Patent Description:
As mentioned therein, submarine power cables utilize a water barrier layer to keep critical components in the interior of the cable dry. The water barrier layer should completely block convection or diffusion of water, as an ingress of moisture can ultimately lead to a failure of the cable. As used herein, the terms "water" and "moisture" may be used interchangeably.

In the event that the water barrier layer fails, and water is able to enter into the cable, it is difficult to identify the precise location of the failure, given the often-times great length of the cables. This is particularly challenging in the case of submarine cables which cannot be easily inspected visually. It is therefore desirable to be able to detect and localize the point of ingress of water or moisture. It is also often the case that an occurrence of ingress of water is only first recognized when the cable fails. It is therefore further desirable to detect an ingress of water quickly, so that corrective measures may be taken before failure of the cable. Knowing the water penetration position limits the time to repair (typically <NUM>% of repair time is due to searching the fault) with faster return to operation of the energy assets.

It is known to utilize an optical fiber arrangement to detect ingress of moisture into an electrical power cable past a water barrier layer. <CIT> describes an arrangement where an optical fiber is wrapped with a water-swellable material, with the wrapped optical fiber placed in the interior of an elongated, rigid or semi-rigid, water permeable, cylindrical sheath. <FIG>, <FIG> and <FIG> show the cylindrical sheath arranged in interstices of a cable, interior to a water barrier layer. In the event water gets past the barrier layer, the water will diffuse to the interior of the cable, where it will enter the rigid sheath, causing the swellable material to expand. As described therein, the rigid sheath is necessary to direct the pressure of the expanding material inwards towards the optical fiber, which then becomes squeezed by the pressure of the expanding material. The site of the resulting deformation of the optical fiber can be localized by means known in the art of optical fibers, by observing attenuation and propagation characteristics of the optical fiber.

Likewise, <CIT> describes a moisture ingress detection device comprising an optical fiber wrapped with a water-swellable material, with the wrapped optical fiber placed in the interior of an elongated, rigid or semi-rigid, water permeable, cylindrical sheath. This publication does not disclose that the device is arranged inside a cable, but rather is utilized to detect moisture in buildings or other structures. As with <CIT>, this publication describes the rigid, cylindrical sheath as necessary to direct the pressure of the expanding swellable material inwards towards the optical fiber.

While perhaps effective at detecting moisture, the devices described in <CIT> and <CIT> cannot be effectively used in a dynamic power cable. As described in <CIT>, a dynamic submarine power cable, a term of art known to those skilled in the art of submarine cables, is one that is subject to mechanical loads imposed during dynamic movement of the cable from wave motions and underwater currents. Such cables are often suspended from offshore structures, such as oil wells or wind power installations. The desired lifetime of a submarine cable is often from <NUM>-<NUM> years, therefore the components of a dynamic cable must be able to withstand sustained exposure to such mechanical loads for long periods of time.

One of the critical components for maintaining the structural integrity of a dynamic submarine power cable is the water barrier layer. A conventional water layer barrier is typically manufactured by a continuous or discontinuous extrusion of a seamless tube, often comprising lead or a lead alloy due to its extrudability and ductility. Such lead water barriers are not optimal for dynamic power cables, however, since lead possesses low fatigue resistance, and is therefore not well suited to the mechanical loads imposed on a subsea dynamic power cable by the cyclic movement of wave motions and underwater currents. <CIT> thus describes alternate materials and alloys suitable for manufacturing water barrier layers for a dynamic power cable. Common among them, however, is the need to tightly arrange the water layer barrier about an underlying intermediate layer in a highly compressed manner, to provide structural rigidity to the cable in the face of high pressure and dynamic forces. Such underlying intermediate layer is often a buffer layer between the cable core and the water barrier constituting a rubber layer, swellable tape or pre-impregnated tape which include a material with higher viscosity compared to the tape. Ideally the buffer layer will comprise a dominating fraction of high bulk modulus material.

In order to be most effective, it would be desirable to arrange a moisture ingress detection device as close as possible to, and preferably in direct contact with, the water barrier layer. Water passing through the layer could then be detected more quickly and precisely. If such a device is arranged in interstices further to the interior of the cable, as is the case with the devices described in <CIT> and <CIT> it will take time for the water to diffuse to the device, and the water may travel longitudinally away from the point of ingress before encountering the device. The devices described in <CIT> and <CIT> are however not suitable to be arranged adjacent to a tightly arranged, highly compressed barrier layer in a dynamic cable. The rigid cylindrical sheath disclosed in these references would cause a deformity or bulging of the water barrier layer that could negatively impact the rigidity and structural integrity of the layer. There is therefore a need for a moisture detecting device that may be arranged in the tight, compressed space between a water barrier layer and an underlying layer in a dynamic power cable.

The present invention provides a cable arrangement comprising a moisture ingress detection device. The cable in which the moisture ingress detection device is arranged is preferably a submarine dynamic power cable, but one skilled in the art will recognize that the moisture ingress detection device may advantageously also be utilized with other cable designs, for example static submarine cables or other cables having a tightly arranged water barrier layer.

As used herein the following terms have the following definitions:.

According to one aspect, the present invention provides a moisture ingress detection device comprising an elongated optical fiber of the type known in the art, surrounded by a water-swellable material. Optical fibers of this type typically comprise a glass filament surrounded by one or more protective acrylate layers. According to one aspect, the water-swellable material is a water-swellable tape.

According to another aspect, the invention provides a cable arrangement in which the moisture ingress detection device of the invention is installed immediately to the interior of, and in direct contact with, a tightly arranged water barrier layer of a submarine cable, between the water barrier layer and a next adjacent intermediate layer of the cable, and in a manner that avoids abrupt bulging of the water barrier layer that could otherwise negatively impact the structural integrity of the cable. The moisture detecting device is arranged such that, if water or moisture passes the water barrier layer, the swellable material will expand and exert pressure on the optical fiber, squeezing and deforming the fiber. The location of the pressure and/or deformity (and thereby the location of moisture ingress) can be localized by techniques known in the art of fiber optic cables by observing attenuation and propagation characteristic of the optical fiber. The cable arrangement of the invention thus further comprises equipment known in the art to propagate and observe signals though the optical fiber. Because the moisture ingress detection device is arranged in direct contact with the water barrier layer, the sensitivity of the device is optimized, as the device does not rely on water diffusing further into the interior of the cable.

In one embodiment, in contrast to the prior art, the optical fiber and surrounding water-swellable material does not utilize a surrounding rigid or semi-rigid sheath to direct pressure from the expanding swellable material towards the optical fiber. Rather, the moisture detecting device relies on the tightness of the wrapping of the water barrier layer about the next adjacent layer to provide the necessary constriction to direct the pressure of the expanding material inward towards the fiber. According to one aspect of this embodiment, a water-swellable tape may be arranged about the optical fiber by folding the tape along its longitudinal axis, or by sandwiching the optical fiber between two layers of tape. Edges of the tape may be closed by an adhesive or by stitching or by other appropriate sealing means. This embodiment of the moisture ingress detection device, having no outer rigid sheath, will therefore be thin enough to avoid any abrupt bulging of the water barrier layer.

In another embodiment, the optical fiber with surrounding swellable material is arranged in an improved pressure-directing sheath, designed to prevent the device from causing an abrupt bulging in the water barrier layer. The improved sheath of this embodiment is in the form of an elongated, shape-following guide element having one or more passages in which the moisture detecting device is arranged. The shape-following guide element has a cross-sectional shape adapted to follow the curvature of the water barrier layer, and tapers from a lateral midpoint of maximum thickness to narrower lateral ends. The shape-following guide element thus disperses the width of the moisture ingress detection device gradually about a portion of the circumference of the water barrier layer in order to avoid an abrupt bulging of the water barrier layer. According to one aspect, the shape-following guide element is made of an elastic material, for example rubber or a synthetic material with similar elastic properties, such that the guide element bends to follow the curvature of the water barrier layer. As is the case with the previously described embodiment, the moisture ingress detection device is arranged in direct contact with the water barrier layer, in this instance with the shape-following guide element of the device being in contact with the water barrier layer. While this embodiment requires moisture to diffuse though that material of the guide element to reach the optical fiber, this embodiment nonetheless optimizes sensitivity by its immediate proximity to the water barrier layer, made possible by the shape-conforming material of the guide element.

According to one aspect, the invention comprises:
A power cable arrangement, comprising a power core comprising one or more conductors, an intermediate layer disposed about the power core, a water barrier layer tightly arranged about the intermediate layer, and a moisture ingress detection device arranged between the water barrier layer and the intermediate layer in direct contact with the water barrier layer, the moisture ingress detection device comprising an elongated optical fiber surrounded by a water-swellable material, the swellable material arranged to expand upon contact with moisture and exert a pressure on the underlying optical fiber sufficient to cause an observable change in the attenuation and/or propagation characteristics of the optical fiber.

A method for detecting the ingress of moisture into a submarine cable having a water barrier layer is also provided. The method comprises the steps of providing a power cable arrangement according to the invention and monitoring the optical fiber for changes in propagation and/or attenuation characteristics indicative of a localized deformation of the optical fiber. Changes in propagation and/or attenuation are indicative of a localized deformation of the optical fiber.

<FIG> illustrate a first embodiment of a moisture ingress detection device <NUM>, for use in a cable arrangement illustrated in <FIG> in which no outer, pressure-directing sheath is employed. An optical fiber <NUM> is surrounded by a length of water-swellable tape <NUM> folded along its longitudinal axis. Optical fiber <NUM> comprises a glass filament <NUM>, surrounded by a first acrylate layer <NUM> and a second acrylate layer <NUM>. Edges <NUM> of tape <NUM> are sealed, by an adhesive <NUM> as shown in <FIG>, or by stitching <NUM> as shown in <FIG>.

<FIG> illustrate a second embodiment of a moisture ingress detection device <NUM>, for use in a cable arrangement illustrated in <FIG> in which no outer, pressure-directing sheath is employed.

According to this embodiment, an optical fiber <NUM> is sandwiched between two pieces of swellable tape <NUM>, the edges <NUM> of which are sealed by adhesive <NUM> or stitching <NUM>.

<FIG> and <FIG> illustrates a submarine cable arrangement <NUM>, comprising the moisture ingress detection device <NUM> of <FIG>. The device <NUM> of <FIG> would be arranged in a similar fashion. As shown, cable arrangement <NUM> comprises a power core <NUM> comprising one or more conductors <NUM>. The power core is arranged in an external sheath <NUM>, with interstices between conductors occupied by filler elements <NUM>. It should be noted that the illustrated arrangement is simplified, as a power cable arrangement may comprise additional layers and structures not illustrated, such as armor layers, signal carrying cables, various insulations layers etc..

Conductor <NUM> is illustrated in more detail in <FIG>. As shown, conductor <NUM> comprises a copper conductor element <NUM> surrounded by a first semiconductor layer <NUM>. Outside the first semiconductor layer <NUM> is a layer of insulation <NUM>. Arranged about the insulation layer <NUM> is a second semiconductor layer <NUM>. Arranged outside the second semiconductor <NUM> is an intermediate layer <NUM>. Intermediate layer <NUM> according to one aspect of the invention is a buffer layer between the cable core and the water barrier comprising a rubber layer, swellable tape or pre-impregnated tape which include a material with higher viscosity compared to the tape. Ideally the buffer layer will comprise a dominating fraction of high bulk modulus material. Tightly arranged about the intermediate layer <NUM> is a metallic water barrier layer <NUM>. According to one aspect of the invention the cable arrangement is a dynamic submarine power cable and the water barrier layer is made of a CuNi alloy.

As further shown in <FIG>, a moisture detection device <NUM>, as illustrated in <FIG> or <FIG>, is arranged between the water barrier layer <NUM> and intermediate layer <NUM>. Because water barrier layer <NUM> is tightly arranged (as that term was previously defined), moisture detection device <NUM> will be firmly engaged and constricted between the water barrier layer and the intermediate layer. While the drawing illustrates device <NUM> in an indented pocket in the intermediate layer, the actual constriction of the device may be more linear or shape conforming depending upon the rigidity of the material of the intermediate layer.

In the event the water barrier <NUM> were to leak, moisture would diffuse along the interface between the water barrier layer and the intermediate layer (or through the material of the intermediate layer itself) until it contacts the water-swellable material of the detection device, causing the swellable material to expand. Because of the constriction of the device caused by the tight arrangement of the water barrier layer, the expanding swellable material will exert pressure on the underlying optical fiber, causing a deformation that can be observed and located according to techniques known in the art of optical fiber signal transmission.

As can be seen, in <FIG>, because no external rigid sheath is used in the embodiments of the moisture ingress detection device <NUM> illustrated in <FIG> and <FIG>, the moisture ingress detection device is, according to one aspect of the invention, arranged such that the water-swellable tape <NUM> is immediately adjacent to, and in direct contact with, water barrier layer <NUM>. This direct contact improves the sensitivity of the moisture ingress detection device as the swellable tape will be able to quickly react to ingress of moisture without the need to rely on diffusion of the moisture further into the interior of the cable. Furthermore, the relatively thin cross-sectional profile of the embodiments from <FIG> and <FIG> do not cause a bulging of the water barrier layer that can negatively impact the structural integrity of the cable. In other words, the relative thickness of the moisture ingress detection device compared to the circumference of the water barrier layer is such that no integrity-degrading bulging is caused in the water barrier layer.

<FIG> and <FIG> illustrate a third embodiment of the moisture ingress detection device, identified as moisture ingress detection device <NUM>, comprising an optical fiber <NUM>, surrounded by a swellable material <NUM>, arranged in an elongated, shape-following guide element <NUM>, for use in a cable arrangement illustrated in <FIG>. The optical fiber <NUM>, as described above, typically comprises a glass filament <NUM>, surrounded by first and second acrylate layers <NUM> and <NUM>.

According to this embodiment of moisture ingress detection device <NUM>, optical fiber <NUM> with surrounding swellable material <NUM> is arranged in one or more passages <NUM> in shape-following guide element <NUM>, the passages dimensioned to directed pressure from expansion of the swellable material towards the optical fiber. <FIG> shows an alternative with two passages <NUM>, while <FIG> shows an alternative with a single passage <NUM>.

As shown in <FIG>, shape-following guide element <NUM> has a cross-sectional shape with a lateral midpoint <NUM> of maximum thickness, tapering to lateral ends <NUM> having a thickness less than the midpoint <NUM>. According to one aspect, shape-following guide element <NUM> is made of an elastic material, such as rubber or a synthetic material with similar elastic properties.

<FIG> illustrates a cable arrangement <NUM> having the same construction as the cable arrangement from <FIG>, except that the moisture detection device that is utilized is the embodiment <NUM> from <FIG> and <FIG>. <FIG> shows a detailed view of a conductor <NUM>, again having the same construction as the conductor shown in <FIG>, with the device <NUM> again being the device illustrated in <FIG> and <FIG>. As shown, the shape following guide element <NUM> is arranged between metallic water barrier <NUM> and intermediate layer <NUM>, preferably in direct contact with water barrier <NUM>. In this instance the constriction required to direct pressure from expanding water-swellable material toward the optical fiber is provided by passages <NUM>, alone or in combination with the tight arrangement of the water barrier layer about the intermediate layer.

When installed between the tightly arranged water barrier layer and the next adjacent intermediate layer, the elastic material of the shape-following guide element <NUM> will bend to conform to the curvature of the water barrier layer. The material of guide element <NUM> is preferably water permeable, such that moisture that passes the water barrier layer may diffuse though the material to the optical fiber <NUM> within passage <NUM>. Alternatively, openings could be provided from the exterior of shape-following guide element <NUM> to the interior of passages <NUM>.

As is the case with the embodiment in <FIG>, the cable arrangement <NUM> shown in <FIG> comprises moisture ingress detection device <NUM> arranged between the tightly arranged water barrier layer and the next adjacent intermediate layer, preferably in direct contact with the water barrier layer in order to optimize the sensitivity of the moisture ingress detection device <NUM>. Whereas the embodiment shown in <FIG> has the swellable tape <NUM> in direct contact with the water barrier layer, in the embodiment shown in <FIG> the shape-following guide element <NUM> of moisture ingress detection device <NUM> will preferably be in direct contact with the water barrier layer. In a similar fashion however, the arrangement of moisture ingress detection device <NUM> in direct contact with the water barrier layer improves the sensitivity of the moisture ingress detection device, albeit the embodiment shown in <FIG> does require moisture to diffuse through guide element <NUM> before encountering the swellable material <NUM>. Shape-following guide element <NUM> bends to conform to the curvature of the water barrier layer, and the tapering cross section of shape-following guide element <NUM> spreads the thickness of the guide element gradually over a portion of the circumference of the water barrier layer to avoid causing a bulging of the water barrier layer that could otherwise negatively impact the structural integrity of the cable.

Claim 1:
A power cable arrangement (<NUM>, <NUM>), comprising a power core (<NUM>) comprising one or more conductors (<NUM>), an intermediate layer (<NUM>) disposed about the power core, a water barrier layer (<NUM>) tightly arranged about the intermediate layer, and a moisture ingress detection device (<NUM>, <NUM>) arranged between the water barrier layer and the intermediate layer in direct contact with the water barrier layer (<NUM>) , the moisture ingress detection device (<NUM>, <NUM>) comprising an elongated optical fiber (<NUM>) surrounded by a water-swellable material (<NUM>,<NUM>), the water-swellable material arranged to expand upon contact with moisture and exert a pressure on the underlying optical fiber sufficient to cause an observable change in the attenuation and/or propagation characteristics of the optical fiber.