Method and apparatus for dredging having bow-stern movement of the suction means

An improved method for dredging by means of a pair of spuds provided on the respective sides of the stern of a dredger and adapted to be alternately driven into the bottom of the water, a ladder projected from the bow portion of the dredger and a suction port member that is mounted at the extreme end portion of the ladder. The improvements are in the repetitive cycles of operation, each consisting of the steps of at first dredging while the suction port member is successively retracted from the foremost position of its stroke to the rearmost position as the ladder is swinging from one side towards the other about the spud on the other side, then driving the spud on the one side and retracting of the spud on the other side, advancing the suction port member to the middle position in its stroke, thereafter dredging as the ladder is swinging towards the one side, then advancing the suction port member to the foremost position of its stroke, and thereafter again dredging as the ladder is swinging from one side towards the other side. Preferable structures of a dredging apparatus to be used in the above method are also disclosed.

The present invention relates to a method for dredging that is suitable for 
dredging of mud and sludge, and a dredging apparatus to be used in the 
method. 
In the method for dredging mud and sludge commonly employed in the prior 
art, the dredging is carried out in the following manner by making use of 
a dredger as illustrated in FIG. 1 (a side view) and FIG. 2 (a plan view). 
That is, in the dredging operation, except for the period when spuds 
S.sub.1 and S.sub.2 provided on the starboard and port, respectively, of 
the stern are alternately driven into the bottom, only one spud on either 
the starboard or the port is driven into the bottom, and the dredging is 
carried out by sucking and conveying the mud and sludge through a suction 
port member U provided at the extreme end of a ladder L while the ladder L 
is swinging (rotating in the left and right directions) jointly with the 
hull about the driven spud. These swinging motions are achieved by winding 
up one of two swing wires J.sub.1 and J.sub.2 on the starboard and the 
port having anchors (not shown) at their extreme ends with a swing winch 
M, while paying out the other swing wire. 
In dredging operations where only the uppermost layer of mud and sludge 
(the depth of mud and sludge equivalent to the height of the suction port) 
is dredged, the operations are carried out, for example, by using the 
starboard spud S.sub.1 as a working spud and the port spud S.sub.2 as an 
advancing spud, as illustrated in FIG. 3. 
At first, starting from the rightmost position A.sub.1 on the starboard in 
FIG. 3, the starboard spud S.sub.1 is driven and the port spud S.sub.2 is 
raised. Under such a condition, if a leftward swing is effected, then 
suction port member U at the extreme end of the ladder moves from the 
position A.sub.1 to the position B.sub.1 while tracing an arc-shaped locus 
having its center at the working spud S.sub.1 on the starboard. 
At the position B.sub.1, the swing motion is stopped, and after the raised 
port spud S.sub.2 has been driven, the driven spud S.sub.1 is drawn up 
alternate. This operation is called reversal of the spuds. Next, when a 
leftward swing is effected, the suction port member U moves from the 
position B.sub.1 to the position C.sub.1 while tracing an arc-shaped locus 
having its center at the advancing spud S.sub.2 on the port. During this 
period, the hull is advanced approximately the length of the side opening 
in the stern-bow direction of the suction port member U. 
Then at the position C.sub.1 the swing motion is stopped, and reversal of 
the spuds is effected. 
Thereafter, when a leftward swing is effected, the suction port member U 
moves from the position C.sub.1 to the position D.sub.1, and the locus 
traced by the suction port member U during this period forms a part of an 
arc having its center at the starboard spud S.sub.1. When the suction port 
member U has reached the leftmost position D.sub.1 on the port, the swing 
motion is stopped, and the motion is switched to a rightward swing motion. 
Subsequently, as the rightward swing is effected, the suction port member U 
moves from the position D.sub.1 through the positions C.sub.1 and E.sub.1 
to the position A.sub.2. 
The locus traced by the suction port member U during this period forms a 
part of an arc having its center at the starboard spud S.sub.1. 
The direction of swing motion is switched at the position A.sub.2, and the 
subsequent sequence of operations is the same as the above-described 
sequence starting from the position A.sub.1. 
More particularly, the sequence is such that between the positions A.sub.2 
and E.sub.1 is effected a leftward swing about the starboard spud S.sub.1, 
and after reversal of the spuds has been effected at the position E.sub.1, 
in the interval between the positions E.sub.1 and C.sub.2 the suction port 
member D is advanced while effecting a leftward swing about the port spud 
S.sub.2, at the position C.sub.2 is effected reversal of the spuds, in the 
interval between the positions C.sub.2 and D.sub.2 is effected a leftward 
swing about the starboard spud S.sub.1, and after the direction of the 
swing has been switched at the position D.sub.2, a rightward swing is 
carried out about the starboard spud S.sub.1 from the position D.sub.2 
through the positions C.sub.2 and E.sub.2 to the position A.sub.3. 
By repeating the aforementioned sequence of operations, the hull is 
intermittently advanced while continuing the dredging work. 
When dredging mud and sludge, it is desirable to continuously dredge a high 
concentration of mud and sludge that is approximately in the state of the 
bottom of the body of water. For that purpose it is necessary to provide 
an apparatus in which only mud and sludge are sucked through a mud and 
sludge inlet opening on the side surface of the suction port member U so 
that excessive water will not be sucked. 
Taking into consideration the aforementioned requirement, we will 
reconsider the above-described sequence of dredging operations illustrated 
in FIG. 3. The rate of filling of the mud and sludge inlet opening by the 
mud and sludge is termed the dredging efficiency, and the relation between 
the dredging efficiency and the positions of the suction port member U in 
the sequence of dredging operations, is diagrammatically shown in FIG. 4. 
In the dredging work, the shifts of the position of the suction port member 
U from A.sub.1 .fwdarw.B.sub.1 .fwdarw.C.sub.1 .fwdarw.D.sub.1 
.fwdarw.C.sub.1 .fwdarw.E.sub.1 .fwdarw.A.sub.2 are repeated, and so, only 
one cycle of the operations from the position A.sub.1 to the position 
A.sub.2 need be investigated. The locus of the suction port member U 
between the positions A.sub.1 and B.sub.1 is the same as the locus between 
the positions A.sub.2 and E.sub.1 in the preceding cycle where the mud and 
sludge have been already dredged, so that the dredging efficiency is equal 
to 0% in this interval. During the period when the spuds are being 
reversed while the swing of the suction port member U is stopped at the 
position B.sub.1, is also the dredging efficiency is also equal to 0%. 
During the period when the suction port member U moves from the position 
B.sub.1 to the position C.sub.1, the dredging efficiency is gradually 
increased from 0% to 100%, but during the period when the spuds are being 
reversed at the position C.sub.1, the dredging efficiency remains at 0%. 
The dredging efficiency is equal to 100% between the positions C.sub.1 and 
D.sub.1, but during the period when the direction of swing motion is being 
reversed at the position D.sub.1, it is equal to 0%. 
Between the positions D.sub.1 and C.sub.1, since dredging has been already 
finished, the dredging efficiency is equal to 0%, and as the suction port 
member D moves from the position C.sub.1 to the position E.sub.1, the 
dredging efficiency is gradually increased from 0% to 100%. Between the 
positions E.sub.1 and A.sub.2 the dredging efficiency is equal to 100%, 
but during the period when the direction of swing motion is being reversed 
at the position A.sub.2 the dredging efficiency is equal to 0%. FIG. 4 
shows the mode of variation of the dredging efficiency during the 
above-described one cycle. 
It is to be noted that in FIG. 3, reference characters P and Q designate 
the positions where the spuds are reversed, and reference character W 
designates the width of dredging. 
As described above, in the prior art method, since the suction port member 
U passes through the area where dredging has been already finished such as 
the intervals between the positions A.sub.1 and B.sub.1 and between the 
positions D.sub.1 and C.sub.1, the suction port member U may possibly suck 
only sea water without sucking any mud and sludge (the dredging efficiency 
then being 0%), or else, in certain areas such as the intervals between 
the positions B.sub.1 and C.sub.1 and between the positions C.sub.1 and 
E.sub.1, as the swing motion proceeds, gradually an increased rate of mud 
and sludge is sucked, but at the same time a large amount of water is also 
sucked in, and consequently, the average value of the dredging efficiency 
is as low as about 40%. 
In the dredging method in the prior art, since a large amount of excessive 
water is sucked in, the prior art method has the disadvantages that the 
amount of mud and sludge (the mud and sludge in the state of the bottom of 
the water) dredged per unit time is reduced, that upon discharging the 
excess sucked water, processing of a large amount of water is necessitated 
for preventing public nuisance, and that the amount of fluid transported 
is increased by the amount of the excess sucked water. 
The present invention seeks a solution to the aforementioned problems 
associated with the dredging of mud and sludge. 
It is one object of the present invention to provide a novel method for 
dredging in which the dredging can be achieved efficiently without sucking 
excessive water during dredging. 
Another object of the present invention is to provide a novel dredging 
apparatus having an excellent performance to be used in said novel method. 
According to one feature of the present invention, there is provided a 
method for dredging, in which a pair of spuds provided on the respective 
end of the stern of a dredger are alternately driven into the bottom of a 
body of water, and while swinging a ladder projecting from the bow portion 
of the dredger jointly with the hull about the spud driven into the 
bottom, dredging is effected through a suction port member that is mounted 
at the extreme end portion of said ladder so as to be movable in the back 
and forth directions; characterized in that said dredging is effected by 
repeating the operations each consisting of the steps of at first dredging 
while said suction port member is successively retracted from the foremost 
position of its stroke to the rearmost position as said ladder is swinging 
from one side towards the other side about the spud on said the other 
side, then driving the spud on said one side into the bottom and 
retracting the spud on said the other side, advancing said suction port 
member to the middle position in its stroke, thereafter dredging as said 
ladder is swinging towards said one side, then advancing said suction port 
member to the foremost position of its stroke, and thereafter again 
dredging as said ladder is swinging from said one side towards said the 
other side. 
According to another feature of the present invention, there is provided a 
dredging apparatus to be used in the above-described method, characterized 
in that said apparatus comprises a ladder projecting from the bow portion 
of a hull so that it be freely inclined up and down, a movable frame 
supported on the extreme end portion of the ladder so as to be rotatable 
about a horizontal lateral axis, and a suction port member for dredging 
held on said movable frame so as to be movable in the back and forth 
directions, and that said suction port member is mounted on a truck that 
is movable in the back and forth directions within said movable frame. 
According to still another feature of the present invention, the 
above-described dredging apparatus is modified in that said suction port 
member for dredging is pivotably mounted to the bottom end of a parallel 
link mechanism that is pivotably mounted on said movable frame so as to be 
rockable in the back and forth directions.

The dredging apparatus to be used in the method according to the present 
invention is principally characterized in that the suction port member U 
for mud and sludge which is provided at the extreme end of the ladder L 
shown in FIGS. 1 and 2 is movable in the bow-stern directions. 
A first preferred embodiment of the present invention is illustrated in 
FIGS. 5 and 6, in which a movable frame K is hinged to the extreme end 
portion of a ladder L by a shaft 3 and bearings 4, so that the movable 
frame K is rotatable about this shaft 3. On the movable frame K is 
provided a traveling truck 5 which is movable in the bow-stern directions, 
and to this travelling truck 5 is fixedly secured a suction port member U 
for mud and sludge by means of a coupling member 6. 
Within the coupling member 6 is disposed a piping system for transporting 
and discharging the mud and sludge collected by the suction port member U 
for mud and sludge, a flexible pipe 8 is connected to the top end of this 
piping system, and the other end of the flexible pipe 8 is connected to a 
pipe 8a which is led to the suction port of a mud and sludge dredging pump 
not shown. 
On the movable frame K are mounted a hydraulic motor 9, a threaded rod 10, 
a thrust bearing 11 and an internally threaded rod receiver 12 threadedly 
mounted on the rod 10, and by rotating the hydraulic motor 9 in the normal 
and reverse directions, the mud and sludge suction port member U can be 
driven in the bow-stern directions by the rod 10, rod receiver 12 and 
coupling member 6. The hydraulic motor 9 can be replaced by an electric 
motor, and depending upon the rotational speed of the hydraulic motor 9, 
if necessary, a reduction gear could be interposed between the hydraulic 
motor 9 and the rod 10. It is to be noted that guide rollers 13 provided 
on the truck 5 could be replaced by sliders made of fluorine group 
synthetic resin having a low coefficient of friction. 
In FIG. 5, reference numeral 14 designates rollers for receiving a lateral 
reaction force exerted upon the suction port member U by the mud and 
sludge, in cooperation with the shaft 3. In addition, reference numeral 15 
in FIG. 6 designates guide plates for the rollers 14, reference numeral 16 
designates a coupling pedestal of a leveling device for always holding the 
suction port member U in a horizontal state, and reference numeral 17 
designates a coupling rod. 
The above-described leveling device carries out leveling control for the 
suction port member U by means of a hydraulic cylinder (not shown) having 
one end connected to the coupling rod 17 and the other end connected to 
the ladder L in response to a signal issued by a level detector (not 
shown) disposed on the movable frame K. 
Alternatively it is also possible to extend the connection rod 17 to form a 
hinged parallelogram structure as shown in FIG. 1 jointly with the ladder 
L and thereby the suction port member U is controlled to maintain a 
horizontal attitude regardless of the inclination of the ladder L. 
In FIG. 6, reference character U' designates the suction port member 
shifted to the stern position, and reference character L' designates the 
ladder positioned for working at substantially the maximum dredging depth. 
A second preferred embodiment of the dredging apparatus to be used in the 
method according to the present invention is illustrated in a side view in 
FIG. 7, in which at the extreme end portion of the ladder L is hinged a 
movable frame K on a shaft 23a in bearings 23, so that this movable frame 
K is rotatable about the shaft 23a. 
In addition, at the bottom ends of mounting legs 25 and 26 which are 
pivotably mounted on the movable frame K at their top ends is pivotably 
supported a suction port member U by pin couplings to form a parallel link 
mechanism, and in order to always maintain the suction port member U in a 
horizontal state, the four coupling pins are at the corners of a 
parallelogram. 
To the top end of the mounting leg 25 is connected a hydraulic cylinder 27 
for shifting the suction port member U back and forth, and the 
construction is such that the suction port member U be shifted in the 
bow-stern directions by extending and retracting the piston rod of this 
hydraulic cylinder 27. 
While FIG. 7 shows the ladder L held in a horizontal state, during dredging 
work in order to lower the suction port member U to the bottom of the body 
of water, the ladder L is inclined depending upon the dredging depth until 
the angle formed between the ladder L and the horizontal level is 
increased to about 45.degree. at the maximum. In order to still maintain 
the suction port member U in a horizontal attitude even in such an 
inclined position of the ladder L, it is necessary to rotate the movable 
frame K depending upon the inclination angle, and to that end a leveling 
cylinder 28 is provided. The extension and contraction of this cylinder 28 
can be controlled by a signal issued from a level detector (not shown) 
mounted on the movable frame K. 
In addition, guide rollers 29 for transmitting the force exerted by the 
suction port member U on the ladder L in cooperation with the shaft 23a 
and bearings 23, are provided on the movable frame K, and guide plates 30 
for the guide rollers 29 are provided on the ladder L. 
It is to be noted that according to the second preferred embodiment, when 
the suction port member U is located at the middle position in its stroke, 
the vertical position of the suction port member U is somewhat lower than 
when the suction port member U is located at a forward position or a rear 
position, but this can be compensated for by moving the ladder L a little 
in the vertical direction. 
Now, the method for dredging according to the present invention which can 
be practiced by means of either one of the above-described dredging 
apparatuses in which the mud and sludge suction port member U is movable 
in the bow-stern direction, will be explained with reference to FIG. 8 
showing the operation steps in a schematic plan view and FIG. 9 which 
shows the relation between the position of the suction port member U and 
the dredging efficiency during the dredging operations. It is also assumed 
in this example of dredging operations that the starboard spud is used as 
a working spud, while the port spud is used as an advancing spud, and the 
stroke of movement in the bow-stern directions of the suction port member 
U is twice as large as the length in the bow-stern directions of the 
opening at the mud and slurry inlet portion of the suction port member U. 
Describing now the dredging work starting from the rightmost position 
A.sub.1 on the starboard shown in FIG. 8, the starboard spud S.sub.1 at 
the stern is driven into the bottom of the body of water while the port 
spud S.sub.2 is kept retracted, and the suction port member U is located 
at the foremost position on the movable frame K. 
Under this condition, when a leftward swing of the ladder L and the hull is 
effected, the suction port member U moves from the position A.sub.1 to the 
position B.sub.1 while tracing an arc-shaped locus having its center at 
the starboard spud S.sub.1. At the position B.sub.1 the swing motion is 
stopped, and after the port spud S.sub.2 has been driven into the bottom 
of the sea, the starboard spud S.sub.1 which has been held in the bottom 
of the sea is retracted. 
Subsequently, when a further leftward swing is effected, the suction port 
member U tends to move from the position B.sub.1 to the position C.sub.2 
while tracing an arc-shaped locus having its center at the advancing spud 
S.sub.2. 
However, while effecting the further leftward swing, the suction port 
member U is gradually shifted backwards or sternwards so that the suction 
port member U will be continuously shifted towards the stern a distance 
twice as large as the length in the bow-stern directions of the opening on 
the side surface of the suction port member U in accordance with the swing 
motion during the period when the suction port member moves from the 
position P of the spud on the starboard to the position Q of the spud on 
the port. Thus the suction port member U traces an arc-shaped locus from 
the position B.sub.1 to the position C.sub.1 rather than the position 
C.sub.2. 
During this period, the hull has advanced a distance twice as large as the 
length of the side opening of the suction port member U, and when the 
suction port member U comes to the position C.sub.1, the suction port 
member U is located at the rearmost position on the movable frame K. At 
the position C.sub.1, the swing motion is stopped, and after the spuds 
have been reversed, the leftward swing is resumed. Then the suction port 
member U moves from the position C.sub.1 to the position D.sub.1 while 
tracing an arc-shaped locus having its center at the starboard spud 
S.sub.1. 
When the suction port member U has reached the leftmost position D.sub.1 on 
the port, the swing motion is stopped, and the suction port member U which 
has been located at the rearmost position on the movable frame K is 
shifted to the center position on the movable frame K. Then the suction 
port member U is located at the position E.sub.1. Subsequently, when a 
rightward swing is effected, the suction port member U moves from the 
position E.sub.1 through the positions F.sub.1 and G.sub.1 to the position 
H.sub.1 while tracing an arc-shaped locus having its center at the 
starboard spud S.sub.1. 
When the suction port member U has reached the rightmost position H.sub.1 
on the starboard, the swing motion is stopped, and the suction port member 
U which has been located at the center position on the movable frame K is 
shifted to the foremost position on the movable frame. Then the suction 
port member U is located at the position A.sub.2. Then a leftward swing is 
started from this position A.sub.2, and the subsequent sequence of 
dredging operations is identical to the above-described sequence of 
operations which was started from the position A.sub.1. 
More particularly, between the positions A.sub.2 and B.sub.2 is effected a 
leftward swing about the starboard spud S.sub.1 with the suction port 
member U at the foremost position on the movable frame K, and after the 
spuds have been reversed at the position B.sub.2, between the positions 
B.sub.2 and C.sub.2 is effected a leftward swing about the port spud 
S.sub.2 while the suction port member U which has been located at the 
foremost position on the movable frame K is gradually shifted towards the 
rearmost position, and during this period the hull is advanced a distance 
equivalent to the shift length in the bow-stern directions of the suction 
port member U. 
After the spuds have been reversed at the position C.sub.2, a leftward 
swing is effected about the starboard spud S.sub.1, at the position 
D.sub.2 the suction port member U is shifted from the rearmost position on 
the movable frame K to its center position, then the suction port member U 
moves from the position E.sub.2 through the positions F.sub.2 and G.sub.2 
to the position H.sub.2 while effecting a rightward swing about the 
starboard spud S.sub.1. At this point of the process the suction port 
member U is shifted from the center position to the foremost position. 
By repeating the above-described sequence of operations, the hull is 
intermittently advanced while continuing the dredging work. 
The relation between the position of the suction port member U and the 
dredging efficiency in the above-described sequence of operations, is 
diagrammatically shown in FIG. 9. In the dredging work, the shifts of the 
position of the suction port member U from A.sub.1 .fwdarw.B.sub.1 
.fwdarw.C.sub.1 .fwdarw.D.sub.1 .fwdarw.E.sub.1 .fwdarw.F.sub.1 
.fwdarw.G.sub.1 .fwdarw.H.sub.1 .fwdarw.A.sub.2 are repeated, and so, only 
one cycle of the operations from the position A.sub.1 to the position 
A.sub.2 need be investigated. 
In the method according to the present invention, it never occurs that an 
area where the dredging has been already finished is again passed over by 
the suction port member U resulting in suction of only water, as is the 
case with the prior art method, nor that during the period when the swing 
motion is being effected about the port spud S.sub.2 the dredging 
efficiency is gradually varied from 0% to 100% and during that period a 
large amount of water is sucked. 
More particularly, according to the present invention, during the periods 
when a swing motion is being effected (A.sub.1 .about.B.sub.1, B.sub.1 
.about.C.sub.1, C.sub.1 .about.D.sub.1, E.sub.1 .about.F.sub.1 G.sub.1 
.about.H.sub.1), the dredging effeciency is always equal to 100%. Suction 
of water occurs only when the swing motion is stopped for the purpose of 
reversing the spuds (B.sub.1, C.sub.1) and when the suction port member is 
being shifted towards the bow at the extreme positions on the starboard 
and on the port (D.sub.1 .about.E.sub.1, H.sub.1 .about.A.sub.2), and 
during these periods the dredging efficiency is equal to 0%. With such 
sequence of operations, the average value of the dredging efficiency is 
raised to as high as about 80%, and accordingly, the dredging efficiency 
is greatly improved in comparison to the average dredging efficiency of 
about 40% in the case of the prior art method. 
While the starboard spud S.sub.1 is used as a working spud and the port 
spud S.sub.2 is used as an advancing spud in the above-described 
embodiments, the roles of the respective spuds could be reversed. 
As a practical method for gradually shifting the suction port member U 
along the movable frame K while the suction port member U is moving from 
the position B.sub.1 to the position C.sub.1 in FIG. 8, the following 
means are known. 
(1) Method for detecting the swing angle from the number of revolutions of 
a swing winch 
Since the swing angle of the hull is substantially proportional to the 
number of revolutions of the wire drum of the swing winch, the amount of 
shift of the suction port member U is controlled in accordance with the 
number of revolutions of the swing winch starting from the time when the 
suction port member U comes to the position B.sub.1. 
(2) Method for detecting the swing angle with a magnetic compass 
On a magnetic compass are preset the angles for alternately reversing the 
spuds, and the amount of shift of the suction port member U is controlled 
in accordance with the swing angle of the hull. 
The effects and advantages of the method for dredging according to the 
present invention as well as the dredging apparatus to be used in said 
method are enumerated in the following: 
(1) When a dredging pump of the same capacity is used, the amount of the 
mud and sludge dredged from the bottom is increased. Expressed in a 
reversed manner, when the same amount of mud and sludge is to be dredged, 
according to the method of the present invention, a smaller pump capacity 
than in the prior art method will suffice, and so, both the initial 
investment and the running cost are reduced. 
(2) When using the dredged mud and sludge for reclamation, in order to 
prevent public nuisance it is necessary to carry out water processing 
(removal of floating and suspended solid particles, harmful substance, 
etc.) before the excessive sucked water is discharged. However, according 
to the present invention, since the amount of excessive sucked water is 
small in comparison to the prior art method, only a small amount of water 
processing is required. 
(3) When transporting the dredged mud and sludge to the reclaimed ground, 
there are different ways of transportation such as pipe line 
transportation, transportation by a sludge transport ship, etc. However, 
in any such way, the mud and sludge dredged by the method according to the 
present invention contains a smaller amount of excessive sucked water in 
comparison to the mud and sludge dredged by the prior art method, and 
therefore, the amount of transportation required is smaller. 
(4) Since the suction port member U is mounted by a truck 5 on the movable 
frame K pivotably mounted on the extreme end portion of the ladder so as 
to be movable in the back and forth directions, the operation of the 
suction port member U can be effected precisely and smoothly. 
(5) In a modified embodiment in which the suction port member U is 
pivotably mounted on the movable frame K on a parallel link mechanism 25 
and 26, the structure is simple and the maintenance is easy.