EoW May 2009

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 .

Temperature Extremes ( °C )

Delta Cold (dB/km)

Delta Hot (dB/km)

Cycle

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

Clamping device

6

-50/+85

-0.003

0.005

Clamping device

Cablelengthundertest

7

-60/+90

0.043

N/A

Load cell

Pulling equipment

Table ▲ ▲ 4 : Temperature Cycle Test Results

Fibremeasuringequipment

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

Figure 9 ▲ ▲

The cable strain and fibre strain results with corresponding attenuation readings can be seen in Figure 10 .

Figure 12 ▲ ▲

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.

Figure 13 ▲ ▲

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 .

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 11 ▼ ▼

57

EuroWire – May 2009