The challenge of launch and recovery
Leading navies are increasingly looking to deploy “toolboxes” of unmanned drones for mine countermeasures (MCM). Their vehicles must be able to enter mined areas, neutralise the mines, and be safely recovered—often by a mothership. Recovery is especially challenging, and consistent success at sea means the LARS is a central concern: careful design thinking is essential.
Conceptual design—the three major options
There are three broad conceptual choices: ramps, where unmanned vehicles approach the stern at speed to be hauled on deck using rollers and cables; davits operated from the ship’s side, with the unmanned vehicle travelling next to the ship while it is attached and hoisted; and floating-cradle designs, where vehicles enter a cradle at the ship’s stern, to be supported by underwater slings before hoisting. The concepts’ relative merits can be compared across three key areas, which must be robustly addressed to guarantee mission success…
Operability is a make-or-break design consideration. Here, both ramps and davits suffer from severe drawbacks—particularly when flexibility to handle varying payloads is required. The key problem is their intrinsic linkage to the mothership’s movements, which sees them move to a very different degree to the drones—especially in rough seas. This can make handling difficult or impossible above Sea State 3, a weakness compounded by a need to physically attach the drone to secure it. Floating cradles solve the movement problem by designing the cradle and unmanned vehicle to be similar in size, as well as restricting surge, sway and yaw. Combined, these features enable the LARS and vehicle to move together, yet semi-independently of the mothership—the essential key to easy handling.
As mines advance, so do the arrays of sensors needed to detect them. While these can be toughened, they remain sensitive, vulnerable instruments. As well as the cost of damage, loss of accuracy at the decisive moment is a risk that navies cannot take. Buffeting of drones during recovery is inevitable, and the most viable solution is robust 360° protection for hull-mounted sensors. Only the floating cradle concept, with its system of well-designed fenders can meet this need; ramps and davits present inherent difficulties, and their large amplitude movements risk sensors sustaining irreparable shocks in agitated seas.
The handling difficulties of both ramps and davits increase risks to personnel. The challenge of motion and the need to attach, result in a high degree of required effort. This may see significant numbers of personnel operating handling equipment at the deck’s edge—an inherent safety risk. In contrast, the underwater slings and hoists used by floating cradles can be tensioned by a single operator from the bridge.
Floating cradles—the concept of choice for consistent mission success
Working with its offshore partner, NOV-BLM, Chantiers de L’Atlantique has analysed in depth the potential of each of these designs to meet navies’ needs. Drawing on long experience of military and hydrographic projects, it has developed a unique floating cradle LARS that enables an unmanned vehicle to be secured and lifted onto the mothership’s deck in a matter of seconds. The floating cradle’s strengths mean it is reliable at Sea State 5 and, in addition to the concept’s superior protection, rapid recovery further minimises exposure to damage.