Performance report of the first deployment of the NGK Automated Elevator System and the Optimare/Sea-Bird Electronics CTD-Logger in the Greenland Sea.

The Automated Elevator System by NGK, Japan, consists of a sophisticated combination of an underwater winch, a control unit, multiple parallel power supplies, and a structure of frame and buoyancy. It is intended for the use in the open ocean where it has to reside in a depth smaller than 300 m and enables measurements right into the ocean surface. This important inclusion of the surface layer poses severe problems to mooring designs, and no commercially available operational alternative to the NGK solution is apparent. When the NGK system pays out rope, a buoyant instrument ascends until it reaches the water surface proper. The winch is halted then and reverses it’s rotational direction, thus moving the instrument back to it’s parking position close to the winch. A tension sensor in the elevator system detects the instrument’s arrival at the surface. Our mooring in the Greenland Sea is the premier use of the NGK system in Europe.

In our mooring, the Optimare/Sea-Bird Electronics CTD-logger is the buoyant instrument which is moved to the surface and back to the winch. It comprises a pumped SBE41 CTD module and a custom designed pressure case containing the special electronics and the electrical supply. The synchronisation of the Elevator System and the CTD is realised by their clocks only, there is no communication between them.

This shallow water yoyo-mooring is added to a site where EP/CC Deep Sea Jojo moorings have been deployed for a number of years. These deep sea yoyos reside in a depth of roughly 100 m and measure daily profiles between this depth and the ocean bottom at about 4 km. The shallow water yoyo complements these measurements by covering the important upper part of the water column with daily measurements, too.

The mooring

 

The mooring which contains the NGK Automated Elevator System and the Optimare/Sea-Bird Electronics CTD-logger has been deployed on 27.07.2007 during a cruise with RV Maria S. Merian. Position is 76°56’N, 4°37’E. Water depth at the mooring site is 3680 m.

Performance

The NGK system came back on deck in an as-new optical condition, and so did the Optimare/Sea-Bird Electronics CTD-logger. However, the two plastic buoyancy spheres, which were added to the logger, were lacking. As they were fixed securely to the rope which holds the CTD-logger (a central hole in the spheres is used to let the rope run through), this was an – unpleasant – surprise and indicated a breakdown of scheduled operation at some time during the deployment period. A quick data check showed that this occurred rather late during mid April 2008, so that the entire winter is covered by measurements.

In detail, the performance history is as follows.  The evaluation is based on the CTD-logger record. The CTD-logger worked faultlessly.

After the start of the mooring period, the CTD-logger resides for several days at the winch. This is owing to the fact that the CTD is attached to the Automated Elevator frame using corrosion links, which release the CTD-logger after 13 days (CTD-logger record number 26). Regular profiles are performed then, with occasional pauses occuring up to day 50 (record 100). From then on, more than 200 daily profiles are performed. Regular performance stops at 17.April 2008. The reason is clear from the depth record: An unexpectedly deep dive to more than 500 dbar (555 dbar) resulted in a collapse of the plastic buoyancy spheres, which had a nominal pressure rating of 300 dbar. This dive is caused by a major current event, which is rare but known in the region. Small scale, fast rotating eddies posess high velocities at their outer rim. The lateral drag on the 4 km long mooring components leads to a related vertical shift of winch and CTD-logger. Subsequently, the CTD-logger remained with it’s own buoyancy only, which is below the tension threshold of the system. Shutting off the profiling action under this condition is as systematical as are the preceding profiles.

 

 

The system’s behaviour in the surrounding of the deepest dive (CTD record 529) is worth noting. During some later instances, the CTD ascends despite the lacking buoyancy spheres. Apparently, the drag by the strong ocean currents is sufficient to exceed the tension threshold of the winch. From very deep positions, the winch pays out all it’s rope (300 m), but the CTD-logger does not reach the ocean surface. The winch retracts the rope as planned under these circumstances. From CTD record no. 600 on, no further current event occurred and the winch remained in its systematical parking mode.

The energy supply’s capacity which was mounted to the winch was sufficient for the described operation under the cold temperatures in the subarctic waters (t about 0°C).  This is as expected, because fewer profiles than originally scheduled have been performed due to the destruction of the CTD-logger’s main buoyancy. We will evaluate the energy remaining in the battery set as soon as possible.

After this one year performance, the rope of the winch was perfectly intact and well spooled. Profiling speed was between 25 and 30 cm/s.

Conclusion

The entire system of NGK Automated Elevator System and Optimare/Sea-Bird Electronics CTD-logger proved to work fully operational for a long work time interval of one year in cold subarctic waters. Energy supply, mechanics, and electronics worked perfectly. More than 200 cycles between a nominal depth of 160 m (exceeded on occasions) and the ocean surface proper were performed. Both the NGK system and the Optimare/Sea-Bird Electronics CTD-logger turned up in an as-new optical condition after the deployment period.

An unprecedented record of Eulerian measurements in the center of the Greenland Gyre has been collected all year round in the complete uppermost layer of the water column which contains the important fresh water signal. The previously impossible extension of moored measurements right into the ocean surface will reveal new insights into the fresh water balance of this climatologically sensible region of deep water formation.