Terminating a knotty problem

Written by: Tom Shelley | Published:

Tom Shelley reports on a dramatic development in an ancient technology that’s still a cornerstone of many types of engineering



By adopting a new method of terminating ropes, they can carry 25% higher loads and last for 40% longer.
The research that led to this finding started out by looking at thin ropes for use with paragliders and then moved on to thicker ropes for use in the offshore industry. Other applications range from yachts to heavy lifting equipment, and it is pertinent to ropes made of traditional materials, high-performance polymers and steel wires.
Dr Hossein Saidpour, a principal lecturer and reader at the University of East London, has spent a number of years investigating ropes, developing improved technologies that are only just now coming to market. His research began while he was at Bournemouth University and a manufacturer asked him to look into rope malfunctions in paragliders, which were leading to four of five failures per year.
“If one rope failed, the other ropes overloaded and also failed,” says Saidpour. “The failures happened at the terminations, in most cases.”
These ropes are made out of high-performance synthetic fibres and are about 2mm in diameter. Over a long period, rain and sun lead to their deterioration.
The problem was given to a then student, Mehran Koohligani – now a senior lecturer and programme leader in design engineering at Bournemouth – as his PhD project. Saidpour explains: “We looked at the properties of the ropes in the laboratory and the process of damage, especially irreversible damage using acoustic emission.”
Sensors were attached to the ropes that were loaded up, and the detected sound waves captured and analysed. Thermography was also used as the ropes were loaded, in order to detect high stress areas where more strain energy leads to higher temperatures. Paraglider ropes from new and used products were studied during the three years of the project.
It was found that the damage resulted from several causes, both along the lengths of the ropes, but more in the region of the terminations. One of the identified problems was the steel eyelets and part of the recommended solution was to make these rings out of something with a similar modulus to the rope.
Another problem area was the stitching used to join the rope ends to the main body of the rope. Again, the recommendation was to use a stitching fibre with a similar modulus to that of the rope and also to adopt a stitching pattern that would not go into the rope and damage it.
It was also recommended that acoustic emission be used to test all ropes with their terminations attached. If any produced more than a certain emission at a certain loading, it should be rejected, states Saidpour. It was further suggested that ropes should be tested every six months to a year, depending on usage. Laboratories have now been set up to do this and, by and large, the recommendations have all been adopted and rope failure-induced accidents virtually eliminated.
This has attracted the interest of the offshore industry, which uses mainly polyester ropes, but with different numbers of strands, braided and non-braided, and with a large number of types of termination. The consequence of failure of mooring lines has the potential to result in major disasters to ships and mooring platforms, apart from the cost of the ropes themselves.
“We looked at abrasion resistance, since ropes rub against each other, and again used acoustic emission, starting with one fibre bundle and then adding more strands,” explains Saidpour. “We looked at ropes at room temperature and high temperatures, and under wet and dry conditions, and under cyclic loading.”
Many of the ropes used today are mostly terminated with what is termed a ‘Parafil’ termination. This depends on inserting the rope into a steel or aluminium alloy socket, which tapers outwards internally away from the direction of tension, with a spike inserted from the free end in such a way that tension on the rope pulls the rope and spike into the taper, securing the rope strands by a wedging action. The design was invented by ICI and patented by the company in 1988. The only problem is that the wedge portion of the spike applies a greater crushing force on trapped fibres and causes fibre damage. Another problem is that the rope becomes abraded between the socket and the spike, substantially reducing the breaking strain of the rope. Also, the fibres can become abraded in the mouth of the socket, which experience has shown is where failure of the rope is most likely to occur. Other, more traditional, methods of rope termination cause even greater damage. One has only to think of the traditional way of securing steel wire ropes with a series of clamps and how broken wire is all too often found immediately adjacent to them.
Modelling ropes themselves is a well-established practice such as by a package called OpTTI Rope, developed by Tension Technology International. Modelling ropes and terminations is less well established, so computer modelling of improved terminations – using Ansys in order to minimise stresses – formed an important part of the study.
In Saidpour’s words, the end result was that the team “came up with an epoxy polymer socket and impregnated the rope in such a way that the termination incorporated a certain amount of give.”
They found several ways of achieving this. One was to introduce reinforcing material inside the rope end portion in the region surrounding the apex part of the spike. This can, if necessary, extend beyond the mouth of the socket. Another is to do away with the spike and pour in epoxy in such a way as to form a spike, but one that is more compliant than steel. For commercial reasons, the precise details of the final design cannot be disclosed, except to say that Yorkshire rope experts Bridon Marine are now implementing it.

Pointers

* It is highly desirable to make rope terminations out of materials that will give them a similar modulus to the rope

* Acoustic emission under loading is an excellent way of testing ropes – which should be undertaken regularly, since they deteriorate over time

* A new rope termination socket has been developed that increases load carrying capacity by 25% and its life by 40%



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