The invention concerns methods for mining sand or gravel from a sand or gravel deposit at the bottom of a body of water. The invention further concerns methods for removing sand or gravel from such a deposit to a different position of the bottom of a body of water, without mining the sand or gravel.

The bottoms underneath both naturally occurring and artificially created bodies of water, including rivers, lakes, sees and oceans as well as harbours, marinas etc., can consists of a variety of materials including bedrock, sand, silt and mud. These materials differ widely in their properties; rock, sand and gravel are usually relatively heavy, high specific weight materials including minerals such as quartz with specific weights up to around 5 p/cm3 or more. By contrast, mud largely consists of decaying organic matter, with a specific weight close to 1.

Particle sizes of materials forming water body bottoms also differ widely. E.g. sand can go from very fine material, with typical grain diameters below 0.2 mm, to coarse sand with average grain size between 1 mm and 2 mm. Generally, gravel has even bigger average particle diameters. Typical sand and gravel particles may be of a more or less rounded, often even almost spherical shape. On the other hand, typical mud particles will be flat or of irregular shape.

These differences have major effects on the mobility of such materials when exposed to e.g. current or gravity. High density, low surface area, round grains of sand are much less capable of floating than low density, large surface area, irregular mud and silt particles. Thus, sand grains and, even more pronouncedly gravel stones or pebbles, will not be carried by a water current to the extent that mud particles are.

Another major difference between these water body bottom materials is their usefulness. Generally, sludge, silt and mud are, at best, bottom materials without any economic usefulness. At worst, they are deposited in areas where their presence is undesired. The silting-up of harbours, "basins, lakes and rivers and even see shore areas is a well known problem in shipping, where a specified depth of the water body is required to accommodate correspondingly sized vessels. It is a well know problem that rivers, harbours, marinas etc. have to be repeatedly dredged, to remove the continuously re-building mud and sludge deposits at their bottom.

While this can be a problem with sand and gravel too, these materials are not just problematic or at best, irrelevant; sand and gravel form desirable raw materials as landfill, for the building industry and so on, and are, for this purpose, mined on an industrial level, worldwide. Submarine sand and gravel deposits are a valuable source of such building materials, and are exploited for this purpose.

Various techniques have been disclosed for producing such materials. Thus, sand and gravel are mined from the sea floor. Established mining methods generally involve the use of mining machinery which can move across the sand or gravel deposit to be mined, in a way comparable to use of machinery in opencast mining on dry land.

Dredging machinery is used for removal of sand, gravel, sludge or mud from places where corresponding deposits hamper shipping operations. Known methods involve the use of dredging machinery such as bucket dredgers, bucket cranes etc. The removed material is often either placed on ships for dumping elsewhere, or is conveyed to some onshore location for deposition.

From US 5,428,908 and US 6,499,239 it is known to mine sand by means of a combined jetting and siphoning device. The device is sunk into a sand layer by forming a cavity through the use of water jets, which cut and disperse the sand. Sand with the desired grain size is siphoned off and primped ashore. The dispersing effect of the jet nozzles is only used for strongly agita_ting the sand in the vicinity of the suction nozzles, so that the desired grain size fraction can be selectively mined. While subsidence may contribute to sand moving to the mining site, no fluidized layer is created for the purpose of conveying sand, by gravitational flow, to the mining device. The device disclosed in this art could not be used as such to create a fluidized layer of sand capable of flowing, under gravity, to a distant location, because the sand layer is completely disintegrated and dispersed, and all mobilized sand is directly siphoned off, or settles back into an immobile state, at the bottom of the hole made by the device.

Where substantially clean sand and gravel are mined, it is generally not a problem that in the course of this mining, some small amount of the mined material is released into the body of water. Sand and gravel settle again very quickly, because of their high specific weight and the generally rounded shape of the individual grains. Any turbidity caused by the mining operation is generally not lasting. Further, sand and gravel do not enter any chemical reactions under such conditions, and thus do not consume oxygen from the body of water before thie sand or gravel settles again.

However, in the removal of silt, sludge and mud, the contamination of the body of water by particles of the removed material, caused by the dredging operation, is a massive problem. On the one hand, the particles are generally very small and also of a specific weight close to that of the surrounding water, so that they settle only very slowly. Further, these materials comprise major amotxnts of organic matter, which is oxidized when dispersed, and the oxygen consumption caused by this process reduces the amount of oxygen available to living organisms, especially fish. While it has been suggested inthe prior art (e.g. DE-A-I 634 O17, GB-A-595 291 and US-A-3,414,862) to deliberately disintegrate mud or sludge bottom layers by the injection of water and/or air, in the hope that a sufficiently strong water flow or a current will then carry the sludge or mud particles away, these methods have not been consistently successful. One reason is of course that their use is limited to such bodies of water, where there is a sufficient flow or current to carry the redispersed particles off to a place far enough away where they may resettle. By definition, such methods are therefore not usable for areas where such flow is insufficient or even absent, e.g. harbours, marinas, lakes etc..

But even where there is sufficient flow or a current, the above -discussed problem of oxygen consumption by the redispersed organic matter remains a problem, and generally prevents the use of these methods, even where the general conditions of flow or current would be favourable.

A suitable method for removing sludge or mud (but not sand or gravel) without redispersal is described in EP-B-O 119 653. Basically, a sludge or mud layer is liquefied by the injection of water. In this, water jet nozzles are inserted directly in the mud layer and the amount of water injected as well as the rate of injection, are selected so that disintegration of the mud layer is prevented. Instead, only the volume of the layer is increased, by the injected water, which makes it possible that the liquefied layer flows under the influence of its (slightly) higher specific weight, along the water body bottom on which it rests, but substantially without mixing with the supernatant water.

Tests have shown that such liquefied mud layers may flow doΛvn suitably sloped riverbeds for more than 30 km without disintegrating, until the liquefied mud layer loses so much of the injection water that it becomes immobile again. In the final stage, the layer basically reverts to its pre-injection state, or reaches an area of the water body where there is sufficient water flow or current, to disintegrate the layer and carry the particles away. This known method is preferably practised where th_e territory, due to its natural inclination, already provides a flow path for the liquefied layer. This is e.g. true in many rivers, or where a harbour basin gets continually deeper towards the harbour mouth. In such cases, the liquefied mud layer can flow away from the original site, to its desired new position, as caused by its specific weight, under the influence of gravity.

Where such a natural flow path is not present, EP 0 119 653 suggest to create a channel, e.g. by dredging, along where the liquefied layer may then flow towards its final destination.

For the relatively rare cases where there is no flow or current above the mud to be removed, and the liquefied layer cannot reach another part of the water body where flow or current is sufficient to disintegrate the layer, EP 0 119 653 suggest to pπrvide wells or recesses in the bottom, wherein a suction pressure pump is placed. The purpose is to make the mud flow towards this pump, which then conveys it towards a storage yard, to some other transportation means, or just to another location where the flow is sufficient.

In the prior art, the water injection method of EP 0 119 653 has been widely practised for the removal of sludge and mud. It appears, however, that it has not been used for moving sand or gravel. The most important reason is probably that the method appears, at first sight, to be unsuitable for such purposes. Sand and gravel have a much higher density than sludge and mud particles, and generally consist of much bigger particles too. The much bigger size, the shape and the much higher specific weight of sand and gravel grains makes it appear almost impossible that these materials could retain injected water to form a liquefied layer, capable of persisting long enough for any useful application.

Further, sand and gravel deposits usually don't create the kind of oxygen depletion and general contamination problems caused by the disintegration of sludge or mucl layers, so that more conventional handling methods, especially the use of customaαy dredging apparatus, is not so problematic.

EP 0 243 994 describes a similar method, wherein additionally, horizontal jet streams can be used to flatten ridges or bumps. Sand and sludge are mentioned in this context. Horizontal jets can, however, only be used to fully disintegrate a deposit, since they can not provide the downward injection effect required to create a fluidized layer capable of flowing under the influence of gravity, substantially without mixing with the overlying body of water.

Generally, the injection method of EP 0 119 653 and EP 0 243 994 involves the use of an injection device (including a horizontally extending tube) which is operated with its injection nozzles inserted into the mud layer. In practise, the whole injection tube carrying the nozzles is at least partly inserted into the mud layer, as e.g. correctly shown in the figures of said references.

Alternatively, as disclosed in EP 0 278 335, it has been suggested to operate a similar injection device with the injection nozzles held at least 10 cm above the surface off the mud layer. EP 0 278 335 discloses that this distance of at least 10 cm is necessary to prevent the injection device from becoming embedded in the mud layer to such an extent as to render the injection ship or vehicle immobile.

Both in the mining and in the removal of sand and gravel, the use of customary machinery, including mechanical excavators, suction pressure dredgers, bucket dredgers, etc., is accompanied by disadvantages, especially in terms of production capacity and correspondingly high operational costs. Thus, in sand and gravel mining, it would be much more cost efficient if the sand or gravel could be moved towards (more or less) stationary mining device, instead of having to move the mining machinery across sand or gravel beds.

Also in the removal of sand or gravel to another place at the bottom at the water body, it would be much more cost efficient if this could be done without having to dredge the material in the conventional fashion, which generally includes transportation by barge and discharge elsewhere, since this implies shipping materials with a high water content, which is inefficient and expensive.

While the apparatus and method of EP 0 119 653 may in principle be used also for moving sand and gravel, it is generally necessary to modify the method for this purpose. Sand and gravel layers can not be rendered mobile (as a flowable or "liquefied" separate layer, capable of gravity flow as e.g. described in EP '653) by exposure to water jets from a distance, as suggested in EP 0 278 335. Also, it is at least very difficult and not generally successful, to use an injection device partly immersed into a sand of gravel layer, as exemplified in EP 0 199 653 and EP 0 278 335, because the injection ship or other vehicle would be immobilized by the layer. It is therefore a primary objective of the instant invention, to provide a more efficient method of mining sand or gravel from the bottom of a body of water.

It is another major objective of the instant invention to provide a more efficient method for removing sand or gravel to a different position at the bottom of a body of water.

Further objectives and advantages of the invention will become apparent from the ensuing description of preferred embodiments.

In the context of this description, "sand" means a more or less loose accumulation, agglomerate or aggregate of separate mineral grains ranging from approximately 0.05 to 2 mm in largest diameter. Most commonly, these grains will consist of quartz, but the usage of "sand" in this invention is preferably not limited to quartz.

In a preferred embodiment of the invention, "sand" comprises fine sand from approximately 0.05 up to 0.2 mm largest grain diameter.

In other preferred embodiments, "sand" means medium size grains, with largest grain diameters up to 0.6 mm. In yet another preferred embodiment, "sand" means coarse sand with largest grain diameters up to 2 mm.

"Diameter" is largest diameter, unless otherwise specified.

The sand of this invention is commonly sand as it naturally occurs at the bottom of river beds, sea beds, lakes etc.. This is often sand that has been shifted by the action of water over extended time periods, so that the individual grains are generally rounded. However, the invention is not limited to this, and in other preferred embodiments also applies to the handling (be it for mining, be it just for removal purposes) of sand that was originally part of solid ground and only became exposed by the removal of overlying materials, e.g. by mining. Thus, the invention also applies to sand with sharp edges and irregular grain shapes.

"Gravel" in the context of this invention is generally also an accumulation, agglomeration or aggregation of individual mineral particles which, however, are generally bigger than the grains of sand. Generally, these particles will be above 2 mm in largest diameter. As with sand, gravel stones or "pebbles" may have been rounded by the action of moving water, but this is not an obligatory requirement of this invention.

Gravel will often comprise quartz particles, but preferably is not limited to quartz.

"Mining" in the context of this invention means the process of extraction and recovery of the material concerned, which in this case is sand or gravel, with the purpose of making it available for applications such as use in the building industry, in road construction, in the fabrication of artificial stones etc.. In the context of the invention, "mining" is used in the context of extraction and recovery of sand or gravel from corresponding deposits which occur at the bottom of a body of water.

A "body of water" in the context of the invention can be any naturally occurring or artificially created body of water, with a size large enough to permit mining and or removal operations of the type discussed herein. Specifically, such bodies of water include rivers, lakes, sea beds, ocean beds and basins, (especially in costal areas), but also harbours, marinas, and artificially created water basins. "Mining machinery" in the context in this invention includes conventional devices capable of conveying sand or gravel, which can be operated for under water excavation of these materials. Thus, mining machinery includes bucket conveyers, bucket cranes, suction pressure pumps and other such known means for excavating and conveying sand and gravel.

"Water injection means" in the context of this invention includes all types of apparatus which can be used to create a liquefied layer, including the liquefied layers disclosed in EP 0119 653, substantially without disintegrating and dispersing the solid material into the supernatant body of water, such that the thus-formed liquefied layer can flow under the influence of gravity, to a different position at the bottom of the body of water, again substantially as described in EP 0 119 653.

In preferred embodiments, water injection means can comprise the kind of water injection device described in EP 0 119 653, especially where the tube and injection nozzles arrangement is concerned, that is used to transport the injection water from the injection pump to the bottom layer into which the water is injected.

The water injection means generally comprises a mobile carrier such as a ship or a float, on which the complete machinery for water injection can be mounted.

Basically, the water injection means can be the same, both for mining and for removal operations. The major difference between these two operations is that for mining, the sand or gravel is caused to flow, as a liquefied layer, to the mining position where the mining machinery is located, whereas in removal operations, the sand or gravel is caused to flow to some position where it can settle and remain, on the bottom of the body of water, without being in the way of shipping etc. or can be dispersed and removed by a current. In both mining and removal operations, the place to which the liquefied sand or gravel layer flows can be determined by the natural inclination of the bottom over which the layer flows. Alternatively, a suitable flow path can be artificially generated by correspondingly preparing the bottom of the body of water, for example by initial excavation of a desired flow path. Since sand and gravel can usually be excavated without creating substantial contamination problems, this is often feasible and can, in a mining operation, in fact be the first step of recovery of the desired sand or gravel.

In such preparatory operations, the methods disclosed in more detail in EP 0 119 653 may be used with advantage when modified in accordance with this invention. Specifically, this concerns the steps or stages in creating one or more paths or strips, be it by excavation, be it by injection, to generate the desired flow.

The apparatus disclosed in EP 0 119 653 may be used for these purposes.

It is an important aspect of the invention that in both mining and removal operations, the injection device is held in contact with the surface of the sand or gravel layer, and is neither (even partly) inserted into the layer, nor kept at a distance above said surface.

While this may be accomplished using sensor technology (of the kind exemplified in EP 0 278 335), it is generally sufficient to control the position of the injection advice in terms of its relative weight and the (resulting) forward speed of the ship.

The (relative) weight of the injection device is most simply expressed in terms of the load on the rope (or other apparatus) used to raise and lower the device, generally by a winch on the ship. Most ships have an indicator device showing the load on the rope, to prevent rope breaking under excessive loads and to indicate when the injection device has been fully lowered onto the bottom. Such an indicator can e.g. be used to monitor operation in accordance with invention.

When the injection device (e.g. a pressure tube provided with downwardly directed injection nozzles) is lowered onto the bottom (i.e. the surface of the layer to be treated by injection), the load on the rope fastened to the device will correspond to the full weight of the device as long as the device remains suspended and is not in contact with said surface. (This is the situation deliberately maintained in the method of EP 0 278 335). This load will remain practically unaffected by the relative speed (over ground) of the ship. The speed of the ship will also be unaffected by the load, before it contacts the bottom.

When the injection device has been lowered onto the layer surface, the layer will support the weight of the device, and the load or the rope will be consequently reduced. For operation in accordance, this is the load to be maintained. It can best be determined while the ship makes no relevant speed. If the ship moves, contact of the device with the bottom will be felt by corresponding (light) braking action.

In operation on mud, as described in e.g. EP 0 119 653, the injection device now begins to dig into the mud layer, and the ship must begin to move, pulling the injection device with it. Even a mud layer offers some resistance to such pulling, and the (relative) weight of the injection device is correspondingly increased.

In operation on sand or gravel, this increase in (relative) weight is much more pronounced. It can in case be read off the load indicator device of the suspension rope; otherwise, it can be derived from the decrease in ship speed caused by the increased pulling resistance of the injection device. In operation according to the invention, the desired position of the injection device, which basically rests on the surface of the sand or gravel layer substantially without digging into said layer, is observable as a minimum in the load on the suspension rope (or any other device used to lift, lower and/or carry the injection device with respect to the ship, or other vehicle) of the injection device at constant forward speed of the ship. This minimum load can thus be used to determine and control the desired position of the device.

The desired position of the injection device on the surface of the bottom layer is observable (more indirectly) by its effect onthe speed of the ship. This, however, does not take the form of a minimum effect, since the ship is faster while the injection device is suspended above the bottom and much slower if the injection device digs into the layer. However, undesired entry of the injection device into the bottom layer is shown by a corresponding drop in forward ship speed, compared to the appropriate operating condition, with the device only in contact with the layer surface.

In preferred embodiments of the invention, on may determine the (relative) weight of the injection device (including e.g. the "injection tubes 7,9" of EP 0 119 653) at contact with the bottom layer, and then control the forward speed of the ship such that this weight is maintained.

In a preferred embodiment relating to the mining of sand from a seafloor deposit, the sand to be mined is found in an area close to the sea shore, at a depth of some meters below the sea surface, and in a generally flat (although preferably inclined) area of the sea floor. Initially, the necessary mining machinery is brought into position. In the embodiment example, the mining machinery comprises a suction pump, the inlet of which is positioned adjacent to the sand deposit, and the outlet of which is connected to a suitable pipe, which extends from the position of the mining pump to the shore and further to an intermediate storage area on the shore, from which the sand can be carried off by means of trucks etc.

Initially, a well is created in the sand deposit, and the pump is placed with its inlet near the bottom of said well, so that the pump sucks in any sand flowing into the well, and conveys it to said storage position on the shore.

A water injection means is provided in the form of a ship, with injection apparatus generally shown in EP 0 119 653. Specifically, the ship has a transverse tube with a row of water j et injection nozzles, which is connected to an injection pump, and which can be lowered down onto the sand deposit. A pump is provided for providing the water injection nozzles with water under sufficient pressure for injection, which the pump takes from the body of water, through an inlet carried by the vessel at a suitable distance from the locus of water injection.

Lifting means are provided for lowering the transfer tubes with the injection nozzles down on to the sand deposit, or lifting it up from said position, e.g. for travel. In operation, the transverse tube with the injection nozzles is lowered, in a generally horizontal position, onto the sand deposit, so that the exit apertures of the injection nozzles are in direct contact with the sand surface. Water is now injected into the sand, in an amount and at a rate suitably controlled by the setting of the pump, so that the sand is transformed into a liquefied layer. In the embodiment example, this water injection starts in the immediate vicinity of the well, where the mining means are placed. This causes that part of the sand closest to the well, to flow into that well, where it is sucked in by the mining pump, and conveyed to the onshore storage position. Subsequently, the ship carrying the water injection means moves away from the well, causing the water injections means to inject water into a part of the sea floor bottom sand which lies adjacent to the area which has already been subjected to water injection, and which now lies at a lower level, because sand has already been removed from it into the well. Therefore, newly injected sand will flow to that lower level and from there into the well, and this will continue while the ships keeps moving away from the well. This is of course provided that the bottom around the well is sufficiently level and horizontal. Depending on the actual shape of the sea floor bottom near the well, the ship may continue moving away from the well, until the difference in level is not sufficient anymore for the liquefied layer to flow to the well under gravity. When this happens, the ship will go back to near the well and start its operation again, either deepening the first flow path it has already cut into the sand deposit, or starting at a new position spaced away from the original starting position, to cut a new flow path.

It will be appreciated that depending on the actual conditions at the mining site, the water injection vessel may perform its injection operation all around the well, or only partly around the well.

In any case, this operation will make it possible to provide the mining machinery with a more or less continuous supply of sand from the vicinity of the well, without having to move the mining machinery. The advantages of this will be the more pronounced, the more difficult to move the mining machinery is. It will be understood that if the sand deposit is sufficiently thick, this operation can be prolonged by suitably deepening the mining well, once the sand deposit around the well has been depleted so much that the original well does not lie at a low enough level anymore, which makes it possible to repeat the water injection procedure and recover more sand.

Surprisingly, it has been found that sand, even coarse sand, is capable of forming a liquefied layer of the type discussed above, capable of flowing over many tens, up to several hundred meters of flow path. This means that areas up to many hundreds of square meters can be mined without changing the position of the mining well.

With gravel, the procedure is basically the same, only the possible length of the flow path is shorter.

In another embodiment example, a sand deposit is to be removed from its position on a river bed, to another position where it does not impede shipping. In this embodiment example, said other position lies naturally at a lower level than the initial position of the sand deposit.

It is therefore possible to use the same type of water injection means as in the above discussed first embodiment example, for initially converting the top layer of the sand deposit into a more liquefied, flowable layer, which can then flow under gravity, to said final position where it can remain.

Again, water injection will start at that end of the sand deposit adjacent to the (natural) flow path of the liquefied layer. Thus, the sand forming this end area of the deposit is initially injected, and the liquefied layer created by this injection then starts to flow down the flow path, to its final destination. From this position, the vessel carrying the water injections means will then move in such a direction, that other parts of the sand deposit, adjacent to the position where the initial injection occurred, will be subjected to water injection, much as above described in the first embodiment example. This procedure will continue until either the vessel has reached the position where further injection would not cause the sand to flow in the desired direction, or if the extreme end of the sand deposit, most remote from the flow path, is reached.

The vessel will, in this embodiment, go back to its original starting position for a repetition of the injection procedure, until enough of the sand has been removed. In this, the vessel may initially deepen the first cut made into the sand deposit further, before moving sideways to remove other parts of the sand deposit, or it may follow a different injection pattern, depending on the shape and place of the sand deposit to be removed. Again, basically the same approach applies to the removal of gravel, only with the possible length of the flow path, of the liquefied gravel layer, being somewhat shorter than the possible flow path of a comparable liquefied sand layer.

In another embodiment, there is no final position for the sand to be removed, to which the sand could flow along a natural inclination. In this embodiment, a flow path has to be initially created, and this is done using conventional machinery such as a dredger. By this procedure, a trench is initially created; if a major amount of sand or gravel is to be removed, a series of such trenches may be created more or less in parallel, or in some other suitable position to each other.

The water injection means is then used to wash the sand or gravel to be removed from its original position into the adjacent trench or trenches, and for this purpose, the sand is transformed into the liquefied layer, as above described in the other embodiments, so that it then can flow under gravity down to the preformed trench. It will be understood that for this purpose, the preformed trenches need to be of a volume and need to be placed in such a position, that they can in fact accommodate the sand or gravel to be removed.

In all described embodiments, care is taken that the water injection means are operated such that the injection device including the nozzle is in contact with the surface of the sand or gravel layer, but substantially neither digs into, or gets embedded in, said layer (as in EP 0 243 994), nor is held at a distance above said layer (as in EP 0 2778 335).

Thus, if in forward motion of the ship the (relative) weight of the injection tube becomes too high, without the speed falling off, the injection device has lifted off the layer, and must be lowered down until the (relative) weight shows contact with the bottom layer.

If however the (relative) weight of the injection tube becomes too high and the speed of the ship is also diminished , the injection device has become inserted into the bottom layer, and must be raised until the ship speed is again in the right relationship with the (relative) weight of the device.

While the invention has been described with reference to several embodiment examples, it will be understood that it is not limited to these embodiment examples, which are just preferred ways of operating the invention.