Grab Excavation System

Seatools supplied this Grab Excavation System (GES) to Deeprock CV, a joint venture between the two renowned dredging contractors Boskalis and Tideway.

The GES underwater tool weighs approximately 75 tons and is essentially a heavy-duty hydraulic clamshell, operated and positioned by remote control down to a water depth of 1000 metres.  The ‘ROV frame’ above the clamshell contains essential subsea utilities, including power packs, an electronics pod and survey equipment. Hydraulic power for the thrusters and the clamshell cylinders is provided by two hydraulic power packs rated at 200 kW each.  All of the equipment on the grab was designed to be robust, durable and easily maintained, taking into account not only common offshore conditions, but also the effects of shock loads and floating debris.  The GES underwater tool can be equipped with various sized clamshells as well as being adapted for operation with other tools, e.g. for underwater mining.  The largely automated GES is positioned via hoist winches and thrusters.  The operator selects the target position for the next bite and the control system automatically places the grab at the desired coordinates.  Despite vessel motions, a positioning accuracy of around 20 cm allows for highly efficient dredging. Extensive simulations were carried out as part of the design process, enabling the correct operation of the motion compensation system to be demonstrated in-house at the Seatools factory before the grab ever touched ground.  These elaborate preparations paid off handsomely, accelerating the progress of the sea trials and facilitating offline optimisation of the system’s performance at a later stage of the project.