It is known to transmit electric power between two places on the coast, the one conductor being a cable on the bottom of the sea and the other conductor being the sea. At the end of the eighties, a direct-current cable was laid between Sweden and Finland, the cable being used for the one pole and the sea water for the other pole.

The sea water pole extends from an anode electrode in the sea in the one place to a cathode electrode in the sea in the other place. The transmitted power is about 500 MW at a voltage of 400 kV. Each electrode has a comparatively large surface, about 20 m2, for good current conduction, and it is most important that the electrodes be protected from mechanical damage, such as damage caused by boat anchors, rolling stone blocks, dumped scrap and the like.

It is also very important that the contact surface of the electrode with the surrounding sea water be as large as possible and that the sea water can flow through the electrode.

An example of a prior art electrode for the one pole in underwater transmission of high-voltage direct current is disclosed in Swedish published application 460,938 and the corresponding PCT application WO89/12334. The elec- trode consists of a titanium netting which is enclosed

between protective structures of a flexible material, and the electrode is adapted to be placed directly on the bottom of the sea and has such a flexibility as to follow to a great extent the contour of the bottom of the sea after being installed. The protective structure, however, cannot prevent anchors and the like from hooking onto the electrode or matter falling down from damaging the tita- nium netting in spite of the protective structure. By the prior art electrode lying directly on the bottom of the sea, the through flow of sea water through the netting will be insufficient, as is also the case with the effec- tive contact surface with the sea water.

An example of a protective structure for installa- tions on the bottom of the sea is disclosed in US patent 5,193,937. The protective structure is made of concrete, consists of a plurality of parts and is adapted to pro- tect a pipeline. The concrete structure closest to the pipeline is tight relative to the surrounding sea water and therefore cannot act as cover for a sea electrode since the sea water would not have access thereto.

Moreover, Swedish patent application 9502263-8 dis- closes an anchor device for anchoring of underwater elec- trodes in HVDC transmission over water. The anchor device is a concrete structure resembling a cable drum. The electrode in the form of a mat is wound onto the drum and thus is not protected against violence caused by e.g. on- hooking boat anchors, stones rolling down and the like.

Moreover, the fluke of an anchor can easily get stuck in the actual drum. The drum construction also results in the amount of surrounding water flowing through the elec- trode mat being limited.

An object of the present invention is to provide a cover frame for an electrode for a device for transmitting heavy current over a sea, which effectively protects the electrode against damage that could be caused by on- hooking boat anchors, attacking stone blocks and the like.

One more object of the invention is to provide an electrode cover frame, which allows good throughflow of surrounding sea water through the electrode.

A further object is to provide an electrode cover frame which to some extent can follow the contour of the bottom of the sea.

According to the invention, these objects are achieved by an electrode cover frame according to the introductory portion, which is characterised in that the cover frame comprises at least two opposite side walls, which are essentially parallel with the longitudinal axis of the cover frame, and a roof, that the cover frame is formed with a plurality of through openings in the side walls and in the roof, and that the electrode is sup- ported by the cover frame at a distance from the bottom of the sea.

The further development of the invention is evident from the distinctive features stated in the subclaims.

A preferred embodiment will now be described by way of an embodiment with reference to the accompanying drawings, in which Fig. 1 is a top plan view of an electrode cover frame according to the invention; Fig. 2 is a longitudinal side view of a pair of electrode cover frames according to Fig. 1 assembled to a coherent protective structure for the electrode and with an end wall mounted; Fig. 3 is a short side view of the construction according to Fig. 2 in the direction of arrows A-A; Fig. 4 is a cross-sectional view of an electrode cover frame along line B-B in Fig. 2; Fig. 5 illustrates on a larger scale the upper por- tion of two adjoining cover frames in a coherent struc- ture with a plurality of electrode cover frames; and Fig. 6 is a top plan view and two side views per- pendicular to each other, illustrating an embodiment of

an electrode which is intended for use together with the cover frame according to the invention.

With reference first to Figs 1, 2 and 4, a cover frame 2 for an electrode 1 according to the invention comprises a shell structure, preferably of concrete with non-conductive or poorly electrically conductive rein- forcement, such as resin, glass fibre etc., or of con- crete with glass fibre added. The cover frame 2 can be made in one piece, but it is preferred from the viewpoint of manufacture, transport and handling that the total cover frame for the electrode 1 be formed of a number of interconnected cover frames 2, as shown in the drawings.

This also results in greater flexibility of the cover frame for adaptation to the contour of the bottom of the sea, as will be described in more detail below.

The cover frame 2, or each identical unit 2', 2", etc. included in the total electrode cover frame, comprises two opposite side walls 6, which are essen- tially parallel with the longitudinal axis of the cover frame, said longitudinal axis being schematically in- dicated by reference numeral 15. A roof 5 connects the side walls 6 to each other, which are inclined outwards away from the roof, i.e. the width of the roof is smaller than the total width of the cover frame. The cover frame has no bottom wall or short side walls, but can suitably be fitted with end walls as will be discussed in more detail below. Owing to the inclination, it will be more difficult for, for instance, boat anchors to catch the cover frame, the cover frame will be steadier on the bottom of the sea which is designated 3 in Fig. 4, and the cover frames are stackable.

As is best shown in Fig. 1, the cover frame 2 is formed with a plurality of through openings 7 in the side walls 6 and in the roof 5 to give the surrounding sea water access to the interior of the cover frame 2 and the possibility of flowing through the same. The total area of the openings 7 suitably is maximised such that the

flow of water round the electrode 1 becomes as great as possible. At the same time the cover frame must withstand the strain to which it may be subjected, such as its dead weight, the weight of the electrode and external stress against which the electrode is to be protected. To satis- fy these requirements, the total area of the openings 7 relative to the total external area of the cover frame (the area of the side walls 6 and the roof 5, including that of the openings 7) is preferably in the range of 25- 708.

The openings 7 are elongate and, for instance, rec- tangular, as shown in the Figures. Preferably, the openings 7 are oriented transversely of the longitudinal axis 15 of the cover frame, see Fig. 1, but they can also be oriented in parallel with the longitudinal axis at an optional angle thereto, if desired. To reduce the risk that an anchor or the like gets stuck with its fluke in the elongate openings, and to improve the throughflow, the sides of the openings taper inwards to the interior of the cover frame or, in other words, the boundary surfaces 16 and 17 of the openings 7 converge from the outside of the cover frame to the inside thereof.

Fig. 4 illustrates that the electrode 1 is supported by the cover frame 2 at a distance from the bottom of the sea 3. The electrode can be supported by a shelf con- struction (not shown) under the roof 5 but is preferably suspended by means of cables, ropes, clamps or similar fixing elements 8, which are laid through suitable openings 7 in the roof 5 and/or in the side walls 6. The electrode 1 can be directly suspended from the fixing elements 8. However, it is preferred that the electrode be supported by a number of beams 20 which are parallel with the longitudinal axis 15 and which in turn are suspended from the fixing elements 8.

A suitable electrode 1, which is known per se, is shown in Fig. 6. The electrode 1 is a flexible metal netting 21, preferably of titanium, which on both sides

is enclosed by a plurality of parallel, interconnected and elongate plastic tubes 22. The plastic tubes 22 are oriented transversely of the longitudinal direction of the netting 21 (and of the cover frame 2), the lower layer of tubes 22, during mounting of the electrode 1, resting on the beams 20 and the upper layer of tubes 22 engaging in a fixed manner projections 23 on the lower portion of the roof 5, see Fig. 4.

Figs 2 and 3 show that one or both short sides 4 or ends of the cover frame 2, or the ends of the outermost cover frames 2' when a plurality of cover frames have been joined together in the longitudinal direction, are suitably sealed by means of end walls 9. The end walls 9 serve to prevent boat anchors and the like from being inserted or penetrating into the cover frame 2 and hooking onto the sides 6 or roof 5 thereof or onto the electrode 1. These end walls can be integrated with the cover frame 2, but are preferably formed as a separate, plate-like element of reinforced concrete or the like and have a shape which essentially conforms with the profile which, transversely of the longitudinal axis 15 of the cover frame 2, is defined by the outside of the side walls 6 and the roof 5 and the bottom of the sea 3. In other words, the shape of the end wall 9 is such as to completely seal the short side 4 of the cover frame 2, see especially Fig. 3. The end wall 9 is formed with a number of through holes 10 at its periphery and is fixed to the cover frame by means of cables, ropes, clamps or similar fixing elements 11, which are laid through the holes 10 and neighbouring openings 7 in the side walls 6 and/or roof 5 of the cover frame. To facilitate said fixing and make it safer, the end wall 9 is formed with a continuous or discontinuous projection 24 which projects perpendicular from the end wall 9 and whose extent con- forms with the outer profile of the cover frame 2 in the transverse direction, such that the projection 24, after mounting of the end wall, rests on the end portion or

short side 4 of the cover frame, as illustrated in Fig. 2.

It has already been mentioned that the electrode cover frame can consist of a single unit 2 or a plurality of interconnected cover frames 2', 2" etc. To provide an electrode cover frame which is easier to handle and which besides is more flexible so as to be able to better fol- low the contour of the bottom of the sea 3, the electrode cover frame is preferably made of a plurality of units, cf. Figs 2 and 5, i.e. a plurality of cover frames 2', 2" etc. are joined to a complete, enclosing cover frame for the electrode (together with the bottom of the sea 3). To allow the cover frames 2', 2" etc. to be aligned with each other in the longitudinal direction, both short sides 4 of each cover frame have two preferably cylin- drical holes each, which are directed in parallel with the longitudinal axis 15, e.g. the holes 12 in Fig. 3 and the holes 12' and 12" in Fig. 5. By means of guide plugs or the like 13 of an electrically non-conductive mate- rial, which are inserted into the opposite alignment holes 12', 12" of two adjoining cover frames 2', 2", the cover frames 2', 2" are aligned with each other, see Fig. 5, and the cover frames 2', 2" are fixed to each other by means of cables, ropes, clamps or similar fixing elements 26, which are laid through neighbouring openings 7 in the side walls 6 and the roof 7 of adjoining cover frames 2', 2", see Fig. 2. Preferably, said openings are formed with recesses 25 extending toward the short side 4 and being aligned with the alignment holes 12', 12" to hold the fixing elements 26 and the guide plugs 13 in place. A spacer or insert of plastic (not shown) is advantageously arranged round, or integrated with, each guide plug 13, thereby making it possible to angle adjoining cover frames somewhat in the vertical direction relative to each other so as to be able to better follow the contour of the bottom of the sea 3. To further increase this flexibility of the assembled cover frame,

also the short sides 4 can be bevelled one or two degrees from the alignment holes 12', 12", as illustrated in Fig. 5.

The cover frame and the electrode can suitably be mounted in the following fashion.

Two longitudinal beams 20 of an electrically non-conductive material are placed on an assembly table which is designed in an appropriate manner. As beams, use can be made of e.g. VP tubes having a diameter of 50 mm.

An electrode netting 21 supplied in rolls where the actual netting is positioned between two tight layers of transverse plastic tubes 22 is unrolled on an assembly table. The electrode netting is thus placed along and on top of the two previously laid-out longitudinal beams.

The cover frames are lifted onto the electrode netting and joined together. After joining all the cover frames, the longitudinal supporting beams are raised by pulling bands 8 round the beams 20 and through openings 7 in the roof 5 of the cover frames. If two VP tubes with a dia- meter of 50 mm are used as longitudinal supporting beams, it is sufficient for the bands to be applied at intervals of 500 mm. The bands and their locking device are not allowed to contain any electrically conductive material.

The complete cover frame 2', 2", 9 and the enclosed electrode 1 are suitably launched by means of a lifting yoke with a number of lifting straps, from which the cover frame is suspended. Before that, the bottom of the sea 3 has suitably been cleaned and levelled out to some extent, and preferably the lower edges of the side walls 6 are designed so as to sink a distance into the bottom of the sea, thereby reducing the risk that, for instance, the fluke of an anchor hooks onto said lower edges.

The invention is not limited to that described above and shown in the drawings, and can be modified within the scope of the appended claims.