[0001] The invention relates to a method to be used in dredging, wherein material present
on the seabed is dredged by means of a dredging unit, which is suspended from a vessel.
[0002] When waterways are dredged to their desired depth, use is frequently made of draghead
suction installations suspended from vessels, whereby a dredging unit suspended from
the vessel is moved across the bottom by means of the vessel and material present
on the bottom is sucked up and deposited into the vessel in question or into a vessel
which is present nearby.
[0003] The working depth is thereby limited to ± 100 m below the water surface.
[0004] Special water craft capable of moving across underwater surfaces have been developed
for operating at great depths. Such craft can only be used, however, if the underwater
surface extends in a regular, more or less horizontal manner and the underwater surface
possesses sufficient bearing capacity to support the craft. Another drawback of using
said craft is the fact that the material can only be transported over a relatively
small distance from the craft.
[0005] Furthermore a jet pump suspended from a vessel is sometimes used for dredging at
great depth. Also when such a jet pump is used the material can only be transported
over a comparatively small distance from the place where the material is dredged,
whilst it is hardly possible to check the direction in which the dredged material
is transported.
[0006] Exploiters of offshore gas and crude oil wells feel an increasing need to be able
to place installations on a solid foundation, also at great depth. An example of this
is the laying of a pipeline, where the pipeline must frequently be laid on an underwater
bed of rubble or the like. When the bottom layer is soft, it is necessary to remove
said soft bottom layer first before depositing the rubble layer, in order to ensure
a stable position of the rubble layer.
[0007] According to the invention an effective transport of material can be achieved also
at great depth in that material dredged by means of the dredging unit is supplied
to a discharge pipe suspended under water from a vessel and coupled to said dredging
unit, via which discharge pipe the material is deposited on the bottom of the sea
at some distance from the dredging place.
[0008] By using the method according to the invention it is possible to dredge material
at a desired location, also at great depth, and then deposit said material at a further
desired location.
[0009] In practice it has been established that it is also possible thereby to carry out
dredging work in an accurate manner at great depths of more than 350 metres.
[0010] Another aspect of the invention relates to a device provided with a dredging unit,
said device comprising a frame which is provided with propulsion means, by which the
device can be moved in any desired direction.
[0011] A dredging unit of this type may be suspended from the vessel, and by using the frame
provided with propulsion means it is possible to effect an accurate positioning of
the dredging unit during operation and thus an accurate execution of the intended
dredging work.
[0012] The invention will be explained in more detail below with reference to the accompanying
Figures.
[0013] Figure 1 is a schematic view of a vessel from which a dredging unit as well as a
discharge pipe are suspended.
[0014] Figure 2 is a larger-scale schematic view of a dredging unit and a frame supporting
said dredging unit, which frame is provided with propulsion means.
[0015] Figure 1 schematically shows a vessel or ship 1, which is provided with at least
one screw 2 for the normal propulsion of the vehicle, as well as bow and stern screws
3 and 4 respectively, by means of which a propulsion force transversely to the longitudinal
direction of the vessel can be exerted on the vessel. Furthermore the vessel is preferably
provided with a positioning system, by means of which the propulsion means in the
shape of screws 2 - 4 in the illustrated embodiment, which are computer-controlled
or the like, are controlled in such a manner that the vessel will remain at a desired
location or is moved along a desired path and at a desired speed.
[0016] A dredging unit comprising a frame 6 is suspended from the vessel by means of cables
5 during operation. Propulsion means, for example in the shape of screws 7 to be driven
by driving means (not shown), are provided in the frame of the dredging unit. The
axes of rotation of the screws 7 schematically shown in Figure 2 extend perpendicularly
to the plane of drawing.
[0017] Besides the two screws 7 shown in the Figure two further screws will be provided
in the frame, which will be arranged symmetrically with respect to the vertically
extending central axis of the frame and whose axes of rotation will extend perpendicularly
to the axes of rotation of screws 7, so that the frame 6 can be propelled in any desired
direction by the propulsion means in the form of the various screws by suitably driving
said screws. Of course it will also be conceivable to provide propulsion means other
than screws, for example jet nozzles or the like. Frames of this type provided with
propulsion means are known per se, as appears for example from EP-A-0 570 034.
[0018] Furthermore a suction pump 8 to be driven by a motor 19 is mounted within frame 6,
said suction pump being connected, via a suction pipe 9, to a suction means 10 or
the like supported by frame 6. Furthermore a jet nozzle 11 is connected to said suction
means 10, which jet nozzle is in communication with a jet pump 12 mounted within frame
6.
[0019] A pressure pipe 13 of suction pump 8 is connected, via a ball joint 14, to the end
of a substantially horizontally extending discharge pipe 15, which is suspended from
vessel 1 by means of cables 16 and 17, which are unwound from winches present on the
ship. A pipe having a comparatively great length can be supported by a few spaced-apart
cables by disposing pipe 15 in the longitudinal direction of the ship 1. It will be
conceivable to use another vessel or a flexible pipe suspended in the water, which
is for example coupled to a vessel and/or anchored in the seabed at one end, to support
pipe 15.
[0020] After ship 1 has arrived at the desired location frame 6 and suction means 10 supported
by frame 6, as well as pipe 15 connected to pump 8, may be lowered by paying out cables
5, 16 and 17 by means of the winches present on ship 1.
[0021] When frame 6 has arrived at a position located ± 3 metres above the bottom surface,
suction means 10 may be placed on the bottom by means of a cable 18 connected to suction
pipe 9, which cable co-operates for example with a setting cylinder (not shown) or
the like hoisting device mounted within frame 6. Water may be squirted into suction
means 10 by means of one or more jet pumps 12 and jet nozzle(s) 11, if necessary,
whilst the material to be dredged can be sucked in by means of suction pump 8 via
suction pipe 9, and be forced out via pressure pipe 13 and pipe 15.
[0022] Signals for controlling the various means provided in frame 6 can be sent to the
respective means from the ship and be received in return via a cable 20. Cable 20
may also be utilized for the power supply of driving means.
[0023] The ship 1 may be moved along a desired path, transversely to its longitudinal direction
during the dredging of the material, so that suction means 10 or the like is moved
in the desired manner across the area to be worked.
[0024] Because not only the ship can be moved in an accurate manner along a desired path
by its propulsion means, but because also the frame 6 of the dredging unit supporting
suction means 10 can be moved in any desired direction and, within certain limits,
over any desired distance across the bottom surface, by its own propulsion means 7,
the dredging unit comprising suction means 10 can be moved in a very accurate manner
with respect to the bottom surface, so that the intended area can be worked in a very
accurate manner by means of the dredging unit. The dredged material is at the same
time moved over a large distance from the area to be dredged via discharge pipe 15
and accurately deposited at the desired location, at a sufficiently large distance
from the area to be dredged.
[0025] The above-described method makes it possible to carry out dredging work with great
accuracy at depths of up to 500 metres or more. Of course it is also possible to use
the method in shallow water.
1. A method to be used in dredging, wherein material present on the seabed is dredged
by means of a dredging unit, which is suspended from a vessel, characterized in that
material dredged by means of the dredging unit is supplied to a discharge pipe suspended
under water from a vessel and coupled to said dredging unit, via which discharge pipe
the material is deposited on the bottom of the sea at some distance from the dredging
place.
2. A method according to claim 1, characterized in that said dredging unit and the discharge
pipe connected to said dredging unit are suspended from a vessel, which is provided
with propulsion means, by which the vessel can be moved both in its longitudinal direction
and in a direction transversely to its longitudinal direction.
3. A method according to claim 1 or 2, characterized in that said discharge pipe is suspended
from said vessel in such manner as to extend at least substantially parallel to the
longitudinal axis of said vessel.
4. A method according to any one of the preceding claims, characterized in that said
dredging unit and said discharge pipe are lowered and/or raised by means of cables
or the like, which are provided between said dredging unit and said discharge pipe
and winches provided on said vessel.
5. A device for carrying out the method according to any one of the claims 1 - 4, characterized
in that said device is provided with a dredging unit comprising a frame which is provided
with propulsion means, by which the device can be moved in any desired direction.
6. A device according to claim 5, characterized in that a pump is provided in said frame,
which pump is connected to a suction means or the like via a suction pipe and which
is connected to said discharge pipe via a pressure pipe.
7. A device according to claim 5, characterized in that said pressure pipe of said pump
and said discharge pipe are coupled via a ball joint.
8. A device according to any one of the claims 5 - 7, characterized in that said suction
means or the like is coupled to said frame in such manner as to be vertically adjustable.