Mooring works (1/2)

We left St Johns some days ago after a short stop of just a couple of hours. Heading towards the Labrador Sea preparing for the work to come. During the last days our focus was on the mooring array at 53°N consisting of 7 mooring.

First, what is a mooring? In very simple words it’s a long wire with measuring devices attached and an anchor weight at the bottom. Mooring are deployed for longer time periods and measure timeseries of various variables at a certain position. Those timeseries are very valuable for us to find variations and trends on different timescales. As the instruments are running by batteries and are exposed to the sea, they need to be replaced every once in a while. In our case this means every two years someone needs to go here. So, we are recovering and servicing the moorings deployed by the MSM74 cruise about two years ago.

When we are at a position of a mooring the recovery follows a clear protocol. First send an acoustic signal to the releaser who let go of the anchor weight enabling the rest of the mooring with hopefully all its instruments to float to the surface. Now we need to spot them. Therefor you may find a lot of people up at the bridge to look for it and try to find it first. The ship approaches the mooring and then the work starts.

Approaching a floating mooring (yellow dots in the middle right)
Photo: Tom Ederleh

Crew members, technicians and scientist everyone is involved in the recovery. With a big winch and a crane meter by meter the wire comes on board and everything that’s attached to it. The floating elements will be demounted and need to be cleaned. Within two years in the sea a lot of algae, mussels or other stuff might be growing on them.  The instruments need to be detached, cleaned and brought to the lab to check if they have been running as they were supposed to. The data will be saved, the batteries changed, and the instruments prepared for calibration. The wire is directly put on another winch to not create a chaos on deck with more then 1000 meters of wire lying on deck. Depending on the length and the number of instruments in a mooring this whole process might take up to a couple of hours. Those are parameters you know before and can plan with it, but while floating at the sea surface such a long wire can get some impressive knots and be wrapped up in itself in a way that adds a lot of complexity to the recovery.

Floating elements and two instruments coming on board.
Photo: Christiane Lösel

That it not always goes the way you have planned the mooring works is what we learnt right at our first station. Having quite nice weather all the way towards the station it turned out to be very foggy, what brings the risk of not finding the mooring when it is floating at the sea surface. So, the very first recovery of the cruise needed to be postponed. 

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