Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
The present invenkion relates to a m~thod of laying
cables and t~e like under water.
In the complete specification of our unite~ Kingdom
patent no. 1466570 (French Utility Model 7439835) (our earlier
patent) we have described and claimed a method of laying a
flexible member below the bed of a body of wa~er comprising
forming a trench in the bed and laying in it a line (for
example, a steel hawser) having a greater strength an~/or a
lower cost than the flexible member; and then in a separate
second operation laying the flexible member from a cable-laying
ship ;n substantially the same trench by submerged apparatus
caused to fo11Ow the path of the line.
As described in our earlier patent, the submerged
apparatus (such as a "plough") may be towed by the cable laying
ship, through a recovered length of ~he line laid in the first
operation or otherwise, or it may have internal trac-tion
means.
Direct towing of the plough has been found difficult
when the water is deep, especially if strong cross-currents are
also encountered, as the taut towing line i9 liable to foul the
cable or other flexible member being laid, and the necessary
alignments are difficult to obtain. Known self-powered ploughs
may not function efficient1y and consistently in varying
conditions of the sea bed~
In accordance with the present invention, a method of
laying a flexible member (or more than one flexible member)
underwater in substantially the same trench as a line already
laia below the bed of the body of water comprises advancing a
submerged cable-positioning device along the path of the line
in a laying direction by traction means on said device pulling
- on the line ahead, and feeding the flexible member from a
cable-laying vessel to said device in such a manner that any
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34~7'
auxiliary lines ex-tending between the vessel and the device
are, relative to the layiny direction, above and behind the
flexible member.
Preferably the required relationship between -the
flexible member and any other llne(s) e~tending be-tween the
submerged device and the surface vessel (whether the previously
laid line or an auxiliary line) i5 obtained by provlding tha-t
the flexible member enters -the submerged device at a level
helow that at which the other line(s) is (or are) connected to
or enter~s) or emerge(s) from the submerged device; or subject
to the effect of tension the flexible member may enter
forwardly of the other line(s). Preferably also any such other
line is supported on the vessel at a height greater -than -that
at which the flexible member is fed from the vessel, but
alternatively (or in addition) the flexible member may be paid
out at the forward side of the vessel while any other line
passes over the rearward side.
In one preferred form of the present invention, the
previously laid line, which acts as a guide or the submerged
device, is received and wound in at the vessel.
In another form of the invention the previously laid
line may be returned to the trench and buried there together
with the flexible member thereby enabling it to act as a guide
line in a subsequent underwater laying or repair operation.
As the existing trench on the sea bed may vary in
depth, because of varying hardness or for other reasons, the
plough preferably includes sensing means responsive to its
depth and capable of controlling the guide(s) positioning the
flexible member so as to place it as close as required to the
bottom of the trench while avoiding the risk of the guiding
surfaces fouling the bottom of the trench and urging the plough
upwards and/or damaging it.
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In order -that the invention may more readily be
understood, a description is now given by way of example only,
reference being made to -the accompanying drawings, in which:-
Figure 1 shows schematically a me-thod of laying a
cable in accordance with the present invention;
Figure 2 is a plan view of thP barge shown in
Figure l;
Figure 3 illustrates a modifiecl method;
Figures 4 and 5 are plan views in the retracted and
-the operating positions respectively of a cable plough in
accordance with the invention;
Figures 6 and 7 are side elevations o:~ the plouyh as
shown in Figures 4 and 5 respectively;
Figures 8 and 9 are Eront elevations of the plough in
the corresponding positions, and
Figures 10 to 12 are a plan and side and front
elevations respectively of another form of cable plouyh in its
operating position.
E'igures 1 and 2 show a line 1 which has been buried
previously within a trench in the surface of the sea bed 2, in
the process of being removed and replaced by ~wo high voltage
cables 3. A cable plough 4 i.s maae to move along the sea
bed 2 following the paths of line 1 in a cable laying direction
L by means of a winch 5 which acts on line 1 therehy
simultaneously advancing plough 4 and paying out line 1 to a
cable-laying barge 6 which is positioned by anchors and
hawsers 7. Paying out of some anchor hawsers while drawing in
other anchor hawsers ensures uniform and controlled motion of
barge 6. A buoyant or semi-buoyant auxiliary line 8 connects
barge 6 and plough 4 in order to convey electrical, hydraulic
and/or pneumatic power to plough 4 and/or control signals in
.. ei-ther directionO The cables 3 are fed at deck level on
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barge 6 Eroln a supply 9, while recovered line 1 and control
line 8 pass over a hlgh leve] gantry 10 on harge 6, thereby
providing an arrarlgement in which the lines 1 and 8 are located
above and behind cables 3, relative to the layiny direction L.
Cables 3 enter near the front of the plough 4 direc-tly below
where the line 1 being recovered under tension emerges from the
plough, and ahead of the place where the relatively slack
umbilical cable 8 is connected to it. Alternatively to the use
of a gantry and as shown in Figure 3, the required re]ationship
between any lines and the cahles can be obtained by paying out
the cable 3 at the forwaxd side of barge 6 and passing
lines 1 and 8 over the rearward siaeO
When the line is recovered from the sea bed, it may
be under as low tension as required (subject to its own weight)
by suitable arrangement of the winch 5.
The plough 4 shown in Figures 4 to 9 comprises a
carrier unit 11 mounted by pivoted arms 12 on a support
chassis 13 tha-t rests on and slides along the sea bed.
Hydraulic cylinders 14 raise and lower the carrier unit 11 wlth
respect to the chassis 13 as required, its upper position
defining the retracted position and its lower the operating
position. A keel 15 depends from the carrier 11 and in the
operating position extends below the chassis 13 and thus into a
trench 16 (Figure 9) fol~ed in the sea bed.
The previously laid line 1 (Figures 5, 7 and 9) is
taken in through an opening at the front of the keel 15 and
passes over suitably-placed guides 17 to the winch 5, around
which it makes several turns. After leaving the winch, the
line 1 passes through a guide 18 and thence upwards towards the
barge.
The plough is advanced by driving the winch 5 to pull
in the line 1, while forwardly and/or laterally directed water
jets 19 clear sediment, etc. from the previously-formed trench
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to allow the keel 15 free passage.
The cables 3 being laid (shown in Figures 7 and 9
only) are received by a smoo-th guide platorm 20 at the front
of the carrier unit 11 and a bell-mouthed guide 21 leading to a
sigmoid passage 22 that deposits it in the trench to the rear
of the keel 15. The upper parts of the guide 21 are hinged and
those of passage 22 are formed by a chain oE plates fle~ibly
connected together and can be removed to release t~e plough
from the cables at the end of the operation.
The plough shown in Figures 10 - 12 is similar in
most respects to the one just described, and only features that
differ will be described separately.
The line 1 is cleaned of sediment, etc. by a stream
of water flowing through duct 23 and forwardly along the line
to ~le front of the keel 15, to reduce abrasion of the
apparatus over which the line is subsequently passed. Line 1
is directed horizontally onto a pulley 17, close to the lower
edge of keel 15. It then passes directly to a winch 24 of the
Wharton quad-capstan type and onward over a pulley 25 and
through guide 18 upwards towards the barge. The quad-capstan
type winch 24 has the advantage that the turns of line 1 are
located in separate grooves thereby avoiding entanglement of
separate turns and unnecessary friction; moreover, no sliding
of the turns occurs. The winch is enclosed as a further
precaution against abrasion.
The positioning of the winch and pulleys 17, 25 is
aesigned to keep the number of pulleys required to a minimum,
since as much as 6% of the traction force exerted on the
line 1 may be lost each time it passes over a pulley under
practical conditions.
Four hydraulic cylinders 26 are located, two at each
end of plough 4, to raise and lower unit 11 with respect to
chassis 13. Carrier unit 11 includes a buoyant or semi-buoyan-t
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frame 27 to reduce the load of plough 4 on the sea bed. Two
elect~ic pumps ~8 are mounted on unit 11 to supply water to
jets 19 and duct 23.
Flat skidplates 29 are provided on the lower surface
of chassis 13.
The keel 15 tapers in width towards the rear of
plough 4 to reduce the risk of the keel jamming in the trench,
and to enable the keel to free itself if such jamming does
occur. Preferably the pressure and flow of the water jets is
such that the width of trench cleared is around 50% greater
than the widest part of the keel; thus with a keel whose width
is 0.4 metres at the front tapering to 0.3 metres at the rear,
a trench 0.6 metres in width is desirable.
As the plough is advanced by the winch a~ting on the
man-made line previously buried in the sea bed, its progress is
consistent and can be readily and accurately monitored;
moreover the present invention provides an improved method of
and apparatus for laying cables underwater which has a lower
incidence of fouling any lines between the barge and plough.
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