Hoists withstand underwater operation

The development of subsea hoists means that underwater as well as on-deck lifting operations for a wide range of functions can be undertaken on jack-up rigs, semi-submersible platforms, drill ships and FPSO vessels. Even undertaking underwater pipeline and wellhead operations down to 70m  can be considered.

The viability of JDN hoists in the underwater environment was demonstrated when its air operated hoists were successfully used for the removal and replacement of a 50 ton rudder on a loaded bulk carrier vessel. After breakdown during a voyage from Brazil to China, the ship was towed to a safe mooring off Venezuela, where the rudder removal and replacement work was undertaken. Three standard air operated JDN Profi 25t capacity hoists were used in conjunction, with the units being subject to full rebuild procedures after the job completion. Work was successfully undertaken at sea, without the need for cargo removal or transfer.

Subsequent to this exercise, the JDN company has introduced purpose designed hoists suitable for subsea operation, incorporating uprated performance and sealing features to withstand continuous underwater operation, even at depths of 70m. They can be supplied for either compressed air or hydraulic operation, using a diver-friendly pendant controller which is compatible with constricted, gloved-hand movements.

This controller is connected by standard length 2m hoses to the hoist which ensures the aquanaut, hoist and load are all located within a close working environment. It is designed to ensure instantaneous and sensitive regulation of the hoist movements, providing infinitely variable speed of the load hook during both the raise and lower hoist operations. The need for voice instructions to/from a surface vessel or rig is obviated, eliminating potential delays or errors created through poor communications. Increased versatility is also provided by the fact that these hook-suspended hoists can also be safely utilised for both horizontal and oblique pulling operations when required. Overload protection is incorporated as standard to cover whatever type of underwater work is undertaken within the demanding inner space environment.

The hoist lift chains provide a 3m raise/lower capacity as standard for variable load options of up to 3.2 or 6.4t respectively. Variations on the length of lift chains or hose feeds to the pendant controller can also be changed for greater versatility. Hoists designed for higher or lower load limits, together with products capable of operating at depths greater than 70m could also be manufactured if required.

The hoist construction uses stainless steel for smaller exposed nuts and bolts together with valve heads, with grease packed covers fitted for larger fixings and wheel bearings. The remaining exposed metalwork is finished with three layers of traffic yellow RAL 1023 marine protective paint to a 300 micron dry thickness, as normally designated for offshore, salt laden operations. These purpose designed hoists can now accommodate heavy duty, high performance operations with minimum maintenance or lubrication over extended periods of subsea operation. The fact that the hoist, diver operator and workpiece being manipulated are all retained within a regulated, visual distance ensures that maximum control and safety is maintained during difficult handling procedures.