DREDGING APPARATUS

Abstract: A dredge head (1) comprising two horizontally-mounted interlacing and counter-rotating archimedean-type screw (6) is moved into silt forming a sea bed (10). The screw (6) are driven so as to deliver a mixture of silt and water at a first pressure to the inlet of a piston pump (2) driven by a hydraulic piston-cylinder assembly (14) to discharge the mixture at a second pressure higher than the first pressure. The mixture is thus caused to flow through a discharge pipe (14) to a remote discharge point (15). The dump valve (5) is operable in response to a density sensor (3) to divert mixture from the discharge line (4) through a dump line (16) when the density of the mixture falls below a predetermined value.

Description: Dredging Apparatus Field of the Invention The invention relates to the excavation of solid/liquid mixtures and, in particular, although not exclusively, to the excavation of solid matter lying below liquid. Apparatus according to the invention, which may form an integral part of a dredger, allows silt and other soft deposits to be excavated and to be fed into and along a pipeline in the form of a solid/liquid mixture. Background Art Grab-crane dredgers excavate silt in its in situ state, but have to load the material into barges alongside for subsequent disposal. They excavate for only about 205- of the time; for the rest of the time, the crane is hoisting, slewing, dumping or lowering the grab- bucket. Bucket-ladder dredgers provide a more continuous operation and excavate silt with little addition of water, but must- generally discharge into barges alongside for subsequent disposal. These dredgers are large, costly, noisy and expensive to run. Vacuum chamber dredgers, such as the dredgers made by the Italian company Pneuma S.P.A., use an evacuated chamber on a river bed or sea bed to suck up silt in almost its in situ state. Once full, the chamber is pressurised with compressed air which forces the silt along a discharge pipeline to the surface. Ihis system has never achieved significant commercial success, being costly, bulky, difficult to handle, and expensive on energy. Various forms of suction dredging apparatus have also been devised for dredging materials and subsequently conveying them in suspension along pipelines. In these cases, the silt or other material is mixed with the overlying water and pumped away in the form of a solid/liquid suspension by a centrifugal pump. As a result, the suction dredging apparatus suffers from the disadvantage that to be pu pable the silt must be diluted to form a mixture comprising about 8θ?2 water and about 20% silt and the eventual necessity for disposal of this collected water often causes problems. Indeed, if the material has eventually to be loaded into barges or vehicles, the effective load carried is only 20S of the whole. A modified suction apparatus, made by the US corporation Mudcat Inc., addresses the problem of excess water collection and this apparatus comprises a dredge head for gathering a solid/liquid mixture and pumping means for transferring the mixture gathered by the dredge head. In this apparatus, the dredge head is fitted with two counter-rotating open augers or screws, provided with blades, which feed towards a centrally placed suction mouth of a centrifugal pump. The augers help convey material towards the suction mouth in a direction at rigjit angles to the direction of travel of the apparatus. However, the rate at which the augers convey a mixture of silt and water is not specifically designed to match the capacity of the centrifugal pump, but rather to ensure that material lying away from the pump mouth can be transported in close enough for it to be sucked up, and that the augers, with their attached blades, can cut up fibrous obstructions such as weeds; the apparatus being extensively used for cleaning out ditches and canals. While the open augers help to laterally transport the silt and water mixture into the general area of the centrifugal pump suction mouth, there is no positive feed pressure to that pump. The suction/flow rate characteristics of the centrifugal pump used are such that clear -water is also sucked in with the sil , so resulting in considerable dilution, although not quite as much as suffered by conventional suction apparatus. Disclosure of the -Invention It is intended, by means of the present invention, to overcome the problems of known dredging apparatus in a simple and convenient manner. m This is achieved by ensuring that the dredge head is operable to deliver the gathered mixture at a first pressure and that the purπping means comprise positive displacement pumping means which are operable to discharge the mixture at a second pressure higher than the first pressure. By this means it is possible to πri_nimise disturbance and dilution of the material being dredged. Thus, according to the invention, there is provided dredging apparatus comprising a dredge head for gathering a solid/liquid mixture and delivering this mixture at a first pressure; and positive " displacement pumping means, for transferring the πiixture, which are operable to discharge the mixture at a second pressure higher than the first pressure. The positive displacement pumping means preferably comprise a hydraulically-σperated piston pump such as those frequently used for pumping thick sludge, moist concrete, and other viscous material. Advantageously, the capacity of the positive displacement pumping means is compatible with the capacity of the dredge head. This means that when the apparatus is operating under optimum or near optimum conditions, solid material is delivered by the dredge head to the positive displacement pumpLng means with a πriz mal addition of the overlying liquid. In spite of this, it is inevitable that occasions will arise, particularly when excavating solid matter lying below liquid, when a high proportion of liquid is delivered by the dredge head to the positive displacement pumping means. This may occur when only a shallow layer of silt is available to the dredge head or when the dredge head moves into deep water. It is therefore useful to provide a density sensor for rreasuring the density of the mixture, transferred by the positive displacement pumping means and a dump valve which is provided on the discharge side of the positive displacement pumping means and which is responsive to the density sensor for diverting the πiixture from a discharge pipeline extending downstream of the positive displacement pumping means when the density of the mixture falls below a predetermined value. An embodiment of the invention is hereinafter described, by way of example, with reference to the accompanying drawing. Brief Description of the Drawing The drawing is a schematic side-elevational view of a dredging apparatus according to the invention. Mode for Carrying Out the Invention As shown in the drawing, a dredge head 1 is mounted on a dredge ladder 7 suspended from a dredger 8 floating in sea water 9 for movement through a silt deposit forming the sea bed 10. Inside the dredge head 1, protected from the ingress of potentially damaging debris by steel guards (not shown), is a screw conveyor comprising two horizontally-mounted, interlacing archimedean-type screws 6 (only one of which is shown). The screws 6 are driven in counter rotation by an electric or hydraulic motor 11 so as to gather silt with m__ni__um of disturbance and dilution and to convey a mixture of silt and water at a low first pressure via a flexible tube 12 and through further debris screens in a stonebox 13 to a positive- displacement piston-pump 2 mounted on the underwater dredge ladder 7 and driven by a hydraulic piston-cylinder assembly 14- This pump 2 discharges the mixture of silt and water at a second pressure, typically up to 70 Bar, through a discharge pipeline to a remote discharge point 15- Behind the pump 2 is a density sensor 3 for measuring the density of the mixture. Thus, if the dredge head 1 moves into an area of deeper water 9, then clear water rather than a mixtixre of silt and water will be passed by the pump 2. To prevent this clear water diluting the mixture which has been already pumped, the density sensor 3 will activate a dump valve 5 which will divert the unwanted water or weak mixture out of the main discharge pipeline 4 through a dump line 16. The prime power source for the underwater-mounted electric or hydraulic motor U and the hydraulic piston-cylinder assembly 14 is mounted on the dredger 8 in conventional manner. It should be noted that the throughput of the positive displacement pump 2 used in this application is nearly constant. This is not true of a centrifugal pump used in known suction dredging apparatus where, in a given situation, throughput increases with a decrease of the mixture's specific gravity, viscosity, or friction in the suction intake zone, so resulting in an even more diluted mixture. The positive displacement pump 2 has available enough power to maintain a constant flow rate at all times and the screw conveyor is matched to it to ensure that the mixture is fed to it at a positive first pressure. This is achieved through a co-ordinated design of the dredge head 1 and the positive displacement pump 2, taking into account specific gravities, viscosities, pipeline velocities, thixotropic behaviour of the silt, and shaft speeds. However, _Ln practice, a given dredger will have only one permanently mounted pump 2 to tackle various materials, but the advantage of this type of chosen pump is its ability to handle without modification a wide variety of materials with differing characteristics. This is not necessarily true of centrifugal pumps. Where enough silt is not available for collection by the dredge head 1 to produce the positive first pressure at the positive displacement pump 2, then the problem of dilution of the discharge is handled by the dump valve 5- This has the ability to control absolutely the ιιιir * irπum silt/water concentration acceptable in the discharge pipeline 4 , a ratio which often differs from one type of work to another. If the dredged silt and water mixture is transferred to the deck of the dredger 8, it can then be discharged along a floating pipeline or along a shore pipeline, as required. Some of the advantages of apparatus according to this invention over existing dredging apparatus are as follows:- (i) silt and similar material is dredged continuously with a πujimum of disturbance and dilution; (ϋ) a mixture of dredged material and liquid can be pumped at high concentration down a pipeline to a barge lying at a convenient location (e.g. outside a harbour) or to the shore. Booster pumps can be used as necessary; (ϋi) pollution of dredged water courses -is πώiimised; (iv) the apparatus is quiet and energy efficient; (v) because the dredged material is m__nimally diluted, disposal barges can be efficiently loaded (i.e. they will carry more solid dredged material, rather than the 20S dredged material and 80 water if loaded by a suction dredger); (vi) because the dredged material is .minimally diluted, the possibility of discharging ashore into farmland, marshes, and other * reclamation areas becomes a practical option not otherwise available. The mixture of silt and water can be disposed of in a small area and, being relatively dry, is much easier to manage; (vii) automatic operation of a dump valve maintains high discharge concentrations; and (vϋi)' the h± . concentration of the mixture enables small diameter pipeline to be used;

Claims: Claims 1. Dredging apparatus comprising:- a dredgehead (1) for gathering a solid/liquid irdxture; and pumping means (2) for transferring the mixture gathered by the dredgehead (1); characterised in that: the dredgehead (1) is operable to deliver " the gathered mixture to the pumping means (2) at a first pressure; and the pumping means (2) comprise positive displacement pumping means 0 operable to discharge the mixture at a second pressure higher than the first pressure. 2. Dredging apparatus, according to Claim 1, in which the puirping means comprise a hydraulically-operated piston pump (2). 3- Dredging apparatus, according to Claim 1, in which the ,5 capacity of the positive displacement pumping means (2) is compatible with the capacity of the dredge head (1). 4- Dredging apparatus, according to Claim 1, in which:- a density sensor (3) is provided for measuring the density of the mixture transferred by the positive displacement pumping means (2); a discharge pipeline (4) extends downstream of the positive displacement pumping means (2); and a dump valve (5) provided on the discharge side of the positive displacement puπping means (2) is responsive to the density sensor (3) for diverting the mixture from the discharge pipeline (4) when the density of the mixture falls below a predetermined value. 5. A dredging apparatus, according to Claim 1, in which the dredgehead (1) comprises horizontally-mounted, interlacing archimedean-type screws (6).

Inventors: ARROL SIMON JAMES, ARROL, SIMON, JAMES
Applicants: ARROL SIMON JAMES, ARROL, SIMON, JAMES
Application year: 1984
Main classification: E02F3
IPC codes: E02F3, E02F7