Answering fibre’s loaded questions
The first thought crossing anyone’s mind on seeing Parkburn’s Deepwater fibre rope capstan winch for the first time will probably be ‘What on earth is it?’
In fact, the initial impression is it looks like a bundle of boards thrown together at odd angles. But that, Sam Bull of Parkburn told MJ, “is where the magic lies”.
To recap, the limitations of steel wire in deep water have been much-discussed, not least that the line itself is heavy enough to significantly reduce payload. Neutrally buoyant fibre rope looks like a good, easy-to-handle alternative with the added advantage of being repairable onboard.
However, there are fundamental differences between the two materials that need to be considered: the tension, especially on a conventional drum winch, is critical as soft fibre rope can pull through multiple layers explained Bull; moreover, splices tend to disrupt the even wind and exacerbate the problem.
A traction winch typically achieves a lower spooling tension by placing two drums in parallel. Each time the rope winds onto a drum, the contact friction reduces the tension by approximately 30%: the line then moves onto the next winch, again reducing the load by 30% and so on.
“That’s a pretty aggressive step down, most of the work being done by the first few turns,” explained Bull: “It results in the generation of frictional heat within the rope as it tensions and relaxes plus there’s the impact of constant bending and straightening as the rope travels from one drum to the other.”
There have been attempts to mitigate the problem: for example, one ingenious multi-sheaved solution automatically takes up the slack on the rope by running the sheaves at slightly different speeds and even it manipulates their surface friction to reduce heat. While this works, it doesn’t overcome the constant bending and straightening of the rope as it moves between the drums; more, it’s a complex solution that comes with a fairly large footprint.
By contrast, Parkburn believes it has hit on an alternative that has an inherent simplicity on its side despite its rather odd look.
Each of those 'boards' on Parkburn’s alien-looking contraption is, in fact, a steel finger tilted at a precise angle. This is achieved by merging two, separate winch drums into one unit, each with its own dedicated drive. Each is a just little offset in both the vertical and horizontal plane, creating an elliptical, helical rope path: the idea is that the rope transfers between the drums at top and bottom dead centre while never leaving the winch.
As Sam Bull explained: “Interleaving the fingers and breaking up the contact surface reduces the wrap angle, resulting in less than 4% de-tensioning per finger.” This way, the necessary ‘step down’ in tension becomes much gentler and generally far less dramatic by spreading the load over many more smaller contact surfaces. “Many hands – or fingers - make light work,” he added.
The meshing of the drums into one reduces the footprint and system weight substantially, allowing for a large drum-to-rope diameter. Plus, he pointed out, the open architecture of the drum allows for efficient air circulation, keeping overall temperatures down.
It’s an interesting technology that has been picked up by MacGregor and integrated into the heart of its FibreTrac 1500 crane: in fact it calls Parkburn’s innovation ‘a game-changing subsea tool’. It's worth noting there are a number of industry ‘firsts’ on this crane including a fibre rope termination provided by Applied Fiber as well as Parkburn's lift-line management tool which captures system parameters, VisionTek’s integrated 3D visual monitoring system and a prediction algorithm provided by DSM producing real time data analysis to maximise rope life.
However, Sam Bull explained, this innovative winch could be a breakthrough for more than deep water fibre rope operations, it could also be useful for handling all sorts of softer products where handling under tension is a big issue, including long-length seismic and undersea cables, plus umbilicals of all kinds.
By Stevie Knight
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