How to catch yourself a mooring

On this research cruise we are deploying and (hopefully!) recovering 13 moorings from west of Scotland out across the Atlantic to the longitude of Iceland. In non-oceanographer speak (we are excellent at jargon which I’m trying my best to avoid) moorings are a set of instruments which are left in the ocean for a period of time, in our case a year. These instruments are attached at different depths to a wire kept vertical by floats and stopped from floating away by a weight on the seabed. They say that pictures are worth a thousand words so there’s a diagram below!

One of the things that makes us scientists nervous (but excited at the same time!) is getting these moorings back after they’ve been in the inhospitable ocean for such a long time. Will they come to the surface? Will all the instruments still be there? Will they have data on? Have I momentously screwed up and accidently programmed the instruments wrong or forgotten to press start?!

The moorings are tethered to the weight on the seabed by a closed hook attached to a sound receiver. When we want to recover the mooring we send a series of sound pulses to the receiver which opens the hook and the mooring (hopefully!) floats to the surface. Although we know the latitude and longitude of the mooring we don’t want to be too close in case the wire gets tangled in the ships propeller. Once we think the mooring is on the surface we slowly move closer with several people on deck trying to spot it. It’s sobering to realise how difficult it is to see even several large yellow floats in anything other than a flat calm ocean. Once the mooring is spotted and the ship has edged along-side, a crew member throws a grappling hook over a recovery rope attached to the mooring and hauls the first small float on board. The rest of the mooring can then be connected to a winch enabling it to be slowly and carefully brought on board. The mooring wire which is up to 2km long on some of these moorings is rolled onto drums and every time an instrument or float comes on board the winch is stopped and the instrument/float removed. All in all the process takes around three hours. So far we have successfully recovered six moorings; it will be a while before we know the full details of the data, but from our quick looks it looks as if we have some really interesting things to keep us busy over the next few months. 😀

PS the weather is better than yesterday but the sea is taking longer than we expected to calm down again.

pic1

Diagram of the simplest mooring in the Rockall Trough. The water depth is nearly 2000m (2km) and it is not to scale!

pic2

Crew member throwing a grappling hook over the mooring recovery line in order to be able to attach it to a winch and recover it. The throw was successful first time and got a round of applause!

pic3

Mooring being recovered. Shown are the mooring wire, floats and an instrument that measures current speed and direction.

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