WO2010081504A1

WO2010081504A1 - Submersible cable arrangement

Info

Publication number: WO2010081504A1
Application number: PCT/EP2009/008538
Authority: WO
Inventors: Paul David Hopewell
Original assignee: Rolls-Royce Plc
Priority date: 2009-01-14
Filing date: 2009-12-01
Publication date: 2010-07-22
Also published as: CA2749169A1; GB0900500D0; GB2478487A; KR20110106910A; GB201111016D0; GB2478487B

Abstract

A submersible cable arrangement (10, 20, 40) for laying on the bed of a body of water, the cable arrangement comprising a base (16, 26) and a cable housing portion (15, 30, 46). The base has a width greater than the width of the cable housing portion such that the base protrudes laterally outwardly from the cable housing portion for supporting the cable housing portion on the bed in use. The base comprises a sloping formation (18, 28) interposed between an outer edge of the base and the cable housing portion so as to streamline the cable arrangement. The base and cable housing may be formed as an integral cable sheath or else may be formed as a separate body, which is shaped to allow insertion of a cable therein.

Description

SUBMERSIBLE CABLE ARRANGEMENT

The present invention relates to a cable arrangement suitable for use underwater and more particularly, although not exclusively, to a cable arrangement which can be laid on the surface of a seabed.

It may be necessary to lay submerged cables for a number of reasons, such as, for example, to provide power lines for offshore power generation equipment. Offshore power generators such as wind and tidal turbines are becoming increasingly important in the bid to exploit renewable energy sources. Approaching 10 GW of offshore wind power facility is currently in place in European waters with over 5 GW of capacity being currently developed in the UK alone.

Offshore wind turbines tend to be deployed on a regular grid pattern covering an area of seabed typically 10-100km². Subsea cables are used to connect the turbines into what are known as 'collection strings' and to transport the produced power to the shore for connection to the power grid.

Most offshore wind farms to date have been situated in relatively shallow seas and where the seabed conditions permit the burial of cables. Burial is considered an effective means of physical protection for the cables. The main export cables are of typically higher strategic value than the in-farm collection strings and tend to be buried deeper to guarantee freedom from mechanical damage. Such cables can be buried at a depth of 1 -2 metres below the sea bed as opposed to a depth of less than 1 metre for collector strings. Where tidal currents are such that scouring occurs, it is possible that the cable may become exposed and/or damaged such that additional protective measures may be required. Significant expense is incurred when burying cables in this way and so the installed cost can exceed 3-5 times the cable cost. This problem is further compounded when one considers that strong tidal currents are a prerequisite for effective tidal power generation. In areas where strong currents exist, sand, silt and other removable materials can be swept away, leaving an exposed rock seabed. This can render impractical the burial of subsea cables in many tidal sites. In areas which permit digging of trenches for cables, the trenches may need to be dug to additional depth to ensure cables remain buried.

If cables are laid on the sea bed without being buried, they can roll or otherwise move under the force of a current such that the cable and any associated connectors come under strain. This can compromise the integrity of cable connections, particularly where the cable is connected to a fixed structure.

Whilst the above problems are described in relation to offshore power generation, the present invention is broadly applicable to any instance in which underwater cables are required.

It is an aim of the present invention to provide a submersible cable arrangement for which the above problems are mitigated in a cost-effective manner.

According to the present invention there is provided a submersible cable arrangement for laying on the bed of a body of water, the cable arrangement comprising a base and a cable housing portion, the base having a width greater than the width of the cable housing portion such that the base protrudes laterally outwardly from the cable housing portion for supporting the cable housing portion on the bed in use, wherein the base comprises a sloping formation interposed between an outer edge of the base and the cable housing portion.

The cable housing and/or base portions may be formed substantially of one or more resilient material. The sloping formation may be arranged such that the height of the cable arrangement is lowest towards its outer edge and increases towards the cable housing portion. The cable housing portion may be located substantially centrally of the base. The cable arrangement may be elongate in form such that the base and cable housing portion are in longitudinal alignment. The base may have laterally opposing edges which extend longitudinally along the cable arrangement. The base may have a longitudinal axis with which the cable housing portion may be aligned. The base may be substantially symmetrical about its longitudinal axis.

In one embodiment, the cable housing may comprise an enclosure for a cable which is integrally formed with the base. The base and cable housing may be formed as a sheath for the cable enclosed therein.

In an alternative embodiment, the base and cable housing portion may be shaped such that a cable is insertable therein. The cable housing portion may have an opening through which a cable is insertable. The opening may be elongate in form and longitudinally aligned with the cable arrangement and/or the base axis. The width of the opening may be slightly smaller than the width of the cable to be inserted such that the cable is a push-fit or friction-fit with the cable housing portion. The base and/or cable housing portion may comprise, or else be formed of, a resilient material.

The base may have grip-enhancing formations thereon on a surface arranged for contact with the bed during use. Such surface typically faces away from the cable housing portion. The formations may comprise a plurality of grooves, lands, furrows, ridges, ribs, castellations or other arrangement of peaks and troughs. The peaks and/or troughs may be elongate in form or else discrete raised protrusions or else a network of interconnected protrusions or recesses. Elongate peaks or troughs may be longitudinally, laterally or obliquely aligned with the base. In one embodiment, the peaks and/or troughs are in a zig-zag arrangement. - A -

The sloping formation may be shaped to provide a curved or straight-edged ramp depending inwardly from a lateral edge of the base. A sloping formation may be provided on either side of the cable housing portion. The cable housing portion may comprise a depression between opposing sloping formations. The cable housing portion may be shaped to closely surround a portion of the cable inserted therein. The cable housing portion may be shaped to closely surround greater than half of the cable in section.

In one embodiment, one or more internal cavities may be defined between the base and the sloping formation. The internal cavities may be in fluid communication with the exterior of the cable arrangement by way of apertures in the base or sloping formation. Additionally or alternatively, one or more internal cavities may be inflatable. The, or each, inflatable internal cavity may be in fluid communication with a valve or other inflation formation in order to allow pressurised gas to be fed thereto. The, or each, inflatable cavity may be defined at least in part by a deformable wall which may be formed of an elastic material such that the cavity can expand in response to the applied inflation pressure.

According to a second aspect there is provided a method of laying a cable arrangement according to the first aspect, comprising attaching the cable within the cable housing portion and subsequently submerging the cable arrangement with the cable therein beneath the surface of a body of water.

The cable arrangement may be preformed as a resilient sheath about the cable. Alternatively the cable may be inserted into a separately formed cable housing arrangement prior to laying. The cable arrangement including the cable may be wound on to a drum and deployed there-from upon laying of the cable.

Any of the preferable features described in relation to the first aspect may be applied to the second aspect and vice versa. One or more working embodiments of the present invention are described in further detail below by way of example with reference to the accompanying drawings, of which:

Figures 1a and 1b show respective plan and cross section views of a cable arrangement according to a first embodiment of the present invention;

Figures 2a and 2b show respective plan and cross section views of a cable arrangement according to a second embodiment of the present invention;

Figures 3a and 3b show respective plan and cross section views of a cable arrangement according to a third embodiment of the present invention; and,

Figures 4a and 4b show respective deployed and undeployed states of a buoyancy arrangement which may be applied to the embodiments of any one of figures 1 to 3.

An economic analysis of the benefits of burying cables versus the cost of doing so has indicated that it may be acceptable to lay cables, such as, for example, collector string cables for offshore power generation, on the bed surface rather than burying them.

Turning now to figure 1 , there is shown a first embodiment of the present invention in plan and in section. The plan view of figure 1a shows only a portion of the cable arrangement and it will be appreciated that the cable arrangement would in fact be elongate in form, having a length dependent on the distance to be spanned by the cable.

The cable arrangement 10 comprises a cable 12 about which is formed a body 14. The body 14 generally takes the form of a sheath which encloses the cable 12 therein. Thus the body has a central cable enclosure 15 which is generally tubular in shape so as to define an opening in which the cable is received. In addition the body 14 is profiled so as to provide a generally flat base 16 and sloping side walls 18 on either side of the cable 12. The sloping walls 18 depend from the base 16 and slope upwardly to the cable enclosure region 15. The width of the base 16 in the embodiment of figure 1 is approximately 3 times the diameter of the cable 12. Thus the base 16 protrudes on either side of the cable which is located substantially centrally of the base. The height of the cable arrangement is greatest at the centre of the cable enclosure and tapers towards its peripheral edges.

The body provides a sheath having lateral projections on either side of the cable. In an alternative embodiment, the sheath may be shaped so as to provide a projection on one side of the cable only. Thus the body may take the general shape of a teardrop in section. Such an embodiment would thus allow the cable to roll by approximately 180° in response to a water current once laid but wouid inhibit further roiling thereafter. The cable could thus roll back and forth by approximately a half turn in response to the varying direction of a tidal flow. Whilst such an embodiment is in many ways less preferred than the embodiment of figure 1 , it may be suitable in certain circumstances, for example when current flows predictably in substantially one direction only.

This shape of cable arrangement of figure 1 provides a means to prevent the movement of cables laid on the surface of the seabed. In particular, the shape of the body 14 streamlines the cable 12 and reduces lateral forces on the cable 12 by the flow of water there-over, such as, for example, by way of tides or other currents. The flat base also provides a footing for the cable and reduces the likelihood that the cable can roll or otherwise move once laid.

The cable arrangement of figure 1 may be produced by extrusion of the body 14 about the cable 12 such that the cable and its streamlined sheath are substantially uniform in section along the entire length of the cable. The moulded body can be made of conventional cable oversheath materials, such as medium density polyethylene (MDPE) which has appropriate mechanical strength and resilience and is known to be resistant to chemical attack from seawater. The benefit of this technique is that conventional sheath production techniques can be used to produce a modified body which provides additional stability for the cable therein.

Given that the lateral protrusions of the body 14 are solid or homogeneous in form, it may be beneficial to use a higher density polyethylene, having a density which is comparable to that of water.

The cable 12 and body 14 are thus formed as an integral cable arrangement which can be handled in a manner similar to conventional cables. Careful positioning of the cable on a cable drum or reel will cause only a relatively small reduction of the cable length that can be accommodated on a reel compared to conventional cables.

Friction or grip-enhancing formations 19 are provided on the underside of the base 16. The formations take the form of a series of longitudinally aligned ribs or channels which are integrally formed with the body 14, for example, by extrusion.

Turning now to figures 2a and 2b, there is shown another embodiment of the present invention, in which the cable arrangement 20 comprises a conventional cable 22 which is insertable into a separate body 24. The cable 22 may have a conventional tubular sheath 25 disposed there-about so as to maintain the cable components in a substantially circular sectional formation.

The body 24 of this embodiment is not formed about the cable 22 but is instead formed separately and shaped to receive the cable therein. The body has a base 26 and sloping side walls 28 on either side of a centrally located depression 30 which extends longitudinally along the body 24. The depression 30 may otherwise be described as a gulley or channel and provides a housing for the cable once inserted therein.

The depression 30 is curved in section and shaped to closely surround the cable 22. The wall of the depression generally follows an arc of a circle which extends about an angle greater than 180°. This ensures that the cable is a close push-fit within the depression and helps to prevent unwanted removal of the cable from the body 24. The angle between the opposing edges of the arc forming the depression wall is between 180 and 220°. The resilience of the body material allows for slight resilient deformation of the body in the vicinity of the opposing edges of the depression to allow for insertion of the cable 22 having a diameter slightly greater than the width of the depression opening. Thus the cable fits snugly and securely within the elongate depression 30.

In an alternative embodiment, the depression profile may be of a size to accommodate a range of cable diameters such that the depression may be greater than the diameter of the cable but having an opening of size sufficient to retain the cable once mounted therein.

The opposing edges of the depression may be considered to be shoulder formations which locate above the widest point of the cable and serve to retain the cable in the depression. It will be appreciated by those skilled in the art that other forms of shoulder or push fit fixing may be used.

As in the embodiment of figure 1 , the width of the base is approximately three times the diameter of the cable to be mounted therein. However it is envisaged that the base in either embodiment may be anywhere between approximately two and four times the cable diameter.

The sloping sides 28 of the body 24 in figure 2b are shown as being straight in profile. However they could equally be curved in a manner similar to that of figure 1.

Also visible in figure 2 are internal cavities 32 formed within the body 24 on either side of the depression 30. The cavities extend along the body 24 in a longitudinal alignment. A series of ports or openings 34 are provided through the wall of the body and open in to the cavity so as to allow fluid communication between the cavities and the outside of the body. As can be seen in figure 2a, the ports 34 are spaced along the length of the sloping walls 28 of the body in a linear array.

The cavities 32 and openings 34 allow water to enter the body when submerged. This serves to help stabilise the cable arrangement and to minimise the amount of material used in production of the body. The cavities may be considered to take the form of interstices within the body. In this embodiment, the cavities are substantially triangular in section to match the angular alignment of the sloping side walls 28 and the base 26.

As in figure 1 , the body of figure 2 has a flat base a series of grip-enhancing formations 36 on the underside thereof. Since the body of figure 2 may or may not be extruded, the formations may take different forms to those of figure 1 and may comprise an array of projections arranged obliquely, laterally or else in a zig-zag fashion relative to the longitudinal axis of the body.

The body of figure 2 is formed in individual sections, one of which is shown in figure 2, to simplify transport and handling. Each section may be a convenient length, such as 1 -2m, such that the overall cable arrangement may be formed by attaching a plurality of body sections to the cable in a longitudinal alignment.

Turning now to figure 3, there is shown respective plan and section views of a cable arrangement 40 in which multiple cables 42 are held in a body 44. The body is substantially as described in relation to figure 2 and the same numerals have been applied to denote like features. The description of figure two may be applied to the embodiment of figure 3 save for the following differences.

The body 44 of figure 3 is wider than that of figure 2 and has a plurality of depressions 46 arranged in a side-by-side arrangement. Three depressions are shown in order to allow three cables to be individually located in a corresponding depression. Three single core cables may be used to form a three phase power circuit. The body is shaped such that each depression is spaced from the adjacent depression by a curved projection in the form of a partition 48 or land. The partitions 48 extend longitudinally along the body and have opposing curved walls which form a portion of the depression 46 wall on either side of the partition. Thus the partitions are shaped to closely fit about a portion of a cable 42 inserted in the corresponding depression.

As with the sloping side walls 28, the partitions extend upwardly from the base to a height above the widest portion of the depression or cable inserted therein. Thus the partitions form shoulders at their upper edge in a manner as described above such that the cables are a push-fit into the depressions.

Where multiple cables are to be laid together, such as in highly-rated power circuits requiring the use of three single core cables instead of a single three core cable, the arrangement shown in Figure 3 may be appropriate. In practice, a body 44 may be provided which is shaped to accommodate two or more cables in a side-by-side arrangement. The width of the base relative to the cable diameters will be dependent on the number of cables to be accommodated and the additional width due to the sloped sides. For example, for a two cable arrangement, the base will be typically between three and five times the width of a single cable. For a three-cable arrangement, the base will be typically between four and six times the width of a single cable.

In any embodiment, the width of the base will be greater than the width of the cable(s) accommodated therein by at least the width of the sloped edges.

Turning now to figure 4, there is shown a further embodiment of the present invention comprising selectively deployable buoyancy means. Such buoyancy means may be applied to any of the above-described embodiments. The body 50 of figure 4 is substantially as described in relation to figure 2 and the same numerals have been applied to denote like features. The description of figure 2 may be applied to the embodiment of figure 4 save for the following differences. In figure 4, inflatable lobes 52 with a highly elastic skin or wall 54 have been applied to the sloping walls 28 of the body. The lobes 52 are in fluid communication with the internal cavities 56 via the ports 34. In this regard, the elastic walls of the lobes are attached to the sloping walls or else integrally formed therewith. A fluid line connection point 58 is provided on one side of the body such that pressurised air can be supplied to the internal cavities 56 of the body when a line 62 is attached. In this embodiment, the internal cavities 56 to which the lobes are attached are not in fluid communication with the surrounding water. Accordingly such cavities are in communication with other internal cavities and the fluid line connection point 58 only and ports from those cavities to the exterior are either blocked or else not present in this embodiment.

In one version of this embodiment, some cavities may be open to the surrounding water and other cavities, which have associated inflation lobes, may be closed. In such an embodiment, a network of open cavities 56 may be provided which are interconnected and which are isolated from the open cavities.

The internal cavities 56 on opposing sides of the body 50 are connected by a lateral passageway 60 such that pressure applied to one cavity 56 is communicated to the opposing cavity.

Controlled inflation and deflation of the buoyancy lobes may be achieved using a suitable air compressor which may be mounted on a ship or other vessel. This allows a very simple method of cable installation and retrieval. During installation the fully inflated cable body is drawn by appropriate vessels between the required points (for example between the locations of two tidal or wind turbines). Once the desired position is achieved the air is gradually let out of the inflation lobes through an air line which terminates on one of the vessels, allowing the cable and stabilising body to slowly settle in a controlled manner onto the seabed. When this is completed, the air line is disconnected from the cable system for example by the use of a suitable remotely operated vehicle (ROV), which, given the nature of the operations it is to perform, need not be unduly large, complex or expensive to operate.

Retrieval of the cable for maintenance, repair or jointing operations is simply the reversal of the installation procedure. A suitable ROV will descend to the desired section of cable and attach an air line, the other end of which is connected to a compressor on board the supporting vessel. Pressurised air is then applied under control until the cable arrangement rises to the surface, so that the cable arrangement can be maintained or removed. As the cable rises towards the surface, the air pressure should be reduced to avoid excessive pressure differential and possible bursting of the lobes.

The system of cable stabilising body and lobes is made in lengths compatible with the length of cable to be installed, or may alternatively be integrated into an extended cable sheath and stabilising base as shown in Figure 1. The lobes are dimensioned such that fully inflating them with air (as shown in figure 4a) will give sufficient buoyancy to lift the stabilising base and cable to the surface.

In an embodiment in which buoyancy lobes are not provided, the cable arrangement may be deployed from vessel in a conventional manner. Care will be required to ensure that the cable arrangement does not twist during deployment. The embodiment of figure 2 is in many ways preferred in this regard since individual body sections attached to the cable may better be able to accommodate any torsion which may occur during installation.

In any of the above-described embodiments, the material used for the cable stabilising body is stable and resistant to seawater attack for a long service life, has a density comparable to or greater than seawater so that the assembled cable arrangement does not float and is suitable for a moulding or extruding manufacturing technique to keep production costs low. A suitably resilient material so as to allow laying of the cable arrangement, including the cable, from a floating vessel or a fixed structure proximate the water surface. The resilient nature of the material typically also allows the cable arrangement to be wound onto and deployed from a drum. Materials such as MDPE may also be recyclable.

The embodiments described above in relation to figures 1 to 4 are stable for tidal currents in all expected directions. That is to say that the reversal of the flow over the cable arrangement would not dislodge the cable due to the symmetry of the cable arrangement about its centre-line. In practice this means that the cable should not rotate 180 and flip back in response to the ebb and flow of the tide. If it did so, repeatedly it could cause excessive wear of the sheath and also subject the armouring and conductors to unnecessary additional mechanical fatigue.

In addition to the potential for reduced cost of cable laying, it has also been found that the seabed surface laying of cables according to the present invention can have a beneficial impact on the cable rating compared to buried cables. The flow of water over the cable arrangement allows for a significant level of thermal conductivity between the cable arrangement and the water such that any heat emanating from the cable is quickly dissipated.

Any features described above in relation to any one embodiment of the invention are interchangeable with any of the other embodiments wherever practicable. For example the cavities and or associated ports of figure 2 may be applied to the embodiment of figure 1.

Claims

A submersible cable arrangement for laying on the bed of a body of water, the cable arrangement comprising:

a base and a cable housing portion formed of a resilient material,

the base having a width greater than the width of the cable housing portion such that the base protrudes laterally outwardly from the cable housing portion for supporting the cable housing portion on the bed in use,

wherein the base comprises a sloping formation interposed between an outer edge of the base and the cable housing portion.

A submersible cable arrangement according to claim 1 , wherein the sloping formation is arranged such that the height of the sloping formation is lowest towards its outer edge and increases towards the cable housing portion.

A submersible cable arrangement according to claim 1 or 2, wherein the cable housing portion is located substantially centrally of the base and longitudinally aligned therewith.

A submersible cable arrangement according to any preceding claim, wherein the cable housing comprises an enclosure for a cable akin to a sheath which is integrally formed with the base.

A submersible cable arrangement according to any one of claims 1 to 3, wherein the cable housing portion is shaped such that a cable is insertable therein. A submersible cable arrangement according to claim 5, wherein the cable housing portion take the form of a depression located between opposing sloping formations.

A submersible cable arrangement according to claim 5 or 6, wherein the cable housing portion has an opening through which a cable is insertable, the opening being elongate in form and having a width smaller than the width of the cable to be inserted such that the cable is a push-fit or friction-fit with the cable housing portion.

A submersible cable arrangement according to claim 5, 6 or 7, wherein the cable housing portion is shaped to closely surround greater than half of the cable in section.

A submersible cable arrangement according to any one of claims 5 to 8, wherein the base is formed as series of base portions which can be longitudinally arranged to accommodate a length of cable.

A submersible cable arrangement according to any preceding claim, wherein the base and cable housing portions are unitarily formed of a resilient material.

A submersible cable arrangement according to any preceding claim, wherein the base has grip-enhancing formations thereon on a surface of the base which is arranged for contact with the bed during use.

A submersible cable arrangement according to claim 11 , wherein the grip- enhancing formations comprise a plurality of peaks and troughs.

A submersible cable arrangement according to any preceding claim wherein a sloping formation is provided on either side of the cable housing portion. A submersible cable arrangement according to any preceding claim, wherein one or more internal cavities are defined between the base and the sloping formation.

A submersible cable arrangement according to claim 14, wherein the internal cavities are in fluid communication with the exterior of the cable arrangement by way of apertures in the base or sloping formation.

A submersible cable arrangement according to claim 14 or 15, wherein the one or more internal cavities have a deformable wall portion such that the internal cavity is inflatable under fluid pressure.

A method of laying a cable arrangement according to any one of claims 1 to 16, comprising:

Locating a cable within the cable housing portion and

subsequently submerging the cable arrangement with the cable therein beneath the surface of a body of water.