Laying Tool for Laying of Submarine Cables Into a Sea-Bottom

Submarine cables for power supply, telecommunications and the like can in the most cases not be laid down directly on the sea-bottom but has to be protected against, for example, floating ice, fishing-tackle, anchors and the like. The cable can either be covered with, for example, iron blocks and cement sacks or the cable can be brought down into the bottom material, for example, by digging and ploughing. The alternative of covering of the cable is mostly very expensive and time-wasting and this method has earlier been used only when it was considered necessary. For digging of cable trenches dredgers have been used. This method is expensive, technically difficult to master and limited to shallow waters. Apparatus for digging cable trenches and simultaneous laying of cable are previously known, for example in Swedish Patent No. 322,279. The known arrangements have, however, been intended to work in loose bottom materials such as sand, mud, soft clay and the like. The aim has principally also been to lay down weaker cables or water conduits in relatively calm waters inside a belt of islands. These arrangements are scarcely applicable for ocean operations, hard and rough bottom and heavy cable constructions as, for example, armoured power cables. Even arrangements for similar operations are previously known, with unsatisfying results in certain cases, among other things the difficulties have been great really to have the cable dug down.

The purpose of the present invention is to asure digging down of power cables in varying sea and bottom states that can be expected in practice. The laying tool is constructed in order to manage all reasonable strains inside as well as outside the belt of islands. The invention is described more in detail here below with the aid of the accompanying drawings.

FIG. 1 is a side view of the laying tool and the pulling vessel,

FIG. 2 shows the drag unit of the laying tool, seen in side view,

FIG. 3 is a longitudinal section through the laying tool,

FIG. 4 is a cross section through the drag unit of the laying tool,

FIG. 5A is a plan view of a ploughing means protruding from the bottom of the drag unit,

FIG. 5B is a side elevational view of the ploughing means,

FIG. 5C is a diagrammatic sectional view taken on line C--C in FIG. 5B,

FIG. 5D is a diagrammatic sectional view taken on line D--D in FIG. 5B,

FIG. 6 is a detail of the plouging means,

FIG. 7 shows a drag unit of the laying tool seen from above, and

FIG. 8 shows a laying tool during refloating from or immersion to the sea-bottom.

The laying tool consists of a drag unit 22 and a pulling tube 26 joined to each other by means of a cardanic suspension. The drag unit 22 comprises a body 34 and a ploughing means 27 similar to a keel and provided with a canal 43 which forms a pulling and a control system together with the pulling tube 26. The cable 24 is immersed from the vessel 23 and enters the drag unit 22 through the tube 26. In the drag unit 22 the pulling tube 26 passes to the canal 43, so designed that it forces the cable 24 down into the bottom of ploughed trench 29 before the ploughing means 27 leaves the trench.

The trench 29 in the sea-bottom 32 is ploughed of the ploughing means 27 under the body 34. Finally the system 22, 23 and 24 is provided with a number of sensors and which allow continuous supervision of the operation of the system and of the depth of the ploughing down of the cable.

The drag unit 22 which is being drawn on the bottom 32 consists of a body 34 on the side of which there is a central notch 42 at the back. In the middle of this notch is placed the ploughing means 27, the side walls 28 of which are plane and vertical. Inside this means 27 is the cable canal 43 and its outlet 44. It is possible to open the canal 43 at its top so that the cable can be placed inside the canal without pulling. The width of the means 27 should at first hand be determined by the diameter of the cable 24. This means 27 is in its front part provided with a ploughing tool consisting of an upper plough 48 and a lower plough 49, the upper plough being placed before the lower plough. This is necessary in order to shape the cable outflow 44 in such a way that the cable 24 is forced down into the bottom of the ploughed trench 29, even when the drag unit 22 does not work in a quite correct position. The ploughs 48, 49 are somewhat broader than the plough 28 of the ploughing means 27 and the upper plough 48 is broader than the lower plough 49. This arrangement reduces the friction and achieves that all the material turned up 31 is pushed aside in a broader trench 29 in order to be finally placed on the bottom 32 aside the dug trench 29. Thus the material 31 will not be pressed into the wall of the ploughed trench. Such a process would lead to essentially larger friction and too quick filling up of the trench. In front of the plough tool 48, 49 there is a plate, a stern frame 40 which serves, on the one hand, to split the bottom material and on the other hand to prevent objects from attaching to the ploughing tool 48, 49 and finally to force up the drag unit 22 entirely if this strikes a bottom material of such a nature that, otherwise, the ploughing means 27 could jam or if the bottom material is too hard to plough through. The shape of the leading edge 41 of the stern frame is thereby important. It is, on the one hand, somewhat broader than the rest of the stern frame in order to reduce the friction and, on the other hand, it is to have a surface which in combination with the weight of the drag unit causes the drag unit to be forced up when the bottom material is too hard, for example granite. Certain parameters based on model tests are to be observed in order to make the laying tool operate in an adequate manner. The edge 41 of the stern frame is to form an angle 7 with the bottom plane 36 of the body within the range of 20.degree.-45.degree. (See FIG. 5). Each plough 48, 49 is to consist of a lower 50 and an upper 51 plough surface, the edges of which form an angle 8, 9 which for the upper plough cutting edge lies within the range of 60.degree.-120.degree. 8 and for the lower one 90.degree.-120.degree. 9. The ploughing depth 12 is essential for the shaping of the other parts of the drag unit. This depth 12 is defined as the difference between the height 33 of the ploughing means 27 and the depth of the central notch 42.

The bottom 36 of the body is mainly plane except for the central notch 42 and the stem 35. The dimensions of the body 34 are also essential for the ploughing capacity. The radius of curvature 13 is to exceed the ploughing depth 12 in the area in which it exceeds the plane bottom surface 36. The line defined by the beginning 15 of this curvature, i.e. the width of the plane bottom, should be at least 0.6 times the distance from said line to the projection of the point 5 of the lower plough on the bottom plane -- a distance which in turn is 5-8 times the ploughing depth 12. Furthermore, the projection of the center of gravity 16 on the bottom plane 36 is to be within the sides of angles which are defined by the projection point of said plough point 5 and the end points of the line which defines the beginning of the curvature 15 of the stem. It is our experience that if these conditions are not fulfilled the laying tool will, for example have difficulties starting the ploughing, will plough unsatisfactorily in a " stem-down-position" and also will push a bank of loose material in front of itself. The stem 35 of the drag unit is, furthermore, to be angled, so as to avoid the risk that the drag unit be stopped by objects protruding from the sea-bottom.

The cable 24 runs through the pulling tube 26 into the canal 43 and out through the outflow 44 in the ploughing means 27. The outflow will force the cable down into the bottom of the ploughed trench, before the ploughing means leaves the same behind. Therefore, it is necessary that the ploughing tool 48, 49 consists of the upper 48 and lower 49 ploughs which are displaced from one another, the upper plough being in front. Moreover, it is necessary that the outflow 44 is shaped as appears from FIG. 6, where

A = 3 d

if D is the width of the ploughing means 27

B = 1.5 d

the radius of curvature R for the bottom 45 of the canal in front of the lower edge 46 of the outflow is to be at least 8 D. The canal bottom surface having this curvature is, as a maximum, to reach forward to the beginning 6 of the ploughing means. The distance between the front edge 46 of the outflow and the projection C of the point 4 of the upper plough on the bottom of the ploughed trench must not be too large and has to be below .sqroot.H (2R - H), where H is the height 33 of the ploughing means. Even the flexibility of the power cables plays an important role concerning these conditions. Therefore the width of the canal 43 is to increase towards the stem and to be at least 1.4 D when passing on to the pulling tube 26.

The suspension device 18 of the pulling and control device 26, 43 is, at the passing on between the canal 43 in the body of the drag unit and the pulling tube 26, to be shaped in a special way so as to make the laying tool operate in the intended manner in certain situations and to make it plough as correct as possible and with regard to the mechanical qualities of a power cable. The suspension device 18 is to be so shaped that it is turnable vertically as well as horizontally and, furthermore, that its cardan joint with the taps of one of the cardan crosses rest in a groove which can be opened. The fastening height over the bottom of the suspension arrangement 18 is to be below the ploughing depth 12 and its position in the horizontal plane is to be located within the distance of one ploughing depth in front of or behind the vertical plane through the line in the bottom plane where the curvature of the stem begins 15.

The laying tool is also provided with floating tanks 38 so adjusted that the tool floats without any cable, but sinks when a cable runs through it. These tanks 38 are, furthermore, provided with back valves 39 so adjusted that they allow automatic submergence, as long as the pressure in the tanks is lower than the pressure of the surrounding water. The tanks are provided with an air valve 52 in order to facilitate a filling of the tanks with air when refloating the laying tool for its recovery. Furthermore, the distribution of weight is to be so adjusted that the center of gravity 17 of the deplacement in the submarine position lies above the center of gravity 16 of the laying tool at a distance which is at least 1/10 of the distance from the line where the curvature of the stem begins 15 to the projection on the bottom plane 36 of the point 5 of the lower plough. In this way, the immersion procedure is automized, the drag unit 22 immerses steadily and no divers need to be nearby during the operation which could be risky to them.