Abstract:
A drag head ( 100 ) for dredging material ( 2 ) from the bed ( 3 ) of a body of water and transporting the material ( 2 ) to a suction tube ( 120 ). The drag head ( 100 ) is arranged to be dragged over the bed ( 3 ) in a dragging direction (D). The drag head ( 100 ) includes a suction section ( 110 ) in which an under pressure can be created to suck up the material ( 2 ) from the bed ( 3 ) through a suction opening ( 113 ) into a suction chamber ( 112 ). A heel section ( 111 ) guides the drag head ( 100 ) along the bed ( 3 ). The suction section ( 110 ) is preferably rotatably connected to the heel section ( 111 ). The suction section ( 110 ) also includes an outlet ( 114 ) for transporting the material ( 2 ) towards the suction tube ( 120 ).

Description:
TECHNICAL FIELD 
     The invention relates to a drag head for dredging purposes and to a trailing suction hopper dredger comprising such a head. The invention further relates to methods of dredging. 
     BACKGROUND 
     Dredging at sea or in open water may be carried out by dredging vessels, such as a trailing suction hopper dredger (TSHD). The dredging vessels comprise a suction tube one end of which can be lowered to the seabed and used to suck up solids such as sand, sludge or sediment, mixed with water. This lower end of the suction tube can be provided with a suction head. The solid material mixed with water is pumped through the suction tube into a hopper of the dredging vessel. 
     Once the hopper is full, the pumping may continue causing an overflow. The overflow will mainly be formed by water, as the solids tend to sink to the bottom of the hopper. The pumping may be stopped when it is no longer efficient to continue, as may be the case when the overflow is becoming too dense. 
     The higher the density of the mixture of solids and water that is pumped through the suction tube, the more efficient the dredging is performed. Dredging with relatively high densities has many advantages. In the first place, dredging can be performed in a more time and cost efficient way. Secondly, more solid material can be pumped into the hopper. Also, overflow losses will be reduced or will even disappear which is advantageous from an energetic point of view. Furthermore, reducing overflow losses will reduce turbidity. 
     One element of the dredging installation that may limit the maximum density is the trailing suction head provided at the lower end of the suction tube. 
     DE214643C discloses a suction tube and a trailing suction head. The suction tube has a bend near the trailing suction head such that the suction opening faces the direction of motion. In the suction opening an adjustable sled member is provided to control the dredging depth. Also, an adjustable plate member may be provided in the suction opening to control the amount of water entering the suction opening. A dragging force is applied directly to the suction head by the suction tube. 
     Other trailing suction heads are known which comprise a body which is arranged to be dragged along the seabed. The body comprises connection means for connecting to a suction tube which may also serve to impart the drag force on the body. A visor having a cutting edge is hingeably connected at a rear side of the body. The angle of orientation and/or the depth of the cutting edge of the visor can be adjusted with respect to the body by means of hydraulic piston/cylinder devices. Jet nozzles are provided in the body to facilitate the dredging process by breaking up the material of the sea bed and fluidizing it for removal via the suction tube. In order to lift the dredged material from the cutting edge towards the inlet to the suction tube, a significant amount of mixing with water is required leading to a reduction in density of the mixture. At present for sand and silt dredging, mixture densities of on average 1350 kg/m 3  are achievable. A drag head of this type is known from EP1653009A1. Similar drag heads are known from EP1108819A1 and AU2005200784A1, the contents of each of which are herein incorporated by reference in their entirety. 
     It would be desirable to provide an alternative to the above discussed drag heads, in particular one which is capable of sucking up mixtures of water and material with a relatively high density in a relatively efficient way whereby excess water transport is minimised. 
     SUMMARY 
     According to the invention, there is provided a drag head for dredging material from a bed of a body of water and transporting the material to a suction tube, the drag head being arranged to be dragged over the bed in a dragging direction by a drag member, wherein the drag head comprises a heel section being connectable to the drag member and having a bed engaging surface arranged to follow the bed and a suction section comprising a suction opening; a suction chamber; and an outlet for connection to the suction tube such that an underpressure can be created in the suction chamber to suck up the material from the bed through the suction opening into the suction chamber, wherein the suction section is adjustably mounted to the heel section such that an orientation of the suction opening can be adjusted relative to the heel section. By providing the suction section separately adjustable from the heel section, the orientation of the suction opening can be set independently of the position of the heel which is being towed along the bottom of the seabed. Such an arrangement is believed to be considerably more versatile in optimizing the direction and/or height of the suction opening. Since the outlet also forms part of the suction section, its orientation may also be adjusted together with the suction opening. In the present context, reference to material is intended to refer to solid or semi solid material including silt, sand, sediment, mud, gravel and fractured rock as may generally be encountered during suction dredging operations. Furthermore, although reference may be made to sea bed, this is equally intended to cover and include beds of rivers, lakes, canals, estuaries and the like. 
     According to the invention the heel section is arranged to be connected to a drag member. The drag member may be a dragging pole, bar, pipe, cable, chain or the like or the suction tube itself, which is connected with the vessel to drag the drag head over the seabed. In the present context, reference to the fact that the heel section is connected to the drag member is understood to mean direct or indirect connection therewith. The dragging force is subsequently applied to the suction section via the heel section. Preferably, the suction section is not connected to the drag member except via the heel section. 
     The suction section may be adjustable in various ways using appropriate mechanical means as will be known to the skilled person. According to a preferred embodiment of the invention, the suction section is rotatable with respect to the heel section about an axis of rotation which is in use substantially horizontal and perpendicular to the dragging direction. Most preferably, this axis lies generally behind the heel section and ahead of the suction section with respect to the direction of movement of the drag head. Preferably too, the axis is positioned relatively low with respect to the bed engaging surface in order to maximize the mass of the suction section that acts downwards. 
     According to a further aspect of the invention, the suction section may comprise a lower edge, e.g. a cutting edge, forming a trailing edge of the suction opening, wherein the lower edge or cutting edge is in use lower than the bed engaging surface of the heel section in order to dig into the material forming the bed. The lower edge or cutting edge is preferably substantially horizontal and substantially perpendicular with respect to the dragging direction and points at least partially in the dragging direction. By providing the lower edge or cutting edge below the bed engaging surface of the heel section, the suction opening will be directed in the dragging direction. By rotating the suction section with respect to the heel section the relative depth of the lower edge or cutting edge with respect to the bed engaging surface of the heel section can be adjusted and thereby the depth of channel dredged by the drag head. 
     The cutting edge may comprise a row of cutting members, which may be formed as (replaceable) teeth being placed in corresponding teeth holders. In general, the width of the cutting edge transverse to the dragging direction may be any appropriate width according to the operation being performed. Nevertheless, in general, the width of the cutting edge will not be more than the width of the bed engaging surface of the heel section. In a most preferred embodiment, both of these sections may have similar widths. It will also be understood that although in general the heel section will lie ahead of the suction section in the direction of movement, this position is not necessarily essential. The heel section may in certain configurations be located to one or both sides or around the suction section. 
     According to one embodiment of the invention, the width of the suction section decreases from the suction opening towards the outlet, most preferably in a gradual way. This smooth transition assists the transport of the dredged material towards the outlet and helps avoid significant energy losses. Preferably, the suction chamber may have a tapered or trumpet like shape to provide a smooth transition between the relatively large suction opening and the smaller outlet towards the suction tube. The term width is used here to indicate the dimension substantially perpendicular to the dragging direction and, in use, substantially horizontal. As an additional or alternative measure, the suction section may have a bottom plate which is at least partially inclined in an upward direction from the lower edge or cutting edge towards the outlet. The bottom plate ensures a smooth flow path for the material that is sucked up, thereby reducing the resistance. The bottom plate may be straight or curved. 
     According to an embodiment the suction section may be connected to the suction tube via a flexible connection. Providing a flexible connection has the advantage that the suction section can be moved with respect to the heel section and the suction tube. The suction tube may be provided on and move with the heel section or may be independent therefrom. The flexible connection may be provided by a flexible reinforced tube or concertina section. Alternatively it may be achieved by telescoping sections of rigid pipe. Preferably, the flexible section is of low-loss design in order to further reduce flow resistance to the dredged mixture, whereby transport of higher mixture densities may be achieved. In a further alternative, the suction tube itself may be flexible. 
     In one embodiment, the suction opening is at least partially bounded by the heel section. In such a configuration, the suction section and heel section may engage together to form the suction chamber. The engagement between the two sections should be sufficiently tight that suction losses and water inflow from between the two sections may be minimal. In a particularly preferred embodiment, the heel section and the suction section comprise two half shells that engage or telescope together to form the suction chamber. The heel section provides the bed engaging surface while the suction section carries the lower edge or cutting edge and forms the suction outlet. 
     The drag head may be provided with means to form a desired mixture density of the dredged material, optimized to achieve transport to the surface with minimal liquid content. The skilled person will be aware of various manners in which this may be achieved using swirl vanes, cutting blades and the like. According to a preferred embodiment the drag head may comprise a plurality of conduits having outlet openings or nozzles for delivering water jets into the suction chamber at or near the outlet. These nozzles may preferably be located on the suction section and most preferably around the outlet. Such water jets may be provided to fluidize the material to make transport of the material easier. 
     According to a further embodiment, the drag head may be provided with means for breaking up or loosening the material of the sea bed at or ahead of the lower edge or cutting edge. In this case too, the choice of measure provided will depend on the particular material being dredged and the skilled person will be aware of the alternatives that may be used. In a preferred embodiment, a plurality of conduits having outlet openings for forming water jets beneath the bed engaging surface of the heel may be provided. Not only do such jets make it easier to remove the material from the bottom but they may also assist in fluidizing it to the desired degree for further transport. 
     According to an embodiment the outlet from the suction chamber is at least partially directed in a direction opposite to the dragging direction. By orientating the outlet from the suction section in this way, the material is initially sucked in a direction at least partially opposite to the dragging direction. This may assist in providing a natural and undisturbed flow path for the material, allowing for an energy efficient suction operation. 
     According to a still further aspect of the invention, the drag head may be provided with an actuator arrangement for displacing the suction section with respect to the heel section. This actuator may be a hydraulic, pneumatic or mechanical actuator and can be automatically operated to set a desired orientation or depth of the suction section or the cutting edge. 
     In an alternative arrangement, the desired orientation may be achieved without actuator by using the natural mass of the suction section. This may be weighted or biased with respect to the heel section to achieve the desired orientation. In one embodiment, the position of the hinge may be adjustable to achieve the desired weighting. In this manner the depth of the lower edge or cutting edge may be adjusted depending e.g. on the dragging speed, seabed consistency and other related factors. 
     According to a further aspect of the invention, the heel section may be provided with a pump to provide suction to the suction chamber via the suction tube. Preferably the pump is a high performance submerged dredge pump for operating with high mixture densities such as a centrifugal pump. The pump may be carried directly on the heel section and may carry the suction tube. Alternatively, the pump and/or the suction tube may be provided at a remote position or may be mounted to the drag member. 
     Preferably the pump is located at a suitable distance above the seabed to avoid damage and for most purposes will be located at about half the water depth in order to most efficiently assist in transport of the mixture. 
     The invention also relates to a vessel, such as a trailing suction hopper dredger, comprising a drag head as generally described above. In its working configuration, the heel section is attached to a drag member trailing from the vessel whereby the drag head may be dragged or towed along the seabed. 
     The invention further relates to a method of suction dredging a mixture of solids and water from the bed of a body of water using a drag head comprising a heel section and a suction section, the method comprising dragging the heel section across the bed in a first direction, positioning the suction section at a desired depth and angle with respect to the heel section such that the suction section at least partially engages and enters the bed, applying suction to the suction section to cause the bed material to be sucked up in a direction at least partially opposed to the first direction and be mixed with water and transporting the mixture to the surface. 
     Most preferably, the method is carried out for a mixture comprising sand and water having a density of more than 1650 kg/m 3 . As a result of the desirable drag head configuration, such densities may be efficiently dredged. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which: 
         FIG. 1  schematically shows a side view of a first embodiment of the invention; 
         FIG. 2  schematically shows a side view of a second embodiment of the invention; 
         FIG. 3  schematically shows a top view of the embodiment of  FIG. 2 ; and 
         FIG. 4  schematically shows a cross-sectional view taken at line  4 - 4  in  FIG. 3 . 
     
    
    
     The figures are meant for illustrative purposes only, and shall not serve as restriction of the scope or the protection as laid down by the claims. 
     DETAILED DESCRIPTION 
     With reference to the figures, embodiments will now be described in more detail. According to  FIG. 1 , there is shown a schematic side view of a drag head  1  according to a first embodiment of the invention being used to dredge sand  2  or other similar material from the seabed  3  and transport it to a vessel  4 . 
     Drag head  1  comprises a heel section  11  in the form of a sled and a suction section  10  having the form of a bucket, articulated together at a generally horizontal hinge  8 . The heel section  11  is attached to a cable  16  via a pair of mounts  18  of which only one is shown. The cable  16  extends to the vessel  4  where it is held fast by a suitable derrick or boom  19  as is conventional in the art. 
     The heel section  11  has a bed engaging surface  22  on its underside. The bed engaging surface  22  is sufficiently long to ensure that the heel section assumes a substantially stable towing position. On its upper surface, heel section  11  carries a suction pump  50  which has a pump outlet  52  connected to a transport tube  54  leading to the surface and into a hopper  5  onboard the vessel  4 . 
     The suction section  10  has a suction chamber  12  within its interior with a suction opening  13  at its lower side. A trailing edge or lower edge of the suction opening  13  forms a cutting edge  15 . The cutting edge  15  may be provided with serrations (not shown). From the cutting edge  15  a bottom plate  17  leads up to an outlet  14  provided at an upper, rear side of the suction section  10 . The outlet  14  connects the interior of the suction chamber  12  to a flexible suction tube  20 . The suction tube  20  is connected to a pump inlet  51  on pump  50 . 
     In use, the drag head  1  is dragged along the seabed  3  by the cable  16  in a direction of motion D. The heel section  11  follows the seabed  3  and the blades  24  on the bed engaging surface  22  cut into the sand  2  and loosen it. The suction section  10  pivots about the hinge  8  due to its mass and causes the cutting edge  15  to dig into the sea bed  3 . The loosened sand  2  is scooped up by the cutting edge and rides up the bottom plate  17  towards the outlet  14 . The pump  50  is operated to generate suction in the suction tube  20  causing water to also be sucked up through the suction opening  113 . As the water and cut sand  2  approach the outlet  14 , the narrowing of the suction chamber  13  causes their velocity to increase whereby the sand  2  becomes entrained with the water. The resulting mixture is pumped via the pump  50  and transport tube  54  to the surface and into the hopper  5 . Due to the advantageous orientation of the suction opening  13  and the upward slope of the bottom plate  17  towards the outlet  14 , the cut sand can be carried away with relatively little entrainment of water and a relatively high density of the mixture. 
     A second embodiment of a drag head  100  according to the invention is shown in  FIG. 2  in which like elements are provided with similar reference numerals preceded by  100 .  FIG. 2  shows a heel section  111  and a suction section  110  which are hinged together at a hinge  108  forming a suction chamber  112  therebetween. The suction section  110  is slightly narrower than the heel section  111 , whereby both sections can partially telescope into each other by rotation about the hinge  108 . A lowermost or trailing edge of the suction section  110  is provided with a cutting edge  115 . The heel section  111  has a lowermost bed engaging surface  122 . Between the cutting edge  115  and the rear edge of the bed engaging surface  122  there is formed a suction opening  113  providing access to the suction chamber  112 . 
     In the embodiment of  FIG. 2 , the heel section  111  further comprises a tubular body  140  rigidly attached to a front surface thereof. The tubular body  140  is in turn connected to a drag member  141  which is towed from the vessel  4  as in  FIG. 1 . The drag member  141  and the tubular body  140  form a relatively rigid arm extending to the surface (although it will be understood that powered joints may be foreseen) which ensures that the angle of the heel section  111  with respect to the seabed remains substantially constant (for a given depth of water). 
     On an upper surface of the tubular body  140  there are provided a pair of actuators  130  (of which one is shown in this view) having piston arms  132  attached to an upper portion of the suction section  110  at a mount  134 . By operating the actuators  130 , the suction section  110  can be pivoted with respect to the heel section  111  to cause the cutting edge  115  to dig deeper into the sea bed. 
     As in the first embodiment, the suction section has a bottom plate  117  which leads upwards to an outlet  114  at an upper rear part of the suction section. Unlike the first embodiment, the outlet  114  is connected to a flexible connection  121  which in turn connects to the suction tube  120 . In this case, the pump  150  is carried by the drag member  141  and has a pump inlet  151  connected to the suction tube  120  and a pump outlet  152  connected to transport tube  154 . 
       FIG. 3  shows a plan view of the embodiment of  FIG. 2  showing heel section  111  and suction section  110  engaging each other with actuators  130  determining the degree of rotation of the sections about hinge  108 . According to  FIG. 3 , it can be seen that the heel section  111  and the suction section  110  have a maximum width W 1  at the position of the cutting edge. From this position, the width of the suction section  110  decreases to a width W 2  at the outlet  114 . 
       FIG. 4  is a sectional view taken on line  4 - 4  in  FIG. 3  showing an interior of the suction chamber  112 . In this view, nozzles  160  can be seen located around outlet  114 . The nozzles  160  are connected to a suitable source of pressure (not shown) and are operated to generate pressurized jets of water within the outlet  114  directed towards the flexible connection  121 . Also visible in  FIG. 4  are further nozzles  162  provided in the bed engaging surface  122  of the heel section  111 . The further nozzles  162  are in communication with a pressure manifold  164  within the heel section  111  into which pressurized water may be supplied from the source of pressure mentioned above. 
     In use, the drag head  100  is dragged along by the dredging vessel in the direction D with the heel section  111  engaging the seabed  3 . Pressurised water is provided to the manifold  164  which causes the formation of jets of water from further nozzles  162  beneath the bed engaging surface  122 . The jets of water loosen and partially break up the sand or silt  2 . The loosened sand  2  is cut and lifted by cutting edge  115  and enters suction chamber  112  through suction opening  113 . The reducing width of the suction chamber  112  and the bottom plate  117  funnel the sand  2  upwards towards the outlet  114 . At this stage, the sand contains a quantity of entrained water due to the further nozzles  162 . Nevertheless, the density is too high for it to be easily transported. As the sand and water mixture enters the outlet  114  additional water jets are injected through nozzles  160 . These jets further loosen the sand  2  and fluidise it to a desired final density of around 1650 kg/m 3  for transport via the pump  150  and transport tube  154  to the surface. Due to the increased density, the vessel  4  can be filled without overflow or further discharge back into the water which is highly advantageous for sensitive environments where such discharge during dredging is prohibited. 
     Thus, the invention has been described by reference to certain embodiments discussed above. It will be recognized that these embodiments are susceptible to various modifications and alternative forms well known to those of skill in the art. In particular, the arrangement of flexible connection of  FIG. 2  may be replaced by a telescoping arrangement. Furthermore, the actual design may be distinct from the schematically illustrated designs. 
     Many modifications in addition to those described above may be made to the structures and techniques described herein without departing from the spirit and scope of the invention. Accordingly, although specific embodiments have been described, these are examples only and are not limiting upon the scope of the invention.