BACKGROUND

Trailing suction hopper dredgers ("TSHD") are vessels which can be used to dredge at sea or in open water. TSHD's typically use a suction tube, one end of which can be lowered to the seabed and used to suck up solids such as sand, sludge, silt or sediment, mixed with water. The lower end of this suction tube can be provided with a suction head or a drag head. The solid material mixed with water is pumped through the suction tube into a hopper of the dredging vessel using a dredge pump. The mixture can then be reduced in speed when in the hopper, and this speed reduction promotes the settling of components suspended in the mixture. Excess water can be overflowed out of the hopper to allow for more load capacity within the TSHD.

The overflow typically contains light fractions of fine sand, clay and silt, as these normally do not settle quickly (or at all in the case of silt). The suspension of particles still in the overflow as well as the interactions between the overflow volume released, the hull, propellers, speed of the vessel and currents; can form a plume in the wake of the dredging process. These can cause negative environmental effects, and therefore when a clean overflow is not able to be obtained the dredger is not always able to overflow excess liquid.

One method to combat a plume generated by overflow is disclosed in WO

2013/119107. A passive overflow device is used to drain away excess water and flow it through a conduit to an outlet abutting the sea bottom to deliver the excess water close to the sea bottom, thereby minimizing the influence on sea life. U.S. Pat. No. 3,975,842 discloses a system which also attempts to minimize the environmental effects by directing overflow to the suction head to be used as the liquid supply for loosening the soil to be suctioned, thus forming a closed system where the overflow is recycled. EP 0642817 introduces a filter formed of plates for helping separate suspended particles within the hopper. The unit is located in the hopper, is separate from the overflow, and works to filter only some of the liquid flowing in the direction of the overflow by screening off some of the liquid so that settlement can occur. Each of these systems typically require large and/or complex installations and can require a large amount of energy. Additionally, they may not be effective with particularly small particles suspended in the dredged mixture, such as silt. SUMMARY

According to an aspect of the invention, a hopper dredger comprises a hopper for receiving a mixture of dredged material; a suction tube extendable from the hopper dredger for suctioning dredge mixture; a dredge pump connected to the suction tube for moving the dredged mixture through the suction tube into the hopper; and a flocculant injector system for injecting flocculants into the dredged mixture. Such a hopper dredger can promote the settling of dredged material with very small particles by using the flocculant injection system to promote the formation of floes which are able to settle in the hopper. This can increase the loading capacity of the hopper and therefore the overall hopper dredger.

According to an embodiment, the flocculant injector system is connected to the suction tube upstream from the dredge pump. Such an embodiment can use the dredge pump to promote mixing of the flocculant with the dredged material, thereby encouraging better mixing for the formation of floes.

According to an embodiment, the flocculant injector system is connected to the suction tube between the dredge pump and the hopper. Such a system can use the movement at that location and/or the motion of flow into the hopper to promote mixing of the flocculant with the dredged material.

According to an embodiment, the flocculant injector system is directly connected to the hopper. Such a system can use the movement and turbulence of flow inside the hopper to promote mixing of the flocculant with dredged material.

According to an embodiment, the hopper dredger further comprises an overflow system in the hopper to allow overflow of liquid from the dredged mixture. By forming floes that are able to settle in the hopper, an overflow system is able to overflow clean liquid. This can increase loading capacity of the hopper while reducing or eliminating negative environmental impacts from overflows with suspended particles.

According to an embodiment, the hopper dredger further comprises a plurality of settling improvement measures in the hopper to promote slowing of the mixture in the hopper. Optionally, these settling improvement measures comprise a plurality of decantation panels.

According to an embodiment, the flocculant injector system further comprises a measurement system for performing one or more measurements on the dredged mixture. Such a measurement system can be used to measure properties of the dredged mixture before and/or after injecting flocculant and/or properties of any overflow. The measured properties can be used to determine the flocculants and/or levels of flocculants to use, validate that the flocculant injection is working as intended, and/or adjust flocculants and/or levels of flocculants injected.

According to an embodiment, the measurement system measures one or more of density, pH, turbidity, flow, redox and temperature. Optionally, the measurement system performs one or more measurements on the dredged mixture before the flocculant injector system injects flocculants into the dredged mixture. Measuring the dredged mixture before injecting flocculants can be used to determine which flocculants and/or the amount of flocculants to inject.

According to an embodiment, the measurement system performs one or more measurements on the dredged mixture after the flocculant injector system injects flocculants into the dredged mixture. Performing measurements after injecting flocculants can be used to adjust the flocculants and/or levels being injected based on measurements and settling characteristics of the mixture.

According to an embodiment, the measurement system performs one or more measurements on overflow liquid from the hopper. Measurements on overflow liquid can be used to validate that overflow is clean. A high level of suspended particles in overflow liquid has the potential to cause negative environmental effects, so performing measurements on the overflow liquid allows for the monitoring of overflow to adjust overflow allowed based on the settling and content of current overflow.

According to an embodiment, the measurement system is used to determine which flocculants and/or the level of flocculants that will be injected into the dredged mixture.

According to an embodiment, the flocculant injector system uses the one or more measurements to adjust the flocculants and/or level of flocculants injected. Adjusting during a dredging operation can allow for settling even when the dredged mixture experiences a change in composition over the course of a dredging operation.

According to a second aspect of the invention, a method of overflowing liquid from a hopper of a hopper dredger includes injecting flocculant with a flocculant injection system into a mixture of dredged material; mixing the dredged material with the injected flocculants to allow the formation of floes; flowing the mixture into the hopper; and overflowing liquid from the hopper. Injecting flocculants into a mixture of dredged material and then flowing the mixture into the hopper can allow for formation of floes in the mixture from small particles. These floes allow for settling in the hopper of particles that may have previously been too small to settle or may have settled very slowly, allowing for a cleaner overflow and therefore higher capacity of the hopper.

According to an embodiment, the step of mixing the dredged material with the injected flocculants comprises mixing the dredged material with the injected flocculants with a dredge pump. Using the dredge pump to mix the flocculants can increase mixture of flocculants and dredged material without needing additional mixing devices or components.

According to an embodiment, the method further comprises measuring one or more of the density, pH, turbidity, flow, redox and temperature of the dredged material.

According to an embodiment, the method further comprises using the one or more measurements to determine the type and/or amounts of flocculants to inject into the dredged material.

According to an embodiment, the method further comprises measuring one or more of the density, pH, turbidity, flow, redox and temperature of the dredged material and flocculant mixture.

According to an embodiment, the method further comprises measuring one or more properties of the overflow liquid.

According to an embodiment, the one or more measurements are used to adjust the types and/or amounts of flocculants injected.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a schematic cross-sectional view of a trailing suction hopper dredger during a dredging operation with a flocculant injection system.

FIG. 2 illustrates a schematic cross-sectional view of an alternative hopper dredger with a flocculant injection system.

DETAILED DESCRIPTION FIG. 1 illustrates a schematic cross-sectional view of trailing suction hopper dredger 10 during a dredging operation on seabed 11. Hopper dredger 10 includes flocculant injection system 12, suction tube 14, pump 16 and hopper 18. Hopper 18 includes decantation panels 20, overflow 22, and settled particles 24. Pump 16 can be, for example, a centrifugal pump.

Suction tube 14 connects to hopper dredger 10 and is extendable from hopper dredger 10 for a dredging operation. Suction tube 14 fluidly connects to hopper 18. Pump 16 connects to suction tube 14, in this case at a location on hopper dredger 10, though in some embodiments, pump 16 is connected to suction tube 14 at a location that is outside hopper dredger 10 and in the sea when suction tube 14 is in place for a dredging operation. Flocculant injection system 12 connects to suction tube 14 upstream from pump 16, though in other embodiments flocculant injection system 12 connects to suction tube 14 at another location or connects directly to hopper 18.

During a dredging operation, hopper dredger 10 works to dredge a mixture of particles and liquid from seabed 11 through suction tube 14. Pump 16 works to pump the dredged mixture through suction tube 14 to hopper 18. Once in hopper 18, decantation panels 20 help to slow the flow of mixture in hopper 18, which promotes settling of particles in the dredged mixture. Extra liquid can be runoff through overflow 22, allowing space for more dredged mixture and settling.

Flocculant injection system 12 injects flocculants into dredged mixture in suction pipe 14 upstream from pump 16. The rotation of pump 16 creates turbulence to enhance the mixing process between the flocculants and the dredged mixture. The injected flocculants cause flocculation to occur in mixture, making the small particles bind together to form larger particles called "floes." These floes are then able to settle in hopper 18, allowing for liquid to be runoff through overflow 22. Flocculants can be injected as a solute and/or powder. Flocculants used can vary, and can be determined by the composition or expected composition of the dredged mixture, and can be, for example, anionic polyacrylamide or other flocculants.

Flocculant injection system 12 can also include a measurement system. The measurement system can be used to perform one or more measurements on the dredged material to determine which types and/or the amount of flocculants to add to the mixture. The measurement system can also be used to perform validation tests, testing the mixture after flocculant has been added. This could be done in suction tube 14, in hopper 18, at overflow 22 or at another location. Measurements can include measurements of density, pH, turbidity, flow, redox, temperature and/or other measurements to determine mixture and/or overflow properties. The measurement system can also be used to adjust the types and/or levels of flocculants being injected during a dredging operation based on the measurements of the dredged material before or after adding flocculants and/or measurements of overflow from hopper 18. The measurement system can include one or more sensors at different locations where measurements are desired.

Flocculant injection system 12 can allow for increased capacity of hopper 18 by promoting settlement of small particles in the dredged mixture. When dredging, some small particles of sand, clay or silt settle very slowly or do not settle at all in hopper 18. This can result in decreased loading capacity of hopper dredger 10. When dredging material is of a type that does not settle at all, for example, silt, dredging must be stopped as soon as hopper 18 reaches a maximum volume. This results in a low concentration of solids in hopper 18, and a lower capacity of hopper dredger 10.

Flocculant injection system 12 adds flocculants to the dredged mixture which creates floes that settle in hopper 18, thereby allowing for excess liquid to be removed through overflow 22 and increased loading capacity of hopper 18. By positioning flocculant injection system 12 upstream from pump 16, the turbulence in pump 16 can enhance the mixing process between the flocculants and the dredged mixture, thereby promoting creation of floes and subsequent settling in hopper 18. Flocculant injection system 12 also helps to promote clean overflow, thereby reducing or avoiding negative environmental impacts from overflow with suspended particles.

FIG. 2 illustrates a schematic view of an alternative embodiment of hopper dredger

10, and includes flocculant injection system 12, suction tube 14, pump 16, hopper 18 and overflow 22. Flocculant injection system 12 can also include a measurement system as discussed in relation to Fig. 1.

In this embodiment, flocculant injection system 12 is placed between pump 16 and hopper 18. In such a system, turbulence to promote mixing of flocculants and therefore formation of floes for settling can be due to the release of mixture into hopper 18 and/or due to the specific geometry and way flocculant injection system 12 injects flocculants. Floes are formed which settle in hopper 18, allowing for use of overflow 22 and a higher capacity hopper 18.

In summary, hopper dredger 10 with flocculant injection system 12 promotes settling of particles in the hopper and therefore reduces the content of solids in any overflow mixture. The flocculants can be injected before dredge pump 16 so that the dredge pump 16 can be used to promote mixture of the flocculants into the dredged mixture. Other embodiments can place flocculant injection system 12 at another location, for example directly connected to hopper 18, and promote mixture in other ways such as through flow into and/or within hopper 18. The flocculants cause the formation of floes, and thus promote settling of even very fine particles in hopper 18, thus promoting a cleaner overflow and allowing for overflow in cases where past systems were not allowed to overflow. The ability to overflow even when dredging low density materials such as silt increases the loading capacity of hopper 18 and therefore the overall loading capacity of hopper dredger 10. A measuring system can also be used to determine which/what amounts of flocculants will be injected and for validation and/or adjustments through the course of a dredging operation.

While hoppers 18 are shown with a rectangular shape and basic overflow 22 (and Fig. 1 with decantation panels 20), other embodiments can have different geometries, overflow features or systems and/or hopper 18 can include other parts that encourage a slowing down of the mixture so that particles have more time to settle.

While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.