Hydraulic and earth engineering – Subterranean or submarine pipe or cable laying – retrieving,... – Submerging – raising – or manipulating line of pipe or cable...
Reexamination Certificate
2000-03-03
2002-08-20
Shackelford, Heather (Department: 3673)
Hydraulic and earth engineering
Subterranean or submarine pipe or cable laying, retrieving,...
Submerging, raising, or manipulating line of pipe or cable...
C405S158000, C405S180000
Reexamination Certificate
active
06435772
ABSTRACT:
The present invention relates to submarine ploughs, and relates particularly, but not exclusively, to submarine ploughs for laying cables and other flexible products under the surface of the sea bed.
A substantial part of the world's international telecommunications is transmitted by means of fibre optic cables laid on the sea bed. A single such cable can carry a very large number of simultaneous telephone calls, and any damage sustained by such cables can result in considerable financial loss. One of the main causes of damage to cables laid on the sea bed is fishing activities, and attempts are generally made to protect cables from such interference by burying them under the surface of the sea bed. The cables are generally buried by means of a submarine plough, which is towed behind a cable laying ship and picks up a cable, laid on the sea bed, into the plough and then buries it in a trench dug in the surface of the sea bed by the plough.
Hitherto, a burial depth of one meter or less has generally been regarded as sufficient, and towing the cable plough at this depth requires a pulling force of up to 50 tonnes.
A conventional plough for burying a cable to a depth of up to one meter in strong or hard soil is shown in FIG.
1
and comprises a plough share
1
which comprises an assembly of parts which cut and move the soil at the deepest part of a trench in the sea bed to bury a cable
2
which passes through the plough and is held down by a movable depressor
3
. The plough share
1
is connected to a pair of depth control skids
4
at the front of the plough by means of a leg
17
and a long beam
5
, the skids
4
being movable up or down relative to the beam
5
by moving skid support arms
6
.
The cable plough runs at a generally constant depth by means of the long beam principle, which will be well known to persons skilled in the art. Under this principle, a cutting edge
7
of the plough share
1
cuts a flat bottom to the trench in the sealed, and a heel
8
supports the weight of the rear of the plough and slides along the soil surface cut by the cutting edge
7
. Any tendency of the plough to alter the running depth, for example by means of the rear of the plough lifting up by pivoting about the front skids
4
, is counteracted by the heel
8
lifting off the soil surface which in turn throws the weight of the rear of the plough on to the share
1
which is unable to support this additional load. As a result, the plough tends to run deeper, counteracting the movement of the rear of the plough. Conversely, it is difficult for the rear of the plough to go deeper because this requires the heel
8
to push down into the soil cut by the share cutting edge
7
.
The plough is pulled by means of a tow rope
9
attached to a cable ship. The cable
2
enters the plough via a bellmouth device
10
designed to prevent the cable from being bent around radii which are too small (a typical minimum radius being of the order of 1.5 meters). If the cable
2
being laid is long it may have one or more joints or repeater amplifiers
11
at intervals along its length and which also have to pass through the plough and be buried with the cable
2
. An auxiliary repeater burial share
12
, which makes the cable trench wider, is used to provide the necessary space for the repeater units
11
.
However, in weak soils, certain aggressive types of fishing gear can penetrate the sea bed to depth in excess of one meter, which exposes buried cables to the risk of damage. It is therefore desirable for cable ploughs to be able to bury cables deeper than one meter, for example up to three meters deep in weak soils such as soft muds. However, cable ploughs of this type, when operated in stronger soils, should also be capable of adjusting their ploughing depth to a shallower value to match the pulling force available from the cable laying ship.
FIG. 2
, in which parts common to the embodiment of
FIG. 1
are denoted by like reference numerals but increased by 100, denotes a conventional cable laying plough adapted to address this problem. The plough is intended to operate as deeply as possible using the 50 tonne pulling force required by a standard cable plough such that its maximum operating depth is not less than 3 meters in a weak soil such as soft mud. The plough of
FIG. 2
is also intended to be not significantly larger or heavier than the existing plough of FIG.
1
.
In effect, the plough of
FIG. 2
consists of the plough of
FIG. 1
, modified to have a longer plough leg
117
and front skid arms
106
to enable the plough to reach greater depths. However, the plough of
FIG. 2
has serious operational limitations which arise from the fact that the separation distance
113
between the horizontal components of the tow rope force
109
and the soil reaction
114
acting on the share
101
is very large. This is particularly so when the cable plough is digging a shallow trench in strong or hard soil, for example as shown in FIG.
2
. The product of the forces
109
,
114
with the separation distance
113
constitutes a couple which tends to lift the rear of the cable plough off the supporting trench surface, thus hampering penetration by the share
101
. In addition, because the lower lip of bellmouth device
110
is high above the sealed, any slack in cable
102
can cause it to bend backwards on itself as shown at
115
, which can cause the cable to wrap around the very small radius formed by the lower lip of the bellmouth
110
. This can result in severe damage to the cable.
Preferred embodiments of the present invention seek to overcome the above disadvantages of the prior art.
According to the present invention, there is provided a submarine plough, the plough comprising:
a plough share having a cutting edge for cutting a bottom of a trench in a sea floor;
a heel portion arranged rearwardly of the cutting edge for engaging the bottom of the trench to at least partially support the plough;
a plough beam having an attachment portion for enabling towing means to be attached thereto to tow the plough to cut a trench in the sea floor, wherein the plough share is pivotable relative to the plough beam to adjust the vertical separation in use between the cutting edge and the attachment portion, sea floor engaging means arranged adjacent a forward end of the plough beam; and
adjustment means for pivoting the plough share relative to the plough beam;
wherein the plough is adapted to pass an elongate flexible product therethrough in an arcuate path into the trench rearwardly of the cutting edge and the heel portion.
By adjusting the working depth of the plough by pivoting the plough share relative to the plough beam, this provides the advantage of avoiding total reliance on adjustment of the height of the sea floor engaging means. This in turn enables the working depth of the plough to be adjusted without the necessity of increasing to unacceptable levels the height of the point of attachment of the towing means above the top of the trench. As result, the risk of the elongate flexible product bending backwards on itself, resulting in damage thereto, is minimised.
In one preferred embodiment, the plough beam is rigidly attached to a plough leg, the plough share is pivotably connected adjacent a rearward portion thereof to the plough leg, and the plough is adapted to receive an elongate flexible product adjacent a forward end of the plough beam and pass the product along a rear portion of the plough leg into the trench.
This provides the advantage of enabling the flexible product upstream of the trench to remain clear of the sea bed even when the plough is at its deepest working position.
The plough share may comprise a substantially part-cylindrical surface substantially coaxial with the pivot axis of said plough share about the plough leg, and adapted to slide against a cooperating substantially part-cylindrical surface on the plough leg.
This provides the advantage of minimising any gaps between the plough share and the plough leg, thus minimising the extent to which soil and other particulate ma
Altera Law Group LLC
Lagman Frederick L.
Shackelford Heather
Soil Machine Dynamics Limited
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