis perpendicular to the flushing tool.

4. A flushing tool according to claims 1 -3, characterized in that the release mechanism is formed by a hollow (20) on a shaft part of one end of the hook, and that the arm is formed with a hole having dimensions which correspond to the outer dimensions of the shaft part, and that inside the hole a transverse hole is provided, having a diameter which is larger than the diameter of the hollow, and that the transverse hole accommodates a piston with a flange, said flange having a diameter which corre- sponds to the diameter of the transverse hole, while the remaining part of the piston has a diameter which corresponds to the diameter of the hollow.

5. A flushing tool according to claim 4, ch a racterized in that the movement of the piston is provided by liquid pressure, supply lines for the supply of pressure being arranged on each side of the flange of the piston.

6. A flushing tool according to claims 1 -5, characterized in that the hook is provided with means, such as one or more metal detectors, to record a hooked undersea cable.

7. A flushing tool according to claims 1 -6, characterized in that the hook is provided with a blocking mechanism, such as a spring-loaded pawl, to fix a hooked undersea cable.

8. A flushing tool according to claims 1 -7, characterized in that the hook is provided with tools, such as knives or scissors, to sever a hooked undersea cable.