METHOD FOR HYDRAULIC SUBSEA DREDGING

Abstract: Method related to the hydraulic dredging of sediment from the sea bottom, the bottom of water dams or the like, inclduding a first operational step where sediment is sucked or pumped through a hose or pipe (2) to a second level located below the natural water surface utilizing the difference in water pressure between the natural water surface and said second level to provide or enhance the required suction or pump capacity. At said second level sediment is received in a container (3) accessible by mechanical equipment located at or over the natural water surface, from which container at least part of the sediment is removable by per se conventional pumps or conventional lifting methods or by releasing sediment to a lower level. At the lower end of said hose or pipe (2) a suction manifold (15) with two inlets or two sets of inlets (16, 17) is arranged, one inlet or set of inlets (16) being arranged at or close to the lower end of the suction manifold and being adapted to suck in sediment from the bottom together with some water, the other inlet or set of inlets (17) being arranged at a vertical distance from said first inlet or set of inlets (16) and being adapted to suck in water only.

Description: Method for the hydraulic subsea dredging. Background It is desirable to be able to remove sediment from areas of the sea bottom in a number of situations. Such areas may be harbours, sailing paths or areas with contaminated sediment. It may be desirable to redeposit the sediment under water ir to place the sediment in deposits ashore, possibly subsequent to purification thereof. There are also water dams, primarily in lands where the inflowing river can deposit large amounts of sediment so that the dam gradually fills with sediment to an extent where the water capacity of the dam becomes undesirably low. Furthermore the sediment may negatively influence the stability of the dam, block sluice gates/ doors or the like and lead to unwanted wear of turbines, if the sediment follows the water into a power station. Sediment may be in the form of large rocks, very fine gram particles like silt and clay and anything therebetween. It is well known in the art that a pump cannot generate an underpressure or a suction larger than one atmosphere if placed at or over the water surface. It is therefore a limitation to how effectively sediment may be dredged from the sea bottom, and from which depths such sediment may be dredged, when pumps are arranged at or over the water surface. It is possible to arrange pressure pumps at the sea bottom, to lift the sediment up, but it is costly to position and reposition such equipment. It is furthermore known m the art of dredging to remove sediment from the sea bottom and place the sediment at an elevated level that is still below the water surface. This way the advantage is achieved that the pressure difference in the water column between the natural water surface and the level at which the sediment is pumped, may be utilized as part of the lifting force for the pumping. Dredging may be performed through nozzles placed in contact with the sea bottom at a certain first position, from which it sucks up an amount of sediment, forming a crater in the sea bottom. Depending on how loose or compact is the sediment at the location in question, the sediment will tend to slide into the crater as it becomes deeper and the crater walls will become steeper At times rather large amounts of sediment will slide out abruptly with no prewarmng, with the consequence that the nozzle becomes plugged and/ or get stuck Traditionally there has been no way to avoid this problem except by frequently moving the nozzle, so that the crater never gets to be very deep or to have crater walls that are very steep. This is, however, not convement if large amounts of sediment are to be removed, as each location in the area in question needs to be treated several times in order to remove the required amount of sediment. Another condition that has lead to problems, is related to the fact that the sea bottom varies a lot with respect to its character or nature, and that a nozzle that is well suited for loose sediment, is less suited for compact sediment. The possibility of adjusting the suction power in accordance with the character of the sea bottom has previously been poor US patent N 3,693,272 descπbes a system (an apparatus) that principally enables dredging at (from) large depths. The solution has the limitation, however, that it relates to a mainly closed system that will be vulnerable to e g. large rocks and other extraneous matter, and it is not easily available for inspection and maintenance. Thus, it is not well suited for the purpose of the present invention where a significant variation in particle size must be expected, with rocks of substantial sizes constituting an essential element. US patent No. 3,815,267 also describes a closed system for sucking up sediment from a sea or ocean bottom, and has in general the same disadvantages or limitations as the above mentioned patent, if used for sediment having a large variation of particle size. US patent No 1 ,468, 199 describes a method and a device for dredging by means of a semi- submersible, open container operating at atmospheric pressure, and a suction pipe that is lowered to the sea bottom, wherein the suction force is provided entirely through the difference in liquid pressure between the surface and the outlet of the suction pipe into said open container. The patent also describes equipment to lift the sediment up to the level of the water surface Objectives It is an object of the present invention to provide a method for dredging/ pumping sediment consisting both fine particles and rocks of significant sizes, from the sea bottom to a level below the natural water surface, and subsequently to transport the sediment further by means of conventional technology. It is a further object of the invention to provide a method that allows a highest possible concentration of sediment, so that the subsequent treatment and deposition of the sediment may be performed with cost as low as possible. It is a further object of the invention to provide a method that is versatile, so in cases where the sediment is to be deposited at a special land fill, it may easily be transferred to such land fill, or the sediment may be pumped to e.g a barge for further transportation It is a further an object of the invention to provide a method that is useful for transporting sediment from a water dam to a location lower than the water surface of the dam, where the transportation is conducted through a pipeline or tunnel to said location in a way m which the need for external energy is kept as low as possible. It is furthermore an object to be able to transport a highest possible concentration of sediment without blocking the pipeline or tunnel with sediment It is further an obj ect of the invention to conduct said method by means of an open system with good accessibility for maintenance and repair work, and with a high degree of operational reliability It is further an object to conduct the method m a way that to a large extent is self-regulatmg with respect to the transport of loose and compact sediment, and by means that are robust m the sense that they will not be blocked by slide-outs commonly arising from the dredging It is further an object to provide a method that allows continuous work independent of the variation in particle size of the sediment to be dredged. It is a still further object of the invention to provide a low-cost method that to a large extent permits use of conventional equipment. The invention The above mentioned objects are achieved by the method according to the invention as defined by the claim 1. Preferred embodiments of the method according to the invention are disclosed by the dependent claims In the following the invention is described in more detail with reference to the accompanying drawings, where Figure 1 shows a schematic view of one embodiment of the invention, Figure 2 and 3 shows different variants of some details of the invention, Figure 4 is a schematic view of an embodiment of the invention for a particular application, Figure 5 shows a particular embodiment of a certain detail of the invention, Figure 6 shows an extra functionality related to the embodiment of the invention illustrated in Figure 2. Figure 1 shows schematically means suited for conducting the invention in relation to dredging an area of the sea bottom 1 or the bottom of a water dam. A hose or pipe 2 is arranged to transport sediment from the bottom 1 to a container 3 that is arranged in a way where the level 4 of water and sediment within the container is lower than the natural water level 5 outside the container. The container 3 is preferably open to the surroundings and is under any circumstances arranged in a way so that maintenance and repair workers have easy access. In connection to the container 3 means are arranged to transport the sediment further in one or more fractions according to particle size. It is an important feature of the invention that rocks and other large particles my be sucked up from the sea bottom without risking blockage, as the pipe 2 is smooth and without any reductions in cross section. The container 3 may, for example, be arranged as part of a barge or have the form of a tank connected to e.g. the leg of an oil platform. It is preferred that the vertical level of the container may be adjusted according to varying requirements. As indicated by Figure 1 a grating 6 is arranged at a level between the pipe outlet 7 and the level 4 of water and sediment in the container. By this arrangement rocks and particles with a smallest diameter larger than the grating openings will be held back on the grating while other sediment will pass therethrough. The sediment consisting of large particles is denoted the coarse fraction 8, while the sediment consisting of smaller particles is denoted the fine fraction 9. Aided by the grating, the fine fraction 9 may be removed separately by means of equipment that need not be dimensioned to handle large rocks or other large particles. Such means may comprise conventional pumps or the like. Figure 1 shows such a pump 10 with a pipe 11 connected thereto and arranged to transport the fine fraction 12 together with a regulated amount of water, to a separate tank 13, which e.g. may be located on a barge. The fine fraction may alternatively be pumped back to a different location under water or to a special land fill, possibly to an intermediate station for purification and subsequent further transportation. The further treatment of the fine fraction and/ or the coarse fraction is, however, not subject of this invention. Furthermore Figure 1 shows a digging or lifting device 14 arranged to take care of the coarse fraction 8 held back on the grating 6. In a similar manner to the fine fraction the coarse fraction may alternatively, in a controlled manner, be dropped back to a convenient location under water or placed in a separate container (not shown) e.g. on a barge or on a land fill. At the lower end of the pipe 2, Figure 1 depicts a particular suction manifold 15 (also denoted a "saxophone head") with a number of openings or slits 16 at its lower end, and with an opening 17 at a vertical distance from the openings 16, arranged at the free end of the saxophone head. While the openings 16 at any time will suck in sediment and varying amounts of water, the opening 17 will always only suck in water. The lower the concentration of sediment in the pipe becomes, the larger the velocity and the larger becomes the sucking forcing. Reversely, at high concentrations of sediment the velocity is reduced and thereby the sucking force is reduced, which leads to a reduction in the amount of sediment being sucked in through the openings 16 compared to the amount of water being sucked in through opening 17. Expressed in another way the saxophone head has the property that the sucking force is determined by the velocity with which the water flows in the pipe. This way the suction manifold 15 is self-regulating and will not easily become blocked. With a convenient dimensioning of the suction manifold 15, i.e. a sufficient vertical distance between the openings 16 and the opening 17, the suction manifold will also function during and subsequent to a slide-out of sediment into the crater that the suction manifold may generate during dredging. This is due to the fact that the opening 17 is elevated to a level where it will always be free and able to suck water, ensuring that the sediment concentration in the nozzle 15 and the pipe 2 will rapidly reduce even subsequent to a near complete blockage. Dependent of the use, a sufficient distance may be in the order of 2-6 metres. The suction manifold shown m Figure 1 provides, in use, the important advantage that to a large extent it may be left alone on the sea bottom for shorter or longer peπods, and does not need to be continuously controlled To maintain the suction manifold in an upright position, buoyancy means (not shown) may be connected to the suction manifold itself and/ or to parts of the pipe 2 When dredging the suction manifold will gradually sink down into a self-created crater m the sea-bottom, while maintaining a self-regulating concentration of sediment as mentioned, the risk of blocking of pipe or suction manifold being as good as eliminated It should be emphasized that the dimensions of the Figure are distorted, as the length of pipe 2 may be several hundred metres while the suction manifold 15 typically is 2-6 metres high Figure 2 shows another embodiment of the container 3 Here there is no grating holding back the largest particles, instead there a pipe 18 is arranged to pump such sediment away This pumping may be effected by means of an ejector pump 19 connected to the pipe 18. An advantage of this way of handling the coarse fraction is that it to a larger extent it may be performed as a continuous process, the disadvantage being that it provides a less sharp distinction between the coarse and the fine fraction, as some fine particles will necessarily follow the coarse fraction The fine fraction will according to Figure 2 be removed by means of a conventional pump 10 Figure 3 shows a still further embodiment of the container 3, where a substantially vertical tubmg 20 with a hatch 21 is arranged at the bottom of the container Normally the hatch is closed, and with the absence of a grating in the container, large rocks will collect close to the hatch, while finer particles will to a larger extent, be dispersed in the water above According to needs or according to regular intervals the hatch is temporarily opened, so that the rocks fall back to the bottom below the container. During opemng of the hatch there is fluid communication for water mside and outside the container 3. Thus, water will flow up the tubmg seeking to balance the levels inside and outside the container It is therefore desirable to hold the hatch open for as short periods as possible, and subsequent to an opening it will normally be required to pump out some water from the container m order to obtain the desired difference between the levels Figure 4 shows a vaπant of the invention in connection with a dam 22 Many of the details corresponds to the details found in Figure 1, like the gratings 6 in the container 3 to hold back the largest particles/ rocks 8. From the container 3 a pipe 23 for transportation of the fine fraction together with a convenient amount of water, extends to a position downstream of the dam 22, which position is one at a lower level than the level 4 in the container 3. Inside the container 3 the pipe 23 is extended with a slotted pipe 24 which has slots allowing particles to be sluiced into the tubing along with a controlled amount of water. This slotted pipe (sediment sluicer) is first described in "Gemini" No. 3, December 1994 and in "Hydropower and Dams", March 1995. By such an arrangement one may sluice in the highest possible concentration of sediment that the pipe 23 is able to transport without risking blockage of the pipe. Excess water may be pumped back to the water dam/ the sea, or extra water may be let into the container if required. It should be noted that the use of a pipe 23 to transport the fine fraction to a lower position, such as outside a dam, is not conditional on the use of such a slotted pipe as described above. Furthermore, it may be convenient, if the pipe 23 is long, to provide it with a conventional pump in order to maintain a desirable capacity of transportation under all conditions. When there is an undesired surplus of water in the container 3, amounts of the least contaminated water may be pumped off from this part of the container by means of a pump (not shown) . If the water level becomes so low that the desired ratio in the pipe 23 cannot be reached, more water may be allowed to enter the container 3. Figure 5 shows an alternative design of a suction manifold 25 with two openings or two sets of openings 16', 17'. The suction manifold is substantially straight, and comprises an outer, cylindrically mantle 26 inside of which is defined a substantially annular void 27. At the upper end of the mantle 26, openings 17' are provided into said annular void. The vertical extension of the mantle 26 is of the same magnitude as the height of the free end of the "saxophone head" 15. The manner of operation for the suction manifold 25 is similar to the manner of operation for the saxophone head 15. Sediment and some water will, during dredging, be sucked into the opening 16' and transported through the pipe 2' to a container (not shown) near the water surface. Water will be sucked into the opening(s) 17' and transported downwards in the annular void 27 to the lower end of the suction manifold, and from there on in a mixture with the sediment up to the container. The amount of water sucked into the opening(s) 17' will, like for the saxophone head, to a large extent be self-regulating dependent of the concentration of sediment in the pipe. The advantage with this head compared to the saxophone head is that it is somewhat less voluminous and that it has an aperture πnrresnnndine to the full cross section nfthe nine, and nan rrpmsnnrt. even large rocks without any risk of blocking. On the other hand, the straight suction manifold can not to the same extent be left alone on the bottom, but needs to be controlled more continuously. Figure 6 shows principally the same embodiment of the invention as Figure 2, but with the added functionality that an ejector pump 28 is connected to the pipe 2 above the suction manifold (not shown) to improve the suction capacity. In order to contaminate as litle water as possible, it is convenient that the ejector pump 28 is supplied with water from the container 3 through a supply conduit 29. It may possibly be convenient to filter this water as it enters the supply conduit 29. Such an ejector pump 28 may also be used in any embodiment of the invention, not just the one depicted in Figure 6. Useful ejector pumps for this purpose are described in PCT patent application No. PCT/NO00/003 9 and in Norwegian Patent Application No. 20001 4843. By the present invention a simple maneuvering and positioning of the suction manifold is obtained, and it can be positioned exactly in a desired location. It may be used for dredging in comparatively deep waters and the risk of blockage of the suction pipe, leading to a shutdown, is very small. All equipment connected to the second level, i.e. to the container 3, is easily accessed for maintenance and repair works etc. In use it should be distinguished between dredging in basins where the depth is typically 30 metres and dredging offshore where the depth is at least 50 metres and more typically 200 - 300 metres. The invention is suited for the entire spectre of depths, but it will naturally be necessary with an adjustment of the height difference between the natural water level and said second level, depending on the depth at which the dredging takes place. The cross section of the pipe 2 will to a large extent be adjusted in accordance with the actual need, but will not generally exceed 50 cm and will, at the other end of the scale, seldom be less than 10 cm. Calculation Example In the table below four calculation examples are shown. It should be noted that these examples are purely theoretical, and that real capacities will depend on the nature of the sediment and how effectively it is sucked into the pipe. Where an ejector is included, it is anticipated that it will be powered by partly contaminated water that has been sucked up earlier. The first two calculation examples are valid for a typical situation where dredging takes place in a harbour basin. It can be seen that the consumption of water is reduced significantly if ejector is used, and in addition the diameter of the suction pipe may be reduced and thereby easier to handle. The last two examples are valid for a typical offshore situation where the depth is significantly larger than in a basin. Also in this situation use of an ejector will render it possible to dredge with significantly higher concentrations of dry material, with the condition that the ejector is powered by already contaminated water. Other advantages of using ejector are the possibility of dredging at larger depths, increasing the total capacity and the ability to reduce the height down to level 2.

Claims: Claims 1. Method related to the hydraulic dredging of sediment from the sea bottom, the bottom of water dams or the like, said sediment comprising particles of a size varying from rocks of a significant size to fine grain particles, said method including a first operational step where sediment is sucked or pumped through a hose or pipe (2) to a second level located below the natural water surface utilizing the difference in water pressure between the natural water surface and said second level to provide or enhance the required suction or pump capacity, sediment received at said second level is landed in a container (3) being accessible by mechanical equipment arranged at or over the natural water surface, from which container at least part of the sediment is removable by per se conventional pumps (10) or methods of elevation or by releasing it to a lower level, characterized in that a suction manifold (15) with two inlets or two sets of inlets (16, 17) is arranged at the lower end of said hose or pipe (2), one inlet or set of inlets (16) being arranged at or close to the lower end of the suction manifold and being adapted to suck in sediment from the bottom together with some water, the other inlet or set of inlets (17) being arranged at a vertical distance from said first inlet or set of inlets (16) and being adapted to only suck in water. 2. Method as claimed in claim 1 , characterized in that the sediment to a desired extent is separated by means of conventional equipment at said second level and transported further on a selective basis to at least one additional level where the sediment is to be delivered, by means of methods adapted to the particle size and concentration of respective fractions. 3. Method as claimed in claim 1 , characterized in that the capacity of pumping of sediment from the sea bottom to said second level is increased by connecting an ejector pump to the hose/ pipe between the bottom and said second level. 4. Method as claimed in claim 3, characterized in that the ejector pump is powered by water pumped from said second level in order that as little water as possible becomes contaminated and/or mixed with the dredged sediment. 5. Method as claimed in claim 1, characterized in that the container receiving the sediment at said second level is open and connected to one or more barges or pontoons, or constitutes a part of a barge or a pontoon, preferably in such a way that the container's vertical position is adjustable relatively to the barge(s) or pontoon(s) in question. 6. Method as claimed in claim 1, characterized in that the sediment at said second level is separated into two or more fractions by means of one or more gratings, the fraction of the largest particles being held back on a grating and lifted further up to a third level by means of a mechanical lifting device. 7. Method as claimed in claim 1, characterized in that the fraction of the finest particles received at said second level, is sucked into a slotted pipe with a longitudinal slot at the bottom of the container. 8. Method as claimed in claim 6, characterized in that the fraction of the finest particles received at said second level is transported together with water to a lower level partly or entirely by means of gravitational forces through a pipe or in a tunnel. 9. Method as claimed in claim 1, characterized in that means are arranged at said second level to ensure that the water level within the container is maintained substantially constant. 10. Method as claimed in claim 1, characterized in that sediment at said second level is separated into two fractions by means of a grating, so that the fraction of the largest particles includes rocks with a smallest diameter down to typically 5 cm. 11. Method as claimed in claim 1, characterized in that the height difference between the natural water surface and the second level typically is in the range 2-30 metres. 12. Method as claimed in claim 1, characterized in that the vertical pumping distance from the sea bottom to said second level typically is in the range of 5-300 m 13. Method as claimed in claim 1, characterized in that the diameter of the pumping pipe typically is 10 - 50 cm.

Inventors: FAGERVOLD GUNNAR, FAGERVOLD TERJE, JACOBSEN TOM, KVALVAAG OLAV, FAGERVOLD, GUNNAR, FAGERVOLD, TERJE, JACOBSEN, TOM, KVALVAAG, OLAV
Applicants: FAGERVOLD GUNNAR, FAGERVOLD TERJE, GTO SUBSEA AS, JACOBSEN TOM, KVALVAAG OLAV, FAGERVOLD, GUNNAR, FAGERVOLD, TERJE, JACOBSEN, TOM, KVALVAAG, OLAV
Application year: 2001
Main classification: E02F3
IPC codes: E02F3, E02F7, F04B15, F04B47, F04F5