Thursday, 9 March 2017

The ups and downs of oceanography

Our first few days at sea were uneventful.   Crew, technicians and scientists quickly got into the routine of recovering moorings, conducting CTD profiles to calibrate the mooring instrumentation, and re-deploying the moorings.  By Sunday we had completed our first 5 moorings and all was on schedule.
On Sunday we had the additional challenge of trying to recover an experimental lander called ‘MYRTLE’ that was first deployed 18 months ago.   This new lander was designed to send data back via ‘data pods’ that rise to the sea surface and then communicate via satellite.  We had not received any messages from it and were keen to get it back so that the fault could be diagnosed.  A few months ago some of our colleagues attempted to recover it but were unable to trigger the ‘release’ that detaches the lander from its anchor enabling it to rise to the sea surface.   We were far from confident that we would succeed this time but conditions were good and Darren persevered with the acoustic communications until the releases fired and three hours later the errant MYRTLE was back on board.   We were all very pleased with our success.  
The MYRTLE-X telemetry lander is lifted on board - more on this in a future post.
The high of our success on Sunday was all too quickly followed by a low the following day.   On Monday afternoon a CTD profile was started with 32 instruments attached to perform the calibrations needed for our next mooring deployment.   Normally the CTD is lowered down on a wire to within a few metres of the sea floor and then winched back on board.    Here the ocean is over 5 km deep and the whole process takes about 5 hours.   As the instruments descend technicians keep an eye on the data and the crew monitor the winch and cable.   But at 16:07 the data stopped.  Without warning the cable had snapped and the CTD package plummeted to the seafloor. 
The breaking of a CTD wire is very rare and everyone was shocked.  Attention soon turned to what should be done next.  There are sufficient spares on board to cover such an eventuality but the value of the equipment of the sea floor was large.  It is possible to ‘drag’ for items on the sea floor using grapples on the end of a wire but manoeuvring a grapple on the seafloor from the ship 5 km above is not easy, and attempts to recover equipment this way are, more often than not, unsuccessful.     

Using acoustic ranging to the instruments on the CTD it was possible to locates its position on the seafloor to within about 10m.  Additionally we had an acoustic beacon that could be attached to the trawl wire and used to track the movement of the grapple as it was dragged over the sea floor.    This would give us a much better chance than usual and preparations for the recovery attempt were started.

Later that day the grapple was lowered to the seafloor.  Guided by data from the beacons instructions were passed to the winch operator and ships navigator to manoeuvre the grapple to the location of the CTD.    Then the trawl line started to be winched in again.   Two hours later the data from the beacon on the CTD indicated that the grapple had worked and the CTD too was rising suspended by 1000m of cable below the grapple.    Once the grapple was brought carefully back up to the ship, the slow process of hauling in the broken CTD wire could begin.     Finally, the CTD with all 32 instruments was back on board again 55 hours after the cable snapped.   A great team achievement by all of the officers, crew and technicians.

We are currently preparing the new CTD for its first profile.  Hopefully this one will be less eventful than the last!

The grapple that pulled the CTD wire back up.
A few hours later the CTD itself is recovered

The successful team

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