GB2345198A

GB2345198A - Subsea cable protection

Info

Publication number: GB2345198A
Application number: GB9828223A
Authority: GB
Inventors: Steven Howlett; Peter Keel
Original assignee: Odebrecht Oil & Gas Services L
Current assignee: Odebrecht Oil & Gas Services L
Priority date: 1998-12-22
Filing date: 1998-12-22
Publication date: 2000-06-28
Anticipated expiration: 2018-12-22
Also published as: GB2345198B; GB9828223D0

Abstract

A protective structure for use in protecting a cable to be laid on a seabed, comprising a body member (2) formed from a structural material. The body member (2) is provided with an elongate culvert (8) which is open at the top for receiving a cable. The structure may be provided with a tunnel (10) which is open at the bottom. The structure is preferably made of concrete. The structure may have a hole (12) for receiving a securing pin.

Description

SUBSEA CABLE PROTECTION This invention relates to a protective structure for use in protecting a cable laid on a seabed, and to a method of laying a cable on a seabed using the protective structure. The invention is of particular use in protecting fibre optic or umbilical cables which cross over existing subsea pipelines. The term"cable" will be used herein to include fibre optic cables and umbilicals.

When subsea'cables are laid, a trench is typically dug in the sea bed to receive the cable and protect it from damage caused by trawlers. It is frequently necessary for cables to cross the paths of subsea pipelines which lie on the sea bed or are buried to only a shallow depth. In these cases it is necessary for the cable to be laid over the pipeline without causing damage to the pipeline and without leaving the cable vulnerable to trawler damage.

One method which is used to achieve this is to lay a series of flexible mats over the region of the pipeline to be crossed. The cable is then laid on top of the mats, and a plurality of further layers of mats is laid on top of the cable. The mats are not over-trawlable, ie, the mats are expendable and are sacrificed to protect the underlying cable when a trawler passes over.

According to a first aspect of the present invention there is provided a protective structure for use in protecting a cable to be laid on a seabed, comprising a body member formed from a structural material, the body member having a lower surface for lying on a sea bed and an upper surface which is provided with an elongate culvert which is open at the top for receiving a cable.

By using a culvert of appropriate depth, so that the cable is disposed beneath the upper surface of the body member, a cable laid in the culvert is protected against damage by trawlers. For maximum protection it is preferred that the culvert extends along the entire length of the upper surface.

The lower surface may be substantially flat, for use in laying a cable over a pipeline which is buried under the sea bed. Alternatively, the lower surface may be provided with an elongate tunnel which is open at the bottom, for locating over a pipeline which lies wholly or partially on the sea bed.

The long axis of the tunnel is preferably substantially at right angles to the long axis of the culvert, but other non-zero angles could also be used.

To provide a platform which is stable for long periods of time on the sea bed, and for reasons of cost, it is preferred the body member is formed from concrete, notably reinforced concrete.

The culvert may be of any suitable depth to receive a cable. The minimum depth of the culvert is preferably in the range 10 to 200 mm, notably 50 to 150 mm.

To enable the culvert to be readily spotted on the sea bed so that a cable can be accurately laid in the culvert, it is preferred that the culvert is of a different colour to its surrounds. We have found that the culvert is particularly easy to see if its surfaces are coloured yellow.

To enable the body member to occupy a large enough crossing corridor or'lay corridor", it is preferred that the body member is at least 3 metres long in the direction of the culvert long axis. In a particularly preferred embodiment, the body member is from 4 to 6 meters long.

Where there is any doubt as to the precise location of a pipeline which is buried shallowly under the sea bed, a particularly long crossing corridor is desirable, to ensure that the structure does in fact bridge the pipeline.

While there is no theoretical limitation to the length of the body member, there are practical limitations.

For example, too big a body member will weigh too much for convenient road transport from its point of manufacture. This problem may be overcome by using a plurality of body members with their culverts lined up end to end.

In a preferred embodiment, means are provided for connecting one body member to another so that the culverts line up substantially end to end. Any suitable connecting means may be used, for example a pin and socket, or a form locking arrangement whereby two interconnectable members lock together by virtue of their shape, in the manner of a dovetail joint. In a preferred embodiment a pair of body members are provided, one with a substantially vertical pin member and the other with a socket or hole for locating around the pin so that the body members are pivotably connected. This enables a first body member to be placed on the sea bed, followed by the second of the pair, disposed so that the pin is located in the socket. The body members may then be pivotably adjusted to align the culverts end to end. Any number of body members may, of course, be connected together in this manner. Connecting the body members in this manner allows for some imprecision in the initial alignment of the pin and socket, for example by using a socket with a belled open end.

Location and adjustment of the body members may be achieved by divers or a remote operated vehicle (ROV) while the body members are lowered by means of a crane or the like.

In a preferred embodiment, one or more regions of the upper surface adjacent to the culvert are provided with a plurality of fronds which function as an anti-scour mattress. The fronds, for example, polypropylene strands, function like seaweed to attract and retain sediment dropout from the sea. Over time, sand and other sediment builds up around the structure, increasing stability and helping to protect against erosion of the local sea bed around the structure.

The body member will usually be heavy enough to sit firmly on the sea bed under its own weight. However, it would also be possible to provide the structure with securing means; for example it could be bolted down or the body member could be provided with a steel or other skirt to penetrate the sea bed.

The protective structure may be lowered and positioned by eye, but it is preferred that it is provided with at least one signal means for indicating its location to a remote controller, for example on board a boat. The or each signal means may comprise a transponder which may be mounted in a pocket in the body member. Preferably, the body member is provided with four signal means to enable it to be remotely located over a pipeline.

According to another aspect of the present invention there is provided a method of protecting a cable to be laid over a subsea pipeline, the method comprising locating a protective structure in accordance with Claim 1 on the sea bed so that the culvert overlies the pipeline with the long axis of the culvert at a nonzero angle with respect to the local axis of the pipeline, and locating the cable in the culvert so that the cable spans the pipeline, the culvert being dimensioned so that when the cable is fully received therein, the cable does not project above the local upper surface of the body member.

For added protection of the pipeline, a known flexible protective mattress may optionally be laid over the pipeline before the protective structure is located over the pipeline.

The invention will now be further described by way of example, with reference to the following drawing in which: Figure 1 is a plan view of a protective structure in accordance with one embodiment of the present invention; Figure 2 is an end elevational view of the protective structure shown in Figure 1, viewed in the direction of the arrow; Figure 3 is a perspective view of the protective structure shown in Figure 1 ; Figure 4 is a plan view of a protective structure in accordance with another embodiment of the present invention; Figure 5 is an end elevational view of the protective structure shown in Figure 4, viewed in the direction of the arrow; Figure 6 is a perspective view of the protective structure shown in Figure 4; Figure 7 is a plan view of a protective structure in accordance with a further embodiment of the present invention; Figure 8 is an end elevational view of the protective structure shown in Figure 7; Figure 9 is a side elevational view of the protective structure shown in Figure 7; Figure 10 is a perspective view of the structure of Figure 7; and Figure 11 is a side elevational view of the structure of Figure 7 in use on a sea bed.

The protective structure shown in Figures 1 to 3 is for use in laying a cable over a pipeline which is buried under the sea bed. The structure comprises a body member 2 formed from a concrete slab weighing about 15 tonnes. The body member 2 has a lower surface 4 which is substantially flat for lying on the sea bed, and an upper surface 6 which is provided with an elongate culvert 8 for receiving a cable. The culvert 8 extends from a first end 3 of the body member 2 to a second end 5. It is about 5 metres long, and 150 mm deep. The culvert 8 is painted bright yellow so that it stands out clearly from the surrounding concrete colour when under water. The end portions of the culvert are flared to facilitate entry of a cable.

The upper surface 6 is provided with a plurality of shackles 20 by which the body member 2 may be raised or lowered on a crane, sling or the like. Pockets 22 are provided, in each of which may be housed a transponder to enable the location of the body member to be remotely sensed.

To lay a cable over a pipeline which is shallowly buried under the sea bed, a trench is dug in the sea bed for receiving the cable, and terminates before the pipeline. The trench digging is resumed starting an equal distance on the other side of the pipeline. The body member 2 is then lowered into position so that one end 3 abuts against an end of the trench and the other end 5 lies over or towards the pipeline. If the body member 2 fully spans the pipeline, so that the other end 5 abuts the trench on the other side, only a single body member 2 is required. However, where there is doubt as to the precise location of the buried pipeline, a wider gap may be required between the end of one trench and the beginning of the other trench (the lay corridor). Such a wider gap may be bridged by the use or more than one body member 2, the body members being connectable together as described below.

The first protective structure shown in Figures 1 to 3 includes a recess 16 which has a vertical steel tube 14 that functions as a pin. The second protective structure shown in Figures 4 to 6 has a projection 18 which is a complementary shape to the recess 16. The projection 18 is provided with a socket 12 for receiving the pin 14. Once the first protective structure is in place on the sea bed, the second protective structure may be lowered into position so that the pin 14 locates in the socket 12. The second structure may then be pivotably adjusted, if necessary, so that the culverts are lined up end to end to permit a cable to be laid along both culverts. The location of the pin in the socket and/or the final adjustment may be effected by one or more divers or by an ROV.

It will be understood that each structure could optionally be provided with a pin at one end and a socket at the other, so that any number of slabs could be hingedly connected end to end if so desired.

Once the lay corridor has been bridged, a cable is laid along one trench, along the culvert (s) 8, and continues to be laid along the other trench. The depth of the culvert 8 is selected so that the cable does not project above any part of the top surface 6 of the body member 2. The cable is therefore protected against over-trawling by the reinforced concrete body member 2.

To ensure that the cable in the culvert remains entirely below the local upper surface of the body member it is preferred to keep the cable, or the cable housing, under some tension.

Turning now to the embodiment shown in Figures 7 to 11, the lower surface 4 is provided with a tunnel 10 for fitting over a pipeline 36 which is wholly or partially disposed above the sea bed 30, as shown in Figure 11.

An optional bitumen mattress 32 has been laid over the pipeline 36, with a segmented concrete mat 34, as added protection for the pipeline. Fronds 24, comprising lengths of polypropylene, are provided on the upper surface of the body member 2. These fronds act as artificial seaweed and trap sand 28, helping to build up a protective barrier against scour.

The fronds 24 are preferably secured under a tarpaulin 26 by means of ties 38 before use, as illustrated in Figures 3,6 and 10. Once the body member 2 is located on the sea bed, the ties 38 are released by a diver or by an ROV, and the tarpaulin is removed to allow the fronds to be deployed.

The invention provides a bridging unit for a simple pre-lay solution for installing cables over subsea pipelines, in which a cable can be installed and immediately protected against trawlers after having been pulled across the bridging unit.

Claims

CLAIMS 1. A protective structure for use in protecting a cable to be laid on a seabed, comprising a body member formed from a structural material, the body member having a lower surface for lying on a sea bed and an upper surface which is provided with an elongate culvert which is open at the top for receiving a cable.
2. A protective structure as claimed in claim 1, wherein the lower surface is provided with an elongate tunnel which is open at the bottom, for locating over a pipeline which lies wholly or partially on the sea bed.
3. A protective structure as claimed in claim 1 or claim 2, in which the body member is formed from concrete.
4. A protective structure as claimed in any one of the preceding claims, wherein the culvert is 10 to 200 mm deep.
5. A protective structure as claimed in claim 4, wherein the culvert is 50 to 150 mm deep.
6. A protective structure as claimed in any one of the preceding claims, wherein the culvert is of a different colour to its surrounds.
7. A protective structure as claimed in claim 6, wherein the culvert is coloured yellow.
8. A protective structure as claimed in any one of the preceding claims, wherein the body member is at least 3 meters long in the direction of the culvert long axis.
9. A protective structure as claimed in claim 8, wherein the body member is from 4 to 6 meters long.
10. A protective structure as claimed in any one of the preceding claims, further including a plurality of fronds provided on one or more regions of the upper surface adjacent to the culvert.
11. A protective structure as claimed in any one of the preceding claims, which is provided with at least one signal means for indicating its location to a remote controller.
12. A protective structure as claimed in any one of the preceding claims, wherein the culvert extends along the entire length of the upper surface.
13. A protective structure as claimed in any one of the preceding claims, further including means for connecting the body member to the body member of another protective structure so that the culverts line up substantially end to end.
14. A pair of protective structures as claimed in any one of the preceding claims, one being provided with a substantially vertical pin member and the other being provided with a socket or hole for locating around the pin so that the body members may be pivotally connected together and arranged when connected so that the culverts line up substantially end to end.
15. A protective structure for use in protecting a cable to be laid on a seabed, substantially as herein described with reference to or as shown in the drawing.
16. A method of protecting a cable to be laid over a subsea pipeline, comprising locating a protective structure in accordance with Claim 1 on the sea bed so that the culvert overlies the pipeline with the long axis of the culvert at a non-zero angle with respect to the local axis of the pipeline, and locating the cable in the culvert so that the cable spans the pipeline, the culvert being dimensioned so that when the cable is fully received therein, the cable does not project above the local upper surface of the body member.
17. A method as claimed in claim 16, further including the step of locating a flexible protective mattress over the pipeline before locating the protective structure over the pipeline so that it overlies the mattress.
18. A method of protecting a cable to be laid over a subsea pipeline, substantially as herein described with reference to the drawing.