A high cycle fatigue test programme up to 40 million cycles has been conducted for the first time on 10, 150 and 250 tonne polyester ropes and a new design TN curve is proposed. The fatigue test duration achieved has surpassed any other work to date and represents the most comprehensive data set of its kind. Polyester rope shows a very significant increase in fatigue perfomance over steel wire rope.
A new fatigue curve has been established for polyester deepwater mooring rope. The establishment of the fundamental fatigue mechanisms will enable proper planning of through life integrity management systems.
In addition to generating fatigue data covering endurances to around 20 million cycles, two ropes (10 tonne and 250 tonne) were each tested to 40 million cycles without failure. Post test examination and textile yarn residual strength tests revealed that no scale effect was apparent in terms of internal wear (or any other fatigue mechanism).
The study has conclusively established the prime fatigue mechanisms that may operate over platform lifetime. The magnitude of any strength loss, if any, is expected to be very small.
Strength loss due to internal abrasion, under low mean and high load range, resulted from the slip between strands within the subropes as the helix angle changes due to changing load.
There was no wear between parallel laid subropes.
Even in the highest loaded fatigue tests, peak 65% of actual break load, no strength loss occurred on the inner yarns, which confirms no creep rupture strength loss had been sustained. This study has confirmed that creep-rupture induced strength loss is not a dominant mechanism in these high load fatigue tests (short duration compared to platform lifetime) and unlikely to be in mooring lines in service over long term cycling.
A laboratory test program investigated the durability of polyester deepwater mooring ropes. Durability topics include; high cycle fatigue, fatigue degradation mechanisms, cumulative damage, strength loss and short term and long term property behaviour. The work presented in this paper concentrated on high cycle fatigue, degradation mechanisms and strength loss (specific to low minimum load fatigue) on 10, 150 and 250 tonne break load rope samples. For the degradation work, run out samples were carefully removed from midspan, eye and splice and extensively examined to establish residual strength and any fatigue mechanisms.
This paper provides the basis of a new fatigue curve for polyester parallel strand ropes that has been proposed in the draft ISO code(1). Fatigue mechanisms have been qualitatively and quantitatively documented which is a significant addition to the industry technical knowledge.
Rope samples were provided by three rope makers, with each supplier being left to offer its preferred construction, provided they were of zero or low torque. One rope maker supplied ropes of parallel strand construction (PSC), another supplied ropes of parallel braid construction (PBC) and the third rope maker supplied ropes in wire rope construction (WRC).
