Cyclic Testing of Continuously Wetted Synthetic Fiber Ropes

ABSTRACT A laboratory study of the cyclic loading endurance properties of continuously wetted synthetic, or manmade, fibre ropes has been conducted on ropes of various materials and forms of construction. The materials investigated were polyamides, polyester and polypropylene. Forms of construction studied were braid-on-braid, eight strand (square) and parallel lay. Rope sizes researched were 100 kN (10 tonf) nominal breaking force and up to 1.0 MN (100 tonf) nominal breaking force. All specimens were single leg. The test results indicate the superior bollard abrasion resistance and interstrand friction wear resistance of polyester and polypropylene over the polyamides. Trends, where observed, are also reported upon for the cyclic endurance performance of the forms of construction investigated. The improvements (or otherwise) in endurance effected by sheathing, coating and thimble protection of the eyes are also discussed. The results and performance trends observed on this test programme will be of interest to designers and users of mooring or tethering systems which operate continuously wet. INTRODUCTION In 1977 the Department of Energy (UK) set up the Wave Energy Steering Committee (WESC) to advise the department on the feasibility of wave energy recovery as an alternative power source. In addition to reviewing the multiplicity of designs and philosophies ensuing from the study, the committee, through its Technical Advisory Group 4 (TAG4), was instructed to report on anchoring and mooring aspects of such offshore structures. This assignment included providing guidance on the possible future use of tethers produced from man-made fibres. Thus the National Engineering Laboratory (NEL) was commissioned to carry out a programme of cyclic tests on continuously wetted man-made fibre ropes of a variety of materials and forma of construction, in the small- and medium-size range. The ropes were obtained from several manufacturers and the selection concentrated on braid-on-braid, eight strand (square)and parallel laid forms of construction. The chosen materials included polyamides, polyester and polypropylene. Studies involving long-term cyclic runs are, of necessity, very time consuming, hence the establishment of useful force/endurance curves for ropes is in its infancy. Previous publications l - 6 have reported mainl on small rope cyclic testing or large polyamide rope cyclic testing, but there is still a great need for cyclic test research over the wide range of man-made fibre ropes. This paper is a brief summary of our S-year test programme for WESC/TAG4 to report on the results and trends discovered this far. TEST PROGRAMME As a datum it was decided that each rope type, size and material should be tested under wet conditions in the unprotected state, hence no sheathing or thimble protection was applied to the eye splices of braid-onbraid or eight strand (square) ropes. Similarly, parallel laid ropes were tested initially with the maker's standard grade terminations. Thereafter, the programme would include cyclic tests with ropes employing eye protection of polyester cloth sheathing or urethane moulding, with or without thimbles. Provision was also made for testing of improved terminations for parallel laid ropes.