By Rita Markina
On 15 July, we recovered the first mooring system at the easternmost point of the OSNAP array. A mooring is an anchored system that consists of many instruments at different depths attached to a long wire. It stays in the ocean for many months and helps us to continuously measure ocean properties like temperature, salinity and the velocity of ocean currents. The mooring is held in a vertical position by several buoys of different sizes located at certain water depths along the wire. During our expedition, we are recovering and deploying eight moorings. The first one that I’ll write about today spent around two years in the ocean.
During the recovery process, the ship first comes close to the location where the mooring was deployed in 2020. The technicians from the National Marine Facility (NMF) in Southampton talk to it from the ship using an acoustic transducer. Once they receive a response from the mooring, the acoustic release is activated and disconnects the mooring from the anchor – and the whole mooring rises to the surface waiting to be taken on board. The wire can get quite tangled on its way to the surface – so the instruments and floats do not necessarily come onboard in the exact same order they have been initially deployed.
Once the mooring is visible on the water surface, the ship comes closer and all sensors and buoys are brought to the deck. After spending two years in the water, the instruments and buoys are covered in seaweed and clams which are being heroically cleaned out by the mooring team on the deck. After this, all instruments are being brought into the lab, where the mooring team downloads all the data.
Buoys and instruments that are closer to the surface are more prone to overgrowing – for example, the buoy that was on 50 meters is all covered in seaweed and some small shells and clams, while the one that has been on 500 meters is almost clean. This happens because no light penetrates to this depth, and there is way less life there.
Following the recovery of the last set of floats, the acoustic release system is also taken out of the water to be redeployed again on a next mooring. To release the mooring wire, instruments and buoys from the anchor, the releaser opens a gate that connects the mooring wire to the anchor. The release mechanism is activated by a specific acoustic signal sent from the ship by the NMF technicians. Once detached from the anchor, the buoyant floats along the mooring allows is to rise to the surface. The acoustic release system consists of two individual releasers to add a measure of redundancy. Each of the releasers is able to open the gate – so even if one of them wouldn’t work, there is a second one that acts as a safety net.
This last video shows how the last chain of floats and the acoustic release are being taken out of the water and the ship sets sail. In one of the next posts, I’ll tell about how to deploy a mooring – this process turned out to be quite nuanced as well. Stay tuned! 🙂