Rope Fuse Technical Requirements
To function as a fuse for rope MoorGuard must have:
The ability to dissipate dangerous potential energy, at a known level, below the minimum breaking point.
The ability to safely hold the load after activation.
Though not necessary for a fuse to work but to be as safe as possible for humans it would be beneficial if the fuse could provide:
Warning of an overload situation.
Extended worktime to allow corrective action.
The ability to release (part) in a manner that prevents snapback.
The technological heart of the MoorGuard fuse is the innovative, proprietary fiber used to make the MoorGuard Rope.
Rope made from this fiber is unique in its ability to elongate, at a nearly constant force, without recovery.
Utilizing the principle of plastic deformation, the rope has four unique characteristics that make the creation of a practical fuse possible.
Under normal loads it behaves as a typical rope until a specific load is applied called the Minimum Elongation Load or MEL. The MEL is usually matched to the SWL or Safe Working Load of the mooring line for the OpenWater series of fuses.
For the LineSaver line of fuses the MEL is calculated to match the minimum capture force required for the system. For more information on LIneSaver fuse design go here.
Once the MEL is exceeded the fiber elongates or stretches until the force on the line drops below the MEL. If the force drops below the MEL anytime after elongation has begun the rope will cease stretching and again behave as a normal rope. If a load is maintained on the rope in excess of the MEL the rope will eventually stretch and collapse.
The second characteristic of the rope is the ultra high stretch – in the range of 8:1. A one meter long section of fuse will stretch to about eight meters before collapse. This extremely high stretch is key to making a practical rope fuse. Unlike an electrical fuse that opens a circuit when overloaded a rope fuse can’t open a circuit, the ship would float away. The ability to stretch and maintain control of the load is crucial to a rope system fuse. The extremely high stretch also gives added warning time to correct unsafe situations.
The third characteristic of the fuse is the uniform resistive force of the rope. High stretch without the ability to hold the load securely makes a rope fuse worthless. Several anti-snapback mooring lines have addressed the safety question. In all cases the warning is very short – but the greatest problem is after the load bearing element has failed the anti-snapback element has as little as 5% of the strength of the original rope. Snapback may be prevented but the ship will be cast free. MoorGuard fuses will hold the load after activation at the MEL which, if engineered to match the Safe Working Load (SWL) of the mooring line, means that while stretching the MoorGuard fuse is holding the ship at the same strength level as the un-activiated system.
The fourth characteristic of the rope is that it does not ‘break’ in the typical sense. By design, every fiber in the rope sees a different stress strain history. As a result the rope does not break in the classical sense rather failure occurs in a cascading fashion. It collapses more than it breaks as a result of millions of individual fibers breaking at different times, sometimes as much as several meters apart. This unique failure mechanism is what dissipates the potential energy in the system during elongation and guarantees that at release a MoorGuard fuse will not snapback at the same force as an unprotected line.