Optical waveguides – Optical transmission cable
Reexamination Certificate
1999-10-01
2002-03-05
Healy, Brian (Department: 2874)
Optical waveguides
Optical transmission cable
C385S099000
Reexamination Certificate
active
06353695
ABSTRACT:
The present invention relates to an underwater cable joint and to a method of joining and deploying two or more lengths of underwater cable.
A conventional method of repairing a damaged underwater optical fibre cable is illustrated in
FIGS. 1
a
-
1
g
. The cable
1
has previously been installed on a seabed
2
. In
FIG. 1
a
the cable
1
is lying on top of the seabed
2
but in practice the cable
1
may be buried. The cable
1
has developed a fault
3
. The approximate location of the fault
3
is determined previously by a known technique (for instance optical or electrical reflectrometry) before a cable repair ship
4
is sent to repair the damaged cable.
When the ship
4
has reached the approximate location of the fault
3
, a cable severing grapnel
5
is deployed via overboard sheave
23
to sever the cable
1
. This separates the cable
1
into a first length
6
(containing the fault
3
) and a second length
7
.
Referring to
FIG. 1
b
, a retrieval grapnel
8
is deployed to retrieve the end
9
of one of the lengths of severed cable (in this case the second length
7
) onto the deck of the ship
4
. The retrieved length of the cable is tested to determine whether it contains the fault. In this case, the second length
7
does not contain the fault
3
, and therefore the second length
7
of the cable is buoyed off using a buoy
10
(
FIG. 1
c
). The retrieval grapnel
8
is used to retrieve the other length of the cable (in this case the first length) as illustrated in
FIG. 1
c.
The cable repair ship
4
pulls in the first length
6
of the cable until the fault
3
is on the deck of the ship as shown in
FIG. 1
d
. The exact location of the fault
3
may be determined using reflectrometry or may be clearly visible (for instance the outer casing of the cable may have been visibly damaged by a ship's anchor or fishing gear) . Once the fault
3
is on the deck of the ship, the section of cable containing the fault
3
is cut out by cutting the first length
6
of the cable at a location
11
on the sea side of the fault
3
.
As illustrated in
FIG. 1
e
, the damaged section of cable is replaced by a third length
12
of cable which is joined to the first length
6
of the cable using a conventional inline cable joint
13
.
The first length
6
and third length
12
of the cable are then paid out until the cable retrieval ship
4
has returned to the buoy
10
. The buoy
10
is retrieved and the second length
7
of the cable is brought onto the deck of the ship as indicated in
FIG. 1
f
At this stage, the third length
12
of the cable is cut at a suitable position
14
and the free end of the third length
12
of cable is joined to the end
15
of the second length
7
of the cable via a conventional inline joint
16
(shown in
FIG. 1
g
). The inline joint
16
is known as a “final splice joint”.
The final splice joint
16
is deployed as illustrated in
FIG. 1
g
. It is not possible to lower the cable on a single deployment rope since this would cause the cable at the point of attachment to bend beyond a minimum bend radius and damage the cable. Therefore two deployment ropes
17
,
18
are used to create a loop having a crown
21
of sufficiently low curvature. The deployment ropes
17
,
18
are attached to the cable via a pair of suitably spaced stoppers
19
,
20
. The cable is lowered overboard using the deployment ropes
17
,
18
and at a suitable point the deployment ropes
17
,
18
are released, either by cutting the ropes
17
,
18
at the ship or by releasing the ropes at the stoppers by actuating a release hook (e.g. an acoustic release hook) Depending on the depth of the water the deployment ropes may be released before the cable has reached the seabed, and may even be released before the stoppers have reached the water.
A similar method of lowering a jointed cable may be employed in a conventional method of installing a cable on an underwater bed between two land masses.
In one example, separate lengths of the cable are first installed in the shallow water near the two respective land masses. The two shallow water lengths of cable are then buoyed. An installation ship joins a third length of cable to one of the shallow water lengths of cable, installs the third length of cable in the deep water between the two land masses, joins the third length of cable to the other shallow water length of cable, and lowers the joined lengths as shown in
FIG. 1
g.
In a second example a first length of the cable is installed from one land mass to a point mid-way between the land masses, and then buoyed off. A second length of cable is then installed from the other land mass until the buoyed-off end of the first length of cable has been located. The first and second lengths are then joined and lowered as shown in
FIG. 1
g
. Alternatively the first and second lengths may be joined before the second length of cable is installed. In this case the cable joint is simply paid off via the overboard sheave as the second length of cable is installed.
The method of deploying the cable illustrated in
FIG. 1
g
has a number of problems.
Firstly the cable tends to develop loops as it returns to the seabed. This problem is illustrated in
FIGS. 7 and 8
which are plan views of a cable on the seabed which has been lowered according to
FIG. 1
g
.
FIG. 7
shows an ideal configuration which is rarely achieved in practice. As the cable is lowered (
FIG. 1
g
), the ship
4
steams away from the line of the cable in the direction indicated at
24
in FIG.
7
. The aim is to ensure that the additional length of cable (which is required due to the depth of the water) forms a single untwisted loop
25
which can be relatively easily reburied. However in practice the cable often tends to develop loops and twists as it is lowered on the deployment ropes
17
,
18
(due to twisting of the deployment ropes and looping of the crown
21
which is not under tension). In addition the cable develops further loops and twists as the cable falls if the deployment ropes are released before the stoppers have reached the seabed. As a result, the cable tends to end up in a configuration of the type shown in FIG.
8
. Instead of lying as a single untwisted loop
25
, the cable develops a number of untwisted loops
26
,
27
and twisted loops
28
,
29
which are difficult or impossible to rebury.
Secondly, if the deployment ropes are released by cutting them onboard the ship then the ropes are left on the seabed attached to the cable via the stoppers
19
,
20
. If the deployment ropes
17
,
18
are later caught by a ship's anchor or fishing gear then the cable can be damaged. To avoid this problem it is common practice to send a Remotely operated Vehicle (ROV) down to the seabed after the cable has been lowered to cut the deployment ropes
17
,
18
from the cable. This adds cost and complexity to the deployment operation.
Thirdly the requirement of two deployment ropes increases the operational complexity of the deployment operation. In some cases, additional deployment ropes may be attached to the cable between the stoppers
19
,
20
to increase the tension on the crown
21
and reduce looping. This increases the complexity of the lowering operation further and increases the number of deployment ropes which must be released and/or cut by an ROV.
Fourthly it may be difficult to achieve sufficient spread at the ship between the deployment ropes
17
,
18
. This is a particular problem where there is limited width available on the deck of the ship (such as on bow-working ships).
In accordance with a first aspect of the present invention there is provided a method of joining and deploying two or more lengths of underwater cable the method comprising
(1) connecting together and securing the lengths of cable in a cable joint whereby each length of cable extends from a cable reception region on one side of the cable joint;
(2) attaching a deployment device to the cable joint;
(3) lowering the cable joint using the attached deployment device; and
(4) releasing the attached deployment device.
In accord
Global Marine Systems Limited
Healy Brian
Knauss Scott
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