Underwater suction dredging unit and method therefor

Abstract: The invention relates to a method and a device to be used in dredging. Material present on the seabed is dredged by means of a dredging unit, which is suspended from a vessel (1). Material dredged by means of the dredging unit is supplied to a discharge pipe suspended (5) under water from a vessel (1) and coupled to the dredging unit, via which discharge pipe the material is deposited on the bottom of the sea at some distance from the dredging place. The device is provided with a dredging unit comprising a frame (6) which is provided with propulsion means, by which the device can be moved in any desired direction.

Description: The invention relates to a method to be used in dredging, wherein material present on the seabed is dredged by means of a dredging unit, which is suspended from a vessel. When waterways are dredged to their desired depth, use is frequently made of draghead suction installations suspended from vessels, whereby a dredging unit suspended from the vessel is moved across the bottom by means of the vessel and material present on the bottom is sucked up and deposited into the vessel in question or into a vessel which is present nearby. The working depth is thereby limited to ± 100 m below the water surface. Special water craft which can be lowered from a vessel and is capable of moving across underwater surfaces have been developed as indicated in AU-B-590125. Such craft can only be used, however, if the underwater surface extends in a regular, more or less horizontal manner and the underwater surface possesses sufficient bearing capacity to support the craft. As described in AU-B-590125 during dredging the vessel is suitable anchored and the dredged material can be discharged e.g. to the shore via a pipe maintained at the water surface by spaced floats. Furthermore a jet pump suspended from a vessel is sometimes used for dredging at great depth. When such a jet pump is used the material can only be transported over a comparatively small distance from the place where the material is dredged, whilst it is hardly possible to check the direction in which the dredged material is transported. Exploiters of offshore gas and crude oil wells feel an increasing need to be able to place installations on a solid foundation, also at great depth. An example of this is the laying of a pipeline, where the pipeline must frequently be laid on an underwater bed of rubble or the like. When the bottom layer is soft, it is necessary to remove said soft bottom layer first before depositing the rubble layer, in order to ensure a stable position of the rubble layer. According to the method of claim 1 an effective transport of material can be achieved also at great depth in that material dredged by means of the dredging unit is supplied to a discharge pipe suspended under water from a vessel and coupled to said dredging unit, via which discharge pipe the material is deposited on the bottom of the sea at some distance from the dredging place. By using the method according to the invention it is possible to dredge material at a desired location, also at great depth, and then deposit said material at a further desired location. In practice it has been established that it is also possible thereby to carry out dredging work in an accurate manner at great depths of more than 350 metres. Another aspect of the invention relates to a combination of a vessel and a dredging unit as claimed in claim 3. As the dredging unit is suspended from the vessel and provided with propulsion means it is possible to effect an accurate positioning of the dredging unit during operation and thus an accurate execution of the intended dredging work. The invention will be explained in more detail below with reference to the accompanying Figures. Figure 1 is a schematic view of a vessel from which a dredging unit as well as a discharge pipe are suspended. Figure 2 is a larger-scale schematic view of a dredging unit and a frame supporting said dredging unit, which frame is provided with propulsion means. Figure 1 schematically shows a vessel or ship 1, which is provided with at least one screw 2 for the normal propulsion of the vehicle, as well as bow and stern screws 3 and 4 respectively, by means of which a propulsion force transversely to the longitudinal direction of the vessel can be exerted on the vessel. Furthermore the vessel is preferably provided with a positioning system, by means of which the propulsion means in the shape of screws 2 - 4 in the illustrated embodiment, which are computer-controlled or the like, are controlled in such a manner that the vessel will remain at a desired location or is moved along a desired path and at a desired speed. A dredging unit comprising a frame 6 is suspended from the vessel by means of cables 5 during operation. Propulsion means, for example in the shape of screws 7 to be driven by driving means (not shown), are provided in the frame of the dredging unit. The axes of rotation of the screws 7 schematically shown in Figure 2 extend perpendicularly to the plane of drawing. Besides the two screws 7 shown in the Figure two further screws will be provided in the frame, which will be arranged symmetrically with respect to the vertically extending central axis of the frame and whose axes of rotation will extend perpendicularly to the axes of rotation of screws 7, so that the frame 6 can be propelled in any desired direction by the propulsion means in the form of the various screws by suitably driving said screws. Of course it will also be conceivable to provide propulsion means other than screws, for example jet nozzles or the like. Frames of this type provided with propulsion means are known per se, as appears for example from EP-A-0 570 034. Furthermore a suction pump 8 to be driven by a motor 19 is mounted within frame 6, said suction pump being connected, via a suction pipe 9, to a suction means 10 or the like supported by frame 6. Furthermore a jet nozzle 11 is connected to said suction means 10, which jet nozzle is in communication with a jet pump 12 mounted within frame 6. A pressure pipe 13 of suction pump 8 is connected, via a ball joint 14, to the end of a substantially horizontally extending discharge pipe 15, which is suspended from vessel 1 by means of cables 16 and 17, which are unwound from winches present on the ship. A pipe having a comparatively great length can be supported by a few spaced-apart cables by disposing pipe 15 in the longitudinal direction of the ship 1. It will be conceivable to use another vessel or a flexible pipe suspended in the water, which is for example coupled to a vessel and/or anchored in the seabed at one end, to support pipe 15. After ship 1 has arrived at the desired location frame 6 and suction means 10 supported by frame 6, as well as pipe 15 connected to pump 8, may be lowered by paying out cables 5, 16 and 17 by means of the winches present on ship 1. When frame 6 has arrived at a position located ± 3 metres above the bottom surface, suction means 10 may be placed on the bottom by means of a cable 18 connected to suction pipe 9, which cable co-operates for example with a setting cylinder (not shown) or the like hoisting device mounted within frame 6. Water may be squirted into suction means 10 by means of one or more jet pumps 12 and jet nozzle(s) 11, if necessary, whilst the material to be dredged can be sucked in by means of suction pump 8 via suction pipe 9, and be forced out via pressure pipe 13 and pipe 15. Signals for controlling the various means provided in frame 6 can be sent to the respective means from the ship and be received in return via a cable 20. Cable 20 may also be utilized for the power supply of driving means. The ship 1 may be moved along a desired path, transversely to its longitudinal direction during the dredging of the material, so that suction means 10 or the like is moved in the desired manner across the area to be worked. Because not only the ship can be moved in an accurate manner along a desired path by its propulsion means, but because also the frame 6 of the dredging unit supporting suction means 10 can be moved in any desired direction and, within certain limits, over any desired distance across the bottom surface, by its own propulsion means 7, the dredging unit comprising suction means 10 can be moved in a very accurate manner with respect to the bottom surface, so that the intended area can be worked in a very accurate manner by means of the dredging unit. The dredged material is at the same time moved over a large distance from the area to be dredged via discharge pipe 15 and accurately deposited at the desired location, at a sufficiently large distance from the area to be dredged. The above-described method makes it possible to carry out dredging work with great accuracy at depths of up to 500 metres or more. Of course it is also possible to use the method in shallow water.

Claims: A method to be used in dredging, wherein material present on the seabed is dredged by means of a dredging unit (16), which is coupled to a vessel (1) which is provided with propulsion means (2), by which the vessel (2) can be moved in its longitudinal direction and wherein material dredged by means of the dredging unit (6) is supplied to a discharge pipe (5) suspended under water from a vessel and coupled to said dredging unit (6), characterized in that said discharge pipe (5) via which the material is deposited on the bottom of the sea at some distance from the dredging place, is suspended from a vessel (1) in such a manner as to extend at least substantially parallel to the longitudinal axis of the vessel (1) and the dredging unit is suspended from the vessel (1) which is provided with propulsion means (3, 4) by which during dredging the vessel (1) can be moved in a direction transversely to its longitudinal axis. A method according claim 1, characterized in that said dredging unit (6) and said discharge pipe (5) are lowered and/or raised by means of cables or the like, which are provided between said dredging unit (6) and said discharge pipe (5) and winches provided on said vessel (1). A combination of a vessel (1), which is provided with propulsion means (2) by which the vessel (1) can be moved in its longitudinal direction, a dredging unit (6) and a discharge pipe (5) connected to said dredging unit (6), characterized in that the vessel (1) has been provided with propulsion means (3, 4) by which the vessel (1) can be moved in a direction transversely to its longitudinal axis as well as with winches whereby said dredging unit (6) and said discharge pipe (5) are suspended and can be lowered and/or raised by means of cables or the like, which are provided between said winches and said dredging unit (6)/discharge pipe (5) which is suspended in such a manner as to extend at least substantially parallel to the longitudinal axis of the vessel (1). Dredging unit for use in a combination according claim 3, characterized in that the dredging unit comprises a frame (6), which is provided with propulsion means (7) by which the dredging unit (6) can be moved in any desired direction and suction means (10) supported by said frame (6). Dredging unit according claim 4, characterized in that a pump (8) is provided in said frame (6), whereby said pump (8) is connected to a suction means (10) via a suction pipe (9) and to a pressure pipe (13) which can be coupled to a discharge pipe. Dredging unit according claim 4, characterized in that there has been provided a ball joint (14) for coupling the pressure pipe (13) of the pump (8) to a discharge pipe (15). Dredging unit according any of the preceding claims, characterized in that there have been provided means (18) for vertically adjusting the suction means (10) or the like with respect to the frame (6).

Inventors: HEIJBOER DIRK, HEIJBOER, DIRK
Applicants: OORD ACZ B V VAN, VAN OORD ACZ B.V.
Application year: 1996
Main classification: E02F3
IPC codes: E02F3