The invention relates to dredging by means of a hopper suction dredger.

With a dredging method of this type, the following functions are integrated in the hopper suction dredger: dredging by means of sucking up the material for dredging, temporary storage (in the well of the hopper suction dredger) , transport of the dredged material and discharging thereof.

On the one hand, a procedure of this type has advantages, which are related to the mobility of a hopper suction dredger. Provided that the body of water in which the dredging work has to be carried out is sufficiently deep, the hopper suction dredger can be used directly. It is not necessary to provide pressure pipes for transport of the dredged material: because of the temporary storage in the well of the hopper suction dredger, the latter is itself able to take care of the removal. On the other hand, the method of dredging by means of a hopper suction dredger is subject to very specific conditions. For instance, the abovementioned functions must be carried out in accordance with a specific fixed pattern. Deviations from that pattern are not possible and, consequently, the flexibility of a hopper suction dredger is limited in this respect.

Moreover, a hopper suction dredger represents a high capital investment. Therefore, the desired return can be achieved only if productivity is relatively high. It is therefore necessary to find a balance between, on the one hand, the time which is needed to suck up the material for dredging and, on the other hand, the time which is needed for sailing backwards and forwards between the dredging location and the discharge site. If the discharge site is a long way away from the dredging location, the productivity is relatively low because of the long sailing times.

The aim of the invention is, therefore, to eliminate the restrictive disadvantages, as mentioned above, from dredging using a hopper suction dredger. This is achieved by means of a method comprising sucking up the material for dredging with the aid of the suction installation of the hopper suction dredger, feeding the sucked-up dredged material to at least the well of an other vessel, sailing the other vessel from the location where the dredged material has been sucked up to a location for discharging said material, and the subsequent discharging of the dredged material from the other vessel at the discharge site.

The method according to the invention may furthermore comprise loading

of the well of the hopper suction dredger, sailing the hopper suction dredger to the discharge site and discharging the hopper suction dredger at the discharge site.

As, with the latter method, the dredged material can be divided between the well of the hopper suction dredger itself and the well of another vessel, the suction installation can be in operation for a relatively long time. Consequently, a more favourable ratio is achieved between the time for which the hopper suction dredger is effectively carrying out dredging work and the sailing time required for removal. The consequence thereof is that the productivity increases, and, consequently, the return also increases. Moreover, the other vessel can represent an appreciably lower capital investment than the hopper suction dredger itself, which is beneficial for the cost price of the work.

Loading of the well of the hopper suction dredger and the other vessel can be carried out in various ways. According to a good option, the well of at least one other vessel is first fully loaded with dredged material, after which, at one and the same time, said vessel sails away and the well of the hopper suction dredger is loaded. In the case of deep beds, it can be more advantageous for the suction process first to load the hopper suction dredger and then to load the other vessels. To this end, the other vessel can be provided with propulsion means of its own, but it is also possible to move said vessel by means of a tug.

According to another possibility, the well of a first other vessel is first fully loaded, after which, at one and the same time, the first other vessel sails away and the well of a second other vessel is fully loaded, after which, at one and the same time, the second other vessel sails away and the well of the hopper suction dredger is loaded. In the same way it is also possible to make use of more than two other vessels. A procedure of this type is particularly advantageous if the sailing speed of the other vessels, on the one hand, and that of the hopper suction dredger, on the other hand, are related in such a way that the hopper suction dredger has arrived back at the location for sucking up the material for dredging at about the same point in time as the first other vessel has arrived back at said location. It is also possible to use the hopper suction dredger exclusively for loading and to allow the transport of the dredged material to be carried out by the other vessels.

In order to be able to load the other vessel while the hopper suction dredger is dredging, said vessel must remain in the vicinity a certain,

constant, distance away. This can be achieved, for example, in that, during loading of the well of the other, vessel, said other vessel sails,- independently of the hopper suction dredger, with the hopper suction dredger and is loaded by means of a dynamically positioned arm. As an alternative it is also possible to tow the other vessel behind the hopper suction dredger while the well of said other vessel is being loaded.

In the abovementioned possibilities for carrying out the method according to the invention, it has always been a question of successive loading of the well of the hopper suction dredger and one or more other vessels. However, it is also possible simultaneously to feed the sucked-up dredged material to the well of both the hopper suction dredger and of one or more other vessels. The advantage of this method is that, for a given output from the suction installation, the load on the well in each of the wells is lower than in the case where only one well is loaded at a time. If the load on the well is low, the dredged material is better able to settle, which is beneficial for productivity. The load on the well is a measure of the rate at which the well is filled and is the ratio between the output Q supplied and the well surface area A _b (= product of width B times length L of the well) , i.e. Q/A _b . If the load on the well is low, the dredged material is better able to settle, with the result that overflow losses are reduced, which is beneficial for loading productivity.

The invention also relates to a hopper suction dredger which is suitable for carrying out the method according to the invention. Said hopper suction dredger is provided with a pressure pipe for loading sucked-up dredged material, the pressure pipe being branched, one branch leading to the well of the hopper suction dredger and it being possible to connect the other branch to the well of another vessel. Preferably, a shut-off device is provided in each branch.

A dynamically positioned boom can also be provided, which carries the branch which can be connected to another vessel or is manoeuvrable above a loading opening.

The invention will be explained in more detail below with the aid of a few illustrative embodiments shown in the figures.

Figure 1 shows a diagrammatic top view of a first variant of the method according to the invention.

Figures 2a and 2b show a diagrammatic top view of a second and third variant.

Figure 3 shows a hopper suction dredger for use with the method

according to the invention.

Figure 4 shows a possible timetable associated with the method according to the invention.

The top view in Figure 1 shows a hopper suction dredger, which is indicated in its entirety by 1, as well as an auxiliary vessel, indicated by 2. The hopper suction dredger 1 has suction arms 3. by means of which the material for dredging is sucked up. The material sucked up is then transferred to the auxiliary vessel 2 with the aid of a pipe 4, which is mounted on a boom which is hingeably mounted on the deck of the hopper suction dredger 1.

Hopper suction dredger 1 and auxiliary vessel 2 each have their own drive and consequently are able to sail independently alongside one another. The boom on which the pipe 4 is mounted is dynamically positioned in a known manner, in such a way that the desired distance between the boom and the auxiliary vessel 2 is always ensured, even if there is some swell.

In the variant shown in Figure 2a, an auxiliary vessel 5 is used, which is towed behind the hopper suction dredger 1. In this case as well, a pipe 6 is provided between hopper suction dredger 1 and auxiliary vessel 5. via which pipe 6 the dredged material sucked up by means of the suction pipes 3 is transferred. In this case, the auxiliary vessel 5 does not have to have its own drive; it can be a barge which, after it has been uncoupled from the hopper suction dredger 1, is towed away by means of a tug.

The third variant in Figure 2b shows other vessels which are loaded on either side of, and behind, the hopper suction dredger. In the first and second variants in Figures 1 and 2a, it is possible that the hopper suction dredger temporarily loads its own well in the period when the loaded vessels 2 or 5 are being replaced by vessels 2 or 5 having an empty well. In the third variant, see Figure 2b, loading of another vessel can be continued if one of the other vessels which has a full well is replaced by one which has an empty well. In that case it is also not necessary that the well of the hopper suction dredger is temporarily loaded while one of the vessels is being exchanged, or in any event the temporary well loading of the well of the hopper suction dredger can then be less than is the case in the first and second variants of Figures 1 and 2a. The side view in Figure 3 shows a hopper suction dredger partially in cross-section. The suction pipe 3 has been lowered in a known manner to the bottom of the body of water in which the hopper suction dredger 1 is located. By means of the pump 7, the sucked-up dredged material can be

transferred, in a known manner, via pipe 8 into the well 9 of the hopper suction dredger 1. The well has an overflow channel 10, via which channel the water dredged up can be discharged.

A perforated pipe 11 is also fitted in the well, by means of which pipe the material for dredging which has been dredged up can be sucked out of the well by pump 7 and can be discharged, via the riser 12, into the branch 13, to which, for example, a pressure pipe can be connected.

According to the invention, the hopper suction dredger has a pipe 4 via which the material for dredging which has been dredged up is not introduced into the well 9 but is fed to an auxiliary vessel. To this end, the pipe 4 is mounted on a boom 14, which is mounted on the deck of the hopper suction dredger 1 so that it is able to swivel.

When the boom 1 is swivelled outwards, as shown in Figure 1, an auxiliary vessel 2 sailing alongside the hopper suction dredger 1 is loaded. The hopper suction dredger 1 also has a pipe 15. running towards the stern, to which pipe the pipe 6 shown in Figure 2 can be connected, by means of which a vessel 5, towed behind the hopper suction dredger or moving under its own power, can be loaded.

Valves 16, with which the various discharge possibilities for the dredged material can be selected, are fitted in the various pipes.

If desired, smaller pipes can also be fitted in order to be able to pump part of the output without the risk of blockages.

The diagram shown in Figure 4 shows an example of the timing of the method according to the invention. Blocks 17, 18 and 19 relate to the dredging operation of the hopper suction dredger. In the timetable shown, a first auxiliary vessel A is loaded with dredged material during period 17• As soon as said auxiliary vessel is fully loaded, it sails away, as indicated by block 20. During period 21 the auxiliary vessel A is emptied and during period 22 the auxiliary vessel A sails back to the hopper suction dredger in order to be loaded again.

After the hopper suction dredger has loaded the first auxiliary vessel

A in period 17, the hopper suction dredger continues its suction activity in period 18 and during this period loads the second auxiliary vessel B.

After said vessel has been fully loaded, it sails away in period 23, discharges in period 24 and sails back in period 25.

The hopper suction dredger C then loads its own well in period 19- Period 26 relates to the hopper suction dredger sailing away, 27 to discharging thereof and 28 to the hopper sailing back again.

The auxiliary vessels A, B will in general be able to develop a lower sailing speed than the hopper suction dredger C itself. The consequence of- this is that the hopper suction dredger C arrives back at the dredging site in period 28, at approximately the same time as the first auxiliary vessel A in period 22. As indicated, the hopper suction dredger can then load the first auxiliary vessel A again in period 29, the entire cycle being repeated.

It is also possible to load two, three or more vessels A and B at the same time and then to load hopper suction dredger C. See Figure 2b. Depending on the distance between the loading site and the discharge site and the sailing speed of vessels A or B and hopper suction dredger C, more auxiliary vessels D, E etc. can be loaded by the hopper suction dredger C before the hopper suction dredger itself is loaded and sails to the discharge site. Another variant is first to fully or partly load the hopper suction dredger itself before loading vessels A, B etc. It is then possible, without adjusting the length of the suction pipe, nevertheless to suck up material for dredging from greater depths.

As described above with respect to the timetable shown in Figure 4, the first and second auxiliary vessel A, B and the hopper suction dredger C are loaded individually. According to an advantageous variant of the method, it is also possible to load one hopper suction dredger, in each case, at the same time as one of the auxiliary vessels. With this procedure, the rate of flow of the pumped medium in each of the wells of said vessels remains restricted, as a result of which very good settling is ensured. In this case, however, the flow chart as shown in figure 4 does not apply.

Another example of the method is that the hopper suction dredger with an empty well or with a partially or completely filled well is used solely to load one or more auxiliary vessels A, B, D, etc. individually and repeatedly one after the other and that, after the relevant dredging work, or part thereof, has been completed, the hopper suction dredger, if loaded, sails to the discharge site where it discharges its load.