The invention relates to a method of loading a suction dredger in which a suspension of sand and water is pumped into the hold of the suction dredger during a loading operation which is carried out in at least two steps, to wit

• a first step in which effluent means are kept at such a high level that the hold is loaded essentially up to its maximum volume and the suction dredger is loaded substantially up to its maximum permissible carrying capacity by pumping a suspension of sand and water into the hold and
• a subsequent second step in which additional suspension is pumped into the hold and in which the effluent means are lowered in dependence upon the weight of the load in the hold to an extent such that the suction dredger remains loaded substantially up to its maximum permissible carrying capacity.

Such a method is known from Dutch patent specification 138,182. Since the quantity of sand in the hold constantly increases, the effluent means are constantly lowered in order to keep the suction dredger at its maximum permissible carrying capacity. Slow adjustment of the effluent means is performed only with difficulty, since they are likely to be retained. The invention provides a more reliable method in which the hold can yet be loaded rapidly. According to the invention the effluent means are intermittently moved to lower and lower levels.

Intermittent lowering of the effluent means can be easily performed in a reliable manner. Although in the method according to the invention the suction dredger is transiently loaded slightly below its maximum permissible carrying capacity, it appears that the suction dredger is loaded largely as rapidly as in the case in which it is endeavoured to hold the suction dredger constantly exactly at its maximum permissible carrying capacity. This may be attributable to the fact that in accordance with the invention there are loading periods in which the flow of effluent is comparatively slight, so that the sand of the suspension contained in the hold has more time to settle down or to the fact that owing to the higher reliability of this method the margin between the standard applied during loading to the maximum permissible carrying capacity and the degree of loading at which the ship may founder can be lower.

The invention relates to and provides, in addition, a suction dredger for carrying out the method in accordance with the invention, said dredger comprising a hold, a pump for pumping a suspension of sand and water into the hold and effluent means to be set in a first loading step at a high level corresponding to the maximum volume of the hold and in a second loading step at a tower level, the proportions of the hold volume being such that in a first step the suction dredger can be loaded with the sand and water mixture substantially up to its maximum permissible carrying capacity, there being provided control-means for bringing the effluent means to lower and lower levels in dependence upon loading measuring means during the second loading step, said dredger being characterized in that said control-means are adapted to lower the effluent means intermittently in steps.

The invention will be described more fully hereinafter by way of example with reference to the accompanying drawing.

In the drawing:

Figure 1 is a schematic longitudinal sectional view of a preferred embodiment of a suction dredger in accordance with the invention in carrying out the method according to the invention, and

Figures 2 and 3 are each a graph representing the volume of the hold and the weight of the load respectively, when the method according to the invention is carried into effect.

The suction dredger 1 shown in Figure 1 is a trawl dredger especially equipped for carrying out the method in accordance with the invention.

The suction dredger 1 comprises a hold 3 in a hull 2, a pump 4 for pumping a suspension of sand and water into the hold 3 and effluent means 9. The pump 4 communicates at one end with a suction pipe 5, a trailing suction head 10 of which is dragged on along the ground 11 below the water 12, when the suction dredger 1 is propelled in the direction of the arrow 14 by means of a propeller 13. At the other end the pump 4 communicates with a loading pipe 6, from which the water and sand suspension gets into the hold 3.

The effluent means 9 comprise a vertical, fixed tube 15 communicating with the water 12, a tube 8 vertically displaceable in the former and having at the top end an overflow dish 7 and a hydraulic cylinder 16, the piston rod 17 of which is connected with the dish 7 for the vertical adjustment of the overflow rim 18 of the overflow dish 7.

The hold 3 has very large proportions as compared with the carrying capacity of the suction dredger 1, that is to say, the hold 3 can be filled with sand only up to the level Z,,,.

In the method according to the invention the loading operation is performed in at least two steps 21 and 22 (see Figures 2 and 3). In the first step 21 the overflow rim 18 is held in its highest position 18"' indicated in Figure 1 by broken lines, at least at such a high level that the hold 3 is loaded essentially up to its maximum volume 19 and up to the maximum carrying capacity 20 of the suction dredger 1 with a suspension of sand and water pumped into the hold 3. In Figures 2 and 3 the graphs drawn in broken lines represent the volume V_t and the weight W, respectively of the total quantities of sand and water contained in the hold 3, whereas the solid lines V_s and W_s represent the volume and the weight respectively of sand settled down in the hold 3. On the abscissa is plotted in Figures 2 and 3 the time. From Figures 2 and 3 it will be apparent that during the first step 21 there is first a period in which the hold 3 is filled with a suspension of sand and water, after which, when the hold 3 is already filled, further suspension of sand and water is pumped into the hold 3 whilst the effluent means 9 are still kept at the highest level so that the total volume V, remains at its maximum. Subsequently the next, second step 22 is carried out, in which additional suspension is pumped into the hold 3 and in which the effluent means are lowered in dependence upon the weight of the load in the hold 3 to an extent such that the suction dredger 1 remains essentially loaded up to its maximum permissible carrying capacity 20. From Figure 3 it will be seen that in the course of time t during the second loading step 22 the graph W_t indicated by broken lines extends substantially horizontally along the dotted line 23. This dotted, straight line 23 and the naturally varying dotted line 24 of Figure 2 would be attained, if the overflow rim 18 were gradually lowered. According to the invention, however, the overflow rim 18 is lowered intermittently rather than gradually. For this purpose the effluent means 9 comprise a control-rod 26 with a head 27, which moves along pulse generators 28 during the upward and downward movements of the overflow rim 18. The pulse generators 28 correspond each to, a level of the overflow rim 18, where it is retained for some time.

From Figure 1 it will be apparent that the level difference of the upper pulse generators 28 may be larger than that of the subsequently lower pulse generator 2P

In order to maintain the suction dredger 1 essentially loaded up to its maximum permissible carrying capacity 20, it is equipped with a draught gauge 29, from which a signal is applied to a comparator 30, in which a value of the maximum permissible carrying capacity is set. When the maximum carrying capacity 20 of the suction dredger 1 is attained, the comparator 30 applies a pulse to a control-means 31 of a hydraulic valve 32, which governs the fluid flow to and from both ends of the hydraulic cylinder 16. The overflow dish 7 then moves down by one level difference until the head 27 of the rod 26 reaches a pulse generator 28, which applies a pulse to a control-member 33 for setting the control-valve 32 in a stop position in order to prevent the overflow dish 7 from moving further downwards. During this downward movement of the overflow dish 7 the supernatant layer of water is at once removed from the hold 3, since it flows across the overflow rim 18 and through the tubes 8 and 15 into the outboard water 12. Then additional suspension is constantly pumped into the hold 3 and after some time the permissible carrying capacity of the suction dredge 1 is again attained or in other words, the broken lines meet the dotted lines of Figures 2 and 3. At this instant the comparator 30 again applies a signal to the control-member 31 and the hydraulic cylinder 16 is reactuated for lowering the overflow dish 7 by a further step. .Thus loading is continued in the second loading step until the hold 3 is substantially filled with sand up to the maximum permissible carrying capacity. During the second loading step 22 the sand level rises from the level Z, via the level Z_II to the level Z_III, to which correspond the positions 18^III, 18" and 18¹ respectively.