Comparison of Fatigue Data for Polyester and Wire Ropes Relevant to Deepwater moorings
Stephen J Banfield, John F Flory, John W S Hearle, Martin S Overington
Polyester ropes are now established in deepwater moorings, but more research is needed to understand long-term durability and to optimise systems. Three categories of rope failures – environmental, surface damage, and structural – are not considered to be likely problems for deepwater polyester moorings which are properly designed and used. Fatigue of fibres in the rope is thus the concern of this paper. The fatigue mechanisms of interest are creep tensile fatigue, compression fatigue and internal abrasion.
Laboratory test data on potential fibre fatigue mechanisms is reviewed with reference to typical mooring system service conditions. For a 20 year life, a typical mooring might experience 60 million cycles, mostly due to small waves. The occasional severe cycling which might be experienced during storm conditions will cause little fatigue damage. Tensile fatigue and creep rupture will not occur in polyester fibres at the applicable low loads. Hysteresis heating is not significant for strain amplitudes less than +0.5%, but might be a problem with the larger amplitudes which might occur in shallow water moorings. Internal abrasion might begin to take effect after millions of cycles. Axial compression fatigue might occur after a large number of cycles in rope elements that go into compression, even though the rope as a whole remains under tension. Existing fatigue data on large spliced ropes is compared with data for steel wire rope given in API 2SK. The demonstrated polyester rope fatigue life is at least as good as spiral strand and better than multistrand steel wire rope.
Computer modelling of rope performance is discussed. Under conditions designed to demonstrate fatigue mechanisms, the model shows an early drop in strength due to hysteresis heating and a second drop as some yarns fail in axial compression fatigue. After a very large number of cycles, internal abrasion takes effect and finally creep rupture occurs.
The general conclusions are that, polyester ropes are suitable for deepwater moorings for expected lifetimes of 20 years and that their potential fatigue performance is at least as good as spiral strand steel wire rope at loads representative for FPS mooring. But more research may be needed to identify and quantify active fatigue mechanisms to enable engineers of mooring systems to satisfy regulatory bodies and certification authorities.