EP2422019B1

EP2422019B1 - Dredging vessel and method for loading a dredging vessel with dredged material

Info

Publication number: EP2422019B1
Application number: EP20100714310
Authority: EP
Inventors: Erik Van Nieuwenhuijzen; Mark Van Den Broeck
Original assignee: Dredging International NV
Current assignee: Dredging International NV
Priority date: 2009-04-22
Filing date: 2010-04-22
Publication date: 2013-04-03
Anticipated expiration: 2030-04-22
Also published as: EP2422019A1; AR076993A1; WO2010122093A1; TWI525236B; AU2010240899A1; TW201114987A; BE1018577A4; AU2010240899B2

Description

The invention relates to a method for loading a dredging vessel with dredged material. The invention likewise relates to a dredging vessel particularly equipped for performing the invented method.
During dredging with a dredging vessel, such as for instance a trailing suction hopper dredger, a mixture of dredged material and water is suctioned up in known manner via a drag head connected to a suction conduit and moving over the water bottom, and carried via an inlet into a hold (or hopper) of the dredging vessel until the hold is completely filled. The maximum filling height of the hold is determined by the height position of an overflow which is present in the hold of the dredging vessel and which forms an open connection with the water, for instance via the underside of the dredging vessel.
The heavier dredged material particles collected in the hold will settle after a period of time and thus form in the hold a settled layer of more or less closely packed coarser particles, this layer increasing in height as the dredging progresses. On top of the settled layer forms a non-settled layer of substantially finer particles free-floating in water. In the known method the overflow is placed in a highest position to enable maximum possible filling of the hold and to avoid non-settled particles re-entering the water via the overflow. This latter must be avoided because there is the chance of the turbidity of the water then increasing too much. This has an adverse effect on the flora and fauna at the location.
As soon as the level of the suctioned-up dredged material/water mixture reaches the upper side of the overflow, mixture runs via the overflow back to the water. During overflow the draught of the dredging vessel can however still increase since an increasing number of heavier particles settle while a mixture of finer particles and water disappears through the overflow. In the known method, as described in US 4 245 932 A and US 4 172 617 A for instance, the overflow remains for a while in the highest position until the trailing suction hopper dredger has reached its maximum allowable draught (also referred to as dredge mark). From this moment the dredge mark is kept constant by optionally lowering the overflow. Dredging is ended at the moment there is no further net settlement of material. The same amount of material then disappears through the overflow as is added to the hold. The dredged material/water mixture entering the water via the overflow can, depending for instance on the current, settle once again (fall back onto the water bottom), erode (fall back onto the water bottom and then be carried away by the current) and/or be transported (carried away by the current without settling). As stated above, it is necessary to ensure that the turbidity of the water is not allowed to increase too much.
US 4 365 509 A discloses a method for measuring the amount of mire and mud present in a dredged mixture collected in the hold of a dredger. The method comprises measuring the height of the settled sand laver of the mixture by a plump weight and measuring the position of the upper surface of the mixture with an acoustic level guide. By combining this (volume) measurement with a number of density measurements taken over the height of the mixture, the total amount of mire is obtained in the dredged mixture.
US 4 206 057 A discloses a dredging vessel, the overflow of which is equipped with a telescopically arranged level-adjustable outlet conduit provided under the vessel. When the head water is polluted, the telescopical part is extended to underneath the main current such that polluted sludge remains on the bottom and is not carried away with the current.
NL 9 201 853 A discloses a method for loading a dredging vessel wherein the dredged slurry is partly contaminated. Dredged slurry collected in the holds is allowed to phase separate in a heavy (clean) fraction and a light (contaminated) fraction. The light fraction is then off-loaded to one dumping site and the heavy fraction to another.
Although the known method delivers a maximum dredging efficiency, the quality of the dredged material which settles in the hold is open to improvement, in particular being not readily controllable.
The present invention has for its object to provide a method for loading a dredging vessel with dredged material which does not have this and other drawbacks, or does so to lesser extent. Another object is to provide a dredging vessel particularly equipped for performing the improved method.
This object is achieved according to the invention by providing a method according to claim 1. According to the invention, a method for loading a dredging vessel with dredged material in a hold provided for this purpose and having a height-adjustable overflow is provided, wherein a dredged material/water mixture is carried via an inlet into the hold and forms a settled layer therein due to settling of the heavier dredged material particles, and wherein the height position of the overflow above the settled layer is adjusted subject to a value of a control parameter measured during filling of the hold. It has been found that by defining a suitable control parameter the quality of the dredged material settled in the hold can be favourably affected. This can be understood as follows. In contrast to the known method, in the method according to the invention the overflow will not be placed in the highest position at the start of dredging, but rather in a lower position. Dredged material mixture, and in particular the finer fraction of the dredged material mixture (since the heavier fraction settles), will hereby overflow via the overflow at a relatively early stage after the start of dredging. It has been found that by adjusting the height position of the overflow the average particle size of the dredged material mixture which overflows via the overflow can be influenced, and therefore also the average particle size of the dredged mixture remaining behind in the hold. Settled material is in this way obtained with a better controlled particle size distribution, in particular a higher average particle size and/or narrower particle size distribution, than is the case in the known method. Such a material has an improved quality. The invented method has the additional advantage that the degree of contamination of the dredged material in the hold and/or of the overflowing dredged material can be influenced. The height position of the overflow is preferably set subject to the momentary value of the control parameter.
A preferred embodiment of the method is characterized in that the height position of the overflow above the settled layer is adjusted so that the measured value of the control parameter remains substantially constant. This embodiment allows a dredged material remaining behind in the hold to be obtained with a predetermined desired average particle size.
According to the invention, the control parameter for use in the method comprises the average horizontal velocity of the non-settled dredged material/water mixture above the settled layer between the inlet and the overflow. It has been found that this control parameter has a high sensitivity to the average particle size of the dredged material mixture overflowing via the overflow, and therefore also for the average particle size of the settled dredged material remaining behind in the hold. In the case of a fixed height position of the overflow above the settled layer it is found for instance that at a higher average horizontal velocity dredged material mixture of a higher average particle size is flushed away via the overflow. A typical average horizontal velocity of the non-settled dredged material/water mixture in the hold amounts to in the order of magnitude of 1 m/sec. The invention is however not limited to such velocities. The average horizontal velocity of the non-settled dredged material/water mixture in the hold is preferably adjusted within a range of 0.1 to 5 m/sec, more preferably within a range of 0.3 to 3 m/sec, and most preferably within a range of 0.5 to 1.5 m/sec.
The invention is based on the insight that the dredged material particles carried via the inlet into the hold will settle (sediment) partially and form a settled layer and, subject to the strength of the current generated above the settled layer, will partially erode (fall back onto the settled layer and then be carried away by the current) and/or will be transported (carried away by the current to the overflow without settling).
The horizontal velocity above the settled layer of the non-settled mixture flowing from the inlet to the overflow can be determined in simple manner by measuring the incoming flow rate of the suctioned-up mixture and dividing it by the width of the hopper and the height of the non-settled mixture. This horizontal velocity is preferably compared to the average fall velocity of the solid particles in the suctioned-up mixture. This fall velocity can easily be determined experimentally, for instance by a sedimentation measurement. It is in principle possible in this way to determine which particles will flow out via the overflow and which particles will settle at the set height (or horizontal velocity of the mixture).
According to the invention the average horizontal velocity V is preferably defined by the following formula: $v = \frac{Q}{B . h}$

wherein Q is the flow rate of the dredged material/water mixture measured at the position of the inlet, B is the width of the hold and h is the height position of the overflow above the settled layer. The flow rate Q at the position of the inlet can be measured in known manner by incorporating a flow meter, for instance in the inlet. The flow rate Q can be controlled by adjusting the dredge pump which pumps the dredged material/water mixture up from the water bottom.
A further preferred embodiment of the method according to the invention is characterized in that the height position h of the overflow above the settled layer is adjusted to a substantially constant value, and that the flow rate Q is adjusted so that the average horizontal velocity V acquires the desired value. The flow rate Q can be adjusted by providing a feedback of the measured flow rate Q to the control of the dredge pump which pumps the dredged material/water mixture up from the water bottom.
An alternative preferred embodiment of the method according to the invention is characterized in that the flow rate Q is adjusted to a substantially constant value (by the pump control), and that the height position h of the overflow above the settled layer is adjusted so that the average horizontal velocity V acquires the desired value. According to the invention the overflow is generally not placed in the highest position at the start of dredging in the method according to the invention, but rather in a lowest position of the overflow. The lowest position of the overflow corresponds to a low load factor of the hold, preferably 25%, more preferably 20%, still more preferably 15%, still more preferably 10% and most preferably 5%. It will be apparent that the lowest position of the overflow will preferably not be lower than the water level outside the dredging vessel, since otherwise water from outside could flow into the hold.
Because contaminants in dredged material are generally bound mainly to the finer particles, for instance to particles with a dimension < 63 µm, and these are preferably discharged according to the invention via the overflow, the settled layer of dredged material comprises fewer contaminants. An additional advantage of the invented method is that the preferably discharged smaller particles are generally carried away more easily with the current, so that they do not settle but, on the contrary, are carried away with the current by erosion and/or transport. The invented method provides the option of adjusting the particle size of the dredged material discharged via the overflow such that such an erosion and/or transport can take place taking into account the strength of the current at the location. In areas with strong current it will generally be possible to discharge an average larger particle size via the overflow without this resulting in an unacceptable increase in turbidity. In areas with a weak current the height of the overflow will preferably be set such that dredged material of an average smaller particle size is discharged via the overflow.
In a preferred embodiment of the method the hold is filled with water to the level of the overflow in the lowest position before dredging begins. Dredged material/water mixture will hereby overflow via the overflow almost immediately after dredging has started.
It is advantageous that in the method according to the invention the control parameter relates to the height of the settled layer and that the overflow is kept above the settled layer, and is more preferably kept at substantially constant height above the settled layer. In this variant the overflow as it were co-displaces with the height of the settled layer, whereby a mixture with well controlled particle size distribution remains behind in the hold. The constant height depends on the conditions at the location and can, if desired, be determined experimentally. It is also possible to arrive at a suitable value for the constant height on the basis of theoretical considerations. The height of the settled layer can be measured in a number of ways. The simplest methods are by measuring the draught of the dredging vessel and/or using a dipstick. It is however also possible to apply other methods.
Yet another preferred embodiment of the method has the feature that the control parameter relates to the particle size distribution of the overflowing mixture close to the overflow. The particle size distribution of a dredged material/water mixture can be determined in different ways. Suitable methods include concentration measurement by photography or film, particle distribution determination of samples taken by means of screening and/or sedimentation scales, x-ray extinction measurement, extinction measurement of light with one or more wavelengths and/or by means of sound waves.
The method according to the invention is preferably characterized in that the control parameter relates to the density of the overflowing mixture close to the overflow. The density can be measured in a manner known to the skilled person, for instance with an extinction measurement. In such a measurement a radioactive source is placed on one side of the mixture flow. A (-detector placed on the other side of the mixture flow measures the amount of radiation passing through the mixture. The (-radiation detected depends on the quantity of solid particles between the source and the detector counter. Devices for measuring density in this way are commercially available, for instance from the firm Berthold Technologies N.V. of Vilvoorde, Belgium.
In a particularly advantageous method the turbidity of the overflowing mixture close to the overflow is applied as control parameter. The turbidity can also be easily measured by means of a density measurement as briefly described above. Turbidity resulting from the removal of the silt has a great ecological effect. Owing to the turbidity less light penetrates to the bottom of the water. The flora and fauna present on the bottom hereby have less opportunity to develop. A very common requirement in dredging operations is that the turbidity during dredging may not rise above 1000 mg per litre. The method according to the present preferred variant provides the option of adjusting the overflow height such that the turbidity (increase) remains below a determined desired value.
It will be apparent that it is likewise possible to apply as control parameter a combination of the above stated control parameters. The position of the overflow can for instance thus be determined in first approximation by the height of the settled layer, wherein corrections to this first approximation are made subject to the value of another control parameter, such as for instance the measured mixture density at inlet and overflow.
For even better control of the particle size distribution of the overflowing mixture, and therefore also of the settled mixture, the overflow is preferably provided with a filter of predetermined mesh width, preferably for the purpose of sampling the particle size distribution of the outflowing mixture.
The invention likewise relates to a dredging vessel adapted to perform the method according to the invention. Particularly provided is a dredging vessel which is provided with a hold with height-adjustable overflow for loading with a dredged material/water mixture, wherein the dredging vessel is further provided with means for measuring a control parameter, as well as with a control device which adjusts the height position of the overflow subject to the measured value of the control parameter. According to the invention, said control parameter relates to the average horizontal velocity of the non-settled dredged material/water mixture above the settled layer between the inlet and the overflow. The advantages of the dredging vessel according to the invention have already been elucidated at length above with reference to the invented method, and will not therefore be repeated here.
In a preferred variant of the dredging vessel according to the invention the measuring means are chosen from the group comprising means for measuring the height of the settled layer, means for measuring the average horizontal velocity of the non-settled mixture between the inlet and the overflow, means for measuring the particle size distribution of the suctioned-up mixture close to the inlet and/or the particle size distribution of the overflowing mixture close to the overflow, means for measuring the density of the suctioned-up mixture close to the inlet and/or the density of the overflowing mixture close to the overflow, means for measuring the turbidity of the overflowing mixture close to the overflow and/or in the water, or combinations of the above stated measuring means.
A particularly advantageous preferred variant relates to a dredging vessel, the measuring means of which comprise a densimeter disposed at the position where the overflowing mixture runs into the overflow.
The method and dredging vessel according to the invention will now be further elucidated on the basis of the following figure, without the invention being limited thereto.
Figure 1 shows a schematic side view of a dredging vessel according to the invention; and
Figure 2 shows schematically the particle size distribution of a dredged material mixture obtainable with the invented method and dredging vessel, compared to the particle size distribution of a dredged material mixture obtainable with the known method.
The trailing suction hopper dredger according to the invention comprises a vessel 1 with a hold 2 and a suction conduit which is constructed from a loading pipe 3 debouching into hold 2, a pump 4, and a suction pipe 5 which is pivotable about an axis 30 relative to vessel 1 and which has on its bottom end a drag head 6 and which is dragged over the bottom 8 located under water 7 while vessel 1 sails in the direction of arrow 9 and bottom material 16, for instance sand, is suctioned up together with water 7 as a suspension from bottom 8 via conduit 5 and carried into hold 2. Drag head 6 is provided with a visor 12 which can be pivoted about a shaft 11 relative to drag head 6 by means of a hydraulic cylinder 13.
Hold 2 is suitable for receiving the mixture of water bottom material and water suctioned up through conduit 5 by means of pump 4. After a time the mixture present in hold 2 will settle and form herein a settled layer 20 with a height 40 relative to the bottom of hold 2. Present on settled layer 20 is a layer 21 comprising an aqueous suspension of non-settled bottom material. The height 41 of layer 21 is determined by the height position of a height-adjustable overflow 10 with conical upper outer end 14. Overflow 10 has an open lower outer end 15 along which mixture overflowing in the direction of arrow 31 can be carried in the direction of arrow 32 to the water 7 via the underside of the trailing suction hopper dredger.
The variant of a trailing suction hopper dredger according to the invention shown in figure 1 is further provided with means for measuring a control parameter, subject to which value the height position of overflow 10 is adjusted. The measuring means comprise a first densimeter 25 which is arranged in the vicinity of the inlet or loading pipe 3 and there measures the density of the suctioned-up mixture. A per se known flow meter 28 is also arranged at substantially the same position. This measures the flow rate of the suctioned-up water, from which the average horizontal velocity of the non-settled part 21 can be determined. The densimeter comprises a radioactive concentration meter obtainable from the firm Berthold Technologies N.V. of Vilvoorde, Belgium. A second densimeter 26 of the same type is arranged at the position of overflow 10 and measures the density of the overflowing mixture. If desired, a third densimeter 27 can be arranged in the water in the vicinity of the trailing suction hopper dredger in order to enable measurement of the turbidity of the overflowing mixture or of the surrounding water. The signals from the above mentioned measuring means are stored in a central computer (not shown), this computer forming part of the control equipment of the trailing suction hopper dredger.
In a preferred method the above described trailing suction hopper dredger is employed to dredge material present in or on water bottom 8. In the method a mixture of water bottom material and water is suctioned up via drag head 6 connected to suction conduit 5 and moving over water bottom 8, and carried into hold 2 via the inlet or suction pipe 3. In a preferred method overflow 10 is set in the lowest position at the start of the method. This preferably corresponds to a load factor of hold 2 of about 10% (of the overall available load in m³). In figure 1 the lowest position of overflow 10 is indicated schematically by the level 100 of the top side of overflow 10. At this position 100 a relatively large amount of suctioned-up mixture will be carried in the direction of arrow 31 through overflow 10 in the direction of arrow 32 to the water 7. This overflow is formed particularly by the finer particles of the suctioned-up mixture, since the coarser fraction has already moved below level 100 under the influence of gravitational force and remains behind in hold 2 as settled layer 20. During dredging the height 40 of settled layer 20 will gradually increase. In the shown variant the height position of overflow 10 is adjusted subject to the height 40 of settled layer 20, this such that overflow 10 is kept at substantially constant height above settled layer 20. This means that the height 41 will have a constant value during a great part of the dredging. The precise value depends on the conditions on location and can amount to for instance 0.75 m. If the suction flow rate measured by flow meter 28 is held constant, the average horizontal velocity of the non-settled part 21 will then also be substantially constant.
Because the height 40 of settled layer 20 increases over time, the height of overflow 10 will also rise until overflow 10 reaches the highest position 101. The highest position 101 is reached when the trailing suction hopper dredger has reached its maximum allowable draught or dredge mark. This situation is shown in figure 1.
The invented method is particularly suitable for dredging access channels for urban areas which run out into the sea or other large stretch of water, and/or for dredging sand, more preferably fine sand, and most preferably fine silt-containing sand. Sand has 65% by volume of particles larger than 63 µm and 50% by volume of particles smaller than 2 mm. Fine sand has between 5-15% by volume of particles with a grain size lying between 2 and 63 µm. Using the invented method more bottom material, and particularly the finer fraction thereof, runs back via the overflow to the water at a low load factor of the hopper than is the case in the known method. This so-called agitation process can be controlled well with the invented method, this being a great advantage in respect of the requirements set for the turbidity of the water. The overflowed finer fraction can be carried away by the current in the access channel from the location of the dredging operations to for instance the sea. In addition, the settled material in the hold has a higher quality, so that it can advantageously be used for sand supply and the like, optionally in the immediate vicinity of the dredging location.
Figure 2 shows schematically the particle size distribution (50) of a dredged material mixture obtained using the invented method and dredging vessel. The obtained particle size distribution is compared to the particle size distribution (51) of a dredged material mixture obtained using the known method. In the shown graphic representation a percentage by weight (52) is plotted against the particle size (53). By allowing a part (54) of the supplied particle size distribution (51) to overflow via overflow (10) during dredging a particle size distribution (50) is obtained which not only has a narrower distribution but also has a larger average particle size (55) than the supplied particle size distribution (56).
The invention is not limited to the above described exemplary embodiments, and modifications can be made hereto to the extent they fall within the scope of the appended claims.

Claims

Method for loading a dredging vessel (1) with dredged material in a hold (2) provided for this purpose and having a height-adjustable overflow (10), wherein a dredged material/water mixture is carried via an inlet (3) into the hold (2) and forms a settled layer (20) therein due to settling of the heavier dredged material particles, and wherein the height position of the overflow (10) above the settled layer (20) is adjusted subject to a value of a control parameter measured during filling of the hold (2), characterized in that the control parameter relates to the average horizontal velocity of the non-settled dredged material/water mixture above the settled layer (20) between the inlet (3) and the overflow (10).
Method as claimed in claim 1, characterized in that the height position of the overflow (10) above the settled layer (20) is adjusted so that the measured value of the control parameter remains substantially constant.
Method as claimed in claim 1 or 2, characterized in that the average horizontal velocity v is defined by the following formula: $v = \frac{Q}{B . h}$

wherein Q is the flow rate of the dredged material/water mixture measured at the position of the inlet (3), B is the width of the hold (2) and h is the height position of the overflow (10) above the settled layer (20).
Method as claimed in claim 3, characterized in that the height position h of the overflow (10) above the settled layer (20) is adjusted to a substantially constant value, and that the flow rate Q is adjusted so that the average horizontal velocity v acquires the desired value.
Method as claimed in claim 3, characterized in that the flow rate Q is adjusted to a substantially constant value and that the height position h of the overflow (10) above the settled layer (20) is adjusted so that the average horizontal velocity v acquires the desired value.
Method as claimed in any of the foregoing claims, characterized in that the control parameter further relates to the particle size distribution of the overflowing mixture close to the overflow (10).
Method as claimed in any of the foregoing claims, characterized in that the control parameter further relates to the density of the overflowing mixture close to the overflow (10).
Method as claimed in any of the foregoing claims, characterized in that the control parameter further relates to the turbidity of the overflowing mixture close to the overflow (10) or in the water (7).
Method as claimed in any of the foregoing claims, characterized in that the control parameter is a combination of the stated control parameters.
Dredging vessel provided with a hold (2) with height-adjustable overflow (10) for loading with a dredged material/water mixture, wherein the dredging vessel is further provided with means (25, 26, 27, 28) for measuring a control parameter and with a control device which adjusts the height position of the overflow (10) subject to the measured value of the control parameter, characterized in that the control parameter relates to the average horizontal velocity of the non-settled dredged material/water mixture above the settled layer (20) between the inlet (3) and the overflow (10).
Dredging vessel as claimed in claim 10, characterized in that the measuring means are chosen from the group comprising means for measuring the height of the settled layer, means for measuring the average horizontal velocity of the non-settled mixture between the inlet and the overflow, means for measuring the particle size distribution of the suctioned-up mixture close to the inlet and/or the particle size distribution of the overflowing mixture close to the overflow, means for measuring the density of the suctioned-up mixture close to the inlet and/or the density of the overflowing mixture close to the overflow, means for measuring the turbidity of the overflowing mixture close to the overflow and/or in the water, or combinations of the above stated measuring means.
Dredging vessel as claimed in claim 11, characterized in that the measuring means comprise a densimeter disposed at the position where the overflowing mixture runs into the overflow (10).