Method And Suction Dredging Installation For Sucking Up Dredging Spoil

Abstract

In dredging sand from an underwater body of sand, the lower end of a suction pipe is inserted into the sand and water under pressure is introduced into the pipe at a level above the lower end of the pipe while suction is maintained at the upper end of the pipe to withdraw a suspension of sand in water having a desired concentration of sand in it which is less than the concentration entering the lower end of the pipe. To lessen the energy required to maintain the suction, the relation between the pressure of the water introduced and the level of its introduction is controlled so that the predetermined concentration of sand exists in the pipe above such level while the level lies well below the top of the body of sand and relatively close to the lower end of the pipe.

Description

The invention relates to improvements in that method of sucking up dredging spoil which involves a suction pipe which is connected at its proximal end to a pump and which has a first suction mouth at its distal end. A second suction mouth is provided to allow water to flow into the pipe at a selected distance and adjustable above the first suction mouth, the first suction mouth sucking essentially only dredging spoil and the second suction mouth sucking essentially only water into the suction pipe. The end of the suction pipe is inserted into a body of the dredging spoil to be sucked up and the distance between the second suction mouth and the first suction mouth being adjusted during operation such that the second suction mouth is disposed essentially at that level to which the static head on the spoil tends to elevate same within the suction pipe. Such a system is disclosed in copending application Ser. No. 866,050, filed Aug. 8, 1969, and in the corresponding Dutch Pat. No. 6,501,405 upon which it is based.

The improvement in this method according to the present invention involves the introduction of water under pressure to the second suction mouth, the distance between the first and second suction mouths being adjusted in dependence on the pressure of the water so introduced. This method results in the advantage that in addition to regulation of the concentration in the suspension sucked up, the total weight of the column in suspension is lighter because the second suction mouth may be adjusted closer to the end of the suction pipe. The suction pump to which the suction pipe is connected produces a higher output in these circumstances.

These and other features of the invention will be illucidated in the following description, with reference to the accompanying drawings, wherein:

FIG. 1 shows diagrammatically the position of a suction dredging installation during the performance of the method according to the invention;

FIG. 2 shows the lower end of a suction pipe of a suction dredging installation further developed according to the invention;

FIG. 3 shows a section, taken along the line III-III in FIG. 2; and

FIG. 4 is a view similar to FIG. 3 but showing the inner pipe retracted.

The suction dredging installation according to the invention consists essentially of a supporting vessel 10, a driven sand pump 11, a suction pipe 12 connected to the intake side of the sand pump 11, a pressure pipe 13 connected to the delivery side of the sand pump 11, a water pump 14, a suction line 15 connected to the intake line of the water pump 14, and a pressure line 16 connected to the delivery side of the water pump 14. The suction pipe 12 is pivotably connected to the vessel 10 about the center line of the sand pump 11 whereas the pressure line 16 is connected to the water pump 14 via a flexible member 17.

At its lower end, the suction pipe 12 includes an inner pipe 18, a jacket 19 which is rigidly connected thereto and which projects beyond the inner pipe 18, and a sliding pipe 20 which can be slid telescopically in relation to the inner pipe 18 by means of a hydraulic cylinder 21 as is shown more particularly in FIG. 2. The pressure line 16 is connected to a jet nozzle 22 of a water jet pump 23 by means of which outside water is inducted in large amounts through inlet apertures 24 of the jet pump and forced into the jacket 19, see particularly FIG. 2. The jacket 19 is open at its lower end to present a first suction mouth 25 through which dredging spoil enters. The water introduced under pressure into the jacket 19 passes into the sliding pipe 20 via an annular aperture between the sliding pipe 20 and the jacket 19, said annular aperture forming a second suction mouth 26. That portion 19a of the jacket 19 which is disposed below the lower end of the sliding pipe 20 contains a relatively high concentration of dredging spoil which, after admixture with water from above the lower end of the pipe 20, is delivered at the desired concentration to the inlet of the sand pump 11.

In the method according to this invention, the end of the suction pipe 12 is inserted deeply below the surface of the body of sand 27 while sand is sucked up. As a result, the sand surrounding and above the lower end of the suction pipe 12, for instance that sand within the chain-dot line 28, is fluidized so that it tends to rise in the suction pipe 12 to that level above the first suction mouth 25 to which the static head thereon forces it. Since the jacket 19 is filled with water under pressure, however, the sand cannot rise to such level, but may rise only to that level established by the differential between the water pressure and the static head. According to the invention, the pressure in the jacket 19 is kept during the dredging operation to a value such that the sand on the one hand cannot rise past that level to which the lower edge of the sliding pipe 20 is adjusted, and on the other hand is able to enter through the first suction mouth 25. Moreover, the telescopic pipe 20 is adjusted so that the level at which water is introduced through the mouth 26, relative to the first suction mouth 25, such that the spacing between these mouths together with the prevailing pressure of the water under pressure in the jacket 19 establish a suspension of sand and water from the lower end of the pipe 20 to the inlet of the pump 11 in which the concentration of sand has the required value. If too much or too little water is sucked into the sliding pipe 20, this can be corrected by its being lowered or raised respectively.

Although the suction pipe is inserted deeply into the sand, the required travel of the sliding pipe 20 is substantially smaller than that required of the telescopic pipe in the prior art suction pipe. The reason for this is that since in both instances the rimer or sliding pipe is extended to or beyond the outer pipe while the suction pipe is being inserted and during initiation of the dredging operation, the pipe must be retracted a much greater distance in the prior art device, i.e. the end of the inner pipe in the present instance is disposed ultimately much closer to the lower end of the outer pipe. In the method according to the present invention, the suction pipe 12 from the sand pump 11 down to the second suction mouth 26-- i.e., substantially over the whole length of the suction pipe-- contains a suspension having the required concentration of dredging spoil, and since the required concentration is much less than the concentration resident in the lower section 19a, the column of suspension which the sand pump 11 must handle is as light as possible, while the column of heavier float sand outside the jacket and above the first suction mouth 25 pushes the light column of suspension upwards.

In the modified suction pipe 112 illustrated in FIGS. 2 and 3, the majority of the components are identical with those described above. In the modification, however, the jacket 119 is narrowed at its lower end as illustrated clearly in FIG. 3, presenting central portions having semicircular duct 129 on either side. The sliding pipe 120 is provided at its lower end with semicircular nozzles 130 communicating with the space within the jacket 119 so that when the nozzles 130 are within the ducts 129, the water introduced into the jacket 119 is constrained to issue only through the nozzles 130 at a relatively high velocity as compared with the velocity of water flow which exists through the mouth 126 when the pipe 120 is retracted as shown in FIG. 4. The FIG. 4 position corresponds to that shown in FIG. 1, the first suction mouth 125 then being defined by the open lower end of the jacket 119 whereas the second suction mouth 126 is defined not only by the areas of the nozzles 130 but also by the annular space surrounding the lower extremity of the pipe 120. During insertion into the sand the telescopic pipe 120 is projected outwardly as shown in FIG. 3. The spout nozzles 130 are thus connected via the ducts 129 to the jacket 119 so that water issues through the nozzles at a high pressure during the inserting of the suction pipe 112 into the sand. Thereafter, the pipe 120 is retracted as in FIG. 4 during the sucking up of sand so that a very large amount of water can be supplied at a relatively low overpressure.

Claims

We claim:

1. The method of dredging sand or like material, which comprises the steps of:

a. inserting the lower end of a pipe into an underwater body of sand whereby a column of sand tends to rise in said pipe due to the static head of said body of sand above said lower end of the pipe;

b. introducing water into said pipe above the lower end thereof at a pressure sufficient to prevent the column of sand from rising in said pipe to that one level to which said static head tends to force it, thereby to depress the top of said column of sand to an operating level within said pipe which is well below said one level and is relatively close to said lower end of the pipe;

c. withdrawing a column of a suspension of sand in water which extends from a point within said pipe upwardly to a point of discharge by maintaining suction on such column of suspension; and

d. adjusting said point within the pipe at which said column of suspension is withdrawn in step (c) to a position near or at said operating level such that a controlled concentration of sand exists in said column of suspension upwardly from said point so that said point lies well below the top of the body of sand and relatively close to said lower end of the pipe whereby to lessen the energy required to maintain said suction.

2. A suction dredging installation comprising, in combination;

a pump having an inlet for inducting a suspension of dredging spoil in water and an outlet for discharging said suspension;

a suction pipe having a lower end defining a first suction mouth for dredging spoil and adapted to be inserted into an underwater body of dredging spoil whereby a column of dredging spoil tends to rise therein due to the static head of the dredging spoil above said lower end of the suction pipe;

means forming a second suction mouth within said suction pipe above said first suction mouth, said means being connected to said inlet of the pump;

means for introducing water into said pipe at a pressure sufficient to prevent the column of dredging spoil from rising in said pipe to that one level to which said static head tends to force it, thereby to depress the top of said column of dredging spoil to an operating level within said pipe which is well below said one level and is relatively close to said first suction mouth;

and means for adjusting said second suction mouth to a position at or near said operating level such that a controlled concentration of dredging spoil exists in said suspension whereby to lessen the energy required of said pump.

3. A suction dredging installation according to claim 2, characterized in that said means for introducing water comprises a water jet pump.