The present invention relates to an apparatus according to Claim 1. An apparatus of this type is disclosed in International Patent Appli- cation WO-A-8400183. The apparatus shown in that patent application is towed underwater with the aid of a steel cable attached to the vessel.

The intention is that, as soon as said apparatus has been installed on the pipeline concerned, that apparatus is no longer detached therefrom until the end of the trench concerned has been excavated. During dredging, freed material is discharged into the immediate surroundings via the out¬ let. As a result there is, on the one hand, a risk that the trench which has just been excavated for the pipeline will immediately be filled in again and, on the other hand, a risk of inadmissible harm to the environ¬ ment. In the latter context, damage to coral reefs, for example, comes to mind.

Moreover, it has been found that the apparatus according to the above entioned International Patent Application WO-A-8400183 is not stable. This apparatus has the tendency to tilt about ths. pipeline. n such a case the pull exerted as a vessel located above the apparatus moves forward can cause damage to the pipeline, which, however, must be avoided at all cost. In order to eliminate this disadvantage, US-A-4 516 880 proposes an apparatus which on one side is laterally clamped by rollers against the pipeline concerned and on the other side is provided with a ballast tank containing backfill material. An apparatus of this type is relatively complex, whilst the risk of damage is high because the pipeline is engaged and enclosed at the side. Moreover, it has been found that, despite the measures indicated in US-A-4 516 880, tilting can still not be completely excluded. Finally, there is the same drawback as in the abovementioned Interna ional Patent λpplica- tion, that is to say the fact that freed material is discharged into the immediate surroundings. This apparatus as well is suitable only for moving along the pipeline after the latter has been laid over a very great length, because otherwise the time taken for fitting the apparatus to the pipeline is too long. The aim of the present invention is to overcome this disadvantage. This aim ir achieved in the case of an apparatus described above by means of the measures according to Claim 1.

By connecting the apparatus according to the invention to a floating

suction device on the surface of the water, it is possible to remove the material freed during dredging. With this arrangement, however, it will be necessary regularly to change the position of the vessel in which the material removed is collected. With this arrangement it will also be neces- sary to remove the actual dredging apparatus which is fitted around the pipe line. In such a case it is extremely important that a dredging appa¬ ratus of this type can be placed rapidly around the pipeline, during which operation it is ensured that no damage to said pipeline can occur. This can be achieved in a simple manner with the aid of at least the one hinging leg. In fact, the dredging apparatus according to the invention makes a "soft" landing on the pipeline. Consequently, damage is precluded. The presence of the conduit between the apparatus which is on the bed and the vessel close to the water surface likewise prevents the apparatus from being able to tilt in the vicinity of the sea surface. The risk of tipping over and causing damage to the pipeline is completely eliminated with the apparatus according to the present invention.

The connecting conduit between the apparatus arranged on the sea bed and the vessel can be a flexible line, but it is likewise possible to construct this conduit of rigid sections with one or more joints, which sections are moveable, optionally to a limited extent, in one or more planes. This move¬ ment can optionally be carried out under the control of hydraulic jacks. As indicated above, positioning on the pipeline can be facilitated by making at least one of the legs such that it can fold out. Moreover, all of the means known from the prior art can be used for this purpose. Examples are cameras fitted to the apparatus and sonar means for determining the position of the pipeline. Control of the apparatus according to the inven¬ tion can, on the one hand, be effected by steering the vessel which tows the apparatus and, on the other hand, by means of propulsion means which are fitted on the apparatus and can be controlled from the vessel concer- ned.

So as not to damage the pipeline and nevertheless to provide optimum guidance, the guide means are preferably constructed as guide rollers. These guide rollers are arranged on the three sections of the U-shaped body. So as to be able to adjust the height of the apparatus with respect to the pipeline, which is important especially if a trench has to be produced in various passes or if local removal of bed material from beneath the pipeline is needed because said pipeline is freely suspended between two

points, the upper rollers are preferably fitted so as to be moveable with respect to the rest of the apparatus. In this way, the height position of the apparatus with respect to the pipeline can be controlled. Moreover, f safety features can be provided in said control of the height position of 5 the rollers in order to prevent overloading of the pipeline during dredg¬ i ing.

According to a further advantageous embodiment of the invention, the dredging means comprise a frame which extends in the longitudinal direction of the pipeline and is provided with jet means at the front and rear in the 10 direction of movement, suction means being arranged between the jet means. The jet from said jet means is preferably directed forwards in the direc¬ tion of movement.

The invention will be explained in more detail below with reference to an illustrative embodiment shown in the drawing. In the drawing: 15 Fig. 1 shows, diagrammatically, positioning of a pipeline on the bed of a sea, and the fitting thereon of the apparatus according to the invention, which is connected to a tug;

Fig. 2 shows, diagrammatically, a side view of the apparatus according to the invention; and 20 Fig. 3 shows a front view of the apparatus according to Fig. 2.

In Fig. 1 the towing vessel is indicated by 1. This vessel can be a conventional trailing hopper suction dredger. This vessel is connected with the aid of a conduit 2 to the apparatus 3 according to the invention. Said connection comprises not only a towing cable but also conduit means which 25 can transport both fluid and signals to and from the apparatus 3. This conduit 2 is, as can be seen from Fig. 1, provided with various hinges, which are indicated by 20-24. Moreover, hinging about an essentially hori¬ zontal centre line parallel to the centre line of the pipeline to be laid is possible to a limited extent. As a result of the presence of conduit 2, 30 apparatus 3 is able to move completely freely in the vertical direction, but is not able, or is barely able, to rotate about pipeline 4. Without additional measures having to be taken, the weight of conduit 2 forms a ballast for stabilisation of apparatus 3.

As can be seen from Fig. 1, apparatus 3 is arranged around a pipeline 4 35 which is positioned on the sea bed 5.

Apparatus 3 according to the invention will be discussed in more detail with reference to Figs 2 and 3.

It can be seen from these figures that apparatus 3 consists of a carrier

7, which is essentially horizontal and at the ends of which side frames 8 are hingeably arranged. The hinges are indicated by 9 and the hinges enable the passage of a fluid, such as water, which is transported to jet nozzles 0 which are present both at the front and at the rear of the side frame 8. Moreover, jet nozzles are present at the rear in order to remove the material beneath the pipeline. The side frame 8 is also provided with a suction head 11, which is connected to a line 12 which ultimately opens into a line 2 for the removal of bed material which has been freed by the jet nozzles 10. This is a conventional means of removal for a trailing hopper suction dredger. Furthermore, guide or side rollers 13 are arranged on the side frames.

Positioning of the side frames with respect to carrier 7 is effected by means of jacks 17. The jacks can be controlled from the bridge of vessel 1.

An auxiliary frame 15 is also present on carrier 7. These auxiliary frame 15 is moveable in the vertical direction with respect to carrier 7 by means of jacks 16. These jacks are likewise controlled from the bridge of the vessel. Top rollers 14 are arranged on the auxiliary frame 15. The pressure to which the top rollers 14 are subjected can be detected (option¬ ally by determining the pressure in jacks 16) and likewise relayed to the bridge of the vessel.

Carrier 7 is also provided on either side with propulsion heads 18. The latter make lateral movement of apparatus 3 with respect to the pipeline possible. Water or another fluid for the jet nozzles 10 is supplied via line 19 from line 2 or via a separate pump which is arranged on carrier 7. Metering of the material flowing from nozzles 10 is controlled by means of valves 20, which are likewise controllable from the bridge.

The apparatus described above is used as follows:

In bed composed of a sandy or clay-like substance in which a trench has to be made for the pipeline by dredging, a pipeline is first installed on the sea bed from a pipe->laying apparatus.

The vessel 1 is then brought into the vicinity of the pipeline and the apparatus 3 according to the invention is lowered down to the vicinity of both the bed and the pipeline. The position of the pipeline is accurately determined with the aid of means known from the prior art, such as sonar means, cameras and the like. During lowering, the side frames 8 are folded out into the position shown by broken lines in Fig. 3, as a result of which the field of view is made as large as possible. In the manner described above, the apparatus 3 according to the invention is placed as accurately

as possible above the pipeline. This is possible, on the one hand, because the vessel 1 in general is provided with propulsion means on all sides, so that said vessel is able to float precisely motionless on the surface of the water even under difficult weather conditions, or can be directed into a specific position. Moreover, as a result of the presence of the propul¬ sion heads 18, "fine adjustments" of the apparatus 3 can take place, after which said apparatus is placed on the pipeline.

By recording the forces acting on the side rollers and reading out the associated signals on the bridge, it is possible to establish on which of the side frames 8 the greatest forces are acting and in which direction said forces are acting, so that more detailed correction of either the vessel or apparatus 3 is possible.

After placing apparatus 3 on the pipeline, the side frames 8 can be folded inward into the position shown by continuous lines. In such a case, the top rollers 14, together with the auxiliary frame 15, will have been moved downwards, generally controlled by jacks 16, that is to say the side frames 8 do not penetrate very deeply into the bed 5.

The dredging operation is then started by supplying water through jet nozzles 10 and removing freed material via suction head 11. During this operation the apparatus automatically settles somewhat into the bed. Ap¬ paratus 3 according to the invention is then towed by the vessel 1 along the pipeline. As a result of the presence of the pipe line 4, accurate guiding of the apparatus 3 according to the invention takes place, so that the trench can be made relatively narrow, as can be seen in particular from Fig. 3.

If the trench has to be made by repeatedly carrying out the tow¬ ing/dredging operation described above, the apparatus is removed from the pipeline at the end of such an operation. This can be effected by simply lifting the apparatus and is optionally preceded by folding out the side frames 8 again.

The apparatus is then returned to the starting position, or the sailing direction is reversed and the position of the top rollers 14 is adjusted such that a deeper trench is formed. As a result of the removal of material at the sides by the suction heads 11 located opposite one another, the mound on which the pipeline 4 is resting is likewise removed.

As indicated above, the information with regard to the lateral and ver¬ tical forces which the apparatus 3 exerts on the pipeline is transmitted to the bridge and measures can be taken if necessary. The detection of such

forces can be measured in a particularly simple manner, for example, by determining the pressure in both jacks 16 and jacks 17.

If, during the operations described above, the collection well of vessel

1 should become full or if detachment from pipeline 4 is desired for some other reason, it is particularly simple, with the construction according to the present invention, for vessel 1 , with apparatus 3 connected, to be sailed away and to return later once more, in which case positioning on the pipeline can be guaranteed without any damage to pipeline 4.

Although the invention has been described above with reference to a preferred embodiment, it must be understood that numerous modifications can be made thereto without going beyond the scope of the present application.

For instance, it is possible to use the apparatus with pipelines which have already been laid, to eliminate local free suspension of such a pipeline.

That is to say, certain high points in the support of the pipeline are removed from the path in the manner described above. This can be carried out in one or more steps. This and other variants are all considered to fall within the scope of the present application.