A new fibre-rope crane dramatically expands depths to which loads can be lifted
Fibre rope weighs very little in water, meaning that regardless of the length of rope paid out, no additional load is experienced by the crane. Wire rope, on the other hand, is heavier and so as more rope is paid out, the permissible load in relation to depth is reduced. A fibre-rope crane with a safe working load capacity of 150 tonnes can replace a wire-rope crane with a safe working load capacity of 250 tonnes when lifting at depths of 3,000 m and can continue to lift loads at even greater depths.
Further, as fibre-rope technology allows a crane to maximise full lifting capacity at any depths, a smaller crane and vessel can be used, enabling owners to bid on a wider range of contracts. And fibre rope lasts longer than wire, further enhancing savings. Using such technology therefore optimises efficiency not just operationally, but also commercially.
Presenting at the 2019 Annual Offshore Support Journal Conference in London, Macgregor’s project engineer and multimedia developer for load handling systems and machine design Scott Garriott outlined the design philosophy behind his company’s new FibreTrac crane, which was officially launched in a two-day event on 13 and 14 February in Kristiansand, Norway.
Mr Garriott explained that the company wanted to combine known, tested and certified technologies, while developing a completely integrated control system and lift-line management system. It aimed to unite all components and manage them smoothly, acknowledging the importance of accurate rope-health feedback. Finally, MacGregor wanted to work hand-in-hand with a class society to develop a plan for requirements and testing.
Mr Garriott said an overarching imperative of the design was the KISS principle, an initialism for “Keep It Simple”. The design team focused on the principal benefit of a fibre-rope crane being a smaller, lighter tool installed on a smaller, lighter vessel. As such, allowing the product to morph into something with extra operational or maintenance complexity would have been a major error, Mr Garriott explained.
Scott Garriott (MacGregor): Fibre-rope allows vessels to do work that traditionally requires heavier vessels and machinery
The rope itself comprises DSM Dyneema fibres with a DM20 XBO coating, which helps reduce the tension required to bend-in the rope, as well as reducing internal heating and abrasion. The rope is produced as LankoDeep rope by Lankhorst in a 12 by 3-strand braid for heavy lifting.
The crane has a safe working load of 150 tonnes, and an 88 mm diameter. The current crane has capacity for lifting at depths of up to 4,000 m, which would require the use of a 500-tonne steel-wire crane, Mr Garriott said.
The rope is terminated by a specially engineered Applied Fiber socket, which contains individual attachment points for all of the rope’s strands to allow balancing and fitting with strain gauges for testing.
While there is a perception that such rope lacks robustness, is prone to salt crystal or grit damage and UV degradation and has poor heat performance, Mr Garriott countered that this rope has been used in many harsh environments, including military, mining and aerospace applications. Further, he noted that testing has proved it can exceed steel in cyclic bend-over-sheave and tension-tension cycles. Mr Garriott went on to cite the example of Skandi Santos, a subsea vessel owned by Avium Subsea that operates a 125-tonne fibre-rope deployment system. The same rope installed on the vessel in 2010 is still in use, and no salt or grit issues have been reported, Mr Garriott said.
“Allowing the product to morph into something with extra operational or maintenance complexity would have been a major error"
The crane’s Parkburn Deep Water Capstan comprises twin drums, each driven from one side and co-located, meshing together with fingers to provide a compact tension winch. Numerous contact surfaces make for smooth de-tensioning, avoiding fatigue and fleeting forces. The capstan can accommodate numerous types and sizes of ropes, including twin ropes. The drum’s two halves are offset slightly, providing a V-angle that automatically spaces the rope. They are also offset in the horizontal plane to provide an elliptical rope path.
Mr Garriott also detailed some of the technology behind the crane control system (CCS), which incorporates a line-lift management system, an autocooling system and an integrated management system (IMS) to gather data, display relevant operations to operators and also process data for service and maintenance purposes.
Information on time, tension and temperature is fed from the crane’s integrated management system into an algorithm that diagnoses rope health and outputs a conservative rope life prediction, not generalised for the rope as a whole, but for every point along the rope.
A VisionTek system monitors the lift-line and takes high-definition images and accurate diameter measurements of all portions of rope passing through it. A baseline is established once the new rope is worked in and can then be conducted at specified intervals, or when there has been a change in rope status or working conditions. This allows the user to build up a database to assess the rope’s condition, based on multiple parameters.
The CCS auto-cooling system receives inputs from all over the machine, including real-time temperature measurements taken at dual sensor-sets on the two high-tension sheaves and capstan. If these measurements exceed a set value, the CCS can output an instruction to automatically spray the rope.
The system can pick out pertinent data to display to the operator in real time, optimised in a simple traffic-light format. The full database of parameters and rope data that track changes can be accessed by service and maintenance personnel locally, remotely or via satellite, allowing for adjustment of IMS or LLMS parameters to maximise efficiency.
MacGregor first started looking into fibre-rope technology in 2010 and launched its fibre-rope range in 2016, developed via a co-operation agreement with UK-based Parkburn Precision Handling Systems, a developer of a fibre-rope tensioning technology. The demonstration in Kristiansand was the culmination of this work.
“We are confident that fibre ropes are the future for offshore cranes,” said MacGregor’s vice president for advanced offshore solutions Hřye Hřyesen.
He added: “The market has been reluctant to order the first of such cranes. Therefore, we had to build a fibre-rope crane ourselves, to showcase and prove that the technology works.”
The FibreTrac design has been tested, verified and stamped under classification society DNV GL’s ST-0378 Standard for Offshore and Platform Lifting Appliances. It is in the final stages of completing the newly developed standard ST-E407 Rope-Based deployment and Recovery Systems for Designated Service.