Login| Sign Up| Help| Contact|

Patent Searching and Data


Title:
A DREDGE VESSEL SYSTEM FOR DEEP-SEA DREDGING
Document Type and Number:
WIPO Patent Application WO/2011/003438
Kind Code:
A1
Abstract:
The application is directed to a system for recovering immerged sediments (3) from a concession and discharging them. From a free sailing extraction ship (1) a mixture of water and sediment is sucked by a submerged pump unit through a flexible hose (7) adapted to be rolled on a rotative reel (5) disposed on said extraction ship, while the ship is sailing over the concession. The sucked mixture is then immediately delivered to one of a series of free sailing carrier and transport units (27) that sails alongside.

Inventors:
WINKELMAN, Marius Otto (Edisonstraat 32, Nijkerk, NL-3861, NL)
RATERING, Hendrik (Edisonstraat 32, Nijkerk, NL-3861, NL)
Application Number:
EP2009/058526
Publication Date:
January 13, 2011
Filing Date:
July 06, 2009
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
DAMEN DREDGING EQUIPMENT B.V. (Edisonstraat 32, NIJKERK gld, NL-3861, NL)
WINKELMAN, Marius Otto (Edisonstraat 32, Nijkerk, NL-3861, NL)
RATERING, Hendrik (Edisonstraat 32, Nijkerk, NL-3861, NL)
International Classes:
E02F3/88; E02F3/90; E02F3/92; E02F7/02; E02F9/06
Domestic Patent References:
WO1995021303A1
Foreign References:
US4373276A
FR2919015A1
US0413091A
GB1422663A
JPS5847835A
US20030000114A1
US3693272A
JPH0539623A
DE2930745A1
US4552082A
Attorney, Agent or Firm:
PICHAT, Thierry et al. (122 rue Edouard Vaillant, Levallois-Perret Cedex, Levallois-Perret Cedex, F-92593, FR)
Download PDF:
Claims:
CLAIMS

1. A floating dredge vessel (1) dedicated to extract sediments (3) from water (3a) and discharge said sediments (3) outboard to have them transported to a determined destination, the dredge vessel (1) having:

- a reel (5) ,

- a flexible suction hose (7) adapted to be rolled onto said reel (5) and unrolled under water to discharge upwardly said sediments (3) ,

- hoisting means (9) for immerging and elevating the flexible suction hose (7),

- a dredging head (11) disposed at a first end (7a) of the flexible suction hose (7), for dredging the sediments (3) ,

pumping means (13) connected to the flexible suction hose (7) for sucking therethrough a mixture of sediments (3) and water (3a) ,

- a discharging unit (15) having a delivering head (15b) adapted for discharging the sucked mixture outside the dredge vessel (1),

- a delivering pipe unit (17) disposed, on said dredge vessel (1), between the flexible suction hose (7) and the discharging unit (15) for delivering the sucked mixture from the flexible suction hose (7) to the discharging unit (15). 2. The dredge vessel (1) of Claim 1, further comprising towing means (21, 21a) for towing the dredging head (11) underwater, from a zone of the vessel (1 located forwardly with respect to the hoisting means (9)

3. The dredge vessel (1) of Claim 1, further comprising towing means (21, 21a) comprising cable means

(21a) having a variable length between the vessel (1) and said dredging head (11), for an underwater steering of the dredging head (11) while said vessel (1) is sailing. 4. The dredge vessel (1) of any preceding Claim, wherein :

- it further has a deck (10) and a hull, and,

- the hoisting means (9) and reel (5) are disposed on said deck so that the flexible suction hose (7), together with the dredging head (11) are adapted to be moved between :

• a first non operative position, above the deck, on a lateral side (10a) thereof, close to a first lateral side (100a) of the hull, and, • a second operative position, within the water, by being hoisted laterally and downwardly along said first lateral side (100a) of the hull.

5. The dredge vessel (1) of Claim 4, wherein the reel (5) has an axis of rotation (5a) around which it rotates for rolling said flexible suction hose (7), and said axis of rotation is substantially parallel to a roll axis (Ia) of the vessel. 6. The dredge vessel (1) of any preceding Claim, wherein the discharging unit (15) comprises a tube (15a) connected at a first end (150b) to the delivering pipe unit (17) and, at a second end (151b), to said delivering head (15b) which extends transversally to the tube (15a) and is adapted to be placed outside the dredge vessel (D•

7. The dredge vessel (1) of any preceding Claim, wherein :

- it further has a deck (10), and

- the discharging unit (15) is adapted to rotate relative to said dredge vessel (1), for having its discharging head (15b):

* stored above said deck (10), in a stored zone,

* and deployed outside the dredge vessel (1), in a deployed zone, when the sucked mixture is to be discharged outboard.

8. The dredge vessel (1) of any preceding Claim, wherein the pumping means (13) comprises a dredge pump

(13a) disposed near the dredging head (11), so that said dredge pump (13) is immerged when the dredging head (11) is dredging the sediments (3) .

9. The dredge vessel (1) of any preceding Claim, further having inboard propulsion motor means (25) for moving the vessel (1) which is a free sailing ship and said inboard propulsion motor means (25) are adapted for positioning said dredge vessel (1) while the sediments

(3) are dredged and the mixture sucked, so that the dredge vessel (1) is free of any riverbed attaching means, seabed attaching means, riverbed anchoring means and seabed anchoring means adapted for temporary maintaining said dredge vessel (1) when said sediments (3) are dredged and mixture sucked, so that the vessel (1) which is a free sailing ship.

10. The dredge vessel (1) of any preceding Claim, wherein the discharging unit (15) is disposed on the dredge vessel (1) for rotating thereon about different axes including a first and a second horizontal axis (23a, 23b) for:

- a rotation between a stored zone over a deck and a deployed zone adapted for delivering the mixture outside the dredge vessel (1) above a sediment (3) loading compartment (29) located on the carrier and transport unit (27) and which is open upwardly, and,

up and down movement in accordance with respective floating levels of the dredge vessel (1) and the carrier and transport unit (27) .

11. An assembly comprising:

- the dredge vessel (1) of any preceding Claim, and,

a floating carrier and transport unit (27) adapted to be disposed alongside the dredge vessel (1) and having a loading compartment (29) above which the discharging unit head (15b) can be disposed, outside the dredge vessel (1), for discharging the sucked mixture in said loading compartment (29).

12. The assembly of Claim 11, wherein the floating carrier and transport unit (27) is a motorised barge.

13. The assembly of Claim 11 or 12, further comprising a sailing positioning system for controlling the respective alongside positions of the dredge vessel

(1) and the floating carrier and transport unit (27), one with respect to the other, when mixture is to be discharged from the dredge vessel (1) to the floating carrier and transport unit (27) .

14. The assembly of Claim 11 to 13, wherein the sailing positioning system comprises an electronic positioning system (33, 34) for dynamically controlling the respective alongside positions of the dredge vessel

(1) and the floating carrier and transport unit (27), one with respect to the other, when mixture is to be discharged from the dredge vessel (1) to the floating carrier and transport unit (27).

15. The assembly of any Claim 11 to 14, wherein the floating carrier and transport unit (27) has a hull (26) and a deck level (28) on which said loading compartment (29) opens upwardly and is adapted to freely communicate with outside, so that water of the discharged sucked mixture progressively overflows outboard as said mixture is discharged in said loading compartment (29) .

16. The assembly of any Claim 11 to 15, wherein the floating carrier and transport unit (27) has a hull

(26) and a deck level (28) on which said loading compartment (29) opens upwardly and is free of any cover means adapted for covering said loading compartment (29). 17. A dredging method (1) for recovering immerged sediments (3) from a concession and discharging them, the method comprising: - from a free sailing extraction ship (1), sucking by a submerged pump unit (13) a mixture of water and sediment (3) through a flexible hose (7) adapted to be rolled on a rotative reel (5) disposed on said extraction ship (1), while the ship is sailing over the concession, immediately delivering the sucked mixture from the extraction ship to one of a series of free sailing carrier and transport units (27) that sails alongside. 18. The method of Claim 17, further comprising the steps of:

sailing a plurality of empty carrier and transport units (27) of said series in the vicinity of the extraction ship (1),

- when said one free sailing carrier and transport unit (27) that sails alongside the extraction ship is full of sucked sediments (3) , substituting one of said empty carrier and transport units (27) for the full one,

- and then controlling the respective speed and course of the dredge vessel (1) and the floating carrier and transport unit (27), one with respect to the other, with no mooring means attaching them together while they are sailing alongside and while mixture is discharged from the dredge vessel (1) to the floating carrier and transport unit (27) .

Description:
A DREDGE VESSEL SYSTEM FOR DEEP-SEA DREDGING

The invention refers to a floating dredge vessel dedicated to extract sediments from water.

Using floating vessels for dredging and recovering sediments from water is already known.

Further, dredgers have encountered problems, including difficulties regarding depth. For example, in operations where the water is not a quiet pond, particularly with offshore dredgers operating in the ocean, swells and waves cannot be avoided. The result has been that dredges could operate offshore only during times when the water was very quiet, and they had to remain idle much of the time.

For taking into consideration such drawbacks, GB 1359 828 discloses a floating deep-digging dredge ship having a hull, a dredging head for dredging materials and an articulated dredge ladder pivotally supported by the hull and comprising a plurality of sections pivoted together for relative swinging movement in a vertical plane .

The vessel also comprises:

- hoisting means for immerging and elevating the dredging head,

- pumping means comprising a suction pump connected to a pipeline having an intake connected to or disposed adjacent to underwater excavating means.

Using an articulated dredge ladder makes deep digging-down to five hundred feet (substantially 150 m) and more below water level-quite practical and economical. GB 1359 828 allows the large cross-sectional ladder girder thicknesses and the excessive weights that would be required by prior art structures to be avoided and made unnecessary by using articulated structures.

However having a fast deployment and retrieval of the dredging head remains difficult, especially under high swells and waves and/or very deep dredging-down . And a ladder made of said plurality of sections requests the ship to hold a large number of such sections, what requires space on the ship, together with time and staff.

The invention intends to overcome at least some of such drawbacks .

More globally, the invention intends to speed up the recovery of the dredged materials, all the more when the vessel has to dredge on an open sea, far away from any harbour where the dredged materials can be delivered ashore.

Speeding up the extraction, on one side, and the delivery of the dredged materials, on another side, is further taken into consideration, in the invention.

For reaching at least some of the above-cited issues, the floating dredge vessel the invention, dedicated to extract sediments from water and discharge said sediments outboard to have them transported to a determined destination, is adapted to be provided with:

- a reel,

- a flexible suction hose adapted to be rolled onto said reel and unrolled therefrom for being immersed under water so as to deliver upwardly the sediments collected by a dredging head,

- hoisting means for immersing and elevating the flexible suction hose,

- the dredging head disposed at a first end of the flexible suction hose, for dredging the sediments, - pumping means connected to the flexible suction hose for sucking therethrough a mixture of sediments and water,

a discharging unit having a delivering head adapted for discharging the sucked mixture outside the dredge vessel,

- a delivering pipe unit disposed, on said dredge vessel, between the flexible suction hose and the discharging unit for delivering the sucked mixture from the flexible suction hose to the discharging unit.

Is furthermore within the scope of the invention an assembly comprising:

- the above-described dredge vessel, and,

- a floating carrier and transport unit adapted to be disposed alongside the dredge vessel and having a loading compartment above which the discharging unit head can be disposed, outside the dredge vessel, for discharging the sucked mixture in said loading compartment .

When the dredge vessel is provided with a loading compartment for stowing the extracted materials, as long as the dredge vessel is sailing, the dredging equipment is dead weight and no material is dredged.

The above-cited solution overcomes this drawback. Is also covered by the invention a dredging method for recovering immerged sediments from a concession and discharging them.

With a view to fulfilling several of the above- cited expectations, such a method comprises, according to the invention:

- from a free sailing extraction ship, sucking by a submerged pump unit a mixture of water and sediment through a flexible hose adapted to be rolled on a rotative reel disposed on said extraction ship, while the ship is sailing over the concession,

immediately delivering the sucked mixture from the extraction ship to one of a series of free sailing carrier and transport units that sails alongside.

Details of the respective issues of the invention are given below, together with additional features and benefits, with reference to the attached diagrammatical illustrations in which, with no limitation:

Figure 1 shows a preferred embodiment of a dredging vessel, viewed from above, together with a symbolic illustration of a loading unit.

- Figure 2 shows said dredging vessel viewed from one lateral side (arrow II),

Figure 3 shows some elements of the dredging vessel (discharging unit essentially) viewed from above,

Figure 4 shows an example of transhipment between the dredging vessel and the so-called floating carrier and transport unit,

- and figure 5 is the same view as figure 2, at a reduced scale.

In figures 1, 2 is illustrated the best mode of a floating dredge vessel 1 dedicated to extract sediments 3, such as sand, from water 3a and discharge said sediments outboard to have them transported to a determined destination.

The vessel 1 has a hull Ia covered by a (rear) deck 10 on which is elevated a frontward bridge deck 12.

On the deck 10 of the dredge vessel 1 are provided the following means: a rotative reel 5 on which a flexible suction hose 7 is adapted to be rolled, hoisting means 9 for immerging and elevating said suction hose, a dredging head 11 disposed at a first end 7a of the flexible suction hose 7, for dredging the sediments 3, pumping means 13 connected to the suction hose, for sucking therethrough a mixture of sediments and water, a discharging unit 15 having a series of delivering heads 15b adapted for discharging the sucked mixture (sediments and water) outside the dredge vessel, and a delivering pipe unit 17 disposed, on said dredge vessel, between the suction hose 7 and the discharging unit 15, for delivering the sucked mixture from the suction hose to the discharging unit.

The reel 5 is rotative around a horizontal axis. From said reel the suction hose 7 can be unrolled under water to discharge upwardly the sediments, when the pumping means 13 are operative.

The suction hose 7 is preferably a tube made of flexible elastic material, possibly mechanically reinforced.

Since the dredging can be operated especially in open sea and the hose 7 in woundable around the reel 5, the length of said hose is higher than the length of the vessel .

For towing the dredging head 11 underwater, from a zone of the vessel located forwardly (FW) with respect to the hoisting means 9 (located at a rearward -RW- location on the deck 10), the dredge vessel further comprises towing means 21. Figure 2 shows that the towing means 21/21a will advantageously extend between a front part 10' a of the hull Ia and the dredging head 11.

A valuable solution can be found by providing the vessel 1 with a single-hulled vessel. And the hoisting means 9 and reel 5 will be disposed on said deck so that the flexible suction hose 7, together with the dredging head 11 will be adapted to be moved between:

• a first non operative position, above the deck, on a lateral side 10a thereof, close to a first lateral side 100a of the hull, and,

• a second operative position, within the water, by being hoisted laterally and downwardly along said first lateral side 100a of the hull.

First lateral side 100a can be starboard or portside .

With a view to making easier the movement of the assembly flexible suction hose/dredging head, the axis 5a of the reel 5 around which it rotates for rolling said flexible suction hose will preferably be substantially parallel to a roll axis Ia of the vessel.

For the global stability of the vessel disposing axis 5a along said roll axis Ia should be a valuable solution (see figure 1) .

As also illustrated figure 2, the towing means preferably comprise cable means 21a having a variable length between the vessel (hull Ia) and the dredging head

11, for an underwater steering of the dredging head while said vessel is sailing.

A role, or drum, 22a is coupled with motor (winch) means 22b for allowing the length of the towing means to be adapted as a function of the water depth, referenced L on figure 2.

If, as advised and illustrated, the assembly 7/11 is hoistable along the lateral side 100a of the hull, drum 22a will preferably be disposed close to said lateral side 10a/100a, frontward. Motor means 22b will preferably be operated from the bridge deck 12 located close to the prow .

A location of the drum 22a on a fore-deck 100 disposed at a higher level than the lower rear deck 10 will be advantageous: Should any problem occur, it would be faster and safer to intervene, all the more since adapting the length of the towing means can be dramatically important.

Fore-deck 100 is located close to the bridge deck 12 and frontwardly therefrom.

As illustrated figure 1, the delivering pipe unit

17 provided on the deck 10 advantageously comprises rigid pipes 17a fixedly attached to the dredge vessel deck, over it, for transferring the sucked mixture from the flexible hose 7 to the rigid discharging unit 15.

For improving the speed and efficiency of the dredging operation, figures 3, 4 show that the discharging unit 15 preferably comprises a rigid tube 15a connected at a first end 170a to the delivering pipe unit

17 and, at a second end 151b, to the delivering head 15b.

As illustrated, said delivering head 15b extends transversally to the tube 15a and is adapted to be placed outside the dredge vessel.

In said illustrations, the discharging tubes 15a and delivering heads 15b are transverse and said heads extend parallel to the longitudinal axis of both the vessel and the adjacent barge (see figure 1) so that a corresponding elongated passage is offered for the sediments.

As shown figure 1, a series of parallel discharging tubes/heads 15a/15b will be provided along the second lateral side 100b of the hull Ia (see also figure 4) .

From the above description and the figures, it can be understood that the flexible hose 7, together with the rigid dredging head 11 and the pump 13 are adapted to be lowered and elevated over one of the vessel sides

(starboard or portside) , 100a in the illustrations.

To that aim, the hoisting means 9 provided with cables 26 and preferably connected to the dredging head 11 are disposed on lateral side 10a of the deck 10.

For avoiding the delivering head to be excessively responsive to the swells and waves, on the open sea, the discharging unit 15 is adapted to rotate relative to said dredge vessel, for having its discharging head:

- stored above the deck 10, in the stored zone SZ; figure 3,

- and deployed outside the dredge vessel, in a deployed zone (DZ figure 3), when the sucked mixture is to be discharged outboard.

Specifically, it is preferred that said discharging unit be adapted to rotate relative to the dredge vessel deck about a first and a second horizontal axis, respectively 23a, 23b, for rotating between said stored and deployed positions, and for an up and down movement of its/each delivering head 15b.

Such a solution is efficient for limiting the space reserved for said discharging unit, on the deck.

Especially, it allows the discharging unit to preferably be stored in an upright position, in the non-operative state (see figures 1,3 dark continuous lines).

Each tube 15a will advantageously comprise at least a first and a second elongated (tube) portions, respectively 153a, 153b, articulated at an intermediate zone of the tube, around the horizontal axis 23b. Further, the first elongated portion 153a will preferably be articulated at a lower end 155 where it is connected to the deck 10 (horizontal axis 23a, figure 1) .

As a consequence, it can be understood from the above and figures 1-3 that, in the non-operative state, said first and second elongated portions 153a, 153b are folded substantially vertically, with axis 23b upward and backward, and discharging head 15b downward (and backward too) , whereas, in the operative state (dotted lines figures 1,3) the so-called first and second elongated portions are deployed (unfolded) frontward (more or less horizontally) , with horizontal axis 23a backward and discharging head 15b frontward, above the barge 27.

With a view to improving the sucking the pumping means 13 comprises a dredge pump 13a disposed near the dredging head 11, so that said dredge pump is immerged when the dredging head is dredging the sediments (see figure 2) . The efficiency is improved compared to a pump located on the deck or in the hull. Further weight and space binds are reduced.

For allowing the vessel to be precisely positioned on the dredging zone, the dredge vessel 1 will preferably be provided with one or more inboard propulsion motor means 25 controlled from the bridge deck 12, for moving the vessel.

More precisely, said inboard propulsion motor means will advantageously be adapted for positioning the dredge vessel while the sediments 3 are dredged from the dredging zone 30 (such as the sea bottom) and the mixture sucked. Consequently, the dredge vessel will be free of any riverbed attaching means, seabed attaching means, riverbed anchoring means and seabed anchoring means (such as traditional anchors) which are adapted for temporary maintaining a dredge vessel when sediments are dredged and mixture sucked.

As a consequence, the vessel will then be a "free sailing" ship.

Figure 4, a floating carrier and transport unit 27 is adapted to be disposed alongside the dredge vessel 1 (see figure 1, too) .

As above-mentioned and illustrated figures 1, 4, such a unit 27 will be situated along said second lateral side 100b of the hull, in front of (below) the discharging unit 15 extended to its deployed operative position .

Said floating carrier and transport unit 27 is a floating unit which is adapted to both transport and carry the sediments. It has a hull 26, a deck level 28 and a loading compartment 29 above which the discharging unit head 15b can be disposed, outside the dredge vessel, for discharging the sucked mixture in said loading compartment .

The loading compartment 29 is open upwardly.

Further, the following system(s) improve (s) the adaptability of the solution of the invention to deliver the mixture outside the dredge vessel 1, above said sediment loading compartment 29.

All the more if vessel 1 is a free sailing ship, it is preferred that unit 27 be a motorised barge. Said barge will be a tug boat, or a self propelled barge. Notably for sake of security with transhipment of the sucked mixture from vessel 1 to unit 27 when swells and/or waves are high, such a solution will allow the assembly of said vessel and unit to sail substantially side by side (side 100a of vessel 1), with no mooring means for removably attaching together the dredge vessel and the floating carrier and transport unit, when said mixture is to be discharged from the dredge vessel to unit 27.

Specifically, a follow-me positioning system will advantageously control the respective alongside positions of the dredge vessel and the floating carrier and transport unit, one with respect to the other.

To that aim, speed and course controls of both the extraction ship and the carrier and transport unit can be synchronised, through the respective motor means 25, 272 connected to the rudders 250, 274 of vessel 1 and unit 27, respectively (see figure 1) . Under the control of the respective tiller man/men, such means will maintain both vessels 1, 27 within a predetermined distance range (laterally and longitudinally) during the dredging and/or transhipment operations.

Alternatively, an electronic positioning system 33 could be installed on the vessel 1, for dynamically controlling the respective alongside positions of said vessel and the floating carrier and transport unit 27, one with respect to the other, when the above-cited mixture is to be discharged from the vessel to unit 27. The positioning system 33 and motor means 25 would then be connected together. Preferably unit 27 will also be provided with a coordinated electronic positioning means 34 wireless connected to the positioning system 33 an adapted to control motor 272 and rudder 274.

During transhipment, the discharging unit head 15b will be located in the so-called deployed zone DZ (figure 1) and will advantageously have its horizontal and/or vertical position (s) adapted as a function of the degree of sediment loading of unit 27 together with the respective alongside positions of the vessel 1 and the floating carrier and transport unit 27, one with respect to the other.

As a consequence, the up/down and/or the frontward/backward movement (s) of the discharging unit head 15b will be controlled in accordance with the respective floating levels of said dredge vessel and carrier and transport unit, through the assistance of the motor means 25-272 and/or electronic positioning system 33.

For improving the speed and the efficiency of charging on unit 27 the sucked mixture, it is preferred that the loading compartment 29 opens upwardly on the deck level 28 and be adapted to freely communicate with outside, so that water of the discharged sucked mixture will progressively overflow outboard as said mixture is progressively charged in the loading compartment 29.

Advantageously, the floating carrier and transport unit 27 will even be free of any cover means adapted for covering loading compartment 29 (see figure 4) .

Furthermore, for speeding the discharging operation of the sucked sediments when unit 27 is full of materials (sediments 3) and water has escaped from the mixture delivered in said compartment 29, the floating carrier and transport unit 27 could be a split-barge adapted to be loaded from an open top and discharged through the bottom of its hull (see zone 32 figure 4 where movable flaps can be disposed in the bottom of compartment 29, as a part of the hull) .

As it can be understood from the above-described system, the invention allows the following dredging method to be operated with a view to recovering the immerged sediments 3 from the underwater concession 30 and discharging said sediments.

Accordingly, it is recommended that:

* from the free sailing extraction ship 1, the so- called mixture of water and sediment 3/3a be sucked by the pump unit 13 - preferably the submerged pump unit 13a - through the suction flexible hose 7, while the ship 1 is sailing over the concession,

* and said sucked mixture be immediately delivered from the extraction ship to one of a series of free sailing carrier and transport units 27, 27a... that sails alongside .

Preferably:

* during the present operations, said plurality of empty carrier and transport units of the series will sail in the vicinity of the extraction ship 1 (see figure 4),

* and, when said one free sailing carrier and transport unit, such as 27, that sails alongside the extraction ship is full of sucked sediments, one empty carrier and transport unit, such as 27a, will be substituted for the full one, as soon as possible.

As a consequence, it is recommended that the above-cited step of sucking the mixture of water and sediment 3/3a, together with the step of delivering the sucked mixture - through the discharging/delivering unit 15/15b - be conducted simultaneously.

In the same way, it is recommended that:

* speed and course controls of both the extraction ship and the carrier and transport unit be synchronised,

* and/or the step of dynamically positioning both the extraction ship and the carrier and transport unit 27 alongside each other be operated as long as the dredging and transhipment operations take place.

Coordination is operated by electronic controlling means 39 coupled with the motor means 25 and/or the electronic positioning system 33-34.

Should vessel 1 be a non self propelled vessel, it would be pushed or pulled by a push or pull tug (non- preferred embodiment) .

Even if the dredging vessel is especially adapted to dredge in open sea, it could also be used in any water, including rivers, lakes....