NL2004442C2

NL2004442C2 - Cutter suction dredger for dredging ground and method for dredging using this cutter suction dredger.

Info

Publication number: NL2004442C2
Application number: NL2004442A
Authority: NL
Inventors: Jan Lefever; Erik Wellen
Original assignee: Baggerwerken Decloedt & Zn N V
Priority date: 2010-03-22
Filing date: 2010-03-22
Publication date: 2011-09-23
Also published as: WO2011117204A1; AR081157A1; TW201139795A

Description

Cutter suction dredger for dredging ground and method for dredging using this cutter suction dredger

The invention relates to a cutter suction dredger for dredging ground under water, and to 5 a method for dredging using this cutter suction dredger A cutter suction dredger is for instance known from NL-A-9200368. The known cutter suction dredger comprises a vessel provided with a ladder to which end is attached a cutter head. The cutter head typically takes the form of a rotation-symmetrical revolving 10 body that is rotatable about a rotation axis by means of drive means and moreover is provided along its peripheral surface with a number of cutting tools for penetrating into the ground. The vessel may be anchored in the ground by means of spud posts that are lowered from the rear deck of the vessel until they take support on the ground. This anchoring creates a means for absorbing and transmitting to the ground the generally 15 considerable reaction forces occurring during dredging. The vessel is further anchored to the ground at the front thereof by providing hauling lines at portside and starboard that may be tightened or rendered by winches, provided on the vessel’s front deck. A suction conduit is connected to the cutter head and typically fixed to the ladder, to remove the ground fragments dredged by the cutter head to a storage location on deck 20 of the vessel or elsewhere.

For dredging, the cutter head with ladder and suction conduit is lowered into the water in front of the vessel under a generally oblique angle with the vessel until it touches the ground. The cutter head is then dragged through the ground by hauling the vessel 25 alternately from portside to starboard side using the winches, whereby the vessel pivots around the spud post that takes support onto the ground. The cutter head makes a lateral sweeping movement through the ground from starboard to port side and back, such that a circular arc is dredged at each passage.

30 The ground surface to be dredged can be covered by moving the cutter suction dredger over a determined distance at a time and repeating the above stated sweeping movement. A limited movement in the longitudinal direction of the vessel is possible by attaching the spud posts to the vessel through a spud carriage that may be moved over rails across the deck. Such a spud carriage is known per se and usually comprises two 2 spud posts. The vessel may be moved over some distance (for instance 15 m) in its longitudinal direction by moving the carriage (and therefore the vessel also) relative to the spud post that remains anchored in a number of steps of for instance 0,5 to 1 m. When reaching the desired position after a step, the carriage is fixated with respect to 5 the spud post. When the maximum stroke of the carriage (for instance 6 m) has been reached, a second spud post is lowered into the water until it takes support onto the ground, where after the first spud post can be moved out of the water and new stroke is available. Moving the cutter suction dredger along a line that is not parallel to the vessel’s longitudinal direction involves taking up the spud posts and sailing the vessel to 10 its desired position.

Using the known dredger and method, the desired depth profile is obtained by controlling and/or measuring the oblique angle of the ladder with respect to the vessel. Knowing the oblique angle as well as the height above ground of the pivot point of the 15 ladder allows to calculate the dredging depth for each position of the vessel (or cutter head). After a complete area has been dredged, the depth profile is typically measured again by a survey vessel or tool that sails the area and measures depths. In case deviations are noticed with respect to the desired depth profile, the cutter suction dredger is reused to dredge the deficient areas.

20

One of the drawbacks of the known cutter suction dredger is that its dredging efficiency is relatively low. Efficiency is understood in the context of this application to mean the average weekly amount of operational hours relative to the available service hours in the same period.

25

One object of the present invention is to provide a cutter suction dredger and method for dredging ground under water that allow a more efficient way of dredging.

The invention to this end provides a cutter suction dredger for dredging ground under 30 water, the dredger comprising a vessel provided with anchoring means to anchor it with respect to the ground, and with a ladder that can be lowered into the water in front of the vessel under an oblique angle with the vessel and to which is attached a cutter head that in operation is moved through the ground in a lateral sweeping movement by hauling the vessel alternately from portside to starboard side whereby the vessel pivots around 3 the anchoring means, and thereby loosens ground fragments that are sucked away by a suction conduit, whereby the vessel further comprises means adapted to measure the depth of the under water ground after this ground has been dredged. Since the means to measure the depth are attached to and/or are part of the vessel, the depth measuring 5 means move with the vessel. The cutter suction dredger of the invention therefore allows to measure the depth of the under water ground after this ground has been dredged by the cutter head by simply moving the vessel to the dredged ground. This is most conveniently done by moving the vessel a step forward as elucidated above. Such a move or step allows to dredge a new circular arc and at the same time allows to 10 measure the depth profile of the circular arc that was dredged in the position of the vessel before taking the step. In this way the lateral depth profile of a dredged circular arc is measured immediately after dredging, i.e. in the next step taken by the vessel. If a deviation in depth is noticed that is too high to be acceptable, the vessel is brought one step or more back and forced to dredge this circular arc again. Bringing the vessel from 15 one position to another by stepping is easily carried out. The cutter suction dredger and associated method may thus provide a depth profile that is accurately defined within a time frame that is substantially shorter than obtained with the known device and method.

20 A preferred embodiment of the cutter suction dredger according to the invention is characterized in that the means adapted to measure the depth are attached to the ladder and positioned between the bow of the vessel and the cutter head in its lowered position. The depth measuring means are therefore located at a distance from the cutter head. Positioning the depth measuring means close to the cutter head (for instance at a 25 distance of 1 m or less) gives rise to difficult to correct measurement disturbances, among others due to turbidity in this area as caused by flying about ground fragments.

Another preferred embodiment of the invention provides a cutter suction dredger, wherein the means adapted to measure the depth are rigidly connected to the vessel. The 30 depth measuring means being rigidly attached to the vessel offers an additional advantage in that the measuring means are less influenced by the massive vibrations of the ladder during dredging. This improves measurement accuracy and reliability.

4

Yet another preferred embodiment of the invention provides a cutter suction dredger, wherein the means adapted to measure the depth are positioned at a distance from the bow of the vessel in the direction of the anchoring means that is less than 50% of the distance between the anchoring means and the bow, more preferably less than 25% of 5 the distance between the anchoring means and the bow, and most preferably less than 10% of the distance between the anchoring means and the bow. For a typical cutter suction dredger, the means adapted to measure the depth are positioned below the vessel at a longitudinal distance from the bow of the vessel that does not exceed 5 m, more preferably 3 m, and even more preferably 1 m. The positions according to these 10 embodiments offer a low disturbance as caused by vibrations and turbidity. Most preferably, a cutter suction dredger is provided wherein the means adapted to measure the depth are positioned below the vessel at bow’s height. The bow of the vessel is considered as the foremost position of the vessel and is readily recognizable by the person skilled in the art. The position of the anchoring means in the longitudinal 15 direction of the ship is defined by the average position of the spud poles of the vessel.

In another preferred embodiment of the invention, the cutter suction dredger according to the invention is characterized in that the means adapted to measure the depth are positioned at port side and at starboard side of the vessel. This allows to measure the 20 depth profile across a substantially complete circular arc.

In another aspect of the invention, a cutter suction dredger is provided wherein the means adapted to measure the depth comprise an ultrasonic and/or an optical camera, in particular an echo sounder. A particularly preferred echo sounder comprises a 25 multibeam camera, as obtainable from Reson® for instance. It is also possible that the means adapted to measure the depth comprise a laser beam.

The cutter suction dredger preferably also comprises monitoring means adapted to monitor the position of the dredger (and therefore also of the cutter head) relative to the 30 ground. In a preferred aspect of the invention, a cutter suction dredger is provided wherein the monitoring means comprise a Global Positioning System. In this way, the movement of the vessel and/or cutter head can be related to the state of the underwater ground as charted by a survey, which state at least includes the depth profile of the ground. Indeed a survey carried out before the actual start of the dredging operation 5 yields an initial depth profile of the natural under water ground. After passage of the cutter head, and with knowledge of the oblique angle of the ladder as well as the height with respect to the ground of the pivot point of the ladder, the new (dredged) local depth can be calculated. By simultaneously monitoring the position of the vessel and cutter 5 head, an updated dredged depth profile is obtained. The cutter suction dredger and method according to the invention then measures the depth profile of a dredged circular arc again ‘immediately’ after the dredging operation. This allows to update the dredged depth profile a second time. It is also possible to repeat the depth measurement another time by providing the vessel with second depth measuring means that are positioned 10 behind first depth measurement means, the forward direction being the direction pointing to the cutter head.

In still another aspect of the invention, the cutter suction dredger comprises display units adapted to display the position of the cutter head, the vessel and/or the state of the 15 bottom, in particular its depth profile. An operator of the cutter suction dredger is then able to oversee the dredging operation and its progress.

The invention also relates to a method for dredging ground under water with a cutter suction dredger according to the invention, the method comprising the steps of 20 anchoring the vessel with respect to the ground by activating anchoring means, lowering the ladder and cutter head into the water in front of the vessel under an oblique angle with the vessel, and moving the cutter head through the ground in a lateral sweeping movement by hauling the vessel alternately from portside to starboard side whereby the vessel pivots around the anchoring means, thereby loosening ground fragments that are 25 sucked away by a suction conduit, and measuring the depth of the under water ground after this ground has been dredged by the means adapted to measure the depth. The advantages of the method have already been elucidated with reference to the description of the cutter head, and are not further repeated here.

30 In a preferred embodiment of the method according to the invention , the depth of the under water ground is measured at a position between the bow of the vessel and the cutter head in its lowered position.

6

In yet another preferred embodiment of the method according to the invention, the depth of the under water ground is measured by means that are rigidly connected to the vessel.

5 Preferably, the method according to the invention is characterized in that the depth of the under water ground is measured at a position at a distance from the bow of the vessel that does not exceed 5 m along the vessel, more preferably does not exceed 3 m, even more preferably does not exceed 1 m, and most preferably is measured at a position at bow’s height.

10

It has advantages to measure the depth of the under water ground is measured at port side and at starboard side of the vessel.

The depth data used to compile a bathymetric map of the bottom may be measured by 15 any means known in the art, but in a preferred method according to the invention is measured by an echo sounder (sonar) mounted beneath or over the side of the cutter suction dredger. Active sonar creates a pulse of sound, and then listens for reflections (echo) of the pulse. This pulse of sound is generally created electronically using a Sonar Projector consisting of a signal generator, power amplifier and electro-acoustic 20 transducer/array. A beamformer is usually employed to concentrate the acoustic power into a beam, which may be swept to cover the required search angles. When active sonar is used to measure the distance from the transducer to the bottom, it is known as echo sounding. Similar methods may be used looking upward for wave measurement.

25 An echo sounder emits a beam of sound downward at the bottom. The amount of time it takes for the sound or light to travel through the water, bounce off the bottom, and return to the receiver of the sounder tells the equipment what the distance to the bottom is. Distance is measured by multiplying half the time from the signal's outgoing pulse to its return by the speed of sound in the water, which is approximately 1.5 kilometres per 30 second. Echo sounding is effectively a special purpose application of sonar used to locate the bottom.

Preferably, a multibeam echo sounder (MBES) is used. Such a multibeam echo sounder features hundreds of very narrow adjacent beams arranged in a fan-like swath of 7 typically 90 to 170 degrees across. The tightly packed array of narrow individual beams provides very high angular resolution and accuracy. The wide swath, which is depth dependent, allows the cutter suction dredger to map more bottom in less time than a single-beam echosounder by making fewer passes. The beams update many times per 5 second (typically 0.1-50 Hz depending on water depth), allowing faster speed while maintaining 100% coverage of the bottom. Preferably, the cutter suction dredger is provided with attitude sensors that allow for the correction of the vessel's roll, pitch and yaw on the water surface, and a gyrocompass that provides accurate heading information to correct for vessel yaw. The Global Positioning System or equivalent is 10 used to position the echo soundings with respect to the surface of the bottom. Sound speed profiles (speed of sound in water as a function of depth) of the water column correct for refraction or "ray-bending" of the sound waves owing to non-uniform water column characteristics such as temperature, conductivity, and pressure. A computer system processes all the data, correcting for all of the above factors as well as for the 15 angle of each individual beam. The resulting sounding measurements are then processed either manually, semi-automatically or automatically to produce a depth profile map of the dredged area. A number of different outputs may be generated, including a sub-set of the original measurements that satisfy some conditions (e.g., most representative likely soundings, shallowest in a region, etc.) or integrated Digital Terrain Models (e.g., 20 a regular or irregular grid of points connected into a surface).

The depth profile or bathymetric data are generally referenced to tidal vertical data. For deep water bathymetry, this is typically Mean Sea Level (MSL) but most data used for nautical charting is referenced to Lowest Astronomical Tide (LAT). Many other data 25 are used in practice, depending on the locality and tidal regime.

The invention will now be further elucidated on the basis of the following figures and description of preferred embodiments, without the invention otherwise being limited thereto. The figures are not necessarily drawn to scale. In the figures: 30 Figure 1 shows a schematic side view of a part of a cutter suction dredger according to the invention with a ladder attached thereto and provided with a cutter head;

Figure 2 shows a schematic top view of a cutter suction dredger according to the invention; Figure 3 shows a schematic representation of a depth profile as measured by the cutter suction dredger according to the invention for one sweep.

8

Figure 4 schematically shows an areal graph of the depths as measured by the cutter suction dredger according to the invention;

Figure 5 shows a schematic representation of a depth profile as measured by the cutter suction dredger according to the invention in the direction of movement; and 5 Figure 6 schematically shows an areal graph of the depths as measured by the cutter suction dredger according to the invention that are less than a predetermined depth level.

Figure 1 shows a cutter suction dredger 10 comprising a vessel 1 on which a ladder 2 is 10 mounted pivotally around a horizontal shaft 3, that acts as pivot point for ladder 2. Ladder 2 is provided on the outer end thereof with a cutter head 20, that carries a number of cutting members 21 on its outer surface, and with a suction pipe 4 which can suction up the loosened ground fragments or parts to a level above water surface 100, after which they are discharged. Ladder 2 (and cutter head 20) may be lowered or lifted 15 under water with respect to the vessel 1 or ground surface 9 by means of cable 8 that runs over winch 5, arranged on the front deck of vessel 1. In order to be able to absorb the forces generated on the ground surface 9 during dredging, the vessel 1 is anchored in the ground by means of a spud posts (101a, 101b) on its rear deck. In figure 1, the left-hand spud post 101a is held in its unanchored position (no contact with the ground 20 9) while the right-hand spud post 101b (in front of left-hand spud post 101a) is held in its anchored position, in which it takes support on the ground. Both spud posts (101a, 101b) are attached to the vessel 1 through a spud carriage 11 that is movable in the longitudinal direction 12 of the vessel 1 over rails (not shown) across the deck over some distance, for instance 6 m. The vessel 1 is moved in the longitudinal direction 12 25 by moving the carriage 11 relative to the spud post 101b that is anchored in the ground 9. Such movement occurs in a number of steps of for instance 0,5 to 1 m. When reaching the desired position after a step 17 (see figure 2), carriage 11 is fixated to spud post 101b. When the maximum stroke of say 6 m of the carriage 11 is reached in a number of consecutive steps, rear spud post 101a is lowered until it takes support onto 30 the ground 9, where after the front spud post 101b is moved out of the ground 9 and new stroke becomes available. By stepping, the cutter suction dredger 10 may be moved forward along a line 13 (see figure 2). For dredging, cutter head 20 is lowered by means of the ladder winch cable 8 and the assembly of vessel 1 and cutter head 20 moved across or in ground surface 9 in a reciprocating, sweeping movement from the port side 9 to the starboard side of cutter suction dredger 1 and back, such that a circular arc 30 is dredged around the lowered spud post. The hauling of vessel 1 from port side to starboard is carried out by hauling cables (not shown) that extend on both sides from the vessel 1 to the ground 9. The cutter suction dredger 10 further comprises means adapted 5 to measure the depth in the form of single beam echo sounders (14a, 14b), positioned below the vessel 1 at the bow thereof, and positioned at port side and at starboard side of the vessel 1. This allows to measure the depth profile 32 across a substantially complete circular arc 31. Multibeam echosounders may also be used but single beam echo sounders offer sufficient information in the context of the invented method. The 10 single beam echo sounders (14a, 14b) are in the embodiment shown positioned at bow’s height. Other embodiments provide echo sounders (14a, 14b) positioned at a distance from the bow of the vessel in the direction of the spud poles 101 of less than 50% of the average distance 50 between the spud poles 101 and the bow 51, more preferably less than 25%, and most preferably less than 10%. The average distance is defined midway 15 between the two spud poles 101.

The cutter suction dredger 10 comprises a Global Positioning System (not shown) to monitor the position of the vessel 1 and of cutter head 20 relative to the ground 9. The movement of vessel 1 and/or cutter head 20 can in this way be related to the depth 20 profile 34 of the ground 9, as charted by a survey, prior to the dredging operation, as shown in figure 2. Such a depth profile typically involves different colors (because of the black and white reproduction these are represented as different gray scales) for different depths. Figure 2 is a typical display of a display unit adapted to display the position of the cutter head 20, the vessel 1 and/or the depth profile 34 of the ground 9, 25 and is typically available to an operator of the cutter suction dredger 1 allowing him to oversee the dredging operation and its progress. The pre-dredging survey yields an initial depth profile 34 of the virgin ground 9. After passage of the cutter head 20, and with knowledge of the oblique angle 15 of the ladder 2 as well as the height 16 with respect to ground 9 of the pivot point 3 of the ladder 2, a dredged local depth can be 30 calculated for each sweeping position of ladder 2. By simultaneously monitoring the position of the vessel 1 and cutter head 20, an updated dredged depth profile 33 along dredged circular arc 30 is obtained. According to the invention, while dredging arc 30, the depth profile 32 of a circular arc 31 that was dredged in a previous position of the vessel 1 before stepping to cutter head position 30 is simultaneously measured by the 10 echo sounders (14a, 14b), not much later after actual dredging of arc 31. This allows to update the dredged depth profile 32 a second time, whereby the depth profile was update a first time by a calculation based on the oblique angle 15, as explained above.

5 With reference to figure 3 the output of the display means may also comprise a chart of the measured depths across a circular arc 30 or 31, of which the center line is represented by line 19. The starboard side is at the left of line 19, while the port side is at the right of line 19. The x-axis 40 corresponds with the deviation across arc 30 or 31 from center line 19. This is a convenient way to assess whether a deviation in depth is 10 too high compared with a pre-set target depth level 35 to be acceptable. This is for instance the case across area 36, where apparently the ground 9 was too hard to be dredged to the required depth. In such case, the vessel 1 is brought only one step back and forced to dredge this part of circular arc 30 or 31 again. Bringing the vessel 1 from one position to another adjacent position by stepping once or twice for instance is easily 15 carried out, and takes not much time.

In an embodiment of the method according to the invention, a chart is made of the measured depths across a number of circular arches (30, 31,...), extending in the direction of movement of the cutter suction dredger 10. i.e. in the direction of line 13.

20 Such a chart is shown in Figure 4. In the chart axis 40 corresponds with the deviation across the dredged arcs from center line 19, while axis 41 corresponds with the direction of advancement of the cutter suction dredger. Each color (or grey scale) corresponds to a particular depth of the ground 9. The depth data shown have been collected by the port and starboard side echo sounders (14a, 14b) respectively during a sweep from port to 25 starboard and back over a total distance 18, which may vary substantially but is typically around 80-100 m. By providing echo sounders on port and starboard, a substantial amount of data is measured twice, for instance when during a sweep from port to starboard, the port side echo sounder 14a travels along a location that was previously sampled by the starboard echo sounder 14b. In the example given in figure 4, 30 the echo sounder date have been collected over a total distance 38 of about 30-40 m, which may correspond to a one day production. This distance can be covered by moving the cutter suction dredger forward a number of times.

11

With reference to figure 5 the output of the display means may also comprise a chart of the measured depths across a particular line in the direction 41 of forward movement. The depth profile shown in figure 5 corresponds to data collected on line 43 (see figure 4) in the movement direction 41. Chart axis 44 represents the depths measured by the 5 two echo sounders (14a, 14b), where datapoints 45 represent the depth data measured by the port side echo sounder 14a, and datapoints 46 represent the depth data measured by the starboard side echo sounder 14b. Also shown is the pre-set target depth level 35 to be acceptable. Collecting and combining the data of figure 5 for a number of lines in the movement direction 41 and keeping only those depths that are less than the pre-set 10 target depth level 35 yields a graph as shown in figure 6. This graph shows the areas where the depth is insufficient with respect to target depth 35 (see the encircled areas). These areas are made visible to the operator of the cutter suction dredger by representing them on a monitoring screen for instance, and he will be able to return to these areas and selectively repeat the dredging operation there.

15

After having dredged one row with a width 18 (the width covered by a circular sweep arc) vessel 1 is freed by moving the spud posts (101a, 101b) out of the ground 9 and sailing the vessel 1 to its desired starting position, where after a new stepping cycle, as described above is repeated.

20

Claims

Cutter suction dredger for dredging ground under water, comprising a vessel provided with anchor means for anchoring the vessel relative to the ground, and a ladder that can be lowered into the water at the front of the vessel at an oblique angle to the vessel, and to which is attached a cutting head which, in use, is moved through the ground in a lateral swinging movement by alternately shifting the vessel from port to starboard, whereby the vessel pivots around the anchor means and thereby loosens 10 soil fragments are sucked away by a suction conduit, the vessel further comprising means adapted to measure the depth of the dredged underwater bottom.

2. Cutter suction dredger according to claim 1, wherein the means adapted to measure the depth are attached to the ladder and are positioned between the bow of the vessel and the cutter head in its lowered position.

The cutter suction dredger according to claim 1, wherein the means arranged to measure the depth are fixedly connected to the vessel. 20

The cutter suction dredger according to claim 3, wherein the means adapted to measure the depth are positioned at a distance from the bow of the vessel in the direction of the anchor means that is less than 50% of the distance between the anchor means and the bow and more preferably less than 25% of the distance between the anchor means and the bow.

The cutter suction dredger according to claim 4, wherein the means adapted to measure the depth are positioned at the height of the bow.

A cutter suction dredger according to any one of the preceding claims, wherein the means adapted to measure the depth are positioned on the port side and starboard side of the vessel.

The cutter suction dredger according to any one of the preceding claims, wherein the means arranged to measure the depth comprise an echo sounder.

8. Cutter suction dredger according to claim 7, wherein the means adapted to measure the depth comprise a multibeam echo sounder.

9. Method for dredging subsoil under water with a cutter suction dredger according to one of the preceding claims, comprising the steps of anchoring the vessel relative to the ground by activating the anchor means, lowering the ladder and cutting head into the water at the front of the vessel at an oblique angle with the vessel, and moving the cutting head by moving the ground in a lateral swinging motion by alternately shifting the vessel from port to starboard whereby the vessel pivots around the anchor means thereby loosening soil fragments are sucked away through a suction line, and measuring the depth of the dredged underwater bottom using the means arranged to measure the depth.

10. Method as claimed in claim 9, wherein the depth of the underwater bottom is measured at a position between the bow of the vessel and the cutting head in its lowered position.

The method of claim 9, wherein the depth of the underwater bottom is measured by means that are rigidly connected to the vessel.

A method according to claim 11, wherein the depth of the underwater bottom is measured at a distance from the bow of the vessel, which distance does not exceed 5 m in the longitudinal direction of the vessel.

The method of claim 12, wherein the depth of the underwater bottom 30 is measured at a position at the height of the bow.

A method according to any of claims 9-13, wherein the depth of the underwater bottom is measured on port side and starboard side of the vessel.

The method of any one of claims 9-14, wherein the depth of the underwater bottom is measured by means of an echo sounder.

The method of claim 15, wherein the depth of the underwater bottom is measured by means of a multibeam echo sounder.