Safe deliveries to the seabed
A subsea system capable of delivering large (and expensive) modules to depths of 3,000m without bouncing them on the bottom has been developed by Cargotec for the benefit of medium sized support craft.
The Module Handling System, powered by an electro-hydraulic unit. has a tower structure with top sheaves and vertical cursor rails. The system is equipped with a main liftline winch located below deck which has a capacity of 120 tons at 2,000m depth. A key feature is the winch’s very precise Active Heave Compensation system, based on a Motion Reference Unit (MRU), giving the winch the ability to keep the module lowering at a fixed rate relative to the seabed rather than the boat.
‘If you are going to place a module worth $10m you need a constant approach, you don’t want it to bounce. You can’t land anything on the seabed if you simply rely on the boat, which could go up and down by over 6m’, explained Øystein Bondevik of Cargotec’s daughter company MacGregor Hydramarine in Norway, adding that this difficulty is compounded by unpredictable wave frequencies, not just heights.
The system also includes three individual Active Heave Compensated guide line winches with a 10 ton capacity, which are located in the top of the tower and can act together or independently. These take a guiding wire to the seabed through the load, so that the module will be able to slide down the lines right into its subsea location, placing it neatly where needed.
Aside from this, the tophook cursor has a lifting capacity of 100 tons to help slide modules easily up and down through the tower and moonpool, reducing the likelihood of any part of the module damaging either itself or the tower in rough seas. There is also a deck skid system including tractors to support loading the module into the tower.
The rest of the €20m contract brings together a 150 ton subsea crane with a 3,000m wire capacity that also benefits from the active heave compensation. It is capable of man riding, and is equipped with an auxiliary winch plus two tugger winches.
There is also a smaller five ton offshore crane capable of 300m water depth, and two ROV launch and recovery units, one of which is operated from a second moonpool in its own integrated superstructure, the other allowing side launching from an A-frame.
