06 February 2022
We're going through some difficult days and this Sunday is a very windy day. Launching operations are impossible with winds of force 9 on the Beaufort scale (75 to 88 km/h). We're taking advantage of this moment to share with you a behind-the-scenes look at the immersion of BathyBot, the underwater robot of the MIO – INSU – OSU (CNRS).
The photo shows BathyBot being lowered into the water in its BathyDock. The boat is launched from the aft deck, just like the other instruments. So far, nothing too different. However, our dear little robot is one of the lightest pieces of equipment sent to the site. It weighs just a few hundred kilos in the BathyDock, unlike the infrastructure of the Scientific Junction Box, which weighs several tonnes. OK, but what difference does that make? It's important to remember that all objects destined for the bottom are sent with a target landing point. The launch is considered to have been successful when the aircraft arrives within 10m of its target point. Remember, the Scientific Junction Box landed 3m from its target, which is a very satisfactory score! Well, when it comes to launching a lighter aircraft, this precision of delivery becomes more complex.
There's another complicating factor for the teams. As you can see, the launching cable is made of textile, which makes it much more sensitive to the current and the movement of the boat than a rigid steel cable - and even more so when it is carrying a light object. However, a steel cable, although more rigid, would be much more dangerous if it broke, as it could crush an instrument on the bottom and damage it. The choice of a flexible cable is therefore a compromise.
The sailors pull out all the stops to make a success of these long and delicate operations...
So how do they manage to position themselves accurately? The team on the aft deck communicates with the control point on the bridge - or wheelhouse. In the photo, we see Eva, the multi-skilled officer in charge of positioning the boat, and the BathyBot scientific team monitoring the operation. Their eyes are riveted on the control screens showing the position of the BathyDock underwater and the ship's trajectory. An important detail is that the Why not? cannot anchor in this zone (anchoring means staying at anchor) because the depth below the boat is 2500m. It therefore has two choices: either to drift away and catch up with its position, or to be constantly sailing. And during the launching operations, he opts for the second option: the ship's engine remains running so that he can adjust and maintain the object's delivery point.
There are many attempts at leadershipThat evening, the textile cable, an impressive 2400m long, was subjected to the west-ligature current. This is a constant current known to the area, but it is quite strong, which slows down the operation. Work started at 5.30pm and finished at 8.30pm, taking 3 hours to release the BathyDock!
On her screen, Eva uses data from the BUC, a positioning system placed on the cable 15m above the BathyDock. The BUC emits a signal that gives her an idea of where the object is underwater. To send the object to the target point, it tries to match the BUC signal with the coordinates of the target point. However, the BUC does not emit a continuous signal: the location is only signalled every 10 seconds. It's not easy to manoeuvre to send an object downstream to a point 2400m below the surface!
Photo credit: Nicolas Fromont - @nicolasfromontphotoThis is how the bridge team communicates with the afterdeck team. The black line that looks like a tangled loop is the ship's track; the small circle a little to the left is the target point to aim for during the launch; and the green dots are the history of the signals emitted by the BUC, where the object is located. The screen shows 7.12pm, they still have 1 hour 15 minutes to go and there are already so many different green dots, it gives you an idea of the object's path underwater... Targeting a point with an object suspended from a 2.4 kilometre soft cable is a real challenge!
Despite these difficult conditions, BathyDock managed to land 10m from its target point. Congratulations to the teams involved! Thanks to them, BathyBot is in place!
Photo credit: Nicolas Fromont - @nicolasfromontphoto