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EuroWire – May 2009
57
technical article
of the cable’s attenuation and strain.
The Instron provided the load data, while
the extensometer provided the strain
data. Since the extra fibre length in this
cable was so insignificant, cable strain and
fibre strain were assumed to be the same.
A schematic of the test set up can be seen
in
Figure 9
.
The cable strain and fibre strain results
with corresponding attenuation readings
can be seen in
Figure 10
.
From the results it can be seen that there
is no large change in attenuation before
30 pounds. All fibre optic cable standards
require that the fibre sees strain no higher
than 60% of the fibre proof level while the
cable is at its maximum rated load. This
proof strain was derived from the study
of the reliability of fibre over a 20-year life
cycle, specifically the propagation of stress
cracks over this time period.
The Deep-Sea ROV cable was only
intended to perform for a short period of
time before it was decommissioned so,
because of the limited life cycle of this
cable, the acceptable load could be much
greater than the 60% fibre proof strain.
A load of 25 pounds appears to be an
acceptable choice.
5.1.4 Temperature cycle to failure
EN-187105 demands the lowest test
temperature at -45°C, while the GR-20
and ICEA-640 call for the highest test
temperature at +70°C.
It was decided to follow a modified
temperature cycle profile that would
cycle the cables to temperature extremes
to initiate cable failure. The temperature
cycle profile used in this test can be seen
in
Figure 11
.
GR-20 requires the most stringent
attenuation requirements for the average
attenuation increase of all of the fibres,
at 0.05dB/km. EN-187105 has the most
stringent requirement for attenuation
increase on an individual fibre, 0.1dB/km.
Testers settled on a modified requirement
that no individual fibre shall have an
attenuation increase greater than 0.1dB/km
and that the average attenuation increase
of all of the fibres shall not be greater
than 0.05dB/km. It was also decided
to follow the more stringent require-
ments of ICEA-640 and GR-20 while taking
attenuation measurements.
All attenuation measurements would be
measured at the temperature extremes and
compared with the baseline measurements
taken at ambient temperature prior to
testing. A schematic of the test set up can
be seen in
Figure 12
.
The results of this testing can be seen in
Figure 13
, where the temperature cycle
was represented on the X-axis and the
fluctuation in attenuation was represented
on the Y-axis. These values represented the
maximum attenuation change of a singular
fibre at every temperature extreme.
From these results it can seen that the
cable was more than capable of handling
large fluctuations in temperature. Even
though the cable is capable of -60°C, it will
most likely never see this temperature as
the sea water in which it operates freezes
at a temperature just below 0°C. The data
is represented in a tabular form in
Table 4
.
5.1.5 Hockle Test
This test was created to test the kink
resistance or hockle resistance of the
variations of the Deep-Sea ROV cable.
Hockling is defined as, “(of a rope) to have
the yarns spread and kinked through
twisting in use.” A benchmark was needed
to judge whether or not the process or
material changes in the design were
helping to improve the hockle effect
on the cable. The test set up included a
twisting bench and a fibre-measuring
device. The cable was strung through
the twisting bench and then connected to
the fibre testing equipment on either end,
as seen in
Figure 14
.
The distance between the crank and
the clamp were set to a predetermined
distance. With the distance set, the
cable was affixed to both the clamp and
the crank. The clamp was then moved
two-thirds of the distance back to the
crank. The crank handle was turned in
10 turn increments starting at 0. Once
twisted through a 10-turn cycle, the clamp
was returned to its designated position.
During the clamp’s return path the cable
would hockle and then remove itself from
a hockle. The fibre was tested following the
release of the hockle.
Figure 9
▲
▲
Figure 10
▲
▲
Figure 12
▲
▲
Figure 13
▲
▲
Figure 14
▲
▲
Figure 11
▼
▼
Pulling
equipment
Fibremeasuringequipment
Load
cell
Clamping
device
Clamping
device
Cablelengthundertest
Cycle
Temperature
Extremes (
°C
)
Delta Cold
(dB/km)
Delta Hot
(dB/km)
1
0/+40
-0.003
0.003
2
-10/+50
-0.002
0.011
3
-20/+60
-0.002
0.010
4
-30/+70
-0.005
0.010
5
-40/+80
-0.004
0.007
6
-50/+85
-0.003
0.005
7
-60/+90
0.043
N/A
Table
▲
▲
4
:
Temperature Cycle Test Results