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(11) | EP 0 655 535 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Method and apparatus for dredging a trench in a water bed |
| (57) The invention relates to a method and an apparatus for dredging a trench in a bed. According to the method the cutter head of a cutter dredger is moved in a reciprocating motion parallel to the course of the trench to be dredged during the dredging, wherein the cutter head is moved perpendicular to the course after every motion. The apparatus comprises a seaworthy cutter dredger ship (100), which is pulled by cables (11-18) along the course of the trench. Advantageously the ship (100) has a shallow draught, and use is made of a dish cutter head on a movable vertical bearing girder. |
[0001] The invention relates to method for dredging a trench in a bed with the help of a cutter dredger with a cutter.
[0002] Using a cutter dredger in dredging and sand dredging activities at sea is commonly known, the cutter head being attached to a ladder, and the ladder being attached to the front of the cutter dredger and extending obliquely forward. At its rear the cutter dredger is rotatably connected to a spud which has been driven into the bed. The cutting motion of the cutter head is carried out using two side wires which run from the cutter dredger via tipping discs, which are attached to the ladder behind the cutter dredger, to two anchors to the left and right at a short distance from the cutter dredger. With the help of winches on the cutter dredger the cutter head is pulled along a part of a circular path around the spud. Such seaworthy cutter dredgers are large and have a draught of 4 to 5 m, a length of approximately 100 m and a width of approximately 15 m.
[0003] These known seaworthy cutter dredgers are especially suited for sand dredging activities, for example, the cutter head being able to follow a large part of a circular path, in principle always moving transverse to the cutter dredger, and dredging taking place up to a relatively great depth along a substantially vertical plane in horizontal paths from top to bottom. With this method of dredging the spud only has to be moved now and again.
[0004] For dredging a trench, this method is not very suitable, however. Because of the high cutting forces on the cutting head, and to retain the positioning accuracy a spud remains necessary, in order to prop up reaction forces.
[0005] With a relatively narrow trench, with a relatively minor depth, the substantially vertical plane along which the cutter head dredges is comparatively small. This means that to be able to dredge another substantially vertical layer, the cutter dredger soon has to be moved again. To dredge the whole length of the trench, the spud has to therefore be moved and to be attached very frequently to the ground and the anchors for the side wires have to be moved regularly. The effective working time with this method is, as a result, low in relation to the total of time needed.
[0006] If, for example, in the North Sea a trench for a pipeline has to be dredged from a depth of 10 m to the shore, the cutter dredger has to dredge a channel for itself as well as dredging the trench. Due to the transverse movement of the cutter dredger around the spud, a trench to be dredged of 90 m in width would easily be necessary for a cutter dredger of 20 m wide.
[0007] For laying a pipeline in a trench 4 m deep, taking account of the formation of a slope on both sides of the trench with a natural inclination of 1 : 3 which is formed in calm water, a trench width of approximately 15 m would, however, suffice.
[0008] As a result of the not particularly effective method of dredging and because the trench has to be made much wider than is necessary for laying a pipeline, dredging a trench of 1 km in length can soon take 1 to 2 weeks. There is the risk that in that time the trench will fill up again due to storm and will therefore have to be cleared out. The dredging time, including mobilization and demobilization, has to therefore be multiplied twice according to a rule of thumb. Seeing as the Department of Public Works requires that activities along the coast have to be finished at the latest by May 15, these activities have to start early in the year, in which season the risk of storms is great. If a storm is nearing it is necessary for security reasons to tow the large cutter dredger away from the coast. The fact is that the cutter dredgers' own drive is not sufficient to manoeuvre the cutter dredger out of the trench. Consequently the tug available for placing and lifting the anchors has to be strong enough to tow the cutter dredger in rough weather to the open sea.
[0009] It is am object of the invention to remove at least one of these disadvantages to a large extent.
[0010] For that purpose the invention according to the present application supplies a method for dredging a trench in a bed with the help of a cutter dredger with a cutter head, the cutter head being moved in a reciprocating motion parallel to the course of the trench to be dredged during the dredging, wherein the cutter head is moved perpendicular to the course after every motion. According to this method dredging now takes place from the bed in a substantially horizontal surface in paths, parallel to the course of the trench to be dredged, from the one side of the trench to the other. This as opposed to the known dredging method with a cutter dredger, in which dredging taking place along a substantially vertical plane. The method according to the invention thus offers the possibility of at least removing the one disadvantage mentioned above.
[0011] Advantageously the invention supplies a method, wherein the cutter head is reciprocated with the help of at least four pre-tensioned cables, which are anchored substantially in the main direction of the trench to be dredged and which are wound and unwound with the help of winches on the cutter dredger. Because use is made of the pretensioned cables it is not only possible to lead the cutting force of the cutter head to the anchoring, but it is also possible to move the cutter dredger along the course in a easy way with the help of the winches. Then the cumbersome and inefficient use of a spud is not necessary.
[0012] The invention alternatively supplies a method wherein the cutter head is reciprocated with the help of at least two pre-tensioned cables, which run substantially along the trench to be dredged and are anchored near the far ends of the trench to be dredged, and along which the cutter dredge is moved with the help of capstans.
[0013] Preferably the invention supplies the method wherein at least two pre-tensioned cables are connected to the cutter dredge in a transverse direction for taking up transverse forces. Current and wave drift forces, as well as transverse forces on the cutter head are taken up by these cables which run in a transverse direction.
[0014] Another aspect of the invention is to provide an apparatus for implementing the inventive method, the apparatus comprising a cutter dredger ship with a cutter head and at least four winches, and at least four pre-tensioned cables which run substantially in the main direction of the trench, which are windable or unwindable with the help of the winches. The apparatus alternatively comprises a cutter dredger ship with a cutter head and at least two capstans, and at least two pre-tensioned cables which run substantially along the trench, along which the cutter dredger ship is movable with the help of the capstans. With the help of this apparatus the cutter head can be moved parallel to the course of the trench to be dredged.
[0015] According to a preferred embodiment the apparatus comprises four winches on the cutter dredger ship and four cables for these winches, these cables being attached in the angular points of an imaginary rectangle, the cables which run substantially in the main direction of the trench forming the short axial line of the rectangle, and the long sides of the rectangle being at least approximately 10% longer than the short sides. In this embodiment the cables to the angular points of the imaginary rectangle can take up the current and wave drift forces well.
[0016] According to another preferred embodiment the apparatus comprises a runner on both sides of the cutter dredge ship, each runner being movable along an anchor cable, which anchor cables are each attached to one side of the trench in the angular points of an imaginary rectangle, with the cables running substantially in the main direction of the trench as axial line, and in which each runner is connected to the cutter dredger ship with the help of at least one cable, which cable can be pre-tensioned with the help of a winch on the cutter dredger ship. In this embodiment each runner describes a portion of an ellipse along its anchor cable, and with the help of the winches the cables between the runners and the cutter dredger ship are lengthened and shortened to such an extent that the cutter dredger ship keeps to the course of the trench. With the help of these cables the current, wave drift and transverse forces are likewise taken up.
[0017] So that the tension variations in the cables do not get too large, the cables which run in the main direction of the trench to be dredged are preferably at least 300 m in length. Their maximum length is preferably approximately 1000 m.
[0018] The invention further provides a seaworthy cutter dredger ship with a draught of at most 1.0 m and main dimensions of approximately 35 x 35 m. This aspect of the invention is based on the understanding that the known method is inefficient, among other things because as a consequence of the deep draught of the known cutter dredgers a very wide channel has to be dredged. With a shallower draught it is possible to dredge a trench which is exactly wide enough for laying a pipe, for example, so that as few m³ as possible are dredged.
[0019] It is noted that dredging in inland waterways with a cutter dredger with a shallow draught, for example 0.5 m, is known per se. These cutter dredgers are, however, not seaworthy and they use a spud.
[0020] Other characteristics of the invention follow from the subclaims. According to a preferred embodiment the invention comprises moreover a dish cutter head, which is attached with the help of a ladder to a vertical bearing girder. The vertical bearing girder extends from a horizontal bridge on the ship, and is vertically and horizontally movable along the horizontal bearing girder. A sliding block is attached to the ladder to allow the dish cutter head to move over the bed. The ladder is connected to the vertical bearing girder rotatably in a horizontal plane to place the dish cutter head in a correct position at the end of the cutting path so as to be able to dredge in the opposite direction.
[0021] According to a preferred embodiment the ship is provided with four retractable screws, so that it can place and lift its anchors itself, and the ship consists of two parts, so that the ship can be transported to inland waterways through locks and can also dredge trenches on, for example, the IJsselmeer Lake.
[0022] Other distinguishing features and advantages of the method and apparatus according to the invention will become apparent in the following figure description.
[0023] Fig. 1A shows a top view of the situation when applying the method and the apparatus near the coast in shallow water, the schematically represented cutter dredging ship being near the coast.
[0024] Fig. 1B shows the situation of fig. 1A, the cutter dredger ship being at the end of the trench.
[0025] Fig. 1C shows a cross section through a dredged trench and the schematically represented cutter dredger ship.
[0026] Fig. 1D shows a cross section approximately halfway through the dredging of the trench and the schematically represented cutter dredger ship.
[0027] Fig. 2 schematically shows a different embodiment of the apparatus, with a runner on both sides of the cutter dredger ship.
[0029] Fig. 3 shows a vertical cross section along the axis of symmetry of a preferred embodiment of the cutter dredger ship, provided with a dish cutter head.
[0032] Fig. 5 shows the use of dredging barges near the cutter dredger ship according to figures 3 and 4.
[0033] Figure 6 shows another preferred embodiment of the cutter dredger ship, provided with a standard cutting head.
[0034] Fig. 7 shows an artist's impression of the cutter dredger ship according to the figures 3 and 4.
[0035] The method will be described on the basis of the figures 1A to 1D, wherein a preferred embodiment of the apparatus is schematically shown.
[0036] Fig. 1A schematically shows a top view of the situation when dredging a trench near the shore. Line A shows the high water mark and line B shows the low water mark. A cutter dredger ship 100 is situated as near as possible to the shore, and is connected with the help of eight cables 11 to 18 to eight anchors 1 to 8. The cables 11 to 18 are wound around winches on the ship 100.
[0037] When carrying out the method according to the invention the ship 100 is first moved seawards, to the right in the figure, by winding up the cables 14 and 18, the cables 11 and 15 having to be unwound. The cables 11, 15 and 14, 18 are held under prestress so that the ship keeps to the course of the trench. The cables 12, 13 and 16, 17 are simultaneously kept under prestress to take up current and wave drift forces. When pulling forward the ship, the cutter head on the ship dredges parallel to the course of the trench, so that the cables 14 and 18 take up the largest part of the reaction forces on the cutting head. The remaining part of the reaction forces, perpendicular to the ship's course, is taken up by the cables 12, 13 and 16, 17.
[0038] The cutter dredging ship is thus moved forward along the whole course of the trench which is to be dredged, until it reaches the position shown in fig. 1B. Here the cutting head is repositioned in order to be able to dredge well in the opposite direction, and the cutting head is moved perpendicular to the course of the trench to be dredged, so as to dredge a new path of earth at the shoreward movement of the ship, to the left in the figure. The ship is moved to the left by winding up the cables 11 and 15 and unwinding the cables 14 and 18. During this the cables 12, 13 and 16, 17 are again held under tension to take up current, wave drift and transverse forces.
[0039] When the ship arrives at the place as shown in fig. 1A, the cutting head is repositioned again and moved perpendicular to the course, after which the above described reciprocal motion is repeated. In this way the upper layer is dredged out of the trench to be dredged, after which the cutting head is moved vertically downwards in order to dredge the next horizontal layer, and so on until the whole trench has been dredged. The speed of the reciprocal motion can be 1 to 1.5 meters per second. All winches are computer-controlled.
[0040] In fig. 1C the situation is shown in which the ship 100 has dredged a trench with a width of 15 m and a depth of 4 m for laying pipeline in it. The trench is shown in cross section. The continuous ground line shows the dredged trench; the dotted lines show the situation after the forming of a slope on both sides of the trench with an inclination of 1:3.
[0041] Fig. 1D shows the situation approximately halfway through the dredging of the trench in cross section. The cutter dredging ship 100 moves in a direction perpendicular to the paper and has already dredged out a few layers from the trench. The right half of the layer which is at present being dredged has already been removed; the cutter head is moved to the left after every reciprocal motion.
[0042] Instead of the apparatus shown in figs. 1A and 1B use can also be made of the apparatus according to fig. 2. The use of the cables 11, 15 and 14, 18 remains the same. However, now one anchor cable 20 goes between the anchors 2 and 3, and also one anchor cable 21 between the anchors 6 and 7. These anchor cables are longer than the distance between their anchors, but can never cross the course of the trench. A runner 22, 23 can move along each anchor cable, which, when the anchor cable is tensioned, describes a part of an ellipse, see figure 2A. Every runner is connected by means of at least one, and in figure 2 with two cables 24, 25 to the cutter dredger ship. By keeping these cables tensioned with the help of winches 26, 27 current, wave drift and transverse forces are taken up.
[0043] When using the apparatus according to figure 2 a wider trench can be dredged in an easy manner by changing the length of the cable(s) to the one runner with regard to the length of the cable(s) to the other runner.
[0044] In both apparatuses described above it is possible to replace the four cables 11, 15 and 14, 18 running in the main direction by two cables running along the course of the trench. The cables 11 and 14 become one continuous cable, just as the cables 15 and 18. Every continuous cable is then wrapped around one or more capstans on the cutter dredger ship, with the help of which the cutter dredger ship is then drawn along the continuous cables.
[0045] The distance shown in figure 1A of 30 m between the cables 14 and 18 and between the cables 11 and 15 applies to a cutter dredger ship of approximately 35 x 35 m. The sizes shown in figure 1B approximately determine the greatest length for the cables. So that the tension variations in the cables as a result of the wave forces do not get too large the cables must be at least 300 m in length.
[0046] The method is very suitable for dredging straight trenches. However, with the apparatus according to figure 1A, B it is also possible to dredge slightly curved trenches by pulling the ship to one side with the help of the cables 12, 13 or 16, 17 during the (recti)linear movement of the shop with the help of the cables 14, 18 or 11, 15. The extent of pulling the ship to one side is determined by the position on the course in which the ship is located. With the apparatus according to figure 2 it is even easier to dredge a curved trench by changing the length of the cables 24, 25.
[0047] The figures 3 and 4 show a preferred embodiment of the cutter dredger ship in a vertical cross section through the axis of symmetry of the ship and in top view, in which only one half of the ship is shown, respectively. The cutter dredger ship 100 comprises a stern 106 with two legs 101 to it, which are connected to each other by a horizontal bridge 110. Eight winches 103 are arranged on the ship, four of which are shown, for winding and unwinding the eight cables 11 to 18, of which the cables 11 to 14 are shown (see figure 4). On the stern there is a pilot house 105 with a control room for the apparatus. In the four corners of the ship four retractable screws (Schottels) are arranged. The cutter dredger ship is 35 x 35 m in size, has a draught of 1 meter and a freeboard of 3 meters.
[0048] A vertical bearing girder 111 extends through the bridge 110, which girder is vertically movable with regard to and horizontally along the horizontal bridge 110. At the bottom of the vertical bearing girder 111 a ladder 113 is attached, which rests on the ground with the help of a sliding block 114 and can slide over it. A disc cutter head 115 is attached to the ladder. The ladder 113 is attached with the help of a bearing 117 vertically rotatably to the vertical bearing girder 111, and a hydraulic apparatus 116 is placed between the vertical bearing girder 111 and the ladder 113 which can serve as swell compensation apparatus, so that in the case of slight vertical movements of the ship the sliding block 114 will always rest on the bed.
[0049] Above a certain load the vertical bearing girder 111 can cant about the horizontal bridge in a direction perpendicular to the centre line of the ship, so that unexpected large transverse forces on the cutter head will not damage the cutter head.
[0050] Fig. 3A shows a cross section through the ladder of fig. 3. A dredge discharge pipe 119 goes through the ladder from the dish cutter head 115 to a pump 118, which pump sucks up the dredge which has been dredged away by the cutter head 115 and discharges it through the dredge pipe 112. The dredge can then be dumped at the side of the ship or removed with the help of barges.
[0051] In fig. 3A it can be seen that the housing 121 in which the pump 118 is arranged and to which the ladder 113 is attached, is attached via a bearing 120 to the vertical bearing girder 111. By means of this bearing 120 the cutter head can easily and quickly be repositioned at the end of each path before cutting a new path when moving the ship in the opposite direction.
[0052] The vertical bearing girder is movable at least 15 m along the horizontal bridge so that the entire width of the trench can be dredged without it being necessary to move the anchors.
[0053] Neither is it necessary to move the anchors for dredging a wider trench, seeing as it is possible to move the centre line of the ship by continuously "adjusting" the cable lengths.
[0054] The vertical bearing girder is movable in a vertical direction to dredge the trench still further, but also to be able to lift the housing 121 to above the water line. A hydraulic apparatus, for example the hydraulic apparatus 116 can then swivel the ladder 113 up, after which, if necessary after rotation of the housing 121 with the help of the bearing 120, the dish cutter head 115 can be reached from the deck, for example for removing dirt from the dish or replacing the dish. This is a big advantage compared to the traditional suction cutters with a cutter head on a ladder, in which after lifting the ladder it had indeed to be climbed to reach the cutter head, which was a dangerous and time-consuming task especially in rough weather.
[0055] The cutter dredger ship is provided with four retractable screws at the corners of the ship, so that the ship can place its anchors itself and hiring a tug or the like for that purpose is not necessary. With this apparatus it is also possible to draw a pipeline into the dredged trench by attaching a 600 ton winch plus a draw-hook to the ship. Thus no time is lost between dredging and drawing the pipeline into the trench. A crane can be placed at the stern of the ship so as to be able to erect dam walls as near as possible to the coast.
[0056] Advantageously use is made in the apparatus of a dish cutter head, seeing as tests to scale have shown that a dish cutter head has demonstrably less cutting loss than a traditional cutter head. This is especially an advantage when dredging polluted soil which has to be removed and of which as little as possible should end up in the surroundings.
[0057] Figure 5 shows that dredge barges 50 can be coupled at the front and back of the dredger for removing (polluted) silt. Because of the exchangeable dredging barges the cutter dredger ship can continue to dredge without interruption.
[0058] It is, however, also possible to use a traditional cutter head, such as shown in fig. 6, the cutter head 130 being attached to a ladder 131, which is attached to the horizontal bridge in the middle between the legs of the ship.
[0059] The method is serviceable when dredging trenches in the sea bed and in, for example, lake and river beds. And although the cutter dredger ship is designed for dredging a trench near the coast, it can also, due to its shallow draught (maximum of 1.0 m), be used in inland waters. Since the ship of 35 x 35 m is too large to fit into a lock, it is divided into two, in a front part consisting of the two legs 101 which are connected by the horizontal bridge 110, and a rear part 106 on which the pilot house stands. Both parts have a maximum width of 17.5 m. It is also possible with this apparatus to dredge a trench in the bed of the IJsselmeer lake, for example. If dredging takes place in very soft ground, it is even not necessary to make use of anchored cables, but the ship can be moved with the help of the four screws alone. It is then also possible to dredge in the traditional manner with the help of this apparatus.
[0060] Because of its shallow draught the cutter dredger ship can ride out a storm at sea after raising the cutter head, by lying as far out of the coast as possible and slackening the cables somewhat. The anchors do not have to be raised, so that dredging can go on for a long time and work can continue directly after the storm.
[0061] With the help of the method and the apparatus according to the invention a trench of 1000 m long, 15 m wide and 4 m deep can now be dredged in a few days, whereas with the help of the traditional method and a traditional cutter dredger several weeks are needed.
[0062] Obviously it is also possible to dredge longer trenches. Then the anchors have to be moved every 1000 metres, which the dredger can do by it's own effort.
[0063] In the inland waters, too, the method and the apparatus according to the invention are advantageous, seeing as it is no longer necessary to always have to drive a spud.
[0064] The apparatus according to the present invention is not limited to the embodiment described above. It will be clear that it is possible to give the cutter dredger ship other dimensions or another draught, another drive or another swell compensation with going beyond the range of protection of the following claims.
1. Method for dredging a trench in a bed with the help of a cutter dredger with a cutter
head, the cutter head being moved in a reciprocating motion parallel to the course
of the trench to be dredged during the dredging, wherein the cutter head is moved
perpendicular to the course after every motion.
2. Method according to claim 1, wherein the cutter head is reciprocated with the help
of at least four pre-tensioned cables, which are anchored substantially in the main
direction of the trench to be dredged and which are wound and unwound with the help
of winches on the cutter dredger.
3. Method according to claim 1, wherein the cutter head is reciprocated with the help
of at least two pre-tensioned cables, which run substantially along the trench to
be dredged and are anchored near the far ends of the trench to be dredged, and along
which the cutter dredger is moved with the help of capstans.
4. Method according to claim 2 or 3, wherein at least two pre-tensioned cables are connected
in a transverse direction with the cutter dredger for taking up transverse forces.
5. Apparatus for implementing the method according to claim 1 or 2, in which the apparatus
comprises a cutter dredger ship with a cutter head and at least four winches, and
at least four pre-tensioned cables which run substantially in the main direction of
the trench, which cables are windable and unwindable with the help of the winches.
6. Apparatus for implementing the method according to claim 1 or 3, in which the apparatus
comprises a cutter dredger ship with a cutter head and at least two capstans, and
at least two pre-tensioned cables which run substantially along the trench, along
which cables the cutter dredger ship is movable with the help of the capstans.
7. Apparatus according to claim 5 or 6, in which the apparatus comprises four winches
on the cutter dredger ship, and four cables for these winches, these cables being
attached to the angular points of an imaginary rectangle, the cables which run substantially
in the main direction of the trench forming the short axial line of the rectangle,
and the long sides of the rectangle being at least approximately 10% longer than the
short sides.
8. Apparatus according to claim 5 or 6, in which the apparatus comprises a runner on
both sides of the cutter dredger ship, in which each runner is movable along an anchor
cable, which anchor cables are each attached to one side of the trench in the angular
points of an imaginary rectangle, with the cables running substantially in the main
direction of the trench as axial line, and in which each runner is connected to the
cutter dredger ship with the help of at least one cable, which cable can be pretensioned
with the help of a winch on the cutter dredger ship.
9. Apparatus according to any one of the claims 5 to 8, in which the length of the cables
which run in the main direction of the trench to be dredged is at least 300 m.
10. Apparatus according to any one of the claims 5 to 9, in which the length of the cables
which run in the main direction of the trench to be dredged is at most approximately
1000 m.
11. Apparatus according to any one of the claims 5 to 10, in which the apparatus comprises
a seaworthy cutter dredger ship with a draught of at most 1.0 m and main dimensions
of approximately 35 x 35 m.
12. Apparatus according to claim 11, in which the ship has substantially the shape of
a U, in which the legs of the U are connected by a horizontal bridge extending above
the water line.
13. Apparatus according to claim 12, in which the distance between the legs of the U is
at least 16 m.
14. Apparatus according to claim 12 or 13, in which a substantially vertically extending
bearing girder projects from the horizontal bridge into the water, in which the cutter
head is attached to the vertical bearing girder.
15. Apparatus according to claim 14, in which the cutter head is a dish cutter head, which
is attached to the vertical bearing girder with the help of a ladder.
16. Apparatus according to claim 15, in which the ladder is rotatable in a horizontal
plane about the vertical bearing girder.
17. Apparatus according to claim 15 or 16, in which the dish cutter head is movable over
the bed with the help of a sliding block.
18. Apparatus according to claim 17, in which means for compensating swell are arranged.
19. Apparatus according to claim 18, in which the means for compensating swell comprise
a hydraulic apparatus between the vertical bearing girder and the ladder.
20. Apparatus according to any one of the claims 14 to 19, in which the vertical bearing
girder can swivel about an axis, parallel to the axis of symmetry of the dredger.
21. Apparatus according to any one of the claims 14 to 20, in which the vertical bearing
girder is vertically movable with regard to the horizontal bridge.
22. Apparatus according to any one of the claims 15 to 21, in which an apparatus is arranged
between the vertical bearing girder and the ladder for swivelling up the ladder with
regard to the vertical bearing girder.
23. Apparatus according to any one of the claims 14 to 22, in which the vertical bearing
girder is horizontally movably along the horizontal bridge.
24. Apparatus according to any one of the claims 5 to 13, in which a conventional cutter
head is attached to the cutter dredger ship with the help of a ladder.
25. Apparatus according to claim 13, in which a ladder with a conventional cutter head
is attached to the horizontal bridge swivellably in a horizontal and vertical direction
in the middle between the legs of the U.
26. Apparatus according to any one of the claims 5 to 25, in which the cutter dredger
ship is provided with it's own drive.
27. Apparatus according to claim 26, in which the cutter dredger ship is provided with
four drivable screws, one at each corner of the ship.
28. Apparatus according to any one of the claims 11 to 27, in which the ship consists
of two substantially equally large parts with a maximum width of 17.5 m, which, in
the case of a ship with substantially the shape of a U, of which the legs are connected
by a horizontal bridge extending above the water line, consist of a front part comprising
the legs of the U with the horizontal bridge and a rear part comprising the rest of
the ship, both parts being disconnectable.