Nylon fibre rope moorings for wave energy converters
The wave energy market has traditionally employed mooring concepts adopted from the mature technology of the oil and gas industry. However, this approach may not always be appropriate to satisfy the particular requirements of Wave Energy Converter (WEC) mooring systems, especially the need for very compliant moorings. Specific challenges are: the huge 1st order motion induced forces generated by storm waves in shallow water; the potential for slack-snap loading on a mooring; and, the fatigue performance of the mooring components. This paper describes the benefits of employing high compliance fibre rope in mooring systems of less than 100 m water depth. Nylon fibre ropes are a very attractive proposition because of their low modulus and cost but have previously been thought to have a poor wet fatigue performance. A specially designed long-lay-length Superline subrope has been tested both for compliance and for wet fatigue performance. The rope compliance is about twice that of parallel-subrope construction polyester rope. The fatigue performance is less than that of parallel-subrope polyester rope but is comparable to that of conventional construction polyester rope and to that of the chain and steel wire ropes which must also be used as mooring components. The high compliance of the nylon subrope construction together with the more than adequate fatigue performance demonstrated in this study qualify such nylon ropes as ideal for WEC mooring systems.
Durability of Polyester Ropes Used as Deepwater Mooring Lines
Four failure modes have concerned others as potentially limiting the service life of polyester ropes used as deepwater mooring lines: axial compression fatigue, creep rupture, hysteresis heating, and internal abrasion. Based on extensive investigations and testing, the first three modes are of no concern with polyester rope. Internal abrasion is the only failure mode of possible concern. It is not a problem in large parallel-strand or jacketed wire-rope construction polyester ropes. Under the relatively high mean loads and relatively low amplitude load patterns normally experienced in typical deepwater mooring lines, the strength reduction caused by internal abrasion is negligible. Internal abrasion might be a problem with poorly designed ropes or rope terminations. Thus prototype rope testing is still necessary. Extensive cyclic load testing shows that the cycles-to-failure (ctf) of polyester rope at moderate load ranges is about fifty times the steel rope ctf curve given in API RP-2SM, "Deepwater Synthetic Mooring Line Guidelines". But the slope of the ctf curve for polyester rope in the present edition of that guideline may be nonconservative at very low load ranges. The axial compression fatigue test now specified in API RP2SM is of no value for polyester ropes. The polyester maximum allowable low-tension axial compression fatigue cycles criterial should be eliminated. The creep rupture criteria specified for polyester in RP2SM is meaningless
Combined Allowable Strength Reduction Factor for Geosynthetic Creep and Installation Damage
The current practice to determine the combined geosynthetic strength reduction due to installation damage and creep is to multiply the individual strength reduction factors for damage and creep together. However, it is difficult to know if this procedure results in conservative design, nonconservative design, or is sufficiently accurate for practical purposes. The effect of synergism on the combined effect of polymeric creep and installation damage is explored in the paper by reviewing published and unpublished constant load (creep) data for both undamaged and installation-damaged geosynthetic specimens. A methodology is developed that uses data from both constant load (creep) tests and index tests to reconstruct installation-damaged isochronous creep curves. Based on the limited data available and the interpretation of reconstructed isochronous curves, it is shown that multiplication of creep reduction and installation damage factors may be conservative and hence results in errors on the safe side for de...