NEPTUNE@Sea Blog Blog from 23-Sep-10

Harsh Realities

The deep-sea is certainly not a friendly environment for complex metal structures hosting delicate scientific instruments. Large pressure differences during the descent, corrosion during long-term deployment and ground faults due to contact with seawater are all significant engineering challenges, but these conditions are hard to reproduce on land. Even more challenging is laying the thin cables, often many kilometres long, that link these platforms and instruments to the junction boxes and nodes that provide power and communications.

Following a cable route survey covering harsh seafloor features, the Regional Circulation Mooring (RCM) North instrument platform was deployed and the 4.6km cable linking it to the Endeavour node was laid without complications. Sadly, after the connection was made, power levels on the cable were good, but our on-land team could not communicate with the junction-box on the instrument platform. Time to go to all out troubleshooting mode!

Team MacGyver

In an attempt to exclude the simplest and most likely possibilities first, both ends of the cable were disconnected and reconnected... No go.

The instrument platform was recovered and the junction-box tested on deck... Working. Time to carry out more complex tests. Our onboard team of MacGyvers did an amazing job at trying to figure out the issue; with only the resources available on the ship, they built various devices to test the different parts of the system in the hopes of narrowing down the problem further.

First, they used an Optical Time Domain Reflectometer (OTDR) provided by the ROPOS team. This system sends light down ROPOS’s umbilical cord, through connectors and all the way through the fibre optic cable on the seafloor measuring wherever the light gets reflected back. If all is well with the cable, light reaches the end with some reflectance only at the connections, but if has damage occurred, the light will be reflected at other points, or not reach the end at all... Results were inconclusive. The system worked on deck, but at depth, it indicated problems on both ends of the cable; an unlikely event. The possibility had to be considered that part of the system could not withstand the pressure and produced an unreliable test underwater. A visual survey of the laid cable also did not detect any clear indications of cable failure.

Maybe something is wrong with the node; time to quickly engineer a shorter version of a node to junction-box cable. Recycling the cables usually used for on-deck instrument testing and with the help of the ship’s engineers, the two cables were joined and the connection placed into a termination can built out of rubber, PVC and galvanized hardware. Once vacuum tested and filled with oil, ROPOS lowered the newly-built cable to the seafloor with the Mothra instrument platform and connected it to the node... Communication was established.

Conclusion

A problem is present in the cable and only recovering it would allow us to discover exactly what the problem is and how to remediate it in the future.

Immediate Solution

The cable planned for Mothra will now serve to link the RCM North site to Endeavour node. This will ensure that the one regional circulation mooring scheduled for this year can be tested before the others are deployed next year. Meanwhile, duplicate instruments to those destined for Mothra are being deployed at RCM North and Main Endeavour Field, so we will receive data from those prior to deploying their twins at Mothra next year.

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