TECHNICAL FIELD The invention relates to systems and methods for dredging. In particular the system relates to such systems and methods where the intention is to remove bottom sediments from body waters.

BACKGROUND ART

There are a number of methods which are used to remove sediments from the bottom of a lake, the sea, streaming water or similar. These methods usually involve that sediment is lifted to the surface by a dredger or sucked up by means of hoses. A disadvantage with these methods is that particles of the sediment with the substances are spread to the surrounding water as the sediment is removed from the bottom. This problem is a reason to why contaminated sediment is oftenly left untouched at the bottom of a water body.

US 6637135 describes a method to reduce the impact of that toxic or hazardous substances is spread. A sea vessel, adapted for the purpose, enables to create a difference in pressure between a dredging zone and the surrounding water. A draw back with this method is that it does not entirely protect the surrounding water and bottom from the unwanted substances to be spread.

In a few cases freeze dredging has been performed in order to remove sediments containing unwanted substances, such as hazardous or toxic substances. Freeze dredging

may be seen as an application area for artificial ground freezing.

WO 9527830 shows a freezing cell having one or more freezing tines may be used to remove contaminated sediment from the beds of water bodies. The freezing tines are to be immersed into the sediment. Refrigerant is supplied to the tines by a refrigeration unit during the freezing cycle to solidify the surrounding sediment into a block held by the cell. The block of frozen sediment is lifted from the water body. A method of freezing marine sediment into blocks is used to recover contaminants from water bodies.

A problem with the previous known methods, freezing cells and systems of freezing cells is that they are unsuitable to be used for freezing of large areas . With large areas is meant areas larger than 20 square meters.

Freeze dredging is performed in order to stabilize the contaminated sediment in situ and to remove it in a frozen form. Freeze dredging methods are used in a limited manner in order to remove contaminated sediments. Freeze dredging may be used for other purposes. Such as to remove sediments of a water body in order to find parts of a crashed aircraft or items at a crime scene, and at the same time being able to establish positions of the found items.

In previous existing methods positioning of the freezing cells may be performed by means of a crane, such as a crane on a barge. The position of the freezing cells is adjusted by a diver, or by a submerged vessel. The

previous known methods consumes substantial resources at positioning of the freezing cells, the methods are also labour intensive, not at least due to the need of divers. Figure 9 shows an overview of a number of freezing cells being positioned at the bottom by means of previous available technology. Each of the freezing cells has a hose connected for incoming refrigerant and another hose connected for the refrigerant returning to the surface. The total number of hoses to handle is a problem.

SUMMARY OF THE INVENTION

An object of the invention is to provide a method enabling an efficient positioning of freezing cells at the bottom of a water body, such as a sea, a lake, a dam or a streaming water, with the purpose of freeze dredging sediment from a large area. This object is achieved by the previous mentioned method, the method involves connection of a number of freezing cells to each other by means of connecting means. Further the method involves submerging and positioning the freezing cells by means of a ramp means. The ramp means may comprise a conveyor.

The invention is intended for freeze dredging of sediments of a water body such as a lake, a river, a sea, a dam, a harbour or similar. The sediment comprises particles of varying sizes, and parts of the sediment are lifted to the surface as frozen sections.

An advantage of the invention is that it enables contaminated sediment of a large area to by lifted up to the surface without particles, such as toxic particles, are spread to the surrounding water during the lifting process. The invention enables this in a cost effective

manner. At traditional dredging methods of large bottom areas, particles contaminating the sediment whirl to the surrounding water.

Another advantage is that compared to previous known freeze dredging methods and systems a substantial less number of connections are needed from a floating platform down to the freezing cells. This approach also decreases energy losses appearing when the refrigerant is transported between the surface and the bottom. This is due to that the invention enables that the total length of connecting means between the surface and the freezing cells are drastically reduced.

Yet another advantage with the invention is that it enables that submerging the freezing cells is performed without divers or underwater vessels are necessary for all critical steps.

In addition to freeze the sediment under each freezing cell, sediment between the freezing cells are frozen in a controlled manner which enables that sediment between the freezing cells is lifted to the surface.

The water that is frozen is the water which is comprised in the sediment. Compared to previous known methods and systems for dredging of large areas a substantial less amount of water is lifted to the surface compared to traditional dredging methods. This also means that the invention enables that far less water is needed to be processed at the surface when dredging large areas. Another advantage is that sediment which has been frozen has other characteristics compared to non-frozen

sediment. Sediment that has been frozen gets other characteristics which make it easier to dewater. This means that less amount of sediment needs to be transported to a dump compared to. previous known methods and systems.

Submerging a set of freezing cells is performed in series. Before submerging the set of freeze cells, at least two of the freezing cells are connected to each other above the surface and then cells are submerged. The set of freezing cells may then be extended by connecting additional freezing cells and this is repeated until all freezing cells of the set is submerged to the bottom in series .

In one embodiment the submerging step is performed by moving the freezing cells on a conveyor like means.

In another embodiment each of the connecting means comprises a first and a second flexible pipe, such as two hoses. The flexible pipes connect the freezing cells and are also used to transport the refrigerant. The flexible pipes may be intended for single use.

Freezing of sediment is performed under at least a number of hours, typically 1-3 days, and begins after all cells are submerged. The refrigerant is led down from at least one refrigerating plant above the surface. Typically the refrigerating plant is positioned on a floating vessel.

The freezing cells are mainly flat bottomed. The cells typically have four main corners. The length and the width of the underside intended to be positioned against

the bottom with sediment are substantially longer than the height of each freezing cell. The four cornered shape facilitates the positioning of the cells in rows. The rows may run next to each other, and one row may comprise one or several sets of cells in series.

The freezing cells may be pushed down in the sediment by means of a pushing device arranged on a lifting means, such as a crane or winch arranged on a floating vessel.

As it is complicated to perform manual tasks in a body of water it is an advantage that the breaking step involves tearing off the connecting means, such as flexible pipes, this as an alternative to dismantle the flexible pipes involving work down at the bottom. Tearing off the flexible pipes is typically integrated in the breaking step. The breaking step is typically performed by means of a lifting device, such as a crane, that is attached to one of the freezing cells by means of a hook and a wire. As the lifting device lifts the freezing cell the flexible pipes are torn apart. And one single freezing cell is then lifted up to the surface. To enable such a procedure it is necessary to be able to control the amount and thickness of sediment and water that is frozen between the freezing cells. The thickness should be sufficient to allow that as much as possible of the upper layer of sediment between the freezing cells are lifted to the surface. Dismantle of the torn flexible pipes is then performed at the surface, for instance on a floating vessel.

Yet another object of the invention is to provide a system enabling an efficient positioning of freezing

cells at the bottom of a water body with the purpose to freeze dredge sediment of a large area. This object is achieved by the previous mentioned system, the system comprises a number of freezing cells intended to be connected to each other in series by means of connecting means. The connecting means intended to connect two neighbor cells typically comprise at least a first flexible pipe intended to transport refrigerant in one direction and second flexible pipe intended to transport refrigerant in the opposite direction. The system comprises comprise a conveyor means similar to a ramp. The conveyor means is intended to transport freezing cells in series from a position at the surface to a position at the bottom of the sea body. It is an advantage if the upper end of the ramp is attached to a floating vessel and the lower part of the conveyor means is possible to submerge down to the bottom or to be hoisted to the surface. The system may also comprise a pushing device intended to push individual freezing cells into the sediment.

The freezing cells are configured to be connected to each other in series.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in more detail in connection with the enclosed schematic drawings.

Figure Ia shows an overview of a method according to the invention.

Figure Ib is a more detailed overview of a flow chart of an embodiment .

Figure 2 is an example of a system for freeze dredging according to the invention. Positioning of freezing cells is performed from a floating vessel, such as a barge, by means of a ramp with a conveyor like means.

Figure 3 is an overview of a set of freezing cells connected in series.

Figure 4 is an overview of a set of freezing cells connected in series with flexible pipes.

Figure 5 shows two freezing cells positioned at the bottom of a body of water.

Figure 6 is a schematic overview of two freezing cells pushed down into the sediment.

Figure 7 shows a schematic overview of two freezing cells pushed down into the sediment at the end of a freeze cycle .

Figure 8 shows breaking of the frozen sediment between two neighbor freezing cells and of the flexible pipes between the freezing cells.

Figure 9 shows a system based on prior art technology. The number of connecting means from the surface down to each freezing is the reason to a number of problems. One such a problem is during the submerging phase when a freezing cell is submerged and placed on the bottom since it may cause damage on hoses to freezing cells already in place .

DETAILED DESCRIPTION OF THE INVENTION Freeze dredging is performed by leading a cold refrigerant to each of a set of freezing cells positioned at the bottom of the water body. The sediment is later lifted in frozen sections. These steps may be referred to as a freeze cycle. The time necessary for a freeze cycle depends on several aspects. The most important aspects being: - the temperature of the refridgerant

- the energy characteristics of the sediment

- the type of used freezing cells

- heat losses to the surrounding water

A freeze cycle may take a few hours, up to several days depending on the size and type of assembly of the freeze dredging system.

Figure 3 shows that connection of a set of freezing cells 10 is performed by means of connecting means 30. Figure 4 indicates that in one embodiment each of the connecting means 30, comprises a first 40 and a second 41 flexible pipe. In addition to connect the freezing cells 10, the pipes 40, 41 are in this type of embodiment intended to transport refrigerant to and from neighboring cells. The refrigerant has at least during parts of a freezing cycle an in-temperature lower than -5 degrees Celsius. The first flexible pipe is intended to transport refrigerant in to the freezing cell, the second flexible pipe 41 is intended to transport the refrigerant out of the same freezing cell. As shown in figure 4 the freezing cells comprise means for connections on two sides. The means for connection of the flexible pipes may comprise a threaded pipe, one for each flexible pipe, sticking out

of the side of the freezing cell 10. The flexible pipes 40, 41 may be at least partly made out of a material used for hoses or flexible tubes. The refrigerant is intended to circulate in a closed system where the freezing plant is situated above the surface, preferably on a vessel. The refrigerant is transported down to a first freezing cell in a series of cells in first connecting hose. The refrigerant is transported up to the surface from a second freezing cell in the series of cells by means of second connecting hose. Compared to prior art systems, as shown in figure 9 with a number of connecting hoses, the system according to the invention is more efficient to handle at submerge of freezing cells. And energy losses are smaller in a system according to the invention due to that the total length of the connecting hoses from the surface down to the bottom is shorter.

Figure Ia shows an overview of a method according to the invention. Connecting 2 of freezing cells 10 is performed above the surface, preferably on the floating vessel 14.

Figure Ia also shows that after connecting 2 a set of freezing cells in series, a submerging step 3 where the freezing cells are transported down to the bottom 12. In figure 2 is shown that submerging is performed by means of a ramp comprising a conveyor like means 16. The conveyor like means 16 has a first end which is possible to submergible to the bottom 12. Figure 2 further shows that the second end is positioned above the surface 19, or at the surface, attached to the floating vessel 15.

During a method according to the invention the submerging 3 of the connected cells 10 is performed in series. In the case where the conveyor like means 16 is utilized,

the freezing cells 10 are placed in series on the conveyor like means 16, which is shown in figure 2.

Further, figure Ia shows that the method may comprise the step of pushing 4 each of the cells down in to the bottom 12. Pushing 4 in to the bottom 12 is performed such the freezing cells 10 are completely or partly pushed down into the sediment 11. An advantage with pushing down the freezing cells 10 is that the time of the freeze cycle is reduced as the energy losses are reduced compared with if the freezing cells are more laying on top of the sediment 11. Figure 5 shows freezing cells before they are pushed down 4 and figure 6 shows the cells 10 after they have been pushed down 4.

Figure 2 shows an embodiment where the cells are pushed 4 by means of a pushing device 13 attached to a lifting device 14 which main body is arranged the floating vessel

15. The floating platform 15 may be a barge or a ship. The lifting device 14 may be a crane or a winch with a wire or similar which may be attached to a pushing device 13 while pushing cells into the sediment. The pushing device 13 is adapted to push down each cell rather than forcing it down by hitting each cell at the time of impact. It is an advantage if pushing 4 is performed in conjunction when a cell leaves the conveyor like means

16. As the cell leaves the conveyor like means 16 it is a good opportunity to observe the position of the cell for instance by measuring depth from the surface to the bottom and the angle of the ramp with the conveyor like means 16 compared to the floating vessel. The pushing device 14 may be embodiment in a multitude of ways. The pushing device 13 may comprise an element with

substantial weight and surface facing the freezing element with a fairly soft and protective material.

Figure Ia also shows that the method 1 comprises the step of freezing 5 of sediment 11 under and between the cells 10. The frozen sediment 61 between the freezing cells 10 may be the upper layer of the sediment. The frozen sediment often forms a bridge between cells 10 in the cracking zone 61. The frozen sediment may often be frozen together with the connecting means. In some cases the frozen sediment, such as in figure 6, forms a bridge that is only to some extent frozen together.

At freeze dredging with mainly flat freezing cells 10 layers of typically 300 - 400 mm of sediment may be lifted up at the end of each freezing cycle. Objects such as stone, scrap or plants does not hinder freeze dredging when applying the invention. The depth of sediment that is frozen may be controlled by adjusting the time of a freeze cycle. In order to calculate the size of the cracking zones 61 and the depth of frozen sediment under the cells, the propagation of the freeze volume in the sediment should be calculated. Propagation of the freeze volume in the sediment may be calculated providing that the water ratio of the sediment, density and thermo- technical properties are known. The deciding properties are the sediment's ability to conduct heat and effective latent heat. There are numerical methods to calculate the propagation of freeze volume but also commercial available simulation programs. In order to study the propagation of the freeze front in multiple dimensions it is necessary to model by means of partial differential equations, the problems are in practice too difficult to

solve by means of classical analytic methods. The finite element method (FEM) is a numerical approach where differential equations are solved by means of approximations. A characteristic of the finite element method is that one does not apply approximation across the total studied area, but rather divides the area into smaller areas called finite elements. As the system is complicated it is not possible to solve by hand, instead it solved by means of advanced computer software. There is commercial software to model freeze propagation. Such a software is TEMP/W (GEOSLOPE) , which the inventors have used successfully.

The method further comprises breaking 6 of the frozen sediment 60 between the freezing cells 10. Figure 8 shows that breaking 6 may be performed when lifting the freezing cell 10. It is typical that a number of freezing cells are frozen together by cracking zones. When lifting a freezing cell 10 splits may occur in a number of these cracking zones 61. Breaking 6, as well as lifting a certain cell, is performed with the frozen sediment 81 frozen and attached to the bottom of the freezing cells 10.

In one alternative embodiment of the method 1 cutting of connecting means 30 is performed in conjunction of breaking the frozen sediment in the cracking zones 61. In the case the connecting means are flexible pipes 40, 41 the cutting is performed before lifting a certain freezing cell 10.

Figure Ib shows a more detailed overview of a flow chart of a method 1 according to the invention. Connecting a set of freezing cells 10 to each other in series, two or

more, by means of connecting means 30 is followed by start of submerging 3 the freezing cells 10 to the bottom. Additional freezing cells 10 may be connected to make the series of cells longer while the cells 10 are submerged on the conveyer means . The conveyer may be paused in a repeated manner as cells are placed on the conveyer and connected to each other. It may be the case that as the first cell reaches the bottom there are still a number of un-connected cells on-board the floating vessel 15. The connecting step 2 and the submerging step 3 may be repeated a number of times.

The pushing step 4 may also be repeated a number of times in a method 1 according to the invention, which is indicated in figure Ib. The freezing step 5 is performed during the same time period for all freezing cells 10. The freezing cells 10 are during the freezing step connected in series to each other. The breaking step 6 is typically repeated, indicated in figure Ib, a number of times as cells are lifted to the surface.

The freezing cells 10 comprise a system of pipes in which the refrigerant circulates during a freezing cycle. Each freezing cell comprises a fastening means, such as a bracket or similar, which is suitable for connecting to the attachment means of the lifting means. An attachment means may for instance comprise a hook. The attachment means may also embodied such that a chain is attached close to each corner of an individual freezing cell 10. The other end of each chain may be connected by a ring or similar to which the attachment means is connected during the lifting step. The lifting step may be performed by

means of a crane or winch, which preferably is identical to the previous mentioned lifting means 14.

The complete method 1 may be repeated several times at a certain dredging area. The number of times the freezing cycle is repeated depends for instance on the depth of sediment 11 that is frozen during a freeze cycle. Such a limitation may depend on the construction of the freezing cell, such as dimensions of the pipe system within the cell or distance between those pipes. It also depends on the freezing equipment positioned above the surface 19. The limitation also depends on the capacity of the lifting means 14.

The method may involve freezing cells 10 are positioned in at least two rows next to each other. In this manner an area corresponding to at least two rows of freezing cells 10 may be frozen in a freezing cycle. The two cells at the end of two neighboring rows are connected with flexible pipes of a longer length than the flexible pipes connecting the cells within one row. The connection of the cells at the end of each row is typically necessary to perform manually by a diver.

It is an advantage if the upper side of the freezing cell 10 is covered with an insolating layer which reduces the energy losses. The bottom side of a freezing cell may be covered by a thermal conducting material .

Frozen sediment is stable, easy and safe to transport. Transportation may be performed by means of a barge. If the lifted sediment is to be transported on land it is advantage the frozen sediment sections are loaded in

containers for further transportation by road. The sediment is typically frozen for several days in a container.

Modifications of the invention will occur to those skilled in the art and to those who make or use the invention. Therefore, it is understood that the embodiments shown in the drawings and described above are merely for illustrative purposes and not intended to limit the scope of the invention, which is defined by the following claims as interpreted according to the principles of patent law, including the doctrine of equivalents .