SE529323C2 - Procedure for automated freeze dredging - Google Patents

Abstract

The invention involves a system (17) and a method (1) for automated freeze dredging of sediment. A set of freeze cells (10) are connected to each other above the surface, preferably on a floating vessel. The freeze cells (10) are submerged to the bottom (12) by means of a conveyor like means (16) where the upper layer of the sediment is frozen such that sections of sediment adhere to the underside of the freeze cells (10). The invention is particular useful to lift up sediment comprising particles that are toxic or otherwise hazardous. The invention is also suitable to be used for crime scene investigation purposes. The invention enables that frozen sediment is transported to the surface without particles flowing out to the surrounding water during transportation of the sediment. Further the invention enables that less sediment needs to be processed in order to reduce the amount of water compared to methods involving traditional dredging.

Description

Artificial soil freezing is a long-known technique.

It has been used for a number of different purposes. GB428423 shows an area of use where freezing technology is used to facilitate repairs of structures that are normally below the water surface. A significant problem with existing processes for soil freezing and sediment freezing is that they are not suitable for efficient freezing of large areas. In this context, large areas refer to areas that are at least 20 sqm.

Freeze dredging is performed to stabilize the contaminated sediment in situ with freezing and to then lift it into frozen form. Freeze dredging is a procedure that has been used to a limited extent for environmental remediation of bottom sediments, e.g. of bottom sediments containing industrial waste. Frigeo AB is a company that conducts research and development in the area. In addition to freeze dredging, Frigeo AB has also carried out freezing and lifting of bottom sediments for forensic purposes.

Objects lying in the bottom sediment around the wreck of a shot down DC3 have been lifted to the surface with technology similar to freezing dredging. A freezer dredger often includes a lifting and placing unit as well as a freezer system.

Freezing dredging is performed by directing a cold brine into freezer cells placed on the bottom, the sediment is lifted after it has frozen in sections. This component forms a freezing cycle. The time required for a freezer cycle depends on many things. The most important factors are; ~ the temperature of the refrigerant - the thermal properties of the sediment - the type of freezer cells used - heat losses to the water 10 15 20 25 30 k 529 523 3 A freezer cycle can take a few hours, but also up to several days depending on how the freezer dredging system is built.

Placement and lifting of the freezer cells is done with the help of a crane, e.g. a pontoon-mounted crane, where the freezing cells are guided in by means of divers, alternatively by an underwater vehicle. The hitherto known procedures for freeze dredging are resource-intensive and labor-intensive. Figure 9 shows an overview of a number of freezer cells placed on the bottom with known technology. Each of the freezer cells has connected hoses in which coolant is transported.

There are a lot of hoses to deal with which is a problem.

An advantage of freeze dredging is that relatively little contaminated material is spread. During uptake, most of the contaminated sediment handled is frozen and therefore stable. A sediment consists of 50-90% water. In the case of uptake with, for example, suction dredging, the sediment is diluted with water and the water ratio increases further, in the case of freezing dredging, only the pore water is taken up. This means that the amount of material to be transported, dewatered and post-treated after a freeze dredging only covers the "original volume" of the contaminated sediment. This can be compared with traditional suction dredging where the occupied sediment is diluted with water during the uptake, the dilution becomes greater the firmer the material. OBJECTS AND SUMMARY OF THE INVENTION An object of the invention is to provide a method for efficiently deploying freezer cells to freeze dredged sediment from a larger area. This object is achieved by the method mentioned in the introduction, characterized in that the method comprises connecting a number of freezing cells to each other with connecting elements. Furthermore, the process is characterized by immersion to the bottom and placement of the freezing cells, as well as depression in the bottom of the freezing cells. The process is further characterized in that when freezing is completed, cracking of the frozen sediment is performed between the freezing cells.

A method according to the invention relates to freezing dredging of sediment from the seabed, from the seabed, from the bottom of running water or the like, the sediment comprises particles of varying size, where parts of the sediment are lifted up as frozen sections.

An advantage of the invention is that it enables sediments containing environmentally hazardous substances to be lifted to the surface without parts of the environmentally hazardous substances spreading during the actual lifting, and this in a cost-effective manner.

In traditional dredging procedures, parts of the environmentally hazardous substances often swirl into the surrounding water.

Another advantage is that a significantly fewer number of connections go from the floating platform down to the freezing cells, compared with already known technology.

This in turn reduces losses of cold.

A further advantage of the invention is that immersion of the freezing cells is carried out without the participation of divers or submarines. An advantage of the invention is that sediment between the freezing cells is frozen under controlled forms, which means that sediment between the freezing cells is also lifted to the surface. This is in contrast to prior art where unintentional freezing of sediment between the freezing cells was mainly a problem.

Since only the water in the sediment is frozen, a smaller amount of water is lifted to the surface compared to traditional dredging procedures such as digging or sucking up the sediment. This in itself means that significantly less water needs to be purified at the surface, which is a significant advantage.

The sediment that has been frozen acquires a changed character which makes it significantly easier to drain.

The amount of sediment that is post-treated after uptake and dewatering is significantly reduced compared to previously known methods for dredging large areas. This in turn means that the amount of sediment that needs to be transported to landfill is reduced compared to previously known methods.

It is advisable to immerse the connected freezer cells in series. This means that typically two or a few freezer cells are connected above the water surface and then immersion of these two or a few freezer cells begins. The series of freezer cells is then built up through further connections of freezer cells, after which immersion of these freezer cells is also started and this is repeated until all the desired freezer cells are immersed to the bottom. In one embodiment, the step of immersion comprises moving the freezing cells on a conveyor belt-like means.

In a further embodiment, each of the connecting elements comprises a first and a second flexible tubular element. The tubular elements connect the freezing cells and are also intended to transport a coolant which during most of a freezing cycle has an in-temperature lower than -5 degrees Celsius. The pipe elements can advantageously be of a disposable nature.

Sediment freezes for at least a number of hours, typically 1-3 days, and begins after all freezing cells have been immersed. The coolant is then led down from at least one freezer unit above the water surface to the freezer cells.

In one embodiment, the freezer cells are designed as plate-like, and mainly square elements.

The measure of the length and width of the surface intended to lie against the bottom is significantly larger than the measure of the height of the freezer cells. The square shape facilitates the placement of the freezer cells in a number of adjacent rows.

Depression of the freezing cells is advantageously done by means of a depression device attached to a lifting device arranged on a floating platform, such as a barge or a ship. The lifting device can be, for example, a crane or a winch device which controls the position of the depressing device. When it is resource-intensive to perform work under water, it is an advantage that the step of buckling comprises the further step of cutting the flexible tubular elements. This is because an alternative disassembly of the flexible pipe elements is costly. Cutting of the flexible pipe elements can, for example, be done with traction.

For example. so the tubular elements can be pulled off when lifting a freezer cell. Alternative concepts are, for example, cutting and tearing.

Another object of the invention is to provide a system for efficiently deploying freezer cells to freeze sediment from a larger area. The object is achieved by a system characterized in that it comprises a number of freezing cells arranged to be connected to each other with connecting elements, the freezing cells are intended to freeze sediment under each and between each other, whereby the sediment which is intended to be frozen between the freezing cells constitutes an intended breaking zone.

The system further comprises a conveyor belt-like means which is intended to transport the freezing cells from a position above or near the surface to the bottom.

The system also comprises a depressing device arranged to depress the individual freezing cells in the sediment.

A system according to the invention relates to freezing dredging of sediment from the seabed, from the seabed, from the bottom of running water or the like, the sediment comprises particles of varying size, where parts of the sediment are lifted up as frozen sections. 10 15 20 25 30 529 323 8 The freezer cells in the system are arranged to be connected to each other in series.

It is an advantage if one end of the conveyor-like means is hoistable down to the bottom from a floating platform, such as a barge or a ship.

DESCRIPTION OF THE FIGURES The invention is explained in more detail with reference to the accompanying figures, in which Figure 1a shows an overview of a method according to the invention.

Figure 1b shows a more detailed overview of a flow chart for an embodiment according to the invention.

Figure 2 shows an example of a freezing dredging system according to the invention. Placement of freezer cells is performed from a floating platform, like a barge, by means of a conveyor belt-like means.

Figure 3 is an overview of a number of freezer cells connected in series.

Figure 4 is an overview of a number of freezer cells connected in series with tubular elements.

Figure 5 is a schematic figure of two bottom-immersed freezer cells.

Figure 6 is a schematic figure of two freezer cells depressed in the sediment. Figure 15 shows a cross-section of two freezing cells after freezing of sediment into sections has taken place.

Figure 8 shows an example of cracking of the frozen sediment between the two freezing cells and of the tubular elements between the freezing cells.

Figure 9 shows the placement of freezer cells according to prior art.

DESCRIPTION OF EMBODIMENTS As previously mentioned, a plurality of freezer cells 10 are connected to each other with connecting elements 30, as shown in Figure 3. Figure 4 shows that in one embodiment each of the connecting elements 30, a first 40 and a second 41 comprises flexible pipe elements. In addition to interconnecting 41 in that embodiment intended to transport the freezing cells 10, the tubular elements 40 are coolant which at least during the main part of the freezing cycle has an in-temperature lower than -5 degrees Celsius. The first tube element 40 is intended to transport the coolant into the freezer cell, the second tube element 41 is intended to transport the coolant out of the freezer cell. As can be seen from Figure 4, the freezer cells 10 comprise connections for tubular elements on two sides.

Connections for the pipe elements may include threaded pipes.

The pipe elements 40, 41 may very well be hose-like.

The liquid solution is intended to circulate in a closed system where the freezer unit above the water surface cools the brine. Figure 1a shows an overview of a method according to the invention. Connection 2 of freezer cells 10 is made above the water surface 19, typically on the floating platform 14.

Figure 1a also shows that after connection 2 of a number of freezer cells 10, the step of immersion 3 to the bottom 12 of the freezer cells 10 follows. Figure 2 shows that immersion 3 is made by means of a conveyor belt-like means 16.

The conveyor belt-like means 16 has an end which is lowerable to the bottom 12. Figure 2 further shows that the other end is positioned above the water surface 19 at or on the floating platform 15. During the method according to the invention, immersion 3 of the connected freezer cells 10 is made in series and means that, in case a conveyor belt-like means 16 is used, the freezing cells 10 are placed one after the other on the conveyor belt-like means 16, which is also shown in figure 2. The invention can advantageously be compared with prior art which is shown in figure 9, where each of the freezer cells are connected to at least two hoses that run from the freezer cells on the bottom up to the surface.

Figure 1a further shows that the method comprises the step depression 4 in the bottom 12. Depression 4 in the bottom 12 is performed so that the freezing cells 10 are completely or partially depressed in the sediment 11. An advantage of depressing the freezing cells 10 is that the freezing cycle is shortened as cooling losses decrease, compared with whether the freezing cells 10 lie on top of the sediment 11. Figure 5 shows freezer cells before depression 4 and Figure 6 shows freezer cells 10 after depression 4. Figure 15 shows that in one embodiment depression 4 is performed by means of a depression device 13 attached to a lifting device 14 arranged on the floating platform 15.

The floating platform 15 can be a barge or a ship. The lifting device 14 can be, for example, a winch or a crane with associated wire or rope. It is an advantage if depression 4 is performed in connection with an individual freezing cell 10 leaving the conveyor belt-like means 16. At that time the position of the freezing cell is observable by e.g. measuring water depth and angle of the conveyor belt-like means 16 relative to the floating platform 15. The depression device 13 can be designed in many different ways. For example. the depressing device 13 can be designed as a weight element adapted for the purpose, comprising a lower part of damping material.

Figure 1a also shows that the method 1 comprises the step of freezing 5 of sediment 11 below and between the freezing cells 10. The frozen sediment 61 between the freezing cells 10 may consist of parts of the highest sediment.

The frozen sediment often forms a continuous bridge in the fracture zone 61. In some cases, the frozen sediment, as in Figure 6, forms only a partially continuous bridge. The frozen sediment between the freezing cells 10 may be frozen together with the connecting elements 30.

For freezer dredging with plate-like freezer cells, bearings up to 300 - 400 mm can be taken up per freezer cycle. Disturbing objects such as stones and scrap or plant parts do not constitute an obstacle to freezing dredging. The freezing depth is regulated with the freezing time - a longer freezing time gives a larger freezing spread. In order to be able to calculate the distribution of fracture zones and the freezing of sediment under the freezing cells, the freezing distribution in the sediment must be calculated.

The freezing distribution in the sediment can be calculated provided that you know the sediment's water ratio, density and thermal properties. The crucial variables here are thermal conductivity and efficient latent heat. There are numerical methods for calculating the freezing spread but also commercially available simulation programs.

To study the distribution of the freezing front in several dimensions, modeling with partial differential equations is required, the problems then become too advanced to be solved with classical analytical methods. The finite element method (FEM) is a numerical approach where the differential equations are solved using approximations. Characteristic of the finite element method is that one does not approximate the entire studied area but instead divides the finite element into smaller areas. As the systems are complicated, it is not possible to solve by hand but requires advanced computers for a solution. There are commercially available programs for modeling frost depth and freezing distribution. The program used successfully by the inventors is TEMP / W (GEOSLOPE).

A method 1 according to the invention further comprises buckling 6 of the frozen sediment 60 between the freezing cells 10. Figure 8 shows that buckling 6 can be done when lifting a freezing cell 10. Typically, a number of freezing cells are frozen together in buckling zones. When lifting a freezer cell 10, fractures and cracks can occur in a number of these crack zones. Cracking 6, as well as lifting, of the frozen sediment between the freezing cells 10 is done with the frozen sections 81 frozen in the lower part of the freezing cells 10.

In an embodiment of the method 1, in connection with buckling 6 of frozen sediment in the buckling zone 61, a cutting of the connecting element 30 is performed. In case the connecting element 30 comprises pipe elements 40, 41, the cutting is carried out, for example, by cutting the pipe elements 40, 41 or they wear off when one of the freezing plates 10 is lifted.

Figure 1b shows a more detailed overview of a flow chart for an embodiment of the method 1 according to the invention. Connection of a number of freezing cells 10 to each other, two or more, with connecting elements 30 is followed by initiating immersion 3 to the bottom of the freezing cells 10. Often additional freezer cells 10 are connected to the previously connected freezer cells 10 while the freezer cells 10 are immersed towards the bottom 12. It may also be the case that when the first freezer cell 10 reaches the bottom 12 there are a number of still unconnected freezer cells 10 on board the floating platform 15. In one embodiment, the steps connection 2 and immersion 3 can be repeated a number of times. The depressing step 4 can also be repeated a number of times in a method 1 according to the invention, which is indicated in figure 1b.

The freezing step 5 is performed simultaneously for all submerged freezing cells 10. The freezing cells are then connected to each other in a system. 10 15 20 25 30 529 323 14 The step buckling 6 can, as indicated in figure 1b, be repeated a number of times.

The freezer cells 10 comprise a piping system where the coolant circulates during the freezing cycle. Each freezer cell includes a bracket for a hooking element. A hook element is, for example, a hook. In one embodiment, a chain is attached to each corner of an individual freezer cell 10 and the four chains are connected, for example, to a ring or hook, which comprises means for attaching the hooking element. After freezing is completed, each individual freezer cell 10 is lifted up, for example with a crane or winch. The crane and the winch can advantageously be identical to the previously mentioned lifting device 14.

The whole process 1 according to the invention can be repeated a number of times at a dredging site. This is partly due to the fact that a freezing cycle has a limit in which depth of sediment 11 can be frozen. The limitation is mainly due to the design of the freezer cell 10. Eg. pipe dimensions in the freezer cell 10 and the distance between the pipes in the freezer cell 10, but also depends on the capacity of the freezer unit or units located above the water surface 19. The limitation also depends on e.g. on the lifting capacity of the lifting device 14.

In one embodiment, freezer cells 10 are immersed in at least two adjacent rows. In this way, sediment corresponding to at least two rows of freezing cells 10 can be frozen in a freezing cycle. It is convenient if each row is connected with flexible tubular elements of longer length than the tubular elements used to connect individual freezer cells 10 within one row. Connection of these tubular elements between the rows may be necessary to perform underwater.

It is an advantage if the top of the freezer cells 10 are covered by an insulating layer which reduces cooling losses. It is an advantage if the lower side of the freezer cells 10 comprises a thermally conductive material.

The frozen sediment is stable and therefore easy and safe to transport. An effective method is transport by barge.

If the occupied sediment is to be transported on land, they are advantageously loaded into containers which are then pulled up on swap bodies for further transport by road. In containers, the sediment typically stays frozen for several days.

The material leaves no odor and volatile compounds do not spread. Before treatment / landfill, the water content should be reduced as far as possible. Common dewatering methods are cycloning and treatment in filter presses. Freezing causes changes in the sediment, which leads to the separation of solid particles from water.

The water is then quickly drained away as the material thaws.

Freezing dredging of oil-contaminated sediment in Lake Vassijaure concentrated the solid from 14 to 40% after the material thawed and the water drained off. This process is called freeze-drying.

Freeze-drying means that no precipitants need to be used for dewatering.

Landfilling is a common method of post-treatment of contaminated sediment. Landfilling can take place in landfills on land but also in underwater landfills. Other examples of finishing techniques are thermal stripping, chemical leaching, biodegradation and soil washing. Freeze-dredged material can be transported directly to landfill, both above and below in 529 323 16 water. The material is consolidated as it thaws.

There is no evidence that freezing adversely affects the ability to use any of the commonly used chemical and physical finishing methods.

The invention should not be limited to the above examples of embodiments. The invention can be varied in many ways within the scope of the following claims.

Claims (1)

Water or the like, the sediment (11) comprises particles of varying size, parts of the sediment (11) being lifted up as frozen sections (81), characterized in that the system (17) comprises, - a plurality of freezing cells (10) arranged to be connected to each other by connecting elements (30), the freezing cells are intended to freeze sediment (11) below each other and between each other, whereby the sediment (60) which is intended to be frozen between the freezing cells constitutes an intended breaking zone 61), - a conveyor belt-like means (16) which is intended to transport the freezing cells (10) from a position above the surface (19) or in the vicinity of the surface (19) to the bottom (12), - a depressing device (13) arranged to depress the the individual freezing cells (10) in the sediment (12). A system according to claim 5, characterized in that the freezing cells (10) are arranged to be connected to each other in series. A system according to claim 6, characterized in that each of the connecting elements (30) comprises a first (40) and a second (41) flexible tubular element, which in addition to connecting the freezing cells (10), are intended to transport a liquid solution which during most of a freezing cycle has an in-temperature lower than ~ 5 degrees Celsius. A system according to claim 5 or 7, characterized in that one end (18) of the conveyor belt-like means (16) is hoistable down to the bottom (12) from a floating platform (15), such as a barge or a ship. 529 323 19 A system according to claim 8, characterized in that the depressing device (13) is attached to a lifting device (14) arranged on the floating platform (15). A system according to claim 5, 7 or 9, characterized in that the freezer cells (10) are designed as plate-like, and substantially square elements where the dimension of the length (50) and the width (51) of the surface which is intended to lie against the bottom (12) is significantly larger than the measure of the height (52) of the freezer cells (10). Use of a system according to claim 5,529,323 i