By Richard Austin-Berry
Preventative maintenance is paramount in the fiercely corrosive environment at sea. No time is wasted once the Autosub6000 has landed back on deck following a deployment. Moving parts are stripped from the vehicle, fully flushed and cleaned, before being packed with environmentally developed grease to preserve components during forthcoming deployments.
Another significant part of the Mechanical Engineer role includes Launch and Recovery System (LARS) Pilot. The deployment and retrieval of the AUV from the ocean is a particularly precarious and risky part of our operations, with the vehicle potentially being subject to forces of up to 6G (6 times that of gravity), and requires the coordination and manual skill of the Captain, crew, and LARS Pilot:
Whilst the Sarmiento de Gamboa can operate on auto-heading and doppler-positioning to hold steady in the water, to do so requires intermittent autonomous use of tunnel thrusters using PID control (a bit like cruise control in a car). These thrusters suddenly firing up can cause eddies along with high and low pressure zones at the side of the vessel, potentially sucking the submarine under the ship and into its exposed propellers, inflicting terminal damage. Controlling the ship manually lowers this risk where control can be more subtly applied.
The ship’s crew are incredibly well rehearsed in overboard operations aboard research vessels. They grapnel the Autosub6000 retrieving recovery lines (i.e., using a pole with a hook on the end to latch onto the wires), attach to the LARS, and hand the control over to the LARS Pilot.
During these operations, the LARS Pilot will carefully observe the swell of the ocean as Autosub6000 patiently awaits recovery. The hydro-dynamicity of Autosub6000 will cause it to head into the swell, and the LARS Pilot needs to identify optimum moments to recover… fuelled by adrenalin and the temporary ability to slow down time!
Once the LARS Pilot has control, Autosub6000 is carefully guided towards the ship, paying close attention to all conditions. Too much exertion on either line can turn the heading of Autosub6000, which, assisted by swell, can send it careering towards the side of the ship. A harsh blow to the ship’s hull can result in significant damage to Autosub6000 and its science payload, causing much undesired system downtime for repairs.
Once Autosub6000 is directly underneath the LARS carriage, it’s the moment to hoist from the water. But, a swell washing over it at this point can lift Autosub6000 faster than the winches can run, creating slack line. As the wave passes and the seawater level drops away, it’s like dropping a 3000kg mass onto a dynamically moving and accelerating floor, with 1400kg (the flooded volume mass) that doesn’t want to be arrested.
Get it right, and it’s as though the ocean gently passes the Autosub6000 back to the ship. Get it wrong, and Autosub6000 can be violently thrown around. It’s a matter of intuition, experience, and clear communication between all parties concerned that governs the safe recovery of Autosub6000.
Millions of pounds have been invested into the Autosub project. Years of planning have gone into the expedition. And here we are, live at sea, with just a handful of opportunities to get everything right.
But it’s the acquired data where the value of Autosub6000 truly lies. It’s this data that can give today’s scientists insight into the health of our ocean, and this data in turn can educate the generation of tomorrow, that we might be able to manage the ocean sustainably and preserve one of the planet’s most precious resources.
So, no pressure then…