It is known that some Single Point Mooring hawsers have failed at sea after being subjected to tensile loadings never higher Than half their original breaking load.
Previous work has shown relationships between rope fatigue life and maximum cyclic load in tension in the form of Sc-Nf curves relating these two parameters. Results from this work are analyzed and compared and the influence of Fop material, rope construction, rope terminations and other factors upon fatigue life discussed.
These results are compared with data recently obtained by ourselves and other workers on the rope yarns and filaments from which SPM hawsers are constructed. This comparison gives further insight into possible mechanisms of man-made fiber rope fatigue and thus on how premature failures may be avoided.
Polyester hawsers are shown to have many times the wet fatigue life of nylon hawsers whether the basis of comparison be equivalent size, equivalent wet strength or equivalent energy absorption. The differences are significant statistically and depend little on rope construction. It is shown that the relatively poor performance of wet nylon hawsers could arise from poor internal abrasion resistance and high internal friction within the rope, and from the high visco-elastic damping within the nylon filaments; and that these effects could assume particular importance at cycling periods corresponding to tanker yaw.
Ropes have many hundreds of different uses. One can distinguish between intermittent uses; such as slings, mooring and towing ropes, fishing ropes and yacht sheets, and permanent applications such as Tethered Leg Platform tethers, guyed tower tethers, antenna stays, navigation buoy tethers and SPM tethers and mooring hawsers. 1,2
The life of intermittently used man-made fibre ropes depends on their resistance to impact, abrasion, fusion, oil, chemicals, heat, cold, seawater and sunlight, and data on these factors is available. Ropes which are permanently deployed in a marine environment must also be resistant to hydrolysis, micro-biological attack and, in certain waters, fish bite. Perhaps the most important factor affecting their life, however, is "fatigue".
Fatigue failure has been defined as "the failure of a structure by repeat loading". This definition embraces failures due to creep, hysteretic, structural realignment and abrasion in cycling as well as failures in the basic molecular structure. Many thousands of fatigue tests have been made on metals and the results reported in hundreds of papers "yet even on metals there has been no full understanding of the phenomena and designers have had to rely largely on test data supplemented by empirical rules and formulae".
Fatigue data on man-made fibre ropes has appeared in less than 20 papers and little progress has been made towards understanding the phenomenon. As the search for oil moves into deeper waters so the need for lightweight tension members will increase. Such members will have to work for longer periods and at greater static and cyclic stresses than today.
The study of rope fatigue will thus become of increasing importance.
