The future is fibre

Interest is slowly picking up in fibre rope and now Rolls-Royce has begun to offer cranes up to 400 tonnes based on this field-proven but still innovative technology.

The most well-known advantage of fibre rope is its neutral buoyancy – as a result the deck gear doesn’t need to take the weight of the lifting line as well as the load, Johnny Aarseth of Rolls-Royce told MJ, a characteristic especially relevant to deep water deployments.

“If you can lift 150 tonnes on deck, you can take that down however far you like, even to 5km,” he explained. This means that given the right equipment, far smaller vessels could be potentially used to carry out deep sea installations: at 2,500m depths a 250 tonne fibre rope crane can do the same work as a 400 tonne steel wire crane and the lighter weight also puts less strain on stability.

However, there’s a potentially even bigger advantage: fibre means operators can stop “working blind” said Mr Aarseth. As wire can’t be inspected internally without specialist equipment, the entire length of wire on a winch gets discarded when the schedule indicates its working life is over: “This is simply not cost effective,” said Mr Aarseth. Further, the management criteria vary widely and are by no means failsafe: despite the precautions “the wire still breaks occasionally” he added.

By contrast, “with fibre you can see every single strand right down to filament level”, said Mr Aarseth. “Someone standing on deck can soften and open the structure by hand, that even applies to ropes that cope with hundreds of tonnes.” Once normal stresses are applied, it simply closes back up again. Further, it’s inherently safer: it’s much lighter and there’s not the same kind of ‘spring’ in fibre braids that, if broken, will send it whiplashing back with devastating consequences for those in its path.

There have been challenges to overcome: one particularly ‘knotty’ issue is that the friction between the fibres causes heat to build up during continual bending over the sheaves, an action typical of Active Heave Compensated systems.

While part of the solution has come from advances in rope manufacture, it’s also partly down to the development of onboard deck kit. “The smaller the radii the more friction, so we have introduced quite large sheaves on this system, larger than you see for a steel wire winch,” said Mr Aarseth. Further, Rolls-Royce has also introduced a water cooling element on each bend which has proved to be effective “even in places like Brazil where you have naturally high deck temperatures”.

Most importantly, the different sheaves in the winch system individually compensate for lengthening of the rope when loaded, controlling the spooling tension by using a cable traction control unit (CTCU), “So, no matter if you have empty hook or a maximum load, you will still see the tension on the storage reel varies little,” said Aarseth. This doesn’t just stop the slip-stick effect or the ‘cutting in’ of unevenly spooled rope, it also means “you can recover the winch with an empty hook, pick up the maximum load and deploy it straight away”, he said. A normal, direct pull winch would have to be rewound with a payload to prepare it for the next operation.

Rolls-Royce also applies a health monitoring system “which allows tracking of each section of rope - telling you the remaining life left in it”. This is based on the number of times each section of rope is bent using a formula derived from line tension, accounting for the differing fatigue from light or heavy loads.

Therefore when the read out indicates that a length of fibre rope has become worn, it’s light enough that crew can lay it out on the deck, find and cut out the damage and splice the still-useful lengths back together: the operation taking just a few hours.

Of course, however neatly done, splices result in a thicker rope. Run that over a traditional two barrel traction system and the result is a localised diameter increase, introducing risk of overloading or even locking the winch system. Usefully, the Rolls-Royce CTCU with its automated tensioning mitigates again uneven, sharp loads being passed down the rope.

Fibre rope does require “a different approach”, admitted Mr Aarseth and at present accepted standards are still lagging behind the reality.

Despite that, this crane development shows the slow, steady pull that fibre is exerting on industry.