25Te AHC Launch and Recovery System

*AHC Lauch and Recovery System

Denith Engineering was requested to design, manufacture, install and commission this Launch and Recovery system. The 45 Ton SWL winch is capable of working to a depth of 6 000 meters on the salvaging vessel MV Endurance. 

Denith Engineering had previously supplied a tower and a passive heave compensated moon pool and winch to the vessel for launch and recovery of their ROV. 

Due to the winch application being of a complex nature an additional requirement was to compensate for vessel motion up to a load of 25 Tons. The requirement allows the winch to work safely with tools close to the seabed ranging from 2 to 25 ton and to be able to lift a much larger payload (45 Ton) when required. At these working depths the use of steel wire ropes becomes problematic as the rope itself has considerable weight thus requiring much larger capacity winches to compensate for the rope weight.

Dyneema fiber rope

Therefore Ø62mm Hampidjan Dyneema fiber rope, which is neutrally buoyant in seawater, was selected to keep the winch as effective as possible. To compensate for vessel motion, an active heave compensation control system was installed with the winch. The first installation was a primary control drive with closed loop control by means of a PLC and input from a motion reference unit installed on the vessel. Two 700 kW Diesel motors were used to drive the hydraulic pumps and thus delivering energy to the system. Primary control means that the control lies at your primary power source i.e. the pumps but due to the size and complexity of this installation and in order to reduce control losses due to pipe lengths a control block with a high response valve was installed onto the winch to control the 8 dual displacement motors installed. 

The installation was complete with an over boarding A-frame with 12 meters available clearance under the sheave and 5 meters between legs to launch and recover loads from and to the deck. The drum has a distance of 5 meters between cheek plates to store the 6000 meters of rope and keep the number of layers effective in regards to power requirements as torque requirements increase. To minimize the fleeting angle on a 5 meter drum with very limited deck space, the complete winch was indexed side to side by hydraulic cylinders thus keeping the rope in-line with the sheave.

Upgrade of control and drives

In 2015 the client requested Denith Engineering to upgrade the control and drives of the system. A higher accuracy requirement was achieved by installing a Bosch Rexroth secondary control drive system onto the winch. Secondary control means that control is achieved at a secondary location or mover. This means that the control was now relocated to the motors on the winch with direct input from the Motion Reference Unit (MRU). The motors were changed out from a dual displacement to variable displacement motors with a high response control valves mounted to each motor. Each motor swash plate is monitored and controlled by a linear transducer and works on a master slave concept for the 8 drives installed.

Decommission and store

The clients’ lease with the vessel ended and Denith Engineering was tasked to decommission and store the winch for repurposing. The decommissioning and removal from the vessel was successfully completed in 14 days as allowed for by the client.

Modifications and reinstallation

Denith was tasked to refit the current winch to accommodate for the new installation onto the Seabed Constructor, a high tech survey and salvaging vessel operated by Ocean Infinity. Due to layout challenges the winch was installed in a space frame. This space frame was fitted with a moving spooling sheave to control the fleeting angle onto the drum and a bend sheave to align the rope with an overboarding frame installed on the opposite side of the vessel. For overboarding a “C” Frame design was required in place of a normal “A” Frame type as space on the vessel was severy constrained.

Data collected from Denith Engineering, Ocean Infinity, and Google research. Compiled by Chantelle Venter at ExecuStreet 

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